BS5950 - Member Design Handbook

CSC Fastrak ™

Structural steelwork analysis and design

.cscworld.com/fastrak

HANDBOOK BS 5950 MEMBER DESIGN

BS 5950 Member Design Handbook page 2

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Friday 7 September 2012 – 15:51

Disclaimer

Disclaimer

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CSC (UK) Ltd does not accept any liability whatsoever for loss or damage arising from any errors which might be contained in the documentation, text or operation of the programs supplied. It shall be the responsibility of the customer (and not CSC) •

to check the documentation, text and operation of the programs supplied,

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to ensure that the person operating the programs or supervising their operation is suitably qualified and experienced,

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to ensure that program operation is carried out in accordance with the user manuals,

at all times paying due regard to the specification and scope of the programs and to the CSC Software Licence Agreement.

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Disclaimer

Table of Contents

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BS5950 Member Design Handbook Chapter 1

Introduction

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Chapter 2

Basic Principles .

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. . . . . . . . . . . . Section classification . Member strength checks .

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . Lateral torsional buckling checks . . Deflection checks . . . . . Web Openings . . . . . Design Properties . . . . . Size Constraints . . . . . Sections for Study (in Fastrak Building Designer) . . Sections for Study (in Simple Beam) . . . . Deflection . . . . . . . . Worked Example . . . . . . . Simple Beam Input (in Fastrak Building Designer) . . Simple Beam Input (when run as a standalone program) Designing a beam . . . . . . . Checking a beam . . . . . . . Further information . . . . . . . Further information – Westok Beams . . . .

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29 29 30 30 30 31 32 32 35 37 39 39 39

Definitions. . Design Method . Deflection checks Error messages .

Chapter 3

Simple Beam

. . . . Steel sections . Web openings .

. . . . . . Restraint conditions . Applied loading . Design checks . . Theory and Assumptions Analysis method . Design method . Introduction Scope . Beam .

Chapter 4

Composite Beam Introduction Scope . Beam .

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Westok sections Westok Technical Support and Design Service Steel sections . . . . . . Web openings . . . . . . Profiled metal decking . . . . Precast concrete slabs . . . . Concrete slab . . . . . . Shear connectors . . . . . Reinforcement . . . . .

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Table of Contents

Fibre Reinforced Concrete . . Construction stage restraint conditions Construction stage loading . . Composite stage loading . . Construction stage design checks . Composite stage design checks .

. . . . . . Theory and Assumptions . . . Analysis method . . . . Design method . . . . . Construction stage . . . . Composite stage . . . . Web Openings . . . . . Theory and Assumptions – Westok beams Construction stage . . . . Composite Stage . . . . Design Aspects . . . . .

. . . . . . . . . . . . . . . . Use of Design Properties to Control Section Selection . Checking the effective width used in the design . . Layout of Studs . . . . . . . . Non-composite design within Composite Beam . . Automatic transverse shear reinforcement design . . Worked Example . . . . . . . . Without transverse shear reinforcement . . . . Design Pass 1 . . . . . . . . Design Pass 2 . . . . . . . . Design Pass 3 . . . . . . . . Composite Beam Input (in Fastrak Building Designer) . Composite Beam Input (when run as a standalone program) Designing a beam . . . . . . . Checking a beam . . . . . . . Further Information . . . . . . . Further information – Bison precast concrete slabs . . Further information – Westok Beams . . . .

Chapter 5

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Simple Column .

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Chapter 6


General Beam

. . . Introduction. . . . Scope . . . . . Limitations and Assumptions.

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Table of Contents

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Limitations . Assumptions .

. . Analysis . . . Building Modeller object . General Beam . . . Ultimate Limit State – Strength Classification . . . Shear Capacity . . Moment Capacity . . Axial Capacity . . . Cross-section Capacity . Ultimate Limit State – Buckling

. . . . . . . . . . . . . . . . . . . . . . . . Lateral Torsional Buckling Resistance, Clause 4.3 . . Lateral Torsional Buckling Resistance, Annex G . . Compression Resistance . . . . . . Member Buckling Resistance, Clause 4.8.3.3.1 . . Member Buckling Resistance, Clause 4.8.3.3.2 . . Member Buckling Resistance, Clause 4.8.3.3.3 . . Serviceability Limit State . . . . . . Member End Fixity and Supports . . . . . General Beam Stand-alone . . . . . Building Designer . . . . . . . Design Procedure . . . . . . . Lateral torsional buckling checks . . . . Combined buckling checks . . . . . Worked Example . . . . . . . Design pass 1 . . . . . . . . Design pass 2 . . . . . . . . Design Pass 3 . . . . . . . . General Beam Input (in Fastrak Building Designer) . . General Beam Input (when run as a standalone program) Designing a beam . . . . . . . Checking a beam . . . . . . .

Chapter 7

General Column

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. . Introduction . . . Scope . . . . Limitations and Assumptions Limitations . . . Assumptions . . . Analysis . . . . Building Modeller Object . Ultimate Limit State – Strength Classification . . . Shear Capacity . . Moment Capacity . . Axial Capacity . . . Cross-section Capacity . Ultimate Limit State – Buckling

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. . . . . . . . . . . . . . . Lateral Torsional Buckling Resistance, Clause 4.3 . Lateral Torsional Buckling Resistance, Annex G . Compression Resistance . . . . . Member Buckling Resistance, Clause 4.8.3.3.2 . Member Buckling Resistance, Clause 4.8.3.3.3 . Serviceability limit state . . . . . Worked Example . . . . . .

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Table of Contents

. . . . . . . . . . . . . . . . . . . . . General Column Input (in Fastrak Building Designer)

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. . Why would you want to refine the original design? . Interaction Effects . . . . . . How to Access Design Refinement . . . . Simple Beam - Check Mode. . . . . . Simple Beam - Design Mode. . . . . Composite Beam - Check Mode . . . . Composite Beam - Design Mode . . . . General Beam - Check Mode . . . . General Beam - Design Mode . . . . General Column - Check Mode . . . . General Column - Design Mode . . . . Effective Use of Order Files in Refined Design . .

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Design pass 1 Design pass 2 Design Pass 3

Chapter 8

Braces .

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. . . Steel sections . End Connections . Applied loading . . Design Forces . . Design checks . . Theory and Assumptions Analysis method . Design method . . Classification . . Axial Tension . . Axial Compression .

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Refining Member Designs

Compression Buckling

Brace Input .

Chapter 9

Chapter 10


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Introduction.

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Chapter 1 : Introduction


BS5950 Member Design Handbook

Chapter 1

Introduction Fastrak Building Designer designs steel members, composite members and connections to a range of international codes. This handbook specifically describes the design methods applied in the software when the BS 5950-1(Ref. 2) and BS 5950-3(Ref. 1) codes are selected. A brief description of the contents follows: Basic Principles — (Chapter 2) terminology and basic principles common to each of the design applications. Simple Beam — (Chapter 3) “non-composite steel beam with pinned ends designed for gravity loads acting through the web” Simple Column — (Chapter 4) ”steel column in a simply designed structure” Composite Beam — (Chapter 5) “composite steel beam with pinned ends designed for gravity loads acting through the web” General Beam — (Chapter 6) “non-composite steel beam designed as a beam/column” General Column — (Chapter 7) “steel column designed as a beam/column” Braces — (Chapter 8) “steel members with pinned ends designed for axial loads only” Refining Member Designs — (Chapter 9) advice to assist you in extracting individual members into each design application for more detailed assessment. References — (Chapter 10) references and further information.


Chapter 2

Chapter 2 : Basic Principles

Basic Principles

Definitions Attributes When a member is first created it’s properties (steel grade, maximum section depth etc.) are taken from the attribute set that is currently active. Once a member has been placed it’s properties can be edited as required. Ensuring the attribute set is correct before placement ensures the minimum amount of member editing.

Design Mode Within Building Designer you can access the member design routines automatically for every member in the building model to choose the smallest section from a list of sections (referred to in the program as an order file)

Check Mode Alternatively you can access the member design routines to check the section size already assigned by you to each member, to determine whether it is able to carry the applied loading.

Order Files Each order file is a list of section sizes of a given type arranged in the sequence in which they will be tried during the design. Undesirable sections can be excluded if required. Caution

If you exclude sections from an order file they will remain excluded for all designs until you decide to include them again.

Interactive Design Within Building Designer you can also extract key members from the model into the appropriate design program for further investigation in either Design Mode or Check Mode, providing you with still greater control over the design: • to enable multiple order files to be considered at the same time to determine a list of alternative sections, all of which can withstand the applied loading.

• to adjust the initial design manually without having to re-design the whole building. Any change to the section size or steel grade can then be passed back to the building model, but only affects the individual beam extracted. If the changes are to be applied to other beams also, you would need to update the building model separately and then re-design it. For further details see “Refining Member Designs”

Design Method Unless explicitly stated all calculations in Building Designer will be consistent with the design parameters as specified in BS 5950-1:2000(Ref. 1).

Chapter 2 : Basic Principles


Deflection checks Building Designer calculates both relative and absolute deflections. Relative deflections measure the internal displacement occurring within the length of the member and take no account of the support settlements or rotations, whereas absolute deflections are concerned with deflection of the structure as a whole. The absolute deflections are the ones displayed in the structure deflection graphics. The difference between relative and absolute deflections is illustrated in the cantilever beam example below.

Relative Deflection

Absolute Deflection

Relative deflections are given in the member analysis results graphics and are the ones used in the member design.

Error messages As you define member data, Fastrak Building Designer continually checks to ensure that the data is valid. If a particular value is not valid, then it will be shown using a colour of your choice in the dialog (default red). If a value is not recommended, then a different colour will be used in the dialog, (default orange for ‘warning’). If you allow the cursor to rest over the error or warning field you will see a tip telling you the acceptable range of input. Until all the information within the dialog is valid (but not free of warnings) you will not be able to save the dialog since OK will be dimmed. Although checking in this way prevents you from defining invalid data there are some cases where particular errors occur that cannot be trapped - for instance where an error occurs due to inconsistencies that have arisen between information covered on different dialogs. In these cases when you attempt to perform a design you will see an error message indicating that data is not suitable for the design to proceed. Each message is self-explanatory. You should take a careful note of the error message and then change the member data to correct the problem. If there are other problems with the design, then you will see a series of warning messages in the results viewer. You should take note of any such warnings and take the action that you deem appropriate. Engineering tips are also available in the results viewer which may give you useful information about the assumptions or approach adopted for the particular calculation or about a particular recommendation of good practice with which we recommend that you comply.


Chapter 3

Chapter 3 : Simple Beam

Simple Beam

Introduction SIMPLE BEAM - “non-composite steel beam with pinned ends designed for gravity loads acting through the web” The Simple Beam design application allows you to analyse and design a structural steel beam or cantilever which may have incoming beams providing restraint, and which may or may not be continuously restrained over any length between restraints. Simple Beam can determine the sizes of member which can carry the forces and moments resulting from the applied loading. Alternatively you may give the size of a beam and Simple Beam will then determine whether it is able to carry the previously mentioned forces and moments and satisfy the deflection requirements. Unless explicitly stated all calculations in Simple Beam are in accordance with the relevant sections of BS 5950-1:2000(Ref. 2).

Scope The scope of the Simple Beam application is as follows:

Beam The beam is designed for gravity loads acting through the web only. Minor axis bending and axial loads are not considered. Note

If either minor axis bending or axial loads exist which exceed the limit below which they can be ignored, a warning will be given in the beam design summary.

Steel sections Simple Beam can handle design for an international range of steel I-sections for many different countries. Plated sections can also be checked.

Web openings If you need to provide access for services, etc., then you can add openings to a designed beam and Simple Beam can then check these for you. You can define rectangular or circular openings and these can be stiffened on one, or on both sides. The checks that are performed are in accordance with the guidelines and design process given in the booklet Design for openings in the webs of simple beams.



We advise you to comply with the following positional recommendations for web openings: • Web openings are designed using the bending moment and vertical shear values at the side of the opening where the moment is lower,

• Openings should preferably be positioned at the mid-height of the section. If not, the depth of the upper and lower sections of web should differ by not more than a factor of two,

• Openings should not be located closer to the support than two times the beam depth or 10% of the span whichever is the greater,

• The best location for any opening is between 1/5 and 1/3 of the span from a support in uniformly loaded beams, or in lower shear zone of beams subject to point loads,

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Openings should be not less than the beam depth, D, apart, Unstiffened openings should not generally be deeper than 0.6D or longer than 1.5D, Stiffened openings should not generally be deeper than 0.7D or longer than 2D, Point loads should not be applied at less than D from the side of the adjacent opening.

You cannot currently automatically design sections with web openings, you must perform the design first to get a section size, and then add and check the openings. This gives you complete control of the design process, since you can add appropriate and cost effective levels of stiffening if required, or can choose a different beam with a stronger web in order to reduce or remove any stiffening requirement. Web openings can be added to a beam by a 'Quick-layout' process or manually. The 'Quick-layout' process, which is activated using the check box on the Web Openings dialog page, adds web openings which meet the geometric and proximity recommendations given above and in SCI Publication P068. The openings so created are the maximum depth spaced at the minimum centres recommended for the beam section size. Web openings can be defined manually in two ways from the Web Openings dialog page. With the Quick-layout check box unchecked, the ‘Add’ button adds a new line to the web openings grid to allow the geometric properties of the web opening to be defined, or alternatively, use of the ‘Add...’ button opens the Web Opening Details dialog page which gives access to more help and guidance when defining the opening. Both methods make use of 'Warning' and 'Invalid' text for data entry checks [the default colours being orange and red respectively] to provide assistance as the opening parameters are defined. On the Web Opening Details dialog page, the Centre button will position the opening on the beam centre whilst the Auto button will position the opening to meet the spacing recommendations given above and in P068. Also on this page tool tips give information on the recommended values for all the opening parameters. As web openings are defined, they are immediately visible in the diagram on the Web Openings dialog page. This diagram displays the results of the geometric and proximity checks that are carried out on the opening parameters using 'Warning' and 'Invalid' display colours to highlight those areas that are outside the recommended limits.



A typical display is shown below:

The areas that are subjected to the checks are end posts, web posts, web opening dimensions and tee dimensions. Using the above example, it can be deduced that:

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The left hand end post is less than the recommended limiting value WO #1 diameter is within the recommended limiting values Internal Web Post #2 is within the recommended limiting values WO #2 dimensions are outside the recommended limiting values Internal Web Post #3 is less than the recommended limiting value but quite close to the limit. As the web post dimension reduces, the left and right triangles overlap to a greater degree at their apexes.

• WO #3 dimensions are invalid and must be adjusted to progress the definition of the opening.

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Internal Web Post #4 is within the recommended limiting values WO #4 dimensions are within the recommended limiting values Internal Web Post #5 is within the recommended limiting values WO #5 dimensions are within the recommended limiting values but the dimensions of the tee(s) are not.

This display helps you to decide whether to make any adjustments to the opening parameters before their design is checked. You should bear in mind that the checks carried out at this stage are geometric checks only and compliance with recommended limits is no guarantee that the opening will pass the subsequent engineering design checks.



Note

Adjustment to deflections. The calculated deflections are adjusted to allow for the web openings. See: Deflection checksin the Basic Principles section.

Note

Dimensional checks. The program does not check that openings are positioned in the best position (between 1/5 and 1/3 length for udl’s and in a low shear zone for point loads). This is because for anything other than simple loading the best position becomes a question of engineering judgment.

Restraint conditions If you need to check the lateral torsional buckling of the beam you can: • define the degree of fixity that the end connections are able to provide and hence an effective length associated with the support,

• position additional restraints at any point along the beam (Simple Beam automatically uses 1.0L and 1.2L as the factors for Normal and Destabilizing loads), Help

For a definition of Destabilizing Loads see BS 5950-1:2000 clause 4.3.4.

• Simple Beam automatically takes the average of the effective length factors for differing supports, or between those for the support and the adjacent sub-beam.

• alternatively you can specify the factors that you want to use for the lengths between restraints, or you can enter the effective length of the sub-beam directly by entering a value (in m).

• specify that any length (or lengths) of the beam should be taken as being fully restrained against lateral torsional buckling, independent of the restraint conditions for the adjacent length(s).

Applied loading You can specify a wide range of applied loading for the simple condition: • uniform distributed loads (over the whole or part of the beam),

• point loads, • varying distributed loads (over the whole or part of the beam), • trapezoidal loads. Design checks When you use Simple Beam to design or check a beam the following conditions are examined in accordance with BS 5950-1:2000: • section classification (Clause 3.5.2),

• shear capacity (Clause 4.2.3), • moment capacity: • (Clause 4.2.5.2 for the low shear condition • Clause 4.2.5.3 for the high shear condition), • lateral torsional buckling resistance (Clause 4.3.6) • web openings, • total load deflection check.



Theory and Assumptions This section describes the theory used in the development of Simple Beam and the major assumptions that have been made, particularly with respect to interpretation of BS 5950-1:2000.

Analysis method Simple Beam uses a simple analysis of a statically determinate beam to determine the forces and moments to be resisted by the beam.

Design method The design methods employed to determine the adequacy of the section for each condition are those consistent with BS 5950-1:2000 unless specifically noted otherwise.

Section classification Cross-section classification is determined using Table 11 and Clause 3.5. The classification of the section must be Plastic (Class 1), Compact (Class 2) or Semi-compact (Class 3). Sections which are classified as Slender (Class 4) are beyond the scope of Simple Beam. Note

Asymmetric Slimflor beams (ASB) For all section types flange classification is only performed for the top flange, because for a simple beam this will be the flange in compression. However, in the case of a cantilever beam the bottom flange goes into compression. Hence for a cantilever beam, for the flange classification to be valid the section must be symmetric about the major axis. As a consequence ASB sections must NOT be specified for cantilever beams.

Member strength checks Member strength checks are performed at the point of maximum moment, the point of maximum shear, the position of application of each point load, and at each side of a web opening as well as all other points of interest along the beam. Shear capacity — is determined in accordance with Clause 4.2.3. Where the applied shear force exceeds 60% of the capacity of the section, the high shear condition applies to the bending moment capacity checks (see below). Bending moment capacity — is calculated to Clause 4.2.5.2 (low shear at point) or Clause 4.2.5.3 (high shear at point) for plastic, compact and semi-compact sections.

Lateral torsional buckling checks BS 5950-1:2000 states that lateral torsional buckling checks are required when any length is not continuously restrained. Simple Beam allows you to switch off these checks by specifying that the entire length between the supports is continuously restrained against lateral torsional buckling.



If you use this option you must be able to provide justification that the beam is adequately restrained against lateral torsional buckling. When the checks are required you can position restraints at any point within the length of the main beam and can set the effective length of each sub-beam (the portion of the beam between one restraint and the next) either by giving factors to apply to the physical length of the beam, or by entering the effective length that you want to use. Each sub-beam which is not defined as being continuously restrained is checked in accordance with clause 4.3.6 and Annex B of BS 5950-1:2000.

Deflection checks Simple Beam calculates relative deflections. (see “Deflection checks”in the Theory and Assumptions section of this chapter.) The ‘Service Factor’ (default 1.0), specified against each load case in the combination is applied when calculating the deflections; the following deflections are available:

• dead load deflections, • imposed load deflections, • total load deflection i.e. the sum of the previous items. Deflection limits can be specified to each of the above, as a fraction of the span, or as an absolute limit, (or both). Web Openings The deflection of a beam with web openings will be greater than that of the same beam without openings. This is due to two effects,

• the reduction in the beam inertia at the positions of openings due to primary bending of the beam,

• the local deformations at the openings due to Vierendeel effects. This has two components - that due to shear deformation and that due to local bending of the upper and lower tee sections at the opening. The primary bending deflection is established by 'discretising' the member and using a numerical integration technique based on 'Engineer's Bending Theory' - M/I = E/R = /y. In this way the discrete elements that incorporate all or part of an opening will contribute more to the total deflection. The component of deflection due to the local deformations around the opening is established using a similar process to that used for cellular beams which is in turn based on the method for castellated beams given in the SCI publication, “Design of castellated beams. For use with BS 5950 and BS 449". The method works by applying a 'unit point load' at the position where the deflection is required and using a 'virtual work technique to estimate the deflection at that position. For each opening, the deflection due to shear deformation, s, and that due to local bending, bt, is calculated for the upper and lower tee sections at the opening. These are summed for all openings and added to the result at the desired position from the numerical integration of primary bending deflection.



Note that in the original source document on castellated sections, there are two additional components to the deflection. These are due to bending and shear deformation of the web post. For castellated beams and cellular beams where the openings are very close together these effects are important and can be significant. For normal beams the openings are likely to be placed a reasonable distance apart. Thus in many cases these two effects will not be significant. They are not calculated for such beams but in the event that the openings are placed close together a warning is given. This will indicate that these effects on the deflection of the beam are not taken into account. This warning is issued when, so < 2.5 * do for rectangular openings so <1.5 * o for circular openings Where so=the clear length of web between adjacent openings do=the depth of a rectangular opening taken as the larger if the adjacent openings differ o=the diameter of a circular opening

Web Openings Circular Openings as an Equivalent Rectangle Each circular opening is replaced by equivalent rectangular opening, the dimensions of this equivalent rectangle for use in all subsequent calculations are: do'= 0.9*opening diameter lo = 0.45*opening diameter Properties of Tee Sections When web openings have been added, the properties of the tee sections above and below each opening are calculated in accordance with Section 3.3.1 of SCI P355(Ref. 10) and Appendix B of the joint CIRIA/SCI Publication P068(Ref. 5). The bending moment resistance is calculated separately for each of the four corners of each opening. Design Checks The following calculations are performed where required for web openings:

• • • • • • • • •

Axial resistance of tee sections Classification of section at opening Vertical shear resistance Vierendeel bending resistance Web post horizontal shear resistance Web post bending resistance Web post buckling resistance Lateral torsional buckling Deflections



Design Properties The Design Properties button provides a means by which you can both speed up the design process and control the design more precisely. Note

When you extract a beam from a Fastrak Building Designer model into Simple Beam for further investigation, Design Properties are accessed via the Design Wizard icon.

Size Constraints Size Constraints are only applicable when in Design Mode. They allow you to ensure that the sections that Simple Beam proposes match any particular size constraints you may have.

Sections for Study (in Fastrak Building Designer) This feature is only applicable when running the program in Design Mode. On the left of the page is a list of available order files, only one of which can be selected. The sections contained within the chosen order file appear in the Section Designation list on the right of the page. Only checked sections within this list are considered during the design process.

The design process commences by starting with the smallest section in the chosen order file. Any section that fails any of the design conditions is rejected and the design process is then repeated for the next available section in the list. On completion of the design process, the first satisfactory section from the Section Designation list is assigned to the beam. Caution

Limiting the choice of sections by unchecking a section within an order file is a global change that affects ALL projects, (not just the currently open one). It is typically used therefore to eliminate unavailable or non-preferred sections from the design process. If design requirements for an individual beam require section sizes to be constrained, (due to, for example depth restrictions), then the choice of sections should be limited instead by using Size Constraints, (as these only affect the current beam).


Note


It is possible to create additional order files using a text editor. If you require to do so, please contact your local CSC Technical Support Team for guidance.

Sections for Study (in Simple Beam) When you extract a beam from a Fastrak Building Designer model into Simple Beam for further investigation, a benefit of doing so is that several order files can be considered at the same time. If a check is placed against an order file the sections contained within it appear in the Section Designation list on the right of the page. Only checked sections within this list are considered during the design process.

Typically, you would uncheck those order files that are unlikely to be appropriate for simple beam design, Doing so speeds up the solution. The design process commences by starting with the smallest section in each order file. Any section that fails any of the design conditions is rejected and the design process is then repeated for the next available section in the list. On completion of the design, all the satisfactory sections from the Section Designation list are displayed and the results for each of these can be examined before one of the sections is assigned to the beam. Caution

Limiting the choice of sections by either unchecking an order file or an individual section is a global change that affects ALL projects, (not just the currently open one). It is typically used therefore to eliminate unavailable or non-preferred sections from the design process. If design requirements for an individual beam require section sizes to be constrained, (due to, for example depth restrictions), then the choice of sections should be limited instead by using Size Constraints, (as these only affect the current beam).

Note

It is possible to create additional order files using a text editor. If you require to do so, please contact your local CSC Technical Support Team for guidance.



Deflection The Deflections page allows you to control the amount of deflection by applying either a relative or absolute limit to the deflection under different loading conditions.

A typical application of these settings might be: • to apply the relative span/360 limit for imposed load deflection, to meet code requirements,

• possibly, to apply an absolute limit to the total load deflection to ensure the overall deflection is not too large.

Worked Example If you want to work through this example you will find the file Engineer’s Example in the \documents and settings \ All Users \Application Data\CSC\Fastrak\ Examples folder. You can open and use this file, but you can not save

it away unless you change its name, this is done to protect the original. Let’s take a simple example of a 9 m span spine beam with 6 m span secondary beams at third points.

The floor loading is:

Condition

Value

giving point load at 3 m and 6 m of

Dry Slab

2.0 kN/m2

36kN

Services

1.0 kN/m2

18kN

Live load

5.0 kN/m2

90kN



Design Pass 1 If you run a design you will find that Simple Beam shows a dialog of acceptable sections. If no one has tailored the sections that Simple Beam investigates, then the list will appear as below.

If you move down the list of Available files, you will see all the Section Designations that can carry the applied loading. These are only the ones that pass the design, Simple Beam has tried all the sections in each of the Available files, to determine the acceptable ones. You may have noticed the different section designations in the progress bar as the design ran. However checking all these sections comes at a price, the more sections there are to investigate, the longer the design takes. Simple Beam allows you to choose just the sections you want to include for the design through its Design Wizard.



Design Pass 2 Remove the tick against all the Available files whose section types you don’t want to investigate, and Simple Beam won’t look at any of these sections during the design process. If you remove the tick against all the Available files other than UBBeamOrder.Eur, and then re-perform the design you will find a significant increase in speed as Simple Beam only investigates the universal beams.

Furthermore Simple Beam investigates the sections in the order that they appear in the Section Designation list. If you scroll down many of the lists, you will find that there is a point at which larger sections give way to smaller ones again.

We have ordered the Section Designation list based on our many years experience of the industry, the sections at the top of the list are the ones we know you prefer to use, whilst those at the bottom are those which you use less frequently if at all. By default all the Section Designations are ticked, but you might want to remove the ticks against some or all of the non-preferred sections. Again this will speed the design process. You may also have other requirements specific to your own company, for instance you may never want to use sections with flanges less than 150 mm wide for erection purposes. If you remove the tick against these section sizes, then Simple Beam will never include them when it is performing a design. Thus you are controlling the design, making Simple Beam look at just the section designations you are likely to accept, and in the process speeding up the design itself.



Design Pass 3 With the tick removed against all the non-preferred sections, and all sections with flanges less than 150 wide, Simple Beam only has to check around 20 sections and the design is instantaneous.

Simple Beam maintains the Sections for Study settings that you make, until you choose to change them again. It is therefore worthwhile taking the time to tailor the list so that Simple Beam picks sections of which you are likely to approve during its designs.



Simple Beam Input (in Fastrak Building Designer) In order to create a simple beam within Fastrak Building Designer, you will first need to define an appropriate set of simple beam attributes. Listed below is the typical procedure for defining these attributes. Items in brackets [] are optional.

Step

Dialog

Page

Instructions

1

none

none

Create a new Beam Attribute Set.

2

Attribute Set

General

Give the Attribute Set a Title

3

Attribute Set

Design

Choose Simple construction type

4

Attribute Set

Design

Check the Automatic Design box if Design Beam Mode is required, else leave it unchecked to work in Check Beam Mode

5

Attribute Set

Design

[Check the Gravity Only Design box* if required]

6

Attribute Set

Design

Click the Design Properties button

7

Beam Design Properties

Size Constraints

[Define the Beam Constraints: • max and min beam size]

8


Sections for Study

If in Design Beam Mode choose the Order File

9


Deflection

Define and apply deflection limits • [dead] • imposed • [total]

10

Attribute Set

Alignment

[No changes are applicable for simple beams]

11

Attribute Set

Type

[Check the Fully Restrained box if required]

12

Attribute Set

Supports

For simple beams, simple connections are required at both ends. For cantilevers, one end must be fully fixed and the other must be free.

13

Attribute Set

Size

Choose the steel grade and, if in Check Beam Mode choose the section size

14

Attribute Set

Restraints

Define the restraint details. Note, this page is not visible if the beam is fully restrained.

*In order to speed the design process a distinction is made between those combinations consisting of gravity loads only and those which contain some components acting laterally (e.g. notional loads and wind loads). Setting simple beams to be designed for gravity loads only can significantly reduce the design time.



Simple Beam Input (when run as a standalone program) The design and check mode input procedures are listed below. Items in brackets [] are optional

Designing a beam Step

Icon

Instructions

15

Launch Simple Beam,

16

Create a new project giving the project name [and other project details],

17

Choose the type of beam as either a Simple Beam or a Cantilever Beam [and give the beam reference details],

18

Set Simple Beam into design beam mode,

19

Define the properties for the beam: • grade; • span.

20

Give the details of the beam restraints.

21

Define the loadcases that apply to the simple beam.

22

Incorporate the loadcases into a series of design combinations,

23

[Make any Design Wizard settings that you want to use to control the design.]

24

Perform the design

25

From the list of suitable sections preview the results for the more desirable sections and then choose the one that you would like to use,

26

Add in any web openings that you need to allow access for services etc.

27

Check the beam with the web openings. [Stiffen the web openings if necessary, or increase the size of the beam until the beam with openings is satisfactory.]

28

Specify the content of the report [and print it].

29

Save the project to disk.



Checking a beam In the typical procedure below items in brackets [] are optional.

Step

Icon

Instructions

1

Launch Simple Beam,

2


3

Choose the type of beam as either a Simple Beam or a Cantilever Beam [and give the beam reference details],

4

Set Simple Beam into check beam mode,

5

Define the properties for the beam: • section size, • grade, • span,

6


7


8

Define the loadcases that apply to the simple beam.

9


10


11

Perform the check, (including any web openings),

12

[Stiffen the web openings if necessary, or increase the size of the beam until the beam with openings is satisfactory.]

13


14




Further information Further information – Westok Beams

For further information or technical literature on Westok Beams please contact Westok Technical Support and Design Service. Westok Structural Services Ltd. Horbury Junction Industrial Estate Horbury Junction Wakefield WF4 5ER Tel: 44-1924 264 121 Fax: 44-1924 280 030 email: [email protected]. You can also view this information while running the program by choosing Help / About Westok Structural Services Ltd… which shows the About Westok Structural Services Ltd. dialog.

You can click on the email link on this dialog to create a new email message to Westok.

Chapter 4 : Composite Beam

Chapter 4


Composite Beam

Introduction COMPOSITE BEAM - “composite steel beam with pinned ends designed for gravity loads acting through the web” The Composite Beam design application allows you to analyse and design a structural steel beam acting compositely with a concrete slab. This slab may be created either by using profile steel decking or by using Bison precast concrete slabs. Composite Beam can determine the size of member which: • acting alone are able to carry the forces and moments resulting from the Construction Stage,

• acting compositely with the slab using profile steel decking or with precast concrete slabs (with full or partial interaction) are able to carry the forces and moments at the Ultimate Limit State,

• acting compositely with the slab using profile steel decking or with precast concrete slabs (with full or partial interaction) are able to provide acceptable deflections, service stresses and natural frequency at the Serviceability Limit State. A list of those sections meeting the above requirements is displayed from which you may prefer to choose a slightly heavier beam with less studs, or a simpler layout of studs, in order to provide a more economical solution, or one that is easier to construct. Alternatively you may give the size of a beam and Composite Beam will then determine whether it is able to carry the previously mentioned forces and moments and satisfy the Serviceability Limit State. Additionally you can also use Composite Beam to check any web openings that are necessary, stiffening them where needed to attain an acceptable result. Unless explicitly stated all calculations in Composite Beam are in accordance with the relevant sections of BS 5950-3.1:1990+A1:2010(Ref. 1). You may find the handbook and commentary to the Code of Practice published by the Steel Construction Institute (Ref. 3 and 4) useful.



Scope The scope of Composite Beam is described in this section:

Beam You can specify and design any simply supported composite beam with the design span taken as that defined in BS 5950-1:2000.

Westok sections

You can either check or design Westok sections in Composite Beam. The top and bottom part of the section can be formed from different beam section sizes, although they must be of the same grade. Help

Westok provide a design service to assist you with designs. For further information see: .’Westok Technical Support and Design Service’

The following points are worthy of note: • If the resistance of the web post is insufficient this will yield a Fail status. To overcome this your best option is to manipulate the cell data, or to make a more effective choice of the parent sections. Caution

Although you can use stiffeners to overcome this we would encourage you not to use this option. Please refer to Westok literature which shows these requirements diagramatically. If you do choose to use stiffeners then you should be aware that there is no design, sizing or graphical representation of these.

• If you define a point load within 0.45Ro of the centre-line of a cell, then the maximum shear at either side of the point load is checked against the shear resistance of the net section. If the shear resistance is inadequate you will be told that a filler plate is required. The filler plate is not checked since the adjacent web post resists the same (or similar) shear and the checks on this will confirm the adequacy of the full web. Note

The program does not check the shear resistance of the minimum upper tee section alone. If you are concerned about this you will need to produce additional hand calculations. Automated calculations for this condition could easily be produced in Tedds.

• In this release of the program you cannot define Westok sections which have different steel grades for the top and bottom section.

• When Composite Beam is calculating the properties of Westok sections the root radii are ignored.



• The size and position of the cells that you define should comply with the following limits to ensure that the design model is valid and for practical reasons (see Westok literature). The limits are generally:

• 1.08  S / Do  1.5 and 1.25  D / Do  1.75. • 1.5 < S / Do  1.8 is also allowed following the findings of the latest research program. • If you do not wish to use the Autospace facility, or accept the Composite Beam defaults

you should ensure that your cells comply with the limit 0.7  Do / h  1.3. You should therefore set the value Do to be the depth of the shallower source beam, hmin, at a pitch of 1.5 Do.

• The design model assumes that the beam spans from centre-line to centre-line (BS 5950-1:2000). However in order to check the web posts or cells Composite Beam needs to know how this relates to the physical length of the beam. You need to define this by giving offsets from the centre-line at each end of the beam to its physical end. These offsets are shown in the Analysis Results window. All shears, moments etc. can thus be taken directly from the analysis results for the span as a whole.

Design span c/c Reaction End shear for physical beam

Shear force diagram Note: the program graphics show the complete beam (design span) with the offset shown as solid.

Beam on span Beam setback

Physical beam Note

When you apply loading you need to position this based on the Design span and not on the Physical beam.

Westok Technical Support and Design Service Cellular beams are not manufactured to standard sizes in the same way as castellated beams. All sections are manufactured to the details given by the specifier to meet the requirements of each particular project.



To help specifiers Westok provide a comprehensive design and advisory service completely free of charge or obligation. If you would like a Westok Engineer to provide a cellular beam design, or require technical support on any matter concerning cellular beams then please contact Westok. Help

For further details see: ’Further information – Westok Beams’.

Steel sections Composite Beam can handle design for an international range of steel I-sections for many different countries and also for many specific manufacturers. Plated sections can also be checked. If required the section can be precambered to counteract the effects of dead load on the deflection of the beam. Caution

If you want to use Bison precast concrete slabs, then you should ensure that a minimum width of flange of 133 mm is achieved to allow sufficient bearing for the slabs (50 mm) whilst still allowing a reasonable gap for stud welding and concrete infilling.

Web openings If you need to provide access for services, etc., then you can add openings to a designed beam and Composite Beam can then check these for you. You can define rectangular or circular openings and these can be stiffened on one, or on both sides. The checks that are performed are in accordance with the guidelines and design process given in the publication Design for openings in the webs of composite beams(Ref. 5). We advise you to comply with the following positional recommendations for web openings: • Web openings are designed using the bending moment and vertical shear values at the side of the opening where the moment is lower,

• Openings should preferably be positioned at the mid-height of the section. If not, the depth of the upper and lower sections of web should differ by not more than a factor of two,

• Openings should not be located closer to the support than two times the beam depth or 10% of the span whichever is the greater,

• The best location for any opening is between 1/5 and 1/3 of the span from a support in uniformly loaded beams, or in the lower shear zone of beams subject to point loads,

• • • •

Openings should be not less than the beam depth, D, apart, Unstiffened openings should not generally be deeper than 0.6D or longer than 1.5D, Stiffened openings should not generally be deeper than 0.7D or longer than 2D, Point loads should not be applied at less than D from the side of the adjacent opening.



You cannot currently automatically design sections with web openings, you must perform the design first to get a section size, and then add and check the openings. This gives you complete control of the design process, since you can add appropriate and cost effective levels of stiffening if required, or can choose a different beam with a stronger web in order to reduce or remove any stiffening requirement. Web openings can be added to a beam by a 'Quick-layout' process or manually. The 'Quick-layout' process, which is activated using the check box on the Web Openings dialog page, adds web openings which meet the geometric and proximity recommendations given above and in SCI Publication P068. The openings so created are the maximum depth spaced at the minimum centres recommended for the beam section size. Web openings can be defined manually in two ways from the Web Openings dialog page. With the Quick-layout check box unchecked, the ‘Add’ button adds a new line to the web openings grid to allow the geometric properties of the web opening to be defined, or alternatively, use of the ‘Add...’ button opens the Web Opening Details dialog page which gives access to more help and guidance when defining the opening. Both methods make use of 'Warning' and 'Invalid' text for data entry checks [the default colours being orange and red respectively] to provide assistance as the opening parameters are defined. On the Web Opening Details dialog page, the Centre button will position the opening on the beam centre whilst the Auto button will position the opening to meet the spacing recommendations given above and in P068. Also on this page tool tips give information on the recommended values for all the opening parameters. As web openings are defined, they are immediately visible in the diagram on the Web Openings dialog page. This diagram displays the results of the geometric and proximity checks that are carried out on the opening parameters using 'Warning' and 'Invalid' display colours to highlight those areas that are outside the recommended limits.



A typical display is shown below:

The areas that are subjected to the checks are end posts, web posts, web opening dimensions and tee dimensions. Using the above example, it can be deduced that:

• • • • •

The left hand end post is less than the recommended limiting value WO #1 diameter is within the recommended limiting values Internal Web Post #2 is within the recommended limiting values WO #2 dimensions are outside the recommended limiting values Internal Web Post #3 is less than the recommended limiting value but quite close to the limit. As the web post dimension reduces, the left and right triangles overlap to a greater degree at their apexes.

• WO #3 dimensions are invalid and must be adjusted to progress the definition of the opening.

• • • •

Internal Web Post #4 is within the recommended limiting values WO #4 dimensions are within the recommended limiting values Internal Web Post #5 is within the recommended limiting values WO #5 dimensions are within the recommended limiting values but the dimensions of the tee(s) are not.

This display helps you to decide whether to make any adjustments to the opening parameters before their design is checked. You should bear in mind that the checks carried out at this stage are geometric checks only and compliance with recommended limits is no guarantee that the opening will pass the subsequent engineering design checks.



Note

Dimensional checks. The program does not check that openings are positioned in the best position (between 1/5 and 1/3 length for udl’s and in a low shear zone for point loads). This is because for anything other than simple loading the best position becomes a question of engineering judgment or is pre-defined by the service runs.

Note

Adjustment to deflections. The calculated deflections at both construction stage and composite stage are adjusted to allow for the web openings. See: ’Web Openings’in the Theory and Assumptions section.

Note

Westok beams. You cannot define other web openings when you are using Westok beams.

Profiled metal decking A wide range of profiled steel decking from all current UK manufacturers and some international ones is included. You may define the profiled metal decking to span at any angle between 0° (parallel) and 90° (perpendicular) to the direction of span of the steel beam. You can also specify the attachment of the decking for parallel, perpendicular and angled conditions giving edge distances for studs and the positions of any laps where these are known. Where you specify that the direction of span of the profiled metal decking to that of the steel beam is >=45°, then there is no need to check the beam for lateral torsional buckling during construction stage. Where you specify that the direction of span of the profiled metal decking to that of the steel beam is 45°, then you are given the opportunity to check the steel beam for lateral torsional buckling at the construction stage. Note

This check is not mandatory in all instances. For a particular profile, gauge and fixing conditions etc. you might be able to prove that the profiled metal decking is able to provide a sufficient restraining action to the steel beam until the concrete hardens. If this is so, then you can specify that the whole beam (or a part of it) is continuously restrained. If you do need to check the beam for lateral torsional buckling during construction then this is in accordance with the requirements of BS 5950-1:2000(Ref. 2).

Where you specify that the direction of span of the profiled metal decking and that of the steel beam are parallel, then you must check the steel beam for lateral torsional buckling at the construction stage. Longitudinal shear and decking The factors that influence the Longitudinal Shear capacity of your composite beam are: • concrete strength, slab depth and slab width – you can not change these independently for the longitudinal shear check, since they apply equally to the entire composite beam design,

• the attachment (or lack of attachment) of the decking and the assumed position of the lap (which applies only to certain configurations),

• the areas of Transverse and Other reinforcement which you provide in your beam.



Attachment of decking and lap position There are six separate cases which are detailed in the following table:

Beam Type

Decking angle Perpendicular Comment

Comment

• Not applicable. The worst lap position i.e. zero distance to lap is assumed and can not be changed.

• Discontinuous but effectively attached, • default edge distance 30 mm.

Angled Comment

• Discontinuous but effectively attached, • default edge distance 30 mm.

“Discontinuous and not effectively attached” would be a more onerous condition than the default.

Parallel Internal

Default setting

The comments for perpendicular and parallel decking angles above apply to the angled condition.

• Discontinuous not effectively attached. If you Perpendicular Comment Edge

choose the effectively attached option the edge distance is set to 30 mm.

Details in publications show the decking continuing at least to the edge of the beam. So although not the most conservative setting, effectively attached is the most practical and most likely.

Parallel

• Not effectively attached.

Angled

• Not effectively attached.

Reinforcement in slab The defaults are: • Transverse – T10 @ 200,

• Other – A142 Mesh.



Precast concrete slabs You may define floors which use Bison Solid and Hollow Core precast concrete slabs. You may also choose the depth of slab that you want to use. The advice that is given below refers equally to both types of Bison slab. You can also access safe-load tables equivalent to those provided by Bison as you are defining the precast concrete slabs. This is an invaluable aid to determining the appropriate type and thickness of precast concrete slab. The self-weights are also included for the precast concrete unit itself – these should be increased by 5% when specifying loading at construction or composite stage to allow for the infill concrete. The design strength of the infill concrete will always be set to 30 N/mm2 when you are using precast concrete slabs, this is a minimum requirement, concrete of greater strength can not be used. The modular ratios are defaulted to values appropriate to grade 30 concrete. You may change these if you can justify any alternative values. The overall slab depth that you specify must also comply with the recommendations given by Bison. The depth of the slab with or without topping is limited to a minimum of 150 mm and a maximum of 250 mm to assure the validity of the design rules. The effective width of the concrete flange must be limited to the minimum of span/4 and 1000 mm for internal beams and span/8 and 500 mm for edge beams. Ongoing research may enable wider widths to be used in the future and these will be included as and when appropriate. Since the use of precast concrete slabs produces a continuous trough along the beam you will find that diagrams in Composite Beam show this trough as parallel to the beam. However the use of precast concrete slabs does not usually require the steel beam to be checked for lateral torsional buckling at the construction stage and this is automatically catered for. For beams with a span greater than 9 m you need to give careful consideration to the construction sequence.

Typical precast concrete slab details



Typical precast concrete slab details (Continued)

Note

See also ’Shear connectors’ and ’Reinforcement’ for special requirements when using Bison precast concrete slabs.

Tip

In order to meet the detailing requirements for minimum bearing and a minimum gap of 65 mm when in design mode set the Beam Size Constraints – Minimum width to 133 mm.

Caution

It is advisable that the position of the plastic neutral axis is such that the majority of the stud is in compression. This is best achieved by investigating the results (Moment, Plastic moment capacity details) and ensuring that the plastic neutral axis is no higher than 50 mm above the top of the beam flange.



Concrete slab You can define concrete slabs in both normal and lightweight concrete provided that you comply with the following constraints: • the slab depth must be between 90 and 500 mm,

• the concrete cube strength must be between 25 N/mm2 and 100 N/mm2 for normal or lightweight concrete. If you are defining an edge beam you can specify the projected distance from the centre-line of the beam to the free edge of the slab up to a maximum of 300 mm. If you use this facility then your construction details will need to justify this.

Shear connectors The shear connection between the concrete slab and the steel beam may be achieved by using normal studs or Hilti™ connectors. For Hilti™ connectors the ultimate strengths used in the design are taken from tables published by Hilti Corporation, FL-9494 Schaan, Principality of Liechtenstein, (Group Headquarters, R & D and Manufacturing). Note To enable the full strength of a Hilti connector to be achieved the minimum distance to the edge of the sheet must be at least 15 mm (3 times the diameter of the shot fired pin). The program assumes that this dimensional constraint is met and uses the full design strength of the connector in the calculations. For other studs you can choose whether the ultimate strengths used in the design are to be those taken from Table 5 of BS 5950-3.1:1990+A1:2010 or those taken from tables published by T.R.W Nelson. Alternatively, if you have another source for the appropriate ultimate strengths you can enter the information directly yourself. If you have chosen to use a precast concrete slab construction, then a particular set of stud information must be used (Diameter = 19 mm, Height = 125 mm, Ultimate Strength = 70 kN). You will not be allowed to change these values. Hilti studs can not be used with Bison precast concrete slabs. You should ensure that a minimum distance of 30 mm from the centre line of the stud to the edge of the precast concrete slab is achievable. All types of stud may be positioned in a range of patterns. However, since the A1 amendment to the code, the maximum number of studs in a group is now 2 (see clause 5.4.7.1). Note

Seethe Caution for ’Shear connectors (ULS)’ in Theory and Assumptions for essential information about the layout of shear connectors.

Reinforcement Since the profile metal decking can be perpendicular, parallel or at any other angle to the supporting beam the following assumptions have been made: • Transverse reinforcement,

• if you use single bars they are always assumed to be at 90° to the span of the beam,



• if you use mesh then it is assumed to be laid so that the main bars1 are at 90° to the span of the beam.

• Other reinforcement • if you use single bars they are always assumed to be laid in the direction that is parallel to the trough of the profile metal decking.

• if you use mesh then it is assumed to be laid such that the main bars(1) are always parallel to the trough. When using Bison precast concrete slabs only Transverse reinforcement can be specified. The default values are the recommended sizes and their spacing is fixed at 300 mm cross centres to coincide with the voids in the precast concrete slab.

Slab depth

Reinforcement

 150

T8 @ 300

 200

T10 @ 300

 250

T12 @ 300

In all cases a suitable bond length should be provided to anchor the reinforcement beyond the position where it is fully utilised. Help

For further information see: Typical precast concrete slab details.

Fibre Reinforced Concrete When using decks from certain manufacturers an option exists to use fibre reinforcement as summarised below:

Deck Manufacturer

Fibre Reinforcement

CORUS

FibreFlor

Kingspan

Dramix

RLSD

Strux

SCFD

Metfloor

SMD

TAB-Deck

Note

Fibre reinforcement can not be used with any other decking manufacturer.

Note

Fibre reinforcement can't be used for edge beams, as these need traditional hooped reinf bars.

Help

For further information see the Fibre Reinforced Concrete Advisory Notes.

Footnotes 1. These are the bars that are referred to as longitudinal wires in BS 4483: 1998 Table 1.



Construction stage restraint conditions If you do need to check the lateral torsional buckling of the beam during construction (in the case where the profiled metal decking is unable to provide an acceptable level of restraint) you can: • define the degree of fixity that the end connections are able to provide and hence an effective length associated with the support,

• position additional restraints at any point along the beam (Composite Beam automatically uses 1.0L and 1.2L+2D as the factors for Normal and Destabilizing loads), Help

For a definition of Destabilizing Loads see BS 5950-1:2000 clause 4.3.4.

• Composite Beam automatically takes the average of the effective length factors for differing supports, or between those for the support and the adjacent sub-beam.

• alternatively you can specify the factors that you want to use for the lengths between restraints, or you can enter the effective length of the sub-beam directly by entering a value (in m).

• specify that any length (or lengths) of the beam should be taken as being fully restrained against lateral torsional buckling, independent of the restraint conditions for the adjacent length(s).

Construction stage loading You may specify a wide range of applied loading at the construction stage including: • uniform distributed loads (over the whole or part of the beam),

• point loads, • varying distributed loads (over the whole or part of the beam), • trapezoidal loads. You define these loads into one or more loadcases as required. The Slab wet loadcase is reserved for the self weight of the wet concrete in the slab. If working within a Fastrak Building Designer model, by clicking the Automatic Loading check box this is automatically calculated based on the wet density of concrete and the area of slab supported. An allowance for ponding can optionally be included. If you uncheck Automatic Loading, or if you are using Composite Beam as a standalone application, the Slab wet loadcase is initially empty - it is therefore important that you edit this loadcase and define directly the load in the beam due to the self weight of the wet concrete. If you do not do this then you effectively would be designing the beam on the assumption that it is propped at construction stage. Having created the loadcases to be used at construction stage, you then include them, together with the appropriate factors in the dedicated Construction stage design combination. You can include or exclude the self-weight of the beam from this combination and you can define the load factors that apply to the self weight and to each loadcase in the combination.



Note

You should include the Slab wet loadcase in the Construction stage combination, it can not be placed in any other combination since it’s loads relate to the slab in its wet state. Conversely, you can not include the Slab dry loadcase in the Construction stage combination, since it’s loads relate to the slab in its dry state. The loads in the Construction stage combination should relate to the slab in its wet state and any other loads that may be imposed during construction.

Tip

If you give any additional construction stage loadcases a suitable title you will be able to identify them easily when you are creating the Construction stage combination.

Composite stage loading You may specify a wide range of applied loading for the composite condition: • uniform distributed loads (over the whole or part of the beam),

• point loads, • varying distributed loads (over the whole or part of the beam), • trapezoidal loads. You define these loads into one or more loadcases which you then include, together with the appropriate factors in the design combinations you create. You can include or exclude the self-weight of the steel beam from any combination and you can define the load factors that apply to the beam self weight and to each loadcase in the combination. The Slab dry loadcase is reserved for the self weight of the dry concrete in the slab. If working within a Fastrak Building Designer model, by clicking the Automatic Loading check box this is automatically calculated based on the dry density of concrete and the area of slab supported. An allowance for ponding can optionally be included. If you uncheck Automatic Loading, or if you are using Composite Beam as a standalone application, the Slab dry loadcase is initially empty - it is therefore important that you edit this loadcase and define directly the load in the beam due to the self weight of the dry concrete. For each other loadcase you create you specify the type of loads it contains – Dead, Imposed or Wind. For each load that you add to an Imposed loadcase you can specify the percentage of the load which is to be considered as acting long-term (and by inference that which acts only on a short-term basis). All loads in Dead loadcases are considered to be completely long-term while those in Wind loadcases are considered totally short-term.

Construction stage design checks When you use Composite Beam to design or check a beam for the construction stage (the beam is acting alone before composite action is achieved) the following conditions are examined in accordance with BS 5950-1:2000: • section classification (Clause 3.5.2),

• shear capacity (Clause 4.2.3),



• moment capacity: • Clause 4.2.5.2 for the low shear condition, • Clause 4.2.5.3 for the high shear condition, • lateral torsional buckling resistance (Clause 4.3.6), Note

This condition is only checked in those cases where the profile decking or precast concrete slab (at your request) does not provide adequate restraint to the beam,

• web openings, • Westok checks, • Shear horizontal, • Web post buckling, • Vierendeel bending, • construction stage total load deflection check. Composite stage design checks When you use Composite Beam to design or check a beam for the composite stage (the beam and concrete act together, with shear interaction being achieved by appropriate shear connectors) the following Ultimate Limit State and Serviceability Limit State conditions are examined in accordance with BS 5950 : Part 3 : Section 3.1 : 1990 (unless specifically noted otherwise). Ultimate Limit State Checks

• section classification (Clause 4.5.2), depending on whether adequate connection is achieved between the compression flange and the slab. The section classification allows for the improvement of the classification of the section if the appropriate conditions are met,

• vertical shear capacity (BS 5950-1:2000 - Clause 4.2.3), • longitudinal shear capacity (Clause 5.6) allowing for the profiled metal decking, transverse reinforcement and other reinforcement which has been defined,

• number of shear connectors required (Clause 5.4.7) between the point of maximum moment and the end of the beam, or from and between the positions of significant point loads,

• moment capacity: • Clause 4.4.2 for the low shear condition, • Clause 5.3.4 for the high shear condition, • web openings. Serviceability Limit State Checks • service stresses (Clause 6.2),

• concrete • steel top flange and bottom flange • deflections (Clause 6.1.2) • self-weight • SLAB loadcase,



• dead load, • imposed load1, • total deflections, • natural frequency check (Clause 6.4).

Theory and Assumptions This section describes the theory used in the development of Composite Beam and the major assumptions that have been made, particularly with respect to interpretation of BS 5950-3.1:1990+A1:2010(Ref. 1). A basic knowledge of the design methods for composite beams in accordance with BS 5950-3.1:1990+A1:2010 is assumed.

Analysis method Composite Beam uses a simple analysis of a statically determinate beam to determine the forces, moments and so on, to be resisted by the beam under the Construction stage, at the Serviceability Limit State and at the Ultimate Limit State.

Design method The design methods employed to determine the adequacy of the section for each condition are those consistent with BS 5950-3.1:1990+A1:2010 unless specifically noted otherwise.

Construction stage Composite Beam performs all checks for this condition in accordance with BS 5950-1:2000(Ref. 2)

Section classification Cross-section classification is determined using Table 11 and Clause 3.5. The classification of the section must be Plastic (Class 1), Compact (Class 2) or Semi-compact (Class 3). Sections which are classified as Slender (Class 4) are beyond the scope of Composite Beam. Member strength checks Member strength checks are performed at the point of maximum moment, the point of maximum shear, the position of application of each point load, and at each side of a web opening as well as all other points of interest along the beam. Shear capacity — is determined in accordance with Clause 4.2.3. Where the applied shear force exceeds 60% of the capacity of the section, the high shear condition applies to the bending moment capacity checks (see below). Bending moment capacity — is calculated to Clause 4.2.5.2 (low shear at point) or Clause 4.2.5.3 (high shear at point) for plastic, compact and semi-compact sections.

Footnotes 1. This is the only limit given in BS 5950 : Part 3 : Section 3.1 : 1990.



Lateral torsional buckling checks BS 5950 : Part 3 : Section 3.1 : 1990 states that lateral torsional buckling checks are not required when the angle between the direction of span of the beam and that of the profile decking is greater than or equal to 45°. When the angle is less than this, then lateral torsional buckling checks will normally be required. Composite Beam allows you to switch off these checks by specifying that the entire length between the supports is continuously restrained against lateral torsional buckling. If you use this option you must be able to provide justification that the beam is adequately restrained against lateral torsional buckling during construction. For Bison precast concrete slabs you can specify whether or not the slabs are able to provide restraint to the beam against lateral torsional buckling. For beams with a span greater than 9 m you need to give careful consideration to the construction sequence. When the checks are required you can position restraints at any point within the length of the main beam and can set the effective length of each sub-beam (the portion of the beam between one restraint and the next) either by giving factors to apply to the physical length of the beam, or by entering the effective length that you want to use. Each sub-beam which is not defined as being continuously restrained is checked in accordance with clause 4.3.8 and Annex B of BS 5950-1:2000. Deflection checks Composite Beam calculates relative deflections. (See ’Deflection checks’in the Basic Principles chapter of this handbook.) The following deflections are calculated for the loads specified in the construction stage load combination: • the dead load deflections i.e. those due to the beam self weight, the Slab Wet loads and any other included dead loads,

• the imposed load deflections i.e. those due to construction live loads, • the total load deflection i.e. the sum of the previous items. The loads are taken as acting on the steel beam alone. The ‘Service Factor’ (default 1.0), specified against each load case in the construction combination is applied when calculating the above deflections. If requested by the user, the total load deflection is compared with either a span-over limit or an absolute value The initial default limit is span/200. Note

Adjustment to deflections. If web openings have been defined, the calculated deflections are adjusted accordingly. See: ’Web Openings’ in the Theory and Assumptions section.

Torsion for ASB and SFB beams In the design of ASB/SFB beams, torsion resulting from out of balance construction stage loading is not considered. If this condition occurs, then you will need to produce independent calculations to check this.



Composite stage Composite Beam performs all checks for the composite stage condition in accordance with BS 5950-3.1:1990+A1:2010 unless specifically noted otherwise. Equivalent steel section - Ultimate limit state (ULS) An equivalent steel section is determined for use in the composite stage calculations by removing the root radii whilst maintaining the full area of the section. This approach reduces the number of change points in the calculations while maintaining optimum section properties. Section classification (ULS) For section classification purposes the true section is used. Composite Beam classifies the section in accordance with the requirements of BS 5950-1:2000 except where specifically modified by those of BS 5950-3.1:1990+A1:2010. There are a small number of sections which fail to meet a classification of compact at the composite stage. Although BS 5950-3.1:1990+A1:2010 covers the design of such members they are not allowed in this release of Composite Beam. Member strength checks (ULS) Member strength checks are performed at the point of maximum moment, the point of maximum shear, the position of application of each point load, and at each side of a web opening as well as all other points of interest along the beam. Shear Capacity (Vertical) — is determined in accordance with Clause 4.2.3 of BS 5950-1:2000. Where the applied shear force exceeds 50% of the capacity of the section, the high shear condition applies to the bending moment capacity checks (see below). Shear Capacity (Longitudinal) — the longitudinal shear resistance of a unit length of the beam is calculated in accordance with Clause 5.6. You can set the position and attachment of the decking and details of the reinforcement that you want to provide. Composite Beam takes these into account during the calculations. The following assumptions are made: • the applied longitudinal shear force is calculated at the centre-line of the beam, and at the position of the lap (if known). If the position of the lap is not known, then the default value of 0 mm should be used (that is the lap is at the centre-line of the beam) as this is the worst case scenario. Note

The sheet cover width is shown in the Floor Construction property sheet for your information.

• the minimum concrete depth is assumed for calculating the area of concrete when the profile decking and beam spans are parallel,

• the total concrete area is used when the profile decking and beam spans are perpendicular, • the overall depth of the slab is used for precast concrete slabs. that is the topping is assumed to be structural and any voids or cores are ignored.



In the calculations of the longitudinal shear resistance on the beam centre-line and at the lap, the areas used for the reinforcement are shown in the following table.

Decking angle

Reinforcement type

Area used

transverse

that of the single bars defined or for mesh the area of the main wiresa

other

that of the single bars defined or for mesh the area of the main wires(a)

transverse

that of the single bars defined or for mesh the area of the main wires(a)

perpendicular

parallel other

single bars have no contribution, for mesh the area of the minor wires(b)

a. These are the bars that are referred to as longitudinal wires in BS 4483: 1998 Table 1 b. These are the bars that are referred to as transverse wires in BS 4483: 1998 Table 1

If the decking spans at some intermediate angle () between these two extremes then the program calculates: • the longitudinal shear resistance as if the sheeting were perpendicular, v1,

• the longitudinal shear resistance as if the sheeting were parallel, v2, • then the modified longitudinal shear resistance is calculated from these using the relationship, v1sin2() + v2cos2(). Moment Capacity — for the low shear condition the plastic moment capacity is determined in accordance with Clause 4.4.2. For the high shear condition the approach given in Clause 5.3.4 is adopted. The overall depth of the slab is used for precast concrete slabs. that is the topping is assumed to be structural and any voids / cores are ignored. In this calculation the steel section is idealised to one without a root radius so that the position of the plastic neutral axis of the composite section can be determined correctly as it moves from the flange into the web. Shear connectors (ULS) Composite Beam checks shear connectors to Clause 5.4.7. It calculates the stud reduction factor based on the number of studs in a group. For Bison precast concrete slabs the stud reduction factor is always 1.0. Composite Beam always uses 2*e (and not br) in the calculation of k for perpendicular profiles, and always uses br for parallel cases.



For angled cases two values of k are calculated and summed in accordance with Clause 5.4.7.4. In this instance Composite Beam uses 2*e for the calculation of k1 and br for the calculation of k2. Caution

The value of e (when used) can have a very significant effect on the value of k. This can have a dramatic effect on the number of studs required for a given beam size. Alternatively for a fixed layout of studs this can have a significant effect on the required beam size.

Caution

During the design process Fastrak Building Designer does not check some stud dimensional constraints. You should confirm that the chosen configuration of decking and studs meets those that you deem appropriate.

Optimise Shear Connection — Stud optimization is a useful facility since there is often some over conservatism in a design due to the discrete changes in the size of the section. If you choose the option to optimise the shear studs, then Composite Beam will progressively reduce the number of studs either until the minimum number of studs to resist the applied moment is found, until the minimum allowable interaction ratio (for example 40% for beams with a span less than 10 m) is reached or until the minimum spacing requirements are reached. This results in partial shear connection. The degree of shear connection is checked at the point of maximum bending moment or the position of a point load if at that position the maximum utilisation ratio occurs. Note

During the selection process, in auto design mode point load positions are taken to be ‘significant’ (i.e. considered as positions at which the maximum utilisation could occur) if they provide more than 10% of the total shear on the beam. For the final configuration and for check mode all point load positions are checked.

To determine if the degree of shear connection is acceptable Composite Beam applies the following rules: • If the degree of shear connection at the point of maximum moment is less than the minimum permissible shear connection, then this generates a FAIL status,

• If the point of maximum utilisation ratio occurs at a point that is not the maximum moment position and the degree of shear connection is less than the minimum permissible shear connection, then this generates a WARNING status,

• If the degree of shear connection at any other point load is less than the minimum permissible shear connection, then this does not affect the status in any way. Note

The percentage degree of shear connection is always calculated by the program as a proportion of the maximum concrete force and not simply Na/Np as in the code.

Section properties - serviceability limit state (SLS) BS 5950-3.1:1990+A1:2010 indicates that the Serviceability Limit State modular ratio for all SLS calculations should be based upon an effective modular ratio derived from the proportions of long term loading in the design combination being considered.



Composite beam therefore calculates the deflection for the beam based on the properties as tabulated below.

Loadcase Type

Properties used

self-weight

bare beam

Slab

bare beam

Dead

composite properties calculated using the modular ratio for long term loads

Live

composite properties calculated using the effective modular ratio appropriate to the long term load percentage for each load. The deflections for all loads in the loadcase are calculated using the principle of superposition.

Wind

composite properties calculated using the modular ratio for short term loads

Total loads

these are calculated from the individual loadcase loads as detailed above again using the principle of superposition

Stress checks (SLS) Composite Beam calculates the worst stresses in the extreme fibres of the steel and the concrete at serviceability limit state for each load taking into account the proportion which is long term and that which is short term. These stresses are then summed algebraically. Factors of 1.00 are used on each loadcase in the design combination (you cannot amend these). The stress checks assume that full interaction exists between the steel and the concrete at serviceability state. Deflection checks (SLS) Composite Beam calculates relative deflections. (See ’Deflection checks’ in the Basic Principles chapter of this handbook.) The composite stage deflections are calculated in one of two ways depending upon the previous and expected future load history:

• the deflections due to all loads in the SLAB loadcase and the self-weight of the beam are calculated based on the inertia of the steel beam alone (these deflections will not be modified for the effects of partial interaction). Note

It is the SLAB deflection alone which is compared with the limit, if any, specified for the SLAB loadcase deflection.

• the deflections for all loads in the other loadcases of the Design Combination will be based on the inertia of the composite section allowing for the proportions of the particular load that are long or short term (see above). When necessary these will be modified to include the effects of partial interaction in accordance with Clause 6.1.4.



Note

It is the deflection due to imposed loads alone (allowing for long and short term effects) which is limited within the code. Composite Beam also gives you the deflection for the SLAB loadcase which is useful for pre-cambering the beam. The beam Self-weight, Dead and Total deflections are also given to allow you to be sure that no component of the deflection is excessive.

Note

Adjustment to deflections. If web openings have been defined, the calculated deflections are adjusted accordingly. See: ’Web Openings’ in the Theory and Assumptions section.

Natural frequency checks (SLS) Composite Beam calculates the approximate natural frequency of the beam based on the simplified formula published in the Design Guide on the vibration of floors(Ref. 6) which states that 18Natural frequency = -----

where  is the maximum static instantaneous deflection that would occur under a load equivalent to the effects of self-weight, dead loading and 10% of the characteristic imposed loading, based upon the composite inertia (using the short term modular ratio) but not modified for the effects of partial interaction.

Web Openings Circular Openings as an Equivalent Rectangle Each circular opening is replaced by equivalent rectangular opening, the dimensions of this equivalent rectangle for use in all subsequent calculations are: do'= 0.9*opening diameter lo = 0.45*opening diameter Properties of Tee Sections When web openings have been added, the properties of the tee sections above and below each opening are calculated in accordance with Section 3.3.1 of SCI P355(Ref. 10) and Appendix B of the joint CIRIA/SCI Publication P068(Ref. 5). The bending moment resistance is calculated separately for each of the four corners of each opening. Design at Construction stage The following calculations are performed where required for web openings:

• • • • • •

Axial resistance of tee sections Classification of section at opening Vertical shear resistance Vierendeel bending resistance Web post horizontal shear resistance Web post bending resistance



• Web post buckling resistance • Lateral torsional buckling • Deflections Design at Composite stage The following calculations are performed where required for web openings:

• • • • • • • • • • •

Axial resistance of concrete flange Vertical shear resistance of the concrete flange Global bending action - axial load resistance Classification of section at opening Vertical shear resuistance Moment transferred by local composite action Vierendeel bending resistance Web post horizontal shear resistance Web post bending resistance Web post buckling resistance Deflections

Deflections The deflection of a beam with web openings will be greater than that of the same beam without openings. This is due to two effects,

• the reduction in the beam inertia at the positions of openings due to primary bending of the beam,

• the local deformations at the openings due to Vierendeel effects. This has two components - that due to shear deformation and that due to local bending of the upper and lower tee sections at the opening. The primary bending deflection is established by 'discretising' the member and using a numerical integration technique based on 'Engineer's Bending Theory' - M/I = E/R = /y. In this way the discrete elements that incorporate all or part of an opening will contribute more to the total deflection. The component of deflection due to the local deformations around the opening is established using a similar process to that used for cellular beams which is in turn based on the method for castellated beams given in the SCI publication, “Design of castellated beams. For use with BS 5950 and BS 449". The method works by applying a 'unit point load' at the position where the deflection is required and using a 'virtual work technique to estimate the deflection at that position. For each opening, the deflection due to shear deformation, s, and that due to local bending, bt, is calculated for the upper and lower tee sections at the opening. These are summed for all openings and added to the result at the desired position from the numerical integration of primary bending deflection.



Note that in the original source document on castellated sections, there are two additional components to the deflection. These are due to bending and shear deformation of the web post. For castellated beams and cellular beams where the openings are very close together these effects are important and can be significant. For normal beams the openings are likely to be placed a reasonable distance apart. Thus in many cases these two effects will not be significant. They are not calculated for such beams but in the event that the openings are placed close together a warning is given. This will indicate that these effects on the deflection of the beam are not taken into account. This warning is issued when, so < 2.5 * do for rectangular openings so <1.5 * o for circular openings Where so=the clear length of web between adjacent openings do=the depth of a rectangular opening taken as the larger if the adjacent openings differ o=the diameter of a circular opening

Theory and Assumptions – Westok beams This section describes the theory used in the development of Composite Beam and the major assumptions that have been made, particularly as these relate to Westok beams and with respect to interpretation of BS 5950-3.1:1990+A1:2010(Ref. 1). A basic knowledge of the design methods for composite beams in accordance with BS 5950-3.1:1990+A1:2010 is assumed.

Construction stage Composite Beam performs all checks for this condition in accordance with BS 5950-1:2000(Ref. 2) and SCI P100(Ref. 7). Classification The same classification rules are applied to Westok cellular beams as to normal steel beams. Although the upper and lower sections of Westok beams are welded together the combined beam is still in essence a rolled section. If a Westok beam is classified as slender, then it will be rejected, as would a normal steel beam. Note

If you define a bottom flange which is very large then it could be partly in compression. However, because of the constraining effect of its tension part, the classification is still assumed to be governed by the upper flange or the web.

Note

The depth of the web for web classification allows for the root radius of the two sections making up the cellular beam and the web thickness is taken as that of the thinner web.

Vertical shear The capacity of the Westok section is checked at: • the first web post using the shear capacity of the gross section,



• the centre-line of the first cell in from the left hand support using the shear capacity of the net section. If the check at this point fails, then you will see a warning message indicating that an infill is required at this position. The next cell is checked, and a warning message given if this also fails. This process continues until the first unfilled cell which passes the shear check is found. A similar process is adopted for the right hand web post and the cells adjacent to it. Horizontal Shear Horizontal shear is developed in the web post due to the change in axial forces in the tee as shown in Figure 15 of SCI Publication 100(Ref. 7) which is reproduced below. Wi Ti Vi

xe (D - Do)/2

Vh

Ti+1 Vi+1 No local Vierendeel moment acts at centre-line of opening

S - Do S

Composite Beam evaluates all web posts (including any endposts) since it is not possible to otherwise ascertain the most critical post given the almost infinite variation of moment. Note

If S1 < S / 2 a half infill plate is assumed to be placed in the first cell. Similarly, if Sn < S / 2 a half infill plate is assumed to be placed in the last cell. Furthermore, if S1 or Sn > S – Do / 2 then this implies that a part cell is adjacent to the end of the beam. This is assumed to be fully infilled.

where S

= cell spacing

Do

= cell diameter

S1

= distance to first cell from left set back position

Sn

= distance to last cell from right set back position

Moment Capacity Composite Beam takes account of the interaction between moment and shear in Westok sections by modifying the thickness of the web of the upper and lower sections as necessary. This approach has been adopted in preference to that from BS 5950-1:2000 for a number of reasons: • It maintains consistency with designs carried out before Westok beams were included in Composite Beam. It is similar to the approach adopted for the design of the upper and lower tee sections in conventional web openings.

• It allows the shear to be distributed preferentially between the upper and lower tee sections.



• For equal flanged Class 1 or 2 Plastic or Compact sections, either approach yields the same answer. The form that this equation takes (see below) is similar to that in BS 5950-1:2000(Ref. 2). For low shear the moment capacity is based on py S  1.2 py Z for Class 1 and 2 sections and py Z  py S for Class 3 sections. The calculation of S is based on the full web thickness when Fv  0.5 Pv and on the effective web thickness when Fv > 0.5 Pv. To ensure the correct change point, the limit above which the high shear condition exists, would need to be taken as 0.5 Pv. In BS 5950 a limit of 0.6 Pv is adopted (the reduction in web thickness is less than 5% at this point). However, for consistency with the reduction formula and the approach adopted for Web Openings in SCI Publication 068, the change point of 0.5 Pv is adopted. Since it is not possible to determine the worst case of interaction of shear and moment otherwise, each cell is checked. The shear force resisted by the upper and lower sections is calculated in proportion to their respective capacities calculated as: d  t  0.6  py The effective web thickness of the upper and lower sections is then evaluated from: te=tw  (1 – (2  Fv /Pv - 1)2) Lateral Torsional Buckling These checks are performed in exactly the same way as for normal steel sections. The plastic and elastic section moduli and the buckling properties are based on the net section over the centre-line of the cell. Deflection This is calculated in accordance with Section 6.3 and Figure 16 of SCI Publication 100(Ref. 7). In order to assess the deflection at any point, the method requires a unit point load to be applied at that point; Composite Beam calculates the deflection at 1/40th points along the beam. The shear and moment at the centre-line of each cell are then evaluated for this unit load. The overall deflection at the point under consideration has five contributory components. Each component is evaluated at each cell and then summed. The five contributions are due to: • bending in the tee, y1,

• • • •

bending in the web post, y2, axial force in the tee, y3, shear in the tee, y4, shear in the web post, y5. Note

In the calculation of deflections filled cells are treated as if they are not filled. This is conservative.



Web Post Flexure and Buckling Composite Beam calculates the moment capacity of the web post, (section A-A in Figure 16 of SCI Publication 100(Ref. 7)). c1, c2 and c3 are evaluated to clause 6.2.5 and the moment capacity is compared with the moment generated by the horizontal shear in the web post. Note

The end post is not checked. Web-post buckling is a lateral torsional effect local to the web post (see page 3 of Publication 100). It occurs on a diagonal from the bottom of the cell furthest away from the support up to the top of the cell closest to the support with centre of rotation being (approximately) on the weld line. At the end web-post position the length over which buckling occurs is reduced considerably. The approach detailed for internal web posts would be very conservative for the end web post. Also the type of connection used at the beam end will be significant. For instance if a partial or full depth end plate is used this check would not be valid.

Vierendeel Bending Composite Beam checks Vierendeel bending of both the upper and lower tee sections for all cells apart from any cells that have been filled to satisfy the shear condition. SCI Publication 100(Ref. 7) gives two alternative approaches to calculating the secondary bending stresses around the cell. These methods give similar results, Composite Beam uses Sahmel’s method. A plastic load distribution is used for sections which are classified as either Plastic or Compact. Both plastic and elastic load distributions are used for sections which are classified as Semi-compact. With reference to Figure 10 of SCI Publication 100(Ref. 7), Composite Beam takes the shear and axial load (from the bending moment) at the centre line of the cell from the beam analysis and proportioned between the upper and lower tee sections. At any cross section through the tee at an angle, , to the vertical centre-line of the cell these forces are transposed to an axial load, a shear and a moment acting on a new cross-section. Since the section properties are changing and the axial load decreases whilst the moment increases, it is unclear at what angle the interaction of bending and axial load becomes critical. An incremental approach is therefore adopted and Composite Beam increments the angle in 5° intervals. Composite Beam takes account of a reduced web thickness if high shear occurs in the tee in exactly the same way as in the moment capacity calculations. Note

The upper limit on Mc for Plastic and Compact sections of 1.2 x py x Ztee is not applied.

Note

The plastic moment capacity is used for both Plastic and Compact sections. With regard this point, Section 6.2.6 of SCI Publication 100(Ref. 7) defines Mp as the plastic moment capacity for Plastic sections but as the elastic moment capacity for all other sections. This infers that Compact sections should be limited to their elastic moment capacity. There appears to be no justification for this and so the inferred requirement of SCI Publication 100(Ref. 7) is therefore ignored.



Composite Stage Composite Beam performs all checks for this condition in accordance with BS 5950-3.1:1990+A1:2010(Ref. 2). Classification The same classification rules are applied to Westok cellular beams as to normal steel beams except that the lesser web thickness of the upper and lower sections is used for the web classification. If a Westok beam is classified as semi-compact or slender, then it is rejected in exactly the same way as a normal steel beam. Vertical shear The shear capacity of the Westok section is checked at: • the first web post using the shear capacity of the gross section,

• the centre-line of the first cell in from the left hand support using the shear capacity of the net section. If the check at this point fails, then you will see a warning message indicating that an infill is required at this position. The next cell is checked, and a warning message given if this also fails. This process continues until the first unfilled cell which passes the shear check is found. A similar process is adopted for the right hand web post and the cells adjacent to it. Unlike normal beams, Section 7.2.2 of SCI Publication 100(Ref. 7) allows the contribution of the concrete to be used. It is most likely that the critical cross-section will be over the cells and therefore this accounting of the concrete resistance is compatible with the treatment of shear over standard web openings. Composite Beam makes the conservative assumption that the concrete flange is limited to the area above the profiled decking. This means that no adjustment is necessary for the various possible angles of span of the decking. Horizontal Shear Horizontal shear is developed in the web post due to the change in axial forces in the tee as shown in Figure 15 of SCI Publication 100(Ref. 7) which is reproduced below. Wi Ti Vi

xe (D - Do)/2

Vh S - Do S

Ti+1 Vi+1 No local Vierendeel moment acts at centre-line of opening



Composite Beam evaluates all web posts (including any endposts) since it is not possible to otherwise ascertain the most critical post given the almost infinite variation of moment. Note

If S1 < S / 2 a half infill plate is assumed to be placed in the first cell. Similarly, if Sn < S / 2 a half infill plate is assumed to be placed in the last cell. Furthermore, if S1 or Sn > S – Do / 2 then this implies that a part cell is adjacent to the end of the beam. This is assumed to be fully infilled.

where S

= cell spacing

Do

= cell diameter

S1

= distance to first cell from left set back position

Sn

= distance to last cell from right set back position

Longitudinal shear In the calculations of the longitudinal shear resistance on the beam centre-line and at the lap, the areas used for the reinforcement are shown in the following table.

Decking angle

Reinforcement type

Area used

transverse

that of the single bars defined or for mesh the area of the main barsa

other

that of the single bars defined or for mesh the area of the main bars(a)

transverse

that of the single bars defined or for mesh the area of the main bars(a)

perpendicular

parallel other

single bars have no contribution, for mesh the area of the minor wiresb

a. These are the bars that are referred to as longitudinal wires in BS 4483: 1998 Table 1. b. These are the bars that are referred to as transverse wires in BS 4483: 1998 Table 1.

If the decking spans at some intermediate angle () between these two extremes then the program calculates: • the longitudinal shear resistance as if the sheeting were perpendicular, v1,

• the longitudinal shear resistance as if the sheeting were parallel, v2, • then the modified longitudinal shear resistance is calculated from these using the relationship, v1 sin2() + v2 cos2(). Moment Capacity Composite Beam performs checks for both the low and high shear conditions.



Low shear The design procedures are the same as used for standard beams except that the additional cases which allow the Plastic Neutral Axis to be in the upper and lower web are evaluated.

High shear The effect of high shear and moment is ignored for Westok cellular beams. The calculations assume that the combination of shear and axial load is accounted for when checking Vierendeel bending in the upper and lower tee sections. Hence this protects against any failure due to reduction in moment capacity from high shear. Web Post Flexure and Buckling Composite Beam calculates the moment capacity of the web post, (section A-A in Figure 16 of SCI Publication 100(Ref. 7)). c1, c2 and c3 are evaluated to clause 6.2.5 and the moment capacity is compared with the moment generated by the horizontal shear in the web post. Note

The end post is not checked. Web-post buckling is a lateral torsional effect local to the web post (see page 3 of Publication 100). It occurs on a diagonal from the bottom of the cell furthest away from the support up to the top of the cell closest to the support with centre of rotation being (approximately) on the weld line. At the end web-post position the length over which buckling occurs is reduced considerably. The approach detailed for internal web posts would be very conservative for the end web post. Also the type of connection used at the beam end will be significant. For instance if a partial or full depth end plate is used this check would not be valid.

Vierendeel Bending Composite Beam checks Vierendeel bending of both the upper and lower tee sections for all cells apart from any cells that have been filled to satisfy the shear condition. SCI Publication 100(Ref. 7) gives two alternative approaches to calculating the secondary bending stresses around the cell. These methods give similar results, Composite Beam uses Sahmel’s method. With reference to Figure 10 of SCI Publication 100(Ref. 7), the shear and axial load (from the bending moment) at the centre line of the cell are taken from the beam analysis and proportioned between the upper and lower tee sections. At any cross section through the tee at an angle, , to the vertical centre-line of the cell these forces are transposed to an axial load, a shear and a moment acting on a new cross-section. Since the section properties are changing and the axial load decreases whilst the moment increases, it is unclear at what angle the interaction of bending and axial load becomes critical. An incremental approach is therefore adopted and Composite Beam increments the angle in 5° intervals. Composite Beam takes account of a reduced web thickness if high shear occurs in the tee in exactly the same way as in the moment capacity calculations. At the composite stage the concrete flange is taken to contribute to the shear capacity of the section. The shear is preferentially given to the upper tee which has the benefit that the lower tee capacity is maximised to resist the larger axial load that exists at that level. However, if, during analysis, the upper tee fails then the shear is shifted by 5% into the lower tee and the section is reanalysed.



Note

The upper limit on Mc for Plastic and Compact sections of 1.2 x py x Ztee is not applied.

Note

The plastic moment capacity is used for both Plastic and Compact sections. With regard this point, Section 6.2.6 of SCI Publication 100(Ref. 7) defines Mp as the plastic moment capacity for Plastic sections but as the elastic moment capacity for all other sections. This infers that Compact sections should be limited to their elastic moment capacity. There appears to be no justification for this and so the inferred requirement of SCI Publication 100(Ref. 7) is therefore ignored.

Deflections Composite Beam calculates the deflections at the composite stage using the properties of the gross (uncracked) composite section making allowance for the effects of partial shear connection by following the procedure given in Section 7.3 of SCI Publication 100(Ref. 7). The deflections due to self-weight and SLAB loads are calculated in accordance with Section 6.3 and Figure 16 of SCI Publication 100(Ref. 7). In order to assess the deflection at any point, the method requires a unit point load to be applied at that point; Composite Beam calculates the deflection at 1/40 th points along the beam. The shear and moment at the centre-line of each cell are then evaluated for this unit load. The overall deflection at the point under consideration has five contributory components. Each component is evaluated at each cell and then summed. The five contributions are due to: • bending in the tee, y1,

• • • •

bending in the web post, y2, axial force in the tee, y3, shear in the tee, y4, shear in the web post, y5. Note

In the calculation of deflections filled cells are treated as if they are not filled. This is conservative.

Service Stresses Composite Beam calculates these using the properties of the gross (uncracked) composite section, and with the steel section properties appropriate to the Westok section. Natural frequency Again Composite Beam calculates these using the properties of the gross (uncracked) composite section in a similar manner to normal steel beams, using the section properties appropriate to the Westok section. However the appropriate allowances for partial interaction are taken from Section 7.3 of SCI Publication 100(Ref. 7).



Design Aspects This section aims to explain how the program can be used effectively with regard to the following design situations:

Use of Design Properties to Control Section Selection The Design Properties button provides a means by which you can both speed up the design process and control the design more precisely. Note

When you extract a beam from a Fastrak Building Designer model into Composite Beam for further investigation, Design Properties are accessed via the Design Wizard icon.

Size Constraints Size Constraints are only applicable when running the program in Design Mode.

Beam constraints These allow you to ensure that the sections that Composite Beam proposes match any particular size constraints you may have. For instance for a composite beam you may want to ensure a minimum flange width of 150mm. If so you would simply enter this value as the Minimum width, and Composite Beam Design would not consider sections with flanges less than this width for the design of this beam.

Optimize shear connection If you choose the option to optimize the shear studs, then Composite Beam will progressively reduce the number of studs either until the minimum number of studs to resist the applied moment is found, until the minimum allowable interaction ratio (for example 40% for beams with a span less than 10m) is reached or until the minimum spacing requirements are reached. This results in partial shear connection. For further details of stud optimization and how the partial interaction rules are applied see ’Shear connectors (ULS)’ in Theory and Assumptions.



Sections for Study (in Fastrak Building Designer) This feature is only applicable when running the program in Design Mode. On the left of the page is a list of available order files, only one of which can be selected. The sections contained within the chosen order file appear in the Section Designation list on the right of the page. Only checked sections within this list are considered during the design process.

The design process commences by starting with the smallest section in the chosen order file. Any section that fails any of the design conditions is rejected and the design process is then repeated for the next available section in the list. On completion of the design process, the first satisfactory section from the Section Designation list is assigned to the beam. Caution

Limiting the choice of sections by unchecking a section within an order file is a global change that affects ALL projects, (not just the currently open one). It is typically used therefore to eliminate unavailable or non-preferred sections from the design process. If design requirements for an individual beam require section sizes to be constrained, (due to, for example depth restrictions), then the choice of sections should be limited instead by using Size Constraints, (as these only affect the current beam).

Note

It is possible to create additional order files using a text editor. If you require to do so, please contact your local CSC Technical Support Team for guidance on how to proceed.



Sections for Study (in Composite Beam) When you extract a beam from a Fastrak Building Designer model into Composite Beam for further investigation, a benefit of doing so is that several order files can be considered at the same time. If a check is placed against an order file the sections contained within it appear in the Section Designation list on the right of the page. Only checked sections within this list are considered during the design process.

Typically, you would uncheck those order files that are unlikely to be appropriate for composite beam design. Doing so speeds up the solution. The design process commences by starting with the smallest section in each order file. Any section that fails any of the design conditions is rejected and the design process is then repeated for the next available section in the list. On completion of the design, all the satisfactory sections from the Section Designation list are displayed and the results for each of these can be examined before one of the sections is assigned to the beam. Caution

Limiting the choice of sections by either unchecking an order file or an individual section is a global change that affects ALL projects, (not just the currently open one). It is typically used therefore to eliminate unavailable or non-preferred sections from the design process. If design requirements for an individual beam require section sizes to be constrained, (due to, for example depth restrictions), then the choice of sections should be limited instead by using Size Constraints, (as these only affect the current beam).

Note




Deflection It is often found that serviceability criteria control the design of normal composite beams. This is because they are usually designed to be as shallow as possible for a given span. The Deflections page allows you to control the amount of deflection by applying either a relative or absolute limit to the deflection under different loading conditions.

A typical application of these settings might be: • not to apply any deflection limit to the SLAB loads, as this deflection can be handled through camber,

• to apply the relative span/360 limit for imposed load deflection, to meet code requirements,

• possibly, to apply an absolute limit to the total load deflection to ensure the overall deflection is not too large. Camber Camber is primarily used to counteract the effects of dead load on the deflection of the beam. This is particularly useful in long span composite construction where the self-weight of the concrete is cambered out. It also ensures little, if any, concrete over pour occurs when placing the concrete.

The amount of camber can be specified either: • As a value

• As a proportion of span • As a proportion of dead load deflection In the latter case, if 100% of the dead load deflection is cambered out, it is also possible to include a proportion of the live load deflection if required. A lower limit can be set below which the calculated camber is not applied, this ensures that impractical levels of camber are not specified.



Checking the effective width used in the design Checking the effective width used in the design (in Fastrak Building Designer) Fastrak Building Designer will calculate the effective width of the compression flange, be, for each composite beam as per section 4.6 of BS 5950 : Part 3 : Section 3.1 : 1990. For each side of the beam, it is taken as the smaller of: • beam span/8 – span taken as the centre to centre of supports

• one half of the distance to the centre line of the adjacent beam (for slabs spanning perpendicular)

• 40% of the distance to the centre line of the adjacent beam (for slabs spanning parallel) • the distance to the edge of the slab Although the program calculates be, it is your responsibility to accept the calculated figure or alternatively to adjust it. Engineering judgement may sometimes be required. For example consider the beam highlighted below:

The program calculates the effective width as the sum of: • to the right of the beam, be(right) = beam span/8

• to the left of the beam, be(left) = one half of the shortest distance to the centre line of the adjacent diagonal beam To the left of the beam, some engineers might prefer to use one half of the mean distance to the adjacent beam. To do so you would need to manually adjust the calculated value via the Floor Construction page of the Beam Properties. Note

The calculation of the effective width to the left of the beam is only carried out if the angled beam lies within the tolerance on rectilinearity set within the Building Designer Design Options/Composite. The default tolerance is 15 degrees. If in the above example the angled beam were at say 20 degrees, you would then be prompted to enter the effective width manually - (unless of course you chose to relax the tolerance to something greater than 20 degrees).



If modifications have been made to the floor layout you can get Fastrak Building Designer to recalculate effective widths, either for selected beams only, or for all beams in the model. Pick Recalculate Effective Widths... from the Building menu to do this. Checking the effective width used in the design (in Composite Beam) When Composite Beam is run as a standalone program it does not know anything about the slab spans and so the limitations with regard the spacing of the beams are not implemented. Note The correct effective width based on beam span or distance to adjacent beams is passed through when you extract a beam from a from Fastrak Building Designer model into Composite Beam for further investigation.

Layout of Studs Studs - Strength page You can allow group sizes of 1 or 2 studs - any group sizes that you don't want to be considered can be excluded.

For example, if you do not set up groups with 2 studs, then in auto-design the program will only try to achieve a successful design with a maximum of 1 stud in a group. For each group that you allow you must enter the 'Distance to nearer side of rib (e) and you also specify the pattern to be adopted (e.g. along the beam, across the beam, or staggered). Note

It is up to you to check that a particular pattern fits within the confines of the rib and beam flange since Fastrak will draw it (and use it in design) anyway.

Connectors-Layout page. The overall layout of studs is controlled from here. When running in Design Mode you may not want to specify the stud layout at the start of the design process. To work in this way check Auto-layout to have the program automatically control how the stud design will proceed. When the beam is subsequently designed Auto-layout invokes an automatic calculation of the required number of studs, which is optimized to provide an efficient design. Note

' uto layout' can actually be checked regardless of whether you are auto designing A the beam size or checking it. The combination of 'Check' design with 'Auto layout' of studs can be used to assist you to rationalise your designs e.g. to force a beam to be the same size as others in the building but have Fastrak determine the most efficient layout of studs

You may choose to perform the initial design with Auto-layout checked and then refine the spacing with Auto-layout unchecked if the spacing is not exactly as you require. This may arise if for instance the theoretical design needs to be marginally adjusted for practical reasons on site.



Auto-layout for Perpendicular decks For perpendicular decks, the Auto-layout dialog provides two options for laying out the studs as shown below:

Uniform — forces placement in ribs at the same uniform spacing along the whole length of the beam.

Whether the stud groups are placed in every rib (as shown above), alternate ribs, or every third rib etc. can be controlled by adjusting the limits you set for Minimum group spacing ( ) x rib and Maximum group spacing ( ) x rib. The number of studs in each group will be the same along the whole length of the beam, this number can be controlled by adjusting the limits you set on the Studs - Strength page. Example

If you set Minimum group spacing 2 x rib and Maximum group spacing 3 x rib, then the program will only attempt to achieve a solution with studs placed in alternate ribs, or studs placed in every third rib. It will not consider a solution in which studs are placed in every rib. Additionally, if on the Studs - Strength page, you have allowed groups of 1 stud and 2 studs; then if 1 stud per group proves to be insufficient the program will then consider 2 studs per group.

Non-uniform — If optimization has been checked (see ’Optimize shear connection’) studs are placed at suitable rib intervals (every rib, alternate ribs, every third rib etc.), in order to achieve sufficient interaction without falling below the minimum allowed by the code. If optimization has not been checked, studs are placed at suitable rib intervals in order to achieve 100% interaction.



Knowing the number of studs necessary to achieve the required level of interaction, it is possible that placement at a given rib interval could result in a shortfall; the program will attempt to accommodate this by working in from the ends, (as shown in the example below). If every rib is occupied and there is still a shortfall, the remainder are 'doubled-up', by working in from the ends once more.

In this example the point of maximum moment occurs one third of the way along the span, this results in an asymmetric layout. If you prefer to avoid such arrangements you can check the box Adjust layout to ensure symmetrical about center line. A redesign would then result in the symmetric layout shown below.

For both Uniform and Non-uniform layouts, if the minimum level of interaction can not be achieved this is indicated on the design summary thus: “Not able to design stud layout”.

Auto-layout for Parallel decks For parallel decks, the Auto-layout again provides Uniform and Non-uniform layout options, but the way these work is slightly different.

Uniform — forces placement at a uniform spacing along the whole length of the beam. The spacing adopted will be within the limits you set for Minimum group spacing distance and Maximum group spacing distance. If the point of maximum moment does not occur at mid span, the resulting layout will still be symmetric.

The number of studs in each group will be the same along the whole length of the beam, this number can be controlled by adjusting the limits you set on the Studs - Strength page. Non-uniform — If optimization has been checked (see ’Optimize shear connection’) studs are placed at a suitable spacing in order to achieve sufficient interaction without falling below the minimum allowed by the code. If optimization has not been checked, studs are placed at a suitable spacing in order to achieve 100% interaction.



If the point of maximum moment does not occur at mid span, the resulting non-uniform layout can be asymmetric as shown below.

For both Uniform and Non-uniform layouts, if the minimum level of interaction can not be achieved this is indicated on the design summary thus: “Not able to design stud layout”. Manual Stud Layout You may prefer to manually define/adjust the group spacing along the beam. This can be achieved by unchecking Auto layout. Caution

If you specify the stud spacing manually, then it is most important to note: • the resulting design may not be the optimal design possible for the beam, or • composite design may not be possible for the stud spacing which you have specified.

To generate groups of studs at regular intervals along the whole beam use the Quick layout facility. Alternatively, if you require to explicitly define the stud layout to be adopted for discrete lengths along the beam use the Layout table.

Manual layout for Perpendicular decks For perpendicular decks, the dialog for manual layouts is as shown:

To use Quick layout, proceed in one of two ways: • Choose to position groups in either every rib, or alternate ribs, then specify the number of studs required in the group and click Generate.

• Alternatively: specify the total number of studs, then when you generate, if the number specified is greater than the number of ribs, one will be placed in every rib and the remainder will be 'doubled-up' in the ribs at each end starting from the supports. Similarly if the number specified is less than the number of ribs, but greater than the number of



alternate ribs, one will be placed in every alternate rib and the remainder will be placed in the empty ribs. The code limits of 600mm or 4 x overall slab depth, (whichever is less), are considered. To use the Layout table: • For each segment you should define the following parameters: No. of studs in length and No. of studs in group; Group spacing x rib.

• Your input for these parameters is used to automatically determine Distance end 2 - this latter parameter can not be adjusted directly, hence it is greyed out.

Note

To make it easier to visualize the effect of adjusting any of the above parameters, you can right click on the diagram below the table to switch from a zoomed in view to a display of the entire span. (right clicking once more returns to the zoomed in view).

• If required click Insert to divide the beam into additional segments. (Similarly Delete will remove segments). You can then specify a different stud layout for each segment.



• We would advise that having entered No. of studs in length, group and spacing and ignoring Distance ends 1 and 2 you click Update, this will automatically fill in the missing fields.

• You can also use the diagram below the table to change the number of studs in individual ribs. Simply hover the cursor over the rib until a hand icon appears as shown below. Click in the rib and the number of studs will increase and the table will be updated accordingly.

Manual layout for Parallel decks For parallel decks, the dialog for manual layouts is as shown:

To use Quick layout, proceed in one of two ways: • Choose to position groups at a set repeat distance, then specify the number of studs required in the group and click Generate.

• Alternatively: specify the total number of studs, then click Generate - the program calculates the repeat distance automatically, subject to the code limits.



To use the Layout table: • The preferred method is to check the box to Define by no. of studs, in which case you can adjust the No. of studs in length and No. of studs in group. Alternatively you could leave this unchecked and then adjust No. of studs in group and Group spacing dist.

• If required click Insert to divide the beam into additional segments. (Similarly Delete will remove segments). You can then specify Distance end 1 for each new segment and it’s own stud layout.

Note

To make it easier to visualize the effect of adjusting any of the above parameters, you can right click on the diagram below the table to switch from a zoomed in view to a display of the entire span. (right clicking once more returns to the zoomed in view).



Non-composite design within Composite Beam Typically, at the outset you will know which beams are to be simple (non-composite) and which are to be composite and you will have specified the construction type accordingly. However, circumstances can arise in which a beam initially intended to be composite proves to be ineffective. Examples might be: • very small beams,

• beams with a significant point load close to a support, • beams where the deck is at a shallow angle to the beam, hence the stud spacing is impractical,

• beams where, for a variety of reasons, it is not possible to provide an adequate number of studs, and

• edge beams, where the advantages of composite design (e.g. reduced depth) are not so clear Where Composite Beam is unable to find a section size which works compositely, you can ask for a non-composite design for the same loading. You will find that this facility is particularly useful when you are using Composite Beam as a stand-alone application, or when you extract a key beam from a Fastrak Building Designer model into Composite Beam for further investigation. To invoke non-composite design in Fastrak Building Designer Edit the properties of the beam (right-click the beam and then pick the Edit option from the context menu that appears), and on the Design page ensure that Treat as non-composite is checked.

To invoke non-composite design in Composite Beam Pick Beam / Non Composite from the main menu, or click the non-composite icon ( ) from the Beam, Loading, Design toolbar.



Automatic transverse shear reinforcement design It is possible to automatically design the amount of transverse shear reinforcement for each beam. This is achieved in Fastrak Building Designer by checking the Autoselect option, which is on the Reinforcement tab of the Composite Beam Properties, (or, if running Composite Beam directly, on the Reinforcement tab of the Floor Construction page as shown below:)

The auto-selected bars can be tied into the stud group spacing as shown above. Alternatively, the spacing can be controlled directly by the user. Irrespective of the method adopted, the user still needs to have control over the design. This is achieved in Fastrak Building Designer by clicking on the Design Properties button and then the Reinforcement tab (or, if running Composite Beam directly, via the Design Wizard). Note

You can only design transverse shear reinforcement automatically when you are designing a beam. If you are checking a beam, then you must specify the transverse shear reinforcement that you will provide, and then check out this arrangement.



Bar spacing as a multiple of stud spacing. When the option Bar spacing as a multiple of stud spacing is checked, the Reinforcement tab provides the user with control on the bar size and the multiples of stud spacing.

These can be used to achieve a selection of say, 12mm diameter bars at 2 times the stud spacing, with a slightly greater area than a less preferable 16mm diameter bars at 4 times the stud spacing. Controlling the bar spacing directly. When the option Bar spacing as a multiple of stud spacing is not checked, the Reinforcement tab provides the user with direct control on the bar size and the bar spacing.

Automatic transverse shear reinforcement design with Fibre Reinforced Concrete If fibre reinforced concrete (FRC) has been specified and the Autoselect button is checked, if the FRC is insufficient to provide adequate longitudinal shear resistance then the transverse shear required will be calculated ignoring the presence of the FRC. Note

The resistances provided by FRC and transverse reinforcement can not be used together to give a total resistance.



Worked Example This worked example illustrates the points covered in the previous section. If you want to work through this example you will find the file Engineer’s Example in the \documents and settings \ All Users \Application Data\CSC\Fastrak\ Examples folder. You can open and use this file, but you can not save

it away unless you change its name, this is done to protect the original. Let’s take a simple example of a 9 m span spine beam with 6 m span secondary beams at third points.

For this example we shall use a Ward Building Systems, Multideck 60 deck with a 130mm slab. The floor loading is:

Condition

Value


Wet Slab

2.5 kN/m2

45kN

Dry Slab

2.0 kN/m2

36kN

Services

1.0 kN/m2

18kN

Live load

5.0 kN/m2

90kN

Wet Slab Imposed

0.5 kN/m2

9kN

The beam is designed for composite and construction stage loading.

Without transverse shear reinforcement If no transverse shear reinforcement is available then composite design of this beam is not possible. This happens since the studs on a composite beam must lie between the end of the beam and the point of maximum moment (on this beam this is the 1/3 point). This compaction of the shear studs increases the transverse shear that has to be carried from the studs to the effective concrete flange. When the studs are spaced widely enough apart so as to not need transverse shear reinforcement they do not achieve the minimum interaction requirement for the beam. (40% on a 9m span beam).



When the studs are placed close enough to achieve the minimum 40% interaction transverse shear reinforcement is required. Thus with no transverse reinforcement composite design is not possible.

Design Pass 1 Ensure the beam is in design mode and then edit the floor construction. For the Reinforcement transverse to beam ensure that you tick Auto select and Bar spacing as a multiple of stud spacing.

and then on the Connectors-Layout page ensure that you tick Auto-layout and choose a Uniform layout.



From the UbBeamOrder.Eur file this should select a 457 x 191 x 67 UB (albeit with a utilisation ratio of 1.00) with 47 shear studs at 191.5 mm centres and transverse shear reinforcement of H10’s at 191.5 mm centres.

The design has recognised that the beam needs transverse shear reinforcement to enable composite design and has consequently introduced suitable transverse reinforcement which (at your request) matches the centres of the studs required to achieve the design.



Design Pass 2 There is nothing wrong with Design Pass 1, however you might feel that placing studs at 191.5 mm centres is not desirable. If so you can set the design pass to work with a specified spacing – such as 200 mm. On the Connectors-Layout page, under Quick-layout set the repeat distance to 200mm and click Generate.

With the stud spacing set to 200 mm, automatic design will select a 457 x 191 x 74 UB (one section weight heavier) with 45 studs at 200 mm centres and transverse shear reinforcement of H10’s at 200 mm centres. Whether or not this is a better design is not really the concern of this example, it is simply a different design based in different engineering input. The key issue (that should not be missed) is that if you had specified a stud spacing of 500mm, then composite design would not be possible for this beam.

Design Pass 3 Select the non-composite icon ( ) and then re-design the beam as a simple non composite beam. This time the design indicates that you need to use a 533 x 210 x 101 UB if you cannot provide the transverse shear reinforcement necessary to allow the beam to be designed for composite action.



Composite Beam Input (in Fastrak Building Designer) In order to create a composite beam within Fastrak Building Designer, you will need to define an appropriate set of composite beam attributes. Listed below is the typical procedure for defining these attributes. Items in brackets [] are optional.

Step

Dialog

Page

Instructions

1

none

none


2

Attribute Set

General


3

Attribute Set

Design

Choose Composite construction type

4

Attribute Set

Design


5

Attribute Set

Design


6

Attribute Set

Design


7


Size Constraints

[Define the Beam Constraints: • max and min beam size • optimize shear connection]

8


Sections for Study


9


Deflection

Define and apply deflection limits • [construction] • [slab dry] • imposed • [total]

10


Camber

[Apply camber]

11


Natural Frequency

[Define a limit for the natural frequency]

12

Attribute Set

Alignment, Type, Supports

[No changes are applicable to these attributes for composite beams]

13

Attribute Set

Size

Choose the steel grade and, if in Check Beam Mode choose the section size

14

Attribute Set

Type

Define construction stage bracing details

15

Attribute Set

Reinforcement

[Define continuity reinforcement over the beam]



Step

Dialog

Page

Instructions (Continued)

16

Attribute Set

Connectors

Choose the type of stud layout • Standard • Non-Standard

17

Attribute Set

Studs-Layout

Define stud length, material strength and stud layout

18

Attribute Set

Group Spacing

Define the stud spacing, or if in Design Beam Mode, you can choose to let the program position the studs automatically

*In order to speed the design process a distinction is made between those combinations consisting of gravity loads only and those which contain some components acting laterally (e.g. NHFs and wind loads). Setting composite beams to be designed for gravity loads only can significantly reduce the design time.



Composite Beam Input (when run as a standalone program) The design and check mode input procedures are listed below. Items in brackets [] are optional

Designing a beam In the typical procedure below items in brackets [] are optional.

Step

Icon

Instructions

1

Launch Composite Beam,

2


3

Choose the edge condition for the beam [and give the beam reference details],

4

Set Composite Beam into design beam mode,

5

Define the properties for the beam: • grade, • span,

6

Give the details of the floor construction: • floor construction type: • profiled metal decking • precast concrete slabs • profiled metal deck details or Bison precast concrete slab details (including slab information) • slab details (for profiled metal deck only) • reinforcement details (transverse and any other reinforcement present in the slab for profiled metal deck, transverse to beam only for Bison precast concrete slabs), • Define the shear connector type • shear studs • Hilti connectors • Define the shear connector layout, • construction stage restraint details (for profiled metal deck spanning parallel to or at less than 45° to direction of span of beam and for precast concrete slabs at your request),

7

Define the loadcases that apply to the beam including self-weight, construction stage loadcases and composite stage loadcases.


Step


Icon


8


9


10

Perform the design

11


12


13

Check the beam with the web openings. [Stiffen the web openings if necessary, or increase the size of the beam until the beam with openings is satisfactory.]

14


15


Checking a beam In the typical procedure below items in brackets [] are optional.

Step

Icon

Instructions

1

Launch Composite Beam,

2


3

Choose the edge condition for the beam [and give the beam reference details],

4

Set Composite Beam into check beam mode,

5

Define the properties for the beam: • section size, • grade, • span,



Step

Icon


6


7

Give the details of the floor construction: • floor construction type: • profiled metal decking • precast concrete slabs • profiled metal deck details or Bison precast concrete slab details (including slab information) • slab details (for profiled metal deck only) • reinforcement details (transverse and any other reinforcement present in the slab for profiled metal deck, transverse to beam only for Bison precast concrete slabs), • Define the shear connector type • shear studs • Hilti connectors • Define the shear connector layout, • construction stage restraint details (for profiled metal deck spanning parallel to or at less than 45° to direction of span of beam and for precast concrete slabs at your request),

8

Define the loadcases that apply to the beam including self-weight, construction stage loadcases and composite stage loadcases.

9


10


11

Perform the check, (including any web openings),

12

[Stiffen the web openings if necessary, or increase the size of the beam until the beam with openings is satisfactory.]

13


14




Further Information

Further information – Bison precast concrete slabs For further information or technical literature on Bison Flooring Systems please contact the Bison Technical Department at the address shown below between the hours of 9:00 am and 5:00 pm, Monday to Friday. Bison Concrete Products Ltd. Amington House, Silica Road Tamworth Staffordshire  England B77 4AZ Tel. 01827 64141 Fax. 01827 69009 For sales enquiries please contact the Sales Department during the same times.

Further information – Westok Beams For further information or technical literature on Westok Beams please contact Westok Technical Support and Design Service. Westok Structural Services Ltd. Horbury Junction Industrial Estate Horbury Junction Wakefield WF4 5ER Tel: 44-1924 264 121 Fax: 44-1924 280 030 email: [email protected]. You can also view this information while running the program by choosing Help / About Westok Structural Services Ltd… which shows the About Westok Structural Services Ltd. dialog.

You can click on the email link on this dialog to create a new email message to Westok.

Chapter 5 : Simple Column

Chapter 5


Simple Column

Introduction SIMPLE COLUMN - “steel column in a simply designed structure” Simple Column is a comprehensive design tool which allows you to analyse and design a structural steel column which complies with the requirements for simple construction. The column may have incoming simple beams which are capable of providing restraint, and may have splices along its length at which the section size may vary. You are responsible for designing the splices appropriately. Unless explicitly stated all calculations in Simple Column are in accordance with the relevant sections of BS 5950-1:2000. You may find the handbook and commentary to the Code of Practice published by the Steel Construction Institute useful.

Scope Simple Column uses the simplified rules for design of columns in simple construction as defined by BS 5950-1:2000 cl 4.7.7. For such columns it is not necessary to consider the effects of pattern loading. Using the entered geometry and the loads and eccentricities of the incoming elements, the program determines the x and y axis moments for the column. The loads include for up to 4 incoming beams at the column section being designed. The optimum section can be determined for the applied loading, or you can check the adequacy of a specified section. An international range of steel I-sections and hollow sections for many different countries can be specified. Concrete filled hollow sections can also be used.

Design Checks The following conditions are examined in accordance with BS 5950-1:2000: Section Classification - Clause 3.5.2 Cross-section classification is determined using Clause 3.5 and Table 11 or Table 12. The section classification must be Plastic (Class 1), Compact (Class 2) or Semi-compact (Class 3). Sections which are classified as Slender (Class 4) are beyond the scope of Simple Column. Local Capacity Checks - Clause 4.2.5.2 Checks are performed at the base and top of each lift and the critical position is identified. Overall Buckling Checks - Clause 4.7.3 & C.1 & B.2.1 Checks are performed at the base and top of each lift and the critical position is identified.



Worked Example If you want to work through this example you will find the file Engineer’s Example in the \documents and settings \ All Users \Application Data\CSC\Fastrak\ Examples folder. You can open and use this file, but you can not save

it away unless you change its name, this is done to protect the original. For an example we shall consider a corner column in a regular multi-storey structure. This has 5 regularly spaced floors so that each lift of column is 4 m as shown below.

The loading transmitted to the column by the incoming beams is:

Condition

Major Axis

Minor Axis

Dead

71 kN

44 kN

Live

44 kN

32 kN

This loading is defined in two load cases which are combined into a single combination.

Design pass 1 Using the Design Wizard to specify that the design is to look at UC sections only, with all sections available and designing the column yields a single stack of size 254 254 UC 73. Simple Column does not present a list of acceptable sections, since columns with splices already have multiple section size possibilities, which would give an inordinately large number of possibilities. Simple Column thus homes in to the first acceptable solution it finds. What would be the effect of adding splices to split the column into a series of different stacks?



Design pass 2 Switch back to design mode, and then access the Restraints dialog. Add restraints at floor levels 2 and 4.

Perform the design again, and this time you get 3 section sizes:

• 203 203 UC 46 for the top stack, • 203 203 UC 60 for the middle stack, • 254 254 UC 73 for the bottom stack. Design Pass 3 This design is OK, but there is a difference in section depth of 44.5 mm between the 203 203 UC 60 and 254 254 UC 73. You might feel that this amount of shimming is excessive, and ask is it possible to use a heavier weight of a 203 UC to reduce this?



Access the Design Wizard, and on the Size Constraints page set a maximum depth of 250 mm.

This means that Simple Column will reject any section whose depth is greater than 250 mm. If no heavier 203 UC section is adequate, then this means that Simple Column’s automatic design process will fail to find an appropriate section. Set automatic design mode and perform the design again. Simple Column now picks a 203 203 UC 86 for the bottom stack of the column. The difference in section depth is now 12.6 mm which is fine for shimming.

Design of concrete filled columns The following information is drawn from A design method for concrete-filled, hollow section, composite columns(Ref. 2). The paper outlines ‘a simple design procedure’ for ‘concrete-filled composite columns suitable for manual calculations, based on the recommendations given in BS 5950 for bare steel columns’.

Proposed method The paper proposes that the properties of the bare steel section be replaced by those of the composite section. BS 5950-1:2000 states that two checks are necessary: • a local capacity check;

• an overall buckling check. For each of these a simplified approach and a more exact approach are given. For the local capacity check, Simple Column adopts the simple approach since the values of Mrx and Mry are not available for composite sections. For the overall buckling check, Simple Column adopts the more exact approach.



Points to Note 1. The partial safety factor for steel, s, is taken as 1.0. 2. As not all hollow sections are plastic or compact, the values of B--t- and D---t are checked. If either value is greater than fifty the status of the check is set to invalid. 3. It is assumed that lateral torsional buckling does not occur. To ensure this is the case, Simple Column checks Table 15 of BS 5950-1:2000. If the section is within the specified limits then the above assumption is valid. Otherwise a warning message will be given indicating that the particular configuration is susceptible to lateral torsional buckling but that this has not been taken into account in the design. 4. The bonding strength between the hollow section and the concrete infill is not checked. 5. The concrete grade is limited to be between C25 and C50.

Simple Column Input (in Fastrak Building Designer) In order to create a simple column within Fastrak Building Designer, you will need to define an appropriate set of attributes. The typical procedure for defining these attributes is listed below. Items in brackets [] are optional.

Step

Dialog

Page

Instructions

1

none

none

Create a new Columns Attribute Set.

2

Attribute Set

General


3

Attribute Set

Design

Choose Simple construction type

4

Attribute Set

Design

Check the Automatic Design box if Design Column Mode is required, else leave it unchecked to work in Check Column Mode

5

Attribute Set

Design


6

Attribute Set

Design


7

Column Design Properties

Size Constraints

[Define any Column Size Constraints: • max and min section size]

8


Sections for Study

If in Design Column Mode choose the Order File

9

Attribute Set

Alignment

The angle and alignment can be set here.

10

Attribute Set

Floors

The number of floors and any splice locations can be set here.



Step

Dialog

Page


11

Attribute Set

Restraints

The major and minor axis restraints and effective lengths at each floor level can be set here.

12

Attribute Set

Eccentricities

Beam end reactions are applied at the offset specified from the column face.

13

Attribute Set

Size

Choose the steel grade and if in Check Column Mode choose the section size

*In order to speed the design process a distinction is made between those combinations consisting of gravity loads only and those which contain some components acting laterally (e.g. NHFs and wind loads). Setting columns to be designed for gravity loads only can significantly reduce the design time.

.Simple Column Input (when run as a standalone program) The design and check mode input procedures are listed below.

Designing a column In the typical procedure below items in brackets [] are optional.

Step

Icon

Instructions

1

Launch Simple Column,

2


3

Choose the type of column as either a Steel Section or a Concrete Filled Section [and give the Column reference details],

4

Set Simple Column into design mode,

5

Define the properties for the column: • the number of floors it carries and either the column lengths between floors, or the floor levels; • the column faces into which beams trim at each level (this can vary on a level by level basis).

6

Give the details of the positions of any splices and the restraint provided to the column by the incoming beams at each level.

7

[Specify any alterations to the standard eccentricities to be used to calculate the column’s eccentricity moments.]


Step


Icon

Instructions

8

[Specify the grade of steel to be investigated for each column stack (length from base–splice, splice–splice or splice–top).]

9

Define the loadcases that apply to the simple column.

10


11


12

Perform the design. Simple Column shows the first set of adequate sections that it finds for the column.

13


14


Checking a column In the typical procedure below items in brackets [] are optional.

Step

Icon

Instructions

1

Launch Simple Column,

2


3

Choose the type of column as either a Steel Section or a Concrete Filled Section [and give the Column reference details],

4

Set Simple Column into check mode,

5

Define the properties for the column: • the number of floors it carries and either the column lengths between floors, or the floor levels; • the column faces into which beams trim at each level (this can vary on a level by level basis).

6

Give the details of the positions of any splices and the restraint provided to the column by the incoming beams at each level.


Step

Icon


Instructions

7

[Specify any alterations to the standard eccentricities to be used to calculate the column’s eccentricity moments.]

8

Specify the size of section that you want to check for each column stack (length from base–splice, splice–splice or splice–top). [Specify the grade of steel for each column stack.]

9

Define the loadcases that apply to the simple column.

10


11

[Make any Design Wizard settings that you want to use to control the check.]

12

Perform the check. Simple Column shows the results for the sizes you have specified.

13


14


References and further information 1. British Standards Institution. BS 5950 : Structural use of steelwork in building; Part 1. Code of practice for design in simple and continuous construction: hot rolled sections. BSI 2000. 2. The Structural Engineer. Volume 75/No.21. A design method for concrete-filled, hollow section, composite columns. Y.C Wang and D.B. Moore. 1997.

Chapter 6 : General Beam

Chapter 6


General Beam

Introduction GENERAL BEAM - “non-composite steel beam designed as a beam/ column” The General Beam design application allows you to analyse and design a structural steel beam or cantilever which may have incoming beams providing restraint and which may or may not be continuously restrained over any length between restraints. In addition to major axis bending, it also considers minor axis bending and axial loads. Unless explicitly stated all calculations in General Beam are in accordance with the relevant sections of BS 5950-1:2000. You may find the handbook and commentary to the Code of Practice published by the Steel Construction Institute useful.

Scope In its simplest form a general beam can be a single member between supports to which it is pinned. It is distinguished from a standard simple beam primarily by the loading it has to resist. It can also be a continuous beam consisting of multiple members that do not, with the exception of the remote ends, transfer moment to the rest of the structure. General beams that share load with columns form part of a rigid moment resisting frame. The design of general beams is carried out for rolled sections, uniform and non-uniform plated sections. Fabsec beams can be specified but cannot be designed within Fastrak Building Designer. Web openings are not permitted. General beams can be connected to supports or to the supporting structure in a number of ways. For the meaning and implementation of the various choices see “Member End Fixity and Supports”. The options are subtly different depending upon whether the general beam is defined in the Building Designer1 or is defined within General Beam directly. Conditions of restraint can be defined in- and out-of-plane for strut buckling and top and bottom flange for lateral torsional buckling (LTB). It is upon these that the buckling checks are based. Where both flanges are provided with LTB restraints at the same position, they are simply considered as top and bottom flange restraints that just happen to be coincident, that is they are not treated as a torsional restraint. This means that, where a beam has one or more pairs of LTB restraints between supports, the checks are set up between supports and not between a support and an internal LTB restraint pair or between internal LTB restraint pairs. Footnotes 1. A general beam that is defined in Building Designer is referred to as a Building Designer general beam object.



When the general beam is an object in the Building Designer the design forces for strength and buckling checks are obtained from analysis of the member using the start forces for the member. These are obtained from the solver results. There can be a difference between the start forces from the Building Designer (analysis of the entire structure) and those obtained within General Beam (analysis of a limited model). Within General Beam a full range of loading is available, from which loadcases and design combinations can be created. General beams can be transferred from Building Designer to General Beam. When a general beam has been transferred from Building Designer in this way its loads and loadcases are editable. However any changes to these will invalidate the start and end forces obtained from the building model. To cater for this, if any load or loadcase is modified, a design in General Beam will reanalyse all the beam’s loadcases. Editing of the design combinations does not require reanalysis since the start and end forces are obtained by superposition. A full range of strength and buckling checks are available including Annex G Elastic to G.2.1. As mentioned above the buckling lengths are based on the restraints along the member. The effective lengths to use in the checks depend on: • the type of restraint particularly at supports,

• whether the loads or one component of the loads is destabilizing, • whether the frame is sway or non-sway in one or both directions – this has little effect on beam design. In all cases, General Beam sets the default effective length to 1.0L, it does not attempt to adjust the effective length (between supports for example) in any way. You are expected to adjust the effective length factor (up or down) as necessary. Any strut or LTB effective length can take the type ‘Continuous’ to indicate that it is continuously restrained over that length. Each span of a continuous beam can be of different section size, type and grade. The entire beam can be set to automatic design or check design.

Limitations and Assumptions Limitations The following limitations apply: • composite beams are excluded,

• continuous general beams (more than one span) must be co-linear in the plane of the web within a small tolerance (sloping in elevation is allowed),

• web openings are excluded, • Fabsec beams (with or without openings) are excluded, • sections with unequal flanges are excluded. This includes plated section beams that have unequal flanges, Slimflor beams and asymmetric Slimflor beams,

• there can be a difference in analysis results between those from Building Designer (analysis of the entire structure) and those when run in stand-alone (analysis of a limited model),

• there is no automatic generation of pattern loads either in the stand-alone or in Building Designer.



Assumptions All supports are considered to provide torsional restraint, that is lateral restraint to both flanges. This cannot be changed. It is assumed that a beam that is continuous through the web of a supporting beam or column together with its substantial moment resisting end plate connections is able to provide such restraint. If, at the support, the beam oversails the supporting beam or column then the detail is assumed to be such that the bottom flange of the general beam is well connected to the supporting member and, as a minimum, has torsional stiffeners provided at the support to Clause 4.5.7 of BS 5950-1: 2000. In the Building Designer model, when not at supports, coincident restraints to both flanges are assumed when one or more members frame into the web of the general beam at a particular position and the cardinal point of the centre-line model of the general beam lies in the web. Otherwise, only a top flange or bottom flange restraint is assumed. Should you judge the actual restraint provided by the in-coming members to be different from to what has been assumed, you have the flexibility to edit the restraints as required. Intermediate lateral restraints to the top or bottom flange are assumed to be capable of resisting the forces given in Clause 4.3.2.2 of BS 5950-1: 2000 and transferring these back to an appropriate system of bracing or suitably rigid part of the structure. Members that provide restraint to major or minor axis strut buckling are assumed to be capable of resisting 1% of the axial force in the restrained member and of transferring this to adjacent points of positional restraint as given in Clause 4.7.1.2 of BS 5905-1: 2000. It is assumed that you will make a rational and ‘correct’ choice for the effective lengths between restraints for both LTB and strut buckling. The default value for the effective length factor of 1.0 may be neither correct nor safe.

Analysis Building Modeller object The member end forces for each unfactored loadcase are obtained by submitting the whole model from the Building Designer to the solver. For a general beam in General Beam an appropriate sub-model is sent to the solver. The results from the Building Designer and those from General Beam may not be exactly the same due to (potential) differences inherent in using the full- and sub-model.

General Beam The capacity or resistance is only calculated when an applied force exists about the relevant axis that is greater than the “ignore forces below” value you have specified.



Ultimate Limit State – Strength The checks relate to doubly symmetric prismatic sections (that is rolled I- and H-sections), to singly symmetric sections i.e. channel sections and to doubly symmetric hollow sections i.e. SHS, RHS and CHS. Other section types are not currently covered. The strength checks relate to a particular point on the member and are carried out at 20th points and ‘points of interest’.

Classification General — The classification of the cross section is in accordance with BS 5950-1: 2000. General beam can be classified as: • Plastic Class = 1

• Compact • Semi-compact • Slender

Class = 2 Class = 3 Class = 4

Class 4 sections are not allowed. Sections with a Class 3 web can be taken as Class 2 sections (Effective Class 2) providing the cross section is equilibrated to that described in Clause 3.5.6 where the section is given an ‘effective’ plastic section modulus, Seff. For rolled I and H sections in the UK, this gives no advantage in pure bending since the web d/t is too small. Hence for general beams there is likely to be little advantage in using this approach since the axial loads are generally small, this classification is therefore not implemented. All unacceptable classifications are either failed in check mode or rejected in design mode. Hollow sections — The classification rules for SHS and RHS relate to “hot-finished hollow sections” only (“cold-formed hollow sections” are not included in this release). Important Note The classification used to determine Mb is based on the maximum axial compressive load in the relevant segment length. Furthermore, the Code clearly states that this classification should (only) be used to determine the moment capacity and lateral torsional buckling resistance to Clause 4.2 and 4.3 for use in the interaction equations. Thus, when carrying out the strength checks, the program determines the classification at the point at which strength is being checked.

Shear Capacity The shear check is performed according to BS 5950-1: 2000 Clause 4.2.3. for the absolute value of shear force normal to the x-x axis (Fvx) and normal to the y-y axis (Fvy), at the point under consideration. Shear buckling — When the web slenderness exceeds 70 shear buckling can occur in rolled sections. There are very few standard rolled sections that breach this limit. General Beam will warn you if this limit is exceeded, but will not carry out any shear buckling checks.



Moment Capacity The moment capacity check is performed according to BS 5950-1: 2000 Clause 4.2.5 for the moment about the x-x axis (Mx) and about the y-y axis (My), at the point under consideration. The moment capacity can be influenced by the magnitude of the shear force (“low shear” and “high shear” conditions). The maximum absolute shear to either side of a point load is examined to determine the correct condition for the moment capacity in that direction. Note Not all cases of high shear in two directions combined with moments in two directions along with axial load are considered thoroughly by BS 5950-1: 2000. The following approach is adopted by General Beam: • if high shear is present in one axis or both axes and axial load is also present, the cross-section capacity check is given a Beyond Scope status. The message associated with this status is “High shear and axial load are present, additional hand calculations are required for cross-section capacity to Annex H.3”. General Beam does not perform any calculations for this condition.

• if high shear and moment is present in both axes and there is no axial load (“biaxial bending”) the cross-section capacity check is given a Beyond Scope status and the associated message is, “High shear present normal to the y-y axis, no calculations are performed for this condition.”

• if high shear is present normal to the y-y axis and there is no axial load, the y-y moment check and the cross-section capacity check are each given Beyond Scope statuses. The message associated with this condition is, “High shear present normal to the y-y axis, no calculations are performed for this condition.”

Axial Capacity The axial capacity check is performed according to BS 5950-1: 2000 Clause 4.6.1 using the gross area and irrespective of whether the axial force is tensile or compressive. This check is for axial compression capacity and axial tension capacity. Compression resistance is a buckling check and as such is considered under “Compression Resistance”.

Cross-section Capacity The cross-section capacity check covers the interaction of axial load and bending to Clause 4.8.2 and 4.8.3.2 appropriate to the type (for example – doubly symmetric) and classification of the section. Since the axial tension capacity is not adjusted for the area of the net section then the formulae in Clause 4.8.2.2 and 4.8.3.2 are the same and can be applied irrespective of whether the axial load is compressive or tensile. The Note in “Moment Capacity” also applies here.



Ultimate Limit State – Buckling Lateral Torsional Buckling Resistance, Clause 4.3 For beams that are unrestrained over part or all of a span, a Lateral Torsional Buckling (LTB) check is required either: • in its own right, Clause 4.3 check,

• as part of an Annex G check, • as part of a combined buckling check to 4.8.3.3.1, 4.8.3.3.2 or 4.8.3.3.3, (see “Member Buckling Resistance, Clause 4.8.3.3.1”, “Member Buckling Resistance, Clause 4.8.3.3.2”, and “Member Buckling Resistance, Clause 4.8.3.3.3”, respectively) This check is not carried out under the following circumstances: • when bending exists about the minor axis only,

• when the section is a CHS or SHS, • when the section is an RHS that satisfies the limits given in Table 15 of BS 5950-1: 2000. For sections in which LTB cannot occur (the latter two cases above) the value of Mb for use in the combined buckling check is taken as the full moment capacity, Mcx, not reduced for high shear in accordance with Clause 4.8.3.3.3 (c), equation 2 (See “Member Buckling Resistance, Clause 4.8.3.3.3”). Effective lengths — The value of effective length factor is entirely at your choice. The default value is 1.0 for ‘normal’ loads and 1.2 for ‘destabilizing loads’. Different values can apply in the major and minor axis.

Lateral Torsional Buckling Resistance, Annex G This check is applicable to I- and H-sections with equal or unequal1 flanges. The definition of this check is the out-of-plane buckling resistance of a member or segment that has a laterally unrestrained compression flange and the other flange has intermediate lateral restraints at intervals. It is used normally to check the members in portal frames in which only major axis moment and axial load exist. Although not stated explicitly in BS 5950-1: 2000, it is taken that the lateral torsional buckling moment of resistance, Mb, from the Annex G check can be used in the interaction equations of Clause 4.8.3.3 (combined buckling). Since this is not explicit within BS 5950-1: 2000 a slight conservatism is introduced. In a straightforward Annex G check the axial load is combined with major axis moment. In this case both the slenderness for lateral torsional buckling and the slenderness for compression buckling are modified to allow for the improvement provided by the tension flange restraints (LT replaced by TB and  replaced by TC). When performing a combined buckling check in accordance with 4.8.3.3 the improvement is taken into account in determining the buckling resistance moment but not in determining the compression resistance. If the incoming members truly only restrain the tension flange, then you should switch off the minor axis strut restraint at these positions. Footnotes 1. Unequal flanged sections are not currently included.



The original source research work for the codified approach in Annex G used test specimens in which the tension flange was continuously restrained. When a segment is not continuously restrained but is restrained at reasonably frequent intervals it can be clearly argued that the approach holds true. With only one or two restraints present then this is less clear.BS 5950-1: 2000 is clear that there should be “at least one intermediate lateral restraint” (See Annex G.1.1). Nevertheless, you are ultimately responsible for accepting the adequacy of this approach. For this check General Beam sets mt to 1.0 and calculates nt. The calculated value of nt is based on Mmax being taken as the maximum of M1 to M5, and not the true maximum moment value where this occurs elsewhere in the length. The effect of this approach is likely to be small. If at any of points 1 - 5, R >11, then General Beam sets the status of the check to Beyond Scope. Reference restraint axis distance, a — The reference restraint axis distance is measured between some reference axis on the restrained member - usually the centroid - to the axis of restraint - usually the centroid of the restraining member. The measurement is shown diagramatically in Figure G.1 of BS 5950-1: 2000. General Beam does not attempt to determine this value automatically, since such an approach is fraught with difficulty and requires information from you which is only used for this check. Instead, by default, General Beam uses half the depth of the restrained section, and you can specify a value to be added to, or subtracted from, this at each restraint point. You are responsible for specifying the appropriate values for each restraint position. The default value of 0 mm may be neither correct nor safe.

Compression Resistance For most structures, all the members resisting axial compression need checking to ensure adequate resistance to buckling about both the major- and minor-axis. Since the axial force can vary throughout the member and the buckling lengths in the two planes do not necessarily coincide, both are checked. Because of the general nature of a general beam, it may not always be safe to assume that the combined buckling check will always govern. Hence the compression resistance check is performed independently from the other strength and buckling checks. Effective lengths — The value of effective length factor is entirely at your choice. The default value is 1.0 for ‘normal’ loads and 1.2 for ‘destabilizing loads’. Different values can apply in the major and minor axis. Beams are less affected by sway than columns but the effectiveness of the incoming members to restrain the beam in both position and direction is generally less than for columns. Hence, it is less likely that effective length factors greater than 1.0 will be required but equally factors less than 1.0 may not easily be justified. Nevertheless, it is your responsibility to adjust the value from 1.0 and to justify such a change. Of more importance in beam design is the possible existence of destabilizing loads. These can affect the effective length for lateral torsional buckling (see “Lateral Torsional Buckling Resistance, Clause 4.3”).

Footnotes 1. Which could happen since R is based on Z and not S.



Please note that the requirements for slenderness limits in (for example l/r  180) are no longer included in BS 5950-1: 2000. Consequently General Beam does not carry out such checks. Accordingly, for lightly loaded members you should ensure that the slenderness ratio is within reasonable bounds to permit handling and erection and to provide a reasonable level of robustness.

Member Buckling Resistance, Clause 4.8.3.3.1 This check is used for channel sections. Such sections can be Class 1, 2 or 3 Plastic, Compact or Semi-compact (Class 4 Slender sections and Effective Class 2 sections are not allowed in this release). Note that, whilst this check could be used for any section type dealt with in the subsequent sections, the results can never be any better than the alternatives but can be worse. Two formulae are provided in Clause 4.8.3.3.1, both are checked; the second is calculated twice – once for the top flange and once for the bottom flange. See also the Important Note at the end of “Member Buckling Resistance, Clause 4.8.3.3.2”. Only one value of F is used, the worst anywhere in the length being checked. If the axial load is tensile, then F is taken as zero. If this check is invoked as part of an Annex G check, and thus Mb is governed by Annex G, then mLT is taken as 1.0.

Member Buckling Resistance, Clause 4.8.3.3.2 This check is used for Class 1, 2 and 3 Plastic, Compact and Semi-compact rolled I- and H-sections with equal flanges (Class 4 Slender sections and Effective Class 2 sections are not included in this release). Three formulae are provided in Clause 4.8.3.3.2 (c) to cover the combined effects of major and minor axis moment and axial force.These are used irrespective of whether all three forces / moments exist. Clause 4.9 deals with biaxial moment in the absence of axial force, Clause 4.8.3.3.2 (c) can also be used in such cases by setting the axial force to zero. All three formulae in Clause 4.8.3.3.2 (c) are checked; the second is calculated twice – once for each flange. Only one value of F is used, the worst anywhere in the length being checked. If the axial load is tensile, then F is taken as zero. Important Note Clause 4.8.3.3.4 defines the various equivalent uniform moment factors. The last three paragraphs deal with modifications to these depending upon the method used to allow for the effects of sway. This requires that for sway sensitive frames the uniform moment factors, mx, my and mxy, should be applied to the non-sway moments only. In this release there is no mechanism to separate the sway and non-sway moments, General Beam adopts the only conservative approach and sets these 'm' factors equal to 1.0 if the frame is sway sensitive (in either direction). This is doubly conservative for sway-sensitive unbraced frames since it is likely that all the loads in a design combination and not just the lateral loads will be amplified.



In such a case, both the sway and non-sway moments are increased by kamp and neither are reduced by the above ‘m’ factors. The calculation of mLT is unaffected by this approach, and thus if the second equation of Clause 4.8.3.3.2 (c) governs, then the results are not affected.

Member Buckling Resistance, Clause 4.8.3.3.3 This check is used for Class 1, 2 and 3 Plastic, Compact and Semi-compact hollow sections (Class 4 Slender sections and Effective Class 2 sections are not included in this release). Four formulae are provided in Clause 4.8.3.3.3 (c) to cover the combined effects of major and minor axis moment and axial force. These are used irrespective of whether all three forces / moments exist. Clause 4.9 deals with biaxial moment in the absence of axial force, Clause 4.8.3.3.3 (c) can also be used in such cases by setting the axial force to zero. The second and third formulae are mutually exclusive – that is the second is used for CHS, SHS and for RHS when the limits contained in Table 15 are not exceeded. On the other hand the third formula is used for those RHS that exceed the limits given in Table 15. Thus only three formulae are checked; the first, second and fourth or the first, third and fourth. Either the second or third (as appropriate) is calculated twice – once for each ‘flange’. Only one value of F is used, the worst anywhere in the length being checked. If the axial load is tensile, then F is taken as zero. See also the Important Note at the end of “Member Buckling Resistance, Clause 4.8.3.3.2”.

Serviceability Limit State Building Designer calculates both relative and absolute deflections. (See “Deflection checks”in the Basic Principles chapter of this handbook.) Relative deflections are given in the member analysis results graphics and are the ones used in the member design. For beams, it is the in-plane deflections that are of most interest. However, both in-plane and out-of-plane deflections are given – ‘local z’ and ‘local y’ deflections respectively. Results are given for all ‘Dead’ loads, all ‘Imposed’ loads and for ‘Total’ loads in a particular design combination. In all cases these are the sum of the deflections for each appropriate unfactored loadcase, that is the load factor is taken as 1.0. Where appropriate the maximum deflection for both the positive and negative local directions is given and compared with the limits specified in the Design Wizard. This comparison is only made for major axis deflections (local z) in the current release. In the stand-alone of General Beam the graphic will show deflection in the ‘local z’ and ‘local y’ for both individual loadcases and for design combinations based on unfactored loadcases.

Member End Fixity and Supports In order to provide a robust design model, the fixity at member ends and the associated supporting structure or supports to ground must be compatible with the type of connection, base and foundation that is to be used.



General Beam Stand-alone Internal supports are designated as Continuous and you cannot edit these. At the remote ends of the beam there are a number of options for the combined end fixity and support conditions. These are given below: • Free end — as in a cantilever,

• Simple connection — pinned to the support or supporting member. This means pinned about the major and minor axes of the section but fixed torsionally,

• Moment connection — major axis moment connection, and pinned about the minor axis. This option requires the size and length to the point of contraflexure of the columns above and below the connection,

• Fully fixed — encastré, all degrees of freedom fixed. Building Designer End fixity in continuous beams — Whilst in the stand-alone program member end fixity and supports are dealt with as one entity, in the Building Designer supports are a separate issue and hence are dealt with separately below. All internal connections are considered Continuous – if a pin were to be introduced at an internal position then there would be two beams, hence you cannot edit this setting. At the remote ends of the beam there are a number of options for the end fixity depending upon to what the end of the beam is connected. These are: • If not connected to a beam or column or to a supplementary support –

• Free end (default!) • If connected to an existing member – • Simple connection (default) • Moment connection • If connected to a Supplementary Support – • Simple connection (default) • Fully fixed. The interpretation of these descriptions in relation to being pinned about a particular axis is the same as in General Beam Stand-alone. Moment connections to supporting beams at the remote ends of general beams are prevented. Similarly, for such connections to the web of an I / H section column or to the face of a hollow section column. If you attempt to use such a connection General Beam issues a warning message. This is to draw your attention to the difficulty and cost of making such a connection and, perhaps more importantly, to the possibility that such a joint will not behave as fully rigid.



Design Procedure Lateral torsional buckling checks The process for carrying out LTB checks is determined by whether the beam has intermediate restraints to the top or bottom flange, or both. The impact of one or more of the segment lengths being continuously restrained is also considered. The principal check is that given in Clause 4.3 of BS 5950-1: 2000 but in certain circumstances an Annex G.2 check is also carried out between torsional restraints. You have full control of whether at a particular position one or both flanges are restrained. Restraint to both flanges that are coincident will be taken as torsional1. More information on restraints and the LTB check itself are given in “Assumptions” and “Lateral Torsional Buckling Resistance, Clause 4.3” respectively. Section types that are not susceptible to LTB for example circular and square hollow sections are not processed. General Beam identifies the relevant checks and the lengths over which these checks are performed – these lengths are termed ‘segment lengths’. There is a segment length for each Clause 4.3 check and each Annex G.2 check. For each individual check the following are determined within the segment length: • maximum moment, Mx,

• uniform moment factor, mLT, based on the moment profile – Clause 4.3 only, • slenderness correction factor, nt, based on the moment ratios – Annex G.2 only. The check process generates a set of checks and their associated segment lengths in accordance with Clause 4.32. As part of the Annex G check each segment length between restraints to the top and bottom flanges is also checked to Clause 4.3 separately3. This is

Footnotes 1. In this release, such ‘torsional restraints’ are simply considered as top and bottom flange restraints that just happen to be coincident. This means that, where a beam has one or more ‘torsional restraints’ between supports, the checks are set up between supports and not between a support and an internal torsional restraint or between internal torsional restraints. 2. These are referred to as ‘proper’ 4.3 checks. 3. These are referred to as ‘Annex G’ 4.3 checks.



irrespective of whether these restraints are to the compression or tension flange. This can result in checks over the same length or different lengths to the ‘proper’ Clause 4.3 checks. An example is given below. ‘proper’ 4.3

‘proper’ 4.3

‘Annex G’ 4.3

‘Annex G’ 4.3

‘Annex G’ 4.3

points of contraflexure

‘Annex G’ 4.3

‘Annex G’ 4.3 Annex G

LTB restraints and checks In this example, the ‘proper’ Clause 4.3 checks that are identified are between the torsional restraint and the first intermediate restraint that restrains the top flange when in compression, and, between this restraint and the final torsional restraint. Three checks to the top flange and two to the bottom flange are carried out as part of the Annex G check over the whole length. These in contrast are between restraints that are sometimes to the compression flange and sometimes to the tension flange.

Combined buckling checks From performing LTB and Strut buckling checks there are a series of segments over which the various checks have been carried out. For LTB, there can be a set of these between each pair of torsional restraints – typically but not exclusively between supports (only between supports in the first release). These sets can differ between the top flange and the bottom flange. at each 20th point along each span General Beam determines the segment in which it lies considering LTB of the top flange, LTB of the bottom flange, in-plane strut buckling and out-of plane strut buckling. Also, for each segment General Beam ascertains the following: • an associated effective length,

• a resistance, • the maximum axial load or moment, • and for LTB the moment profile for determining ‘m’ or ‘nt’. For LTB there can be up to three segment lengths for each point, • that associated with a ‘proper’ Clause 4.3 check,

• that associated with an Annex G check, • that associated with intermediate restraint Clause 4.3 check carried out as part of the Annex G check.



An example illustrating how the checks are applied to I- and H-sections with equal flanges (4.8.3.3.2 (c)) is given below. 1.0

3.5

1.5

points of contraflexure

2.6

3.4

LTB and strut buckling checks The beam (span) is 6.0 m long and has torsional restraints at each end. The top flange is restrained out-of-plane at 1.0 m and 4.5 m – these provide restraint to the top flange for LTB and to the beam as a whole for out-of-plane strut buckling. The bottom flange has one restraint at 2.6 m and this restrains the bottom flange for LTB and the beam as a whole for in-plane strut buckling. (This is probably difficult to achieve in practice but is useful for illustration purposes.) General Beam identifies the following lengths and checks. (in this example all the effective length factors are assumed to be 1.0 for simplicity.)

Top flange segment

Bottom flange segment

In-plane strut segment

Out-of-plan e strut segment

length

check

length

check

length

check

0 – 4.5

Proper 4.3

0 – 6.0

Annex Ga

0 – 2.6

0 – 1.0

4.5 – 6.0

Proper 4.3

0 – 2.6

Annex G 4.3

2.6 – 6.0

1.0 – 4.5

0 – 6.0

Annex Ga

2.6 – 6.0

Annex G 4.3

0 – 1.0

Annex G 4.3

1.0 – 4.5

Annex G 4.3

4.5 – 6.0

Annex G 4.3

4.5 – 6.0

a. Only one Annex G is reported – that with the smallest value of nt.

Worked Example If you want to work through this example you will find the file Engineer’s Example 1 in the \documents and settings \ All Users \Application Data\CSC\Fastrak\ Examples folder. You can open and use this file, but you can not save it away unless you change its name, this is done to protect the original.



Let’s take a simple example of a continuous 2 x 9 m span spine beam with 6 m span secondary beams at third points.


Condition

Value

giving point load at the middle support, and at 3 m and 6 m on each beam of

Dead

2.0 kN/m2

36kN

Services

1.0 kN/m2

18kN

Live

5.0 kN/m2

90kN

For the purposes of this example the point load on the middle support is specified by defining half the load at the end of beam span 1 and half at the start of beam span 2. The incoming beams are such that they provide restraint against lateral torsional buckling to both flanges, but they don’t provide restraint against strut buckling. The ends of the beam have simple supports onto other beams.



Design pass 1 Performing a design for UB sections only, and with all the non-preferred sections excluded from the design process (see the example in the Simple Beam Engineer’s Handbook), the first section presented is a 610 210 UB 101. If you look at the analysis results you will see that all the results are symmetric.

General Beam does not automatically consider pattern loading. If you want to do so, then you must specify the appropriate load cases and combinations yourself.

Design pass 2 If you want to work through this example you will find the file Engineer’s Example 2 in the \documents and settings \ All Users \Application Data\CSC\Fastrak\ Examples folder. You can open and use this file, but you can not save it away unless you change its name, this is done to protect the original. For this example you determine that the pattern that you want to consider is: • full service load on one span, with 50% service load on the other, and

• full live load on one span with no live load on the other. This means that you need six new loadcases: • Full Service Span 1,

• • • • •

Full Service Span 2, 50% Service Span 1, 50% Service Span 2, Live Span 1, and Live Span 2.

The loadings that these require are easy to derive from the loading given above. You also need 2 new combinations for the pattern loading: • Pattern 1 - Dead + Full Service Span 1 + 50% Service Span 2 + Live Span 1, and



• Pattern 2 - Dead + 50% Service Span 1 + Full Service Span 2 + Live Span 2. You can review the analysis results for these two combinations immediately.



If you want to review the design results, then you need to re-perform the design. This time the design is virtually instantaneous, General Beam simply checks the existing section, since after the first design it automatically changed from designing sections to checking them.

As expected it is the deflections that are affected by the pattern loading, and the shears at the ends of the beam (as they apply to the supports).

Design Pass 3 If you click the Design Beam icon again, and then perform the design and you will find that this takes significantly longer than the initial one (Design Pass 1). This is because General Beam now has to work with three combinations, rather than then initial one. The design checks have to thus run three times for each section which General Beam investigates. On completion of the design you will find that you are presented with the same list of acceptable sections as for Design Pass 1.



General Beam Input (in Fastrak Building Designer) In order to create a general beam within Fastrak Building Designer, you will need to define an appropriate set of attributes. Listed below is the typical procedure for defining these attributes. Items in brackets [] are optional.

Step

Dialog

Page

Instructions

1

none

none


2

Attribute Set

General


3

Attribute Set

Design

Choose General construction type

4

Attribute Set

Design


5

Attribute Set

Design


6


Size Constraints

[Define the Beam Constraints: • max and min beam size]

7


Sections for Study


8


Deflection

Define and apply deflection limits • [dead] • live • [total]

9

Attribute Set

Alignment

[Change the rotation/alignment of the beam as required]

10

Attribute Set

Beam

Choose the steel grade and, if in Check Beam Mode choose the section size for each span

11

Attribute Set

Supports

Define the supports required at the remote ends of the beam.

12

Attribute Set

Braced (LTB)

Specify if the beam top flange is continuously braced.

General Beam Input (when run as a standalone program) The input procedures for running the program in either design or check mode are listed in the tables below.



Designing a beam Items in brackets [] are optional

Step

Icon

Instructions

1

Launch General Beam.

2

Create a new project giving the project name [and other project details].

3

Give the beam reference details.

4

Set General Beam into design beam mode.

5

Define the properties for the beam: • number of spans, span lengths, section types and section grades; • end support conditions.

6

Give the details of the beam restraints for lateral-torsional- and strut-buckling.

7

Define the loadcases that apply to the beam.

8

Incorporate the loadcases into a series of design combinations.

9


10

Perform the design.

11

From the list of suitable sections preview the results for the more desirable sections and then choose the one that you would like to use.

12

Check the beam.

13


14




Checking a beam Items in brackets [] are optional.

Step

Icon

Instructions

1

Launch General Beam,

2


3

Give the beam reference details,

4

Set General Beam into check beam mode,

5

Define the properties for the beam: • number of spans, span lengths, section sizes and section grades; • end support conditions.

6


7

Define the loadcases that apply to the beam.

8


9


10

Perform the check,

11


12


Chapter 7 : General Column

Chapter 7


General Column

Introduction GENERAL COLUMN - “steel column designed as a beam/column” The General Column design application allows you to analyse and design a structural steel column which can have moment or simple connections with incoming member, and which can have fixity applied at the base. The column can have incoming beams which may also be capable of providing restraint, and may have splices along its length at which the section size may vary. You are responsible for designing the splices appropriately. Unless explicitly stated all calculations in General Column are in accordance with the relevant sections of BS 5950-1: 2000. You may find the handbook and commentary to the Code of Practice published by the Steel Construction Institute useful.

Scope In its simplest form a general column can be a single member between ‘construction levels’ that are designated as floors. It is distinguished from a standard simple column primarily by the loading it has to resist and by the use of a more rigorous approach to design than the formulae adopted for columns in simple construction. More typically a general column will be continuous past one or more floor levels, the whole forming one single entity typically from base to roof. General columns that share moments with general beams form part of a rigid moment resisting frame. The design of general columns is carried out for I-sections, H-sections and hollow sections only. Concrete filled hollow sections are not permitted. The top and bottom of each stack in the general column can be either pinned or fixed which means that the member is pinned about both the major and minor axes in the first case, or fixed about both axes in the second. The floors that define the stacks can be designated either as ‘to be’ or ‘not to be’ included in the determination of the imposed load reductions through a “Count as supported” check box. This feature enables what appears to be a roof to be counted as a floor, or conversely allows a mezzanine floor to be excluded from the number of floors considered for any particular general column. Also, floors can be designated to not have their imposed loads reduced, for example if they are storage or plant floors. In this case the full loading on that floor will be used in determining the reactions onto the column. The moments from fixed ended beams framing into a column are never reduced. Splices are allowed at floor levels only and must be placed at changes of angle between two adjacent stacks and at changes of section size or type. A validation error will result if this is not the case. The splice can be given an offset from the floor level - the default of 500mm is



considered not to be structurally significant. You must detail the splice to resist the applied forces and moments. The detail should provide continuity of stiffness and strength or if designated as pinned in analysis terms be capable of acting as such i.e. has only nominal moment capacity and sufficient rotation capacity. Splices given considerable offset should take account of the P- moment at the position as well as the forces from the analysis. See Clause 6.1.8.2. of BS 5950-1: 2000. Design forces are obtained from the Building Designer. Individual loads. loadcases and combinations can only be added through the Building Designer. The design combinations can be edited since the start and end forces are obtained by superposition. You may find this useful for re-combining the basic loadcases to account for pattern loading although the constituent loadcases would need to be separated in the building model. The (subtle) difference between a column acting as a beam-column and a beam acting as a beam-column is the predominance of axial force in the former. Thus the main design criteria are those given in Clause 4.8 of BS 5950-1: 2000 (although individual capacity and bucking checks are also carried out). Restraints to strut buckling are determined from the incoming members described within the Building Designer. The buckling checks are based on these. Restraining members framing into either Face A or C will provide restraint to major axis strut buckling. Members framing into either Face B or D will provide restraint to minor axis strut buckling. Building Designer determines the strut buckling restraints and you cannot edit these. Similarly members framing into Face B or D will provide lateral torsional buckling (LTB) restraint. General Column always assumes full restraint at the base and at the roof level when carrying out LTB design checks – you are warned on validation if your LTB restraint settings do not reflect this. Restraints are considered effective on a particular plane providing they are within ±45° to the local coordinate axis system. A full range of strength and buckling checks are available including Annex G Elastic to G.2.1. As mentioned above the buckling lengths are based on the restraints along the member. The effective lengths to use in the checks depend on: • the type of restraint particularly at floor levels,

• whether the frame is sway or non-sway in one or both directions – this has a significant effect on the choice of effective length factor. Non-sway effective length factors are likely to vary from 0.7 to 1.0 whereas for sway directions the variation could be from 1.0 to infinity! (see Table 22 and Annex E of BS 5950-1: 2000). In all cases General Column sets the default effective length to 1.0L, it does not attempt to adjust the effective length in any way. You are expected to adjust the effective length factor (up or down) as necessary. Any strut or LTB effective length can take the type ‘Continuous’ to indicate that it is continuously restrained over that length. Each lift (length between splices) of a general column can be of different section size and grade. Different section types within the same column are not allowed due to the particularly complex design routines that general columns require. You are responsible for guaranteeing that the splice detail ensures that the assumptions in the analysis model are achieved and that any difference in the size of section between lifts can be accommodated practically. The entire column can be set to automatic design or check design.



A sway assessment is performed although this can optionally be de-activated for those columns for which it would be inappropriate, (by unchecking the Alpha Crit Check box on the Column Properties dialog).

Limitations and Assumptions Limitations The following general limitations apply: • concrete filled hollow section columns are excluded,

• sections with unequal flanges are excluded. This includes plated section beams that have unequal flanges,

• there is no automatic generation of pattern loads. Sloping General Columns The following additional limitations apply: • the web of each stack of a sloping column must lie in the same plane,

• sloping general columns are limited to having either their web, or flanges in a vertical plane.

• eccentricity moments are not taken into account in design, • there is no imposed load reduction. Assumptions 1. The program assumes that any member framing into the major or minor axis of the column provides restraint against strut buckling in the appropriate plane. These restraints are set and cannot be changed. If you believe that a certain restraint in a particular direction is not effective then you can adjust the effective length to suit – to 2.0L for example. 2. The program assumes that any member framing into Face B or D provides restraint against lateral torsional buckling at that level. There are a number of practical conditions that could result in torsional restraint not being provided at floor levels. At construction levels this is even more likely given the likely type of incoming member and its associated type of connection. You must consider the type of connection between the incoming members and the column since these can have a significant influence on the ability of the member to provide restraint to one, none or both column flanges. For example, consider a long fin plate connection for beams framing into the column web where the beam stops outside the column flange tips to ease detailing. The fin plate is very slender and the beam end is remote from the column flanges such that it may not be able to provide any restraint to LTB. The fact that a slab is usually present may mitigate this. You are expected to adjust the effective lengths to allow for ineffective restraint. 3. Where you provide torsional restraint, it is assumed that you will also provide some system of restraint to both flanges, and that this is taken back to an independent bracing system which is capable of resisting the force couple given in Clause 4.3.3 of BS 5950-1: 2000. An example of such a system is sheeting rails defined at a construction level (they



are not floors by definition) with stays to the inside flange of the column, the sheeting rails being taken back in a continuous line to a braced bay and checked for the appropriate restraint forces. 4. Intermediate lateral restraints to the flange on Face A or on Face C are assumed to be capable of resisting the forces given in Clause 4.3.2.2 of BS 5950-1: 2000 and transferring these back to an appropriate system of bracing or suitably rigid part of the structure. 5. Members that provide restraint to major or minor axis strut buckling are assumed to be capable of resisting 1% of the axial force in the restrained member and of transferring this to adjacent points of positional restraint as given in Clause 4.7.1.2 of BS 5905-1: 2000. 6. It is assumed that you will make a rational and ‘correct’ choice for the effective lengths between restraints for both LTB and strut buckling. The default value for the effective length factor of 1.0 may be neither correct nor safe.

Analysis Building Modeller Object The member end forces for each unfactored loadcase are obtained by submitting the whole model from the Building Designer to the solver. There is a particular difficulty with general columns in that there may exist both ‘real’ moments and eccentricity moments from beam end reactions. The effect of the eccentricity moments can reduce those real moments from frame action that are design critical. However, there are cases where this is not true and so the eccentricity moments are included to prevent over- or under-design due to their presence. General Column checks strength and buckling against the maximum moment due to the algebraic sum of real and eccentricity moments in two directions. General Column determines the uniform moment factors for use in the buckling interaction equations only from the profile of real moments and these factors are applied only to the real moments. Note that the eccentricity moments only apply at the ends of the stack and not at intermediate positions.

Ultimate Limit State – Strength The checks relate to doubly symmetric prismatic sections i.e. rolled I- and H-sections and to doubly symmetric hot-finished hollow sections i.e. SHS, RHS and CHS. Other section types are not currently covered. The strength checks relate to a particular point on the member and are carried out at 10th points and ‘points of interest’. 1

Classification General — The classification of the cross section is in accordance with BS 5950-1: 2000.

Footnotes 1. ‘Points of interest’ are such positions as maximum moment, maximum axial etc.



General columns can be classified as: • Plastic Class = 1

• Compact • Semi-compact • Slender

Class = 2 Class = 3 Class = 4

Class 4 sections are not allowed. Sections with a Class 3 web can be taken as Class 2 sections (Effective Class 2) providing the cross section is equilibrated to that described in Clause 3.5.6 where the section is given an ‘effective’ plastic section modulus, Seff. This approach is not adopted in the current version of General Column. All unacceptable classifications are either failed in check mode or rejected in design mode. Hollow sections — The classification rules for SHS and RHS relate to “hot-finished hollow sections” only (“cold-formed hollow sections” are not included in this release). Important Note The classification used to determine Mb is based on the maximum axial compressive load in the relevant segment length. Furthermore, the Code clearly states that this classification should (only) be used to determine the moment capacity and lateral torsional buckling resistance to Clause 4.2 and 4.3 for use in the interaction equations. Thus, when carrying out the strength checks, the program determines the classification at the point at which strength is being checked.

Shear Capacity The shear check is performed according to BS 5950-1: 2000 Clause 4.2.3. for the absolute value of shear force normal to the x-x axis and normal to the y-y axis, Fvx and Fvy, at the point under consideration. Shear buckling — When the web slenderness exceeds 70 shear buckling can occur in rolled sections. There are very few standard rolled sections that breach this limit. General Column will warn you if this limit is exceeded, but will not carry out any shear buckling checks.

Moment Capacity The moment capacity check is performed according to BS 5950-1: 2000 Clause 4.2.5 for the moment about the x-x axis and about the y-y axis, Mx and My, at the point under consideration. The moment capacity can be influenced by the magnitude of the shear force (“low shear” and “high shear” conditions). The maximum absolute shear to either side of a point of interest is used to determine the moment capacity for that direction. High shear condition about x-x axis — The treatment of high shear is axis dependent. In this release for CHS, if high shear is present, the moment capacity about the x-x axis is not calculated, the check is given a Beyond Scope status and an associated explanatory message. High shear condition about y-y axis — For rolled and plated sections in this release, if high shear is present normal to the y-y axis then the moment capacity about the y-y axis is not calculated, the check is given a Beyond Scope status and an associated explanatory message.



For hollow sections, there is greater potential for the section to be used to resist the principal moments in its minor axis. Of course for CHS and SHS there is no major or minor axis and so preventing high shear arbitrarily on one of the two principal axes does not make sense. Nevertheless, if high shear is present normal to the y-y axis then in this release the moment capacity about the y-y axis is not calculated, the check is given a Beyond Scope status and an associated explanatory message. Note Not all cases of high shear in two directions combined with moments in two directions along with axial load are considered thoroughly by BS 5950-1: 2000. The following approach is adopted by General Column: • if high shear is present in one axis or both axes and axial load is also present, the cross-section capacity check is given a Beyond Scope status. The message associated with this status is “High shear and axial load are present, additional hand calculations are required for cross-section capacity to Annex H.3”. General Beam does not perform any calculations for this condition.

• if high shear and moment is present in both axes and there is no axial load (“biaxial bending”) the cross-section capacity check is given a Beyond Scope status and the associated message is, “High shear present normal to the y-y axis, no calculations are performed for this condition.”

• if high shear is present normal to the y-y axis and there is no axial load, the y-y moment check and the cross-section capacity check are each given Beyond Scope statuses. The message associated with this condition is, “High shear present normal to the y-y axis, no calculations are performed for this condition.”

Axial Capacity The axial capacity check is performed according to BS 5950-1: 2000 Clause 4.6.1 using the gross area and irrespective of whether the axial force is tensile or compressive. This check is for axial compression capacity and axial tension capacity. Compression resistance is a buckling check and as such is considered under “Compression Resistance”.

Cross-section Capacity The cross-section capacity check covers the interaction of axial load and bending to Clause 4.8.2 and 4.8.3.2 appropriate to the type (for example – doubly symmetric) and classification of the section. Since the axial tension capacity is not adjusted for the area of the net section then the formulae in Clause 4.8.2.2 and 4.8.3.2 are the same and can be applied irrespective of whether the axial load is compressive or tensile. The Note in “Moment Capacity” also applies here.

Ultimate Limit State – Buckling Lateral Torsional Buckling Resistance, Clause 4.3 For columns that are unrestrained over part or all of a span, a Lateral Torsional Buckling (LTB) check is required either: • in its own right, Clause 4.3 check,



• as part of an Annex G check, • as part of a combined buckling check to 4.8.3.3.2 or 4.8.3.3.3, (see “Member Buckling Resistance, Clause 4.8.3.3.2”, and “Member Buckling Resistance, Clause 4.8.3.3.3”). This check is not carried out under the following circumstances: • when bending exists about the minor axis only,

• when the section is a CHS or SHS, • when the section is an RHS that satisfies the limits given in Table 15 of BS 5950-1: 2000. For sections in which LTB cannot occur (the latter two cases above) the value of Mb for use in the combined buckling check is taken as the full moment capacity, Mcx, not reduced for high shear in accordance with Clause 4.8.3.3.3 (c), equation 2 (see “Member Buckling Resistance, Clause 4.8.3.3.3”). Destabilising loads are excluded from General Column, this is justified by the rarity of the necessity to apply such loads to a column. If such loads do occur, then you can adjust the ‘normal’ effective length to take this into account although you can not achieve the code requirement to set mLT to 1.0. Effective lengths — The value of effective length factor is entirely at your choice. The default value is 1.0.

Lateral Torsional Buckling Resistance, Annex G This check is applicable to I- and H-sections with equal or unequal1 flanges. The definition of this check is the out-of-plane buckling resistance of a member or segment that has a laterally unrestrained compression flange and the other flange has intermediate lateral restraints at intervals. It is used normally to check the members in portal frames in which only major axis moment and axial load exist. Although not stated explicitly in BS 5950-1: 2000, it is taken that the lateral torsional buckling moment of resistance, Mb, from the Annex G check can be used in the interaction equations of Clause 4.8.3.3 (combined buckling). Since this is not explicit within BS 5950-1: 2000 a slight conservatism is introduced. In a straightforward Annex G check the axial load is combined with major axis moment. In this case both the slenderness for lateral torsional buckling and the slenderness for compression buckling are modified to allow for the improvement provided by the tension flange restraints (LT replaced by TB and  replaced by TC). When performing a combined buckling check in accordance with 4.8.3.3 the improvement is taken into account in determining the buckling resistance moment but not in determining the compression resistance. If the incoming members truly only restrain the tension flange, then you should switch off the minor axis strut restraint at these positions. The original source research work for the codified approach in Annex G used test specimens in which the tension flange was continuously restrained. When a segment is not continuously restrained but is restrained at reasonably frequent intervals it can be clearly argued that the approach holds true. With only one or two restraints present then this is less clear.BS 5950-1: Footnotes 1. Unequal flanged sections are not currently included.



2000 is clear that there should be “at least one intermediate lateral restraint” (See Annex G.1.1). Nevertheless, you are ultimately responsible for accepting the adequacy of this approach. For this check General Column sets mt to 1.0 and calculates nt. The calculated value of nt is based on Mmax being taken as the maximum of M1 to M5, and not the true maximum moment value where this occurs elsewhere in the length. The effect of this approach is likely to be small. If at any of points 1 - 5, R >11, then General Column sets the status of the check to Beyond Scope. Reference restraint axis distance, a — The reference restraint axis distance is measured between some reference axis on the restrained member - usually the centroid - to the axis of restraint - usually the centroid of the restraining member. The measurement is shown diagramatically in Figure G.1 of BS 5950-1: 2000. General Column does not attempt to determine this value automatically, since such an approach is fraught with difficulty and requires information from you which is only used for this check. Instead, by default, General Column uses half the depth of the restrained section, and you can specify a value to be added to, or subtracted from, this at each restraint point. You are responsible for specifying the appropriate values for each restraint position. The default value of 0 mm may be neither correct nor safe.

Compression Resistance For most structures, all the members resisting axial compression need checking to ensure adequate resistance to buckling about both the major- and minor-axis. Since the axial force can vary throughout the member and the buckling lengths in the two planes do not necessarily coincide, both are checked. Because of the general nature of a column, it may not always be safe to assume that the combined buckling check will always govern. Hence the compression resistance check is performed independently from all other strength and buckling checks. Effective lengths — The value of effective length factor is entirely at your choice. The default value is 1.0. Different values can apply in the major and minor axis. The minimum theoretical value is 0.5 and the maximum infinity for columns in rigid moment resisting (RMR) frames. Practical values for simple columns are in the range 0.7 to 2.0. Values less than 1.0 can be chosen for non-sway frames or for sway frames in which the effects of sway are taken into account using the amplified moments method. However, there is a caveat on the value of effective length factor even when allowance is made for sway. In particular for RMR frames, the principal moments due to frame action preventing sway are in one plane of the frame. There will often be little or no moment out-of-plane and so amplification of these moments has little effect. Nevertheless the stability out-of-plane can still be compromised by the lack of restraint due to sway sensitivity in that direction. In such cases a value of greater then 1.0 (or substantially greater) may be required. Similarly, in simple construction where only eccentricity moments exist, it is only the brace forces that ‘attract’ any amplification. Thus for the column themselves the reduced restraining effect of a sway sensitive structure may require effective length factors greater than 1.0 as given in Table 22 of BS 5950-1: 2000. Footnotes 1. Which could happen since R is based on Z and not S.



Member Buckling Resistance, Clause 4.8.3.3.2 This check is used for Class 1, 2 and 3 Plastic, Compact and Semi-compact rolled or plated Iand H-sections with equal flanges (Class 4 Slender sections and Effective Class 2 sections are not included in this release). Three formulae are provided in Clause 4.8.3.3.2 (c) to cover the combined effects of major and minor axis moment and axial force.These are used irrespective of whether all three forces / moments exist. Clause 4.9 deals with biaxial moment in the absence of axial force, Clause 4.8.3.3.2 (c) can also be used in such cases by setting the axial force to zero. All three formulae in Clause 4.8.3.3.2 (c) are checked; the second is calculated twice – once for Face A and once for Face C. Only one value of F is used, the worst anywhere in the length being checked. If the axial load is tensile, then F is taken as zero. Important Note Clause 4.8.3.3.4 defines the various equivalent uniform moment factors. The last three paragraphs deal with modifications to these depending upon the method used to allow for the effects of sway. This requires that for sway sensitive frames the uniform moment factors, mx, my and mxy, should be applied to the non-sway moments only. In this release there is no mechanism to separate the sway and non-sway moments, General Column adopts the only conservative approach and sets these 'm' factors equal to 1.0 if the frame is sway sensitive (in either direction). This is doubly conservative for sway-sensitive unbraced frames since it is likely that all the loads in a design combination and not just the lateral loads will be amplified. In such a case, both the sway and non-sway moments are increased by kamp and neither are reduced by the above ‘m’ factors. The calculation of mLT is unaffected by this approach, and thus if the second equation of Clause 4.8.3.3.2 (c) governs, then the results are not affected.

Member Buckling Resistance, Clause 4.8.3.3.3 This check is used for Class 1, 2 and 3 Plastic, Compact and Semi-compact hollow sections (Class 4 Slender sections and Effective Class 2 sections are not included in this release). Four formulae are provided in Clause 4.8.3.3.3 (c) to cover the combined effects of major and minor axis moment and axial force. These are used irrespective of whether all three forces / moments exist. Clause 4.9 deals with biaxial moment in the absence of axial force, Clause 4.8.3.3.3 (c) can also be used in such cases by setting the axial force to zero. The second and third formulae are mutually exclusive – that is the second is used for CHS, SHS and for RHS when the limits contained in Table 15 are not exceeded. On the other hand the third formula is used for those RHS that exceed the limits given in Table 15. Thus only three formulae are checked; the first, second and fourth or the first, third and fourth. Either the second or third (as appropriate) is calculated twice – once for Face C and once for Face A. Only one value of F is used, the worst anywhere in the length being checked. If the axial load is tensile, then F is taken as zero. See also the Important Note at the end of “Member Buckling Resistance, Clause 4.8.3.3.2”.



Serviceability limit state The column is assessed for sway and the following values are reported for each stack: • Sway X (mm) and critx

• Sway Y (mm) and crity • Sway X-Y (mm) Depending on the reported crit the column is classified as Sway or Non sway accordingly. Note

A sway assessment is only performed for the column if the Lambda Crit Check box is checked on the Column Properties dialog. If very short columns exist in the building model these can distort the overall sway classification for the building. For this reason you may apply engineering judgement to uncheck the Lambda Crit Check box for those columns for which a sway assessment would be inappropriate.

Worked Example For an example we shall consider an edge column in a regular multi-storey structure. This has 5 regularly spaced floors supporting profiled metal decking and a concrete slab. Each stack of column is 4 m as shown below.

This general column forms part of a moment resisting frame in a structure. The forces and moments that the column has to resist are thus dependent on the other members of the frame, and to a lesser extent on the rest of the structure. These forces and moments are thus not determined by loading on the column itself directly, but are calculated by a 3D frame analysis, and applied to the column directly.



Design pass 1 In the first pass design, the Design Wizard is used to specify that the design looks at UC sections only, with all sections available and designing the column yields a single lift of size 203 203 UC 71. (S275) General Column does not present a list of acceptable sections, since columns with splices already have multiple section size possibilities, which would give an inordinately large number of possibilities. General Column thus homes in to the first acceptable solution it finds.

If you look at the summary of results shown above, you will see that it is the Combined Buckling check that controls the design. (It has the highest Capacity Ratio.) If the restraints details for the column were reviewed you would find that General Column shows strut buckling restraints applied at each floor level and in addition lateral torsional buckling restraints at each floor level (irrespective of the method of connection).

This is intentional - Building Designer, and hence General Column assume the incoming beams are capable of providing lateral torsional buckling restraint to both flanges of the column. Typically this is acceptable because the floor construction itself will provide the necessary restraint. However, if this is not the case and LTB restraint is not provided by the beam end connections, you are required to adjust the effective lengths accordingly.



Design pass 2 In the second pass it is assumed that the incoming beams and floor construction do not provide restraint against lateral torsional buckling at level 1. The column is switched back to design mode, and then the Restraints dialog accessed. The new assumption is modelled by increasing the LTB effective lengths to 2L for stack 1 and 2.

When the design is performed again, the column size increases to a 254 254 UC 73.

Design Pass 3 The column is again switched back to design mode. Splices are added at floors 2 and 4. A redesign now yields 3 section sizes: • 152 152 UC 23 for the top stack,

• 203 203 UC 46 for the middle stack, • 254 254 UC 73 for the bottom stack.

It is extremely important to note that the initial change of section size, and the latter change to a three stack column have had no effect whatsoever on the design forces that the column has been designed for. These are locked in from the Building Designer analysis. The changes made in General Column affect the Building Designer model, and these would need to be



subsequently investigated. In practice you would need to return the changed column to Building Designer and re-perform a design check (which includes a re analysis of the changed structure) to determine the actual effects both on this column and on the rest of the model. This investigation is beyond the scope of this example, however, after a design check in Building Designer it is quite possible that the sections calculated above could be found to be not satisfactory.

General Column Input (in Fastrak Building Designer) In order to create a general column within Fastrak Building Designer, you will need to define an appropriate set of attributes. Listed below is the typical procedure for defining these attributes. Items in brackets [] are optional.

Step

Dialog

Page

Instructions

1

none

none

Create a new Columns Attribute Set.

2

Attribute Set

General


3

Attribute Set

Design

Choose General construction type

4

Attribute Set

Design

Check the Automatic Design box if Design Column Mode is required, else leave it unchecked to work in Check Column Mode

5

Attribute Set

Design


6

Attribute Set

Design


7


Size Constraints

[Define any Column Size Constraints: • max and min section size]

8


Sections for Study

If in Design Column Mode choose the Order File

9

Attribute Set

Alignment

The angle and alignment can be set here.

10

Attribute Set

Floors

The number of floors and any splice locations can be set here.

11

Attribute Set

Releases

Set the end conditions (pinned or fixed) here.

12

Attribute Set

Braces(LTB)

The degree of LTB bracing provided in each direction at each floor level. can be set here.

13

Attribute Set

Braces (Comp)

The degree of Comp bracing provided in each direction at each floor level. can be set here.

14

Attribute Set

Eccentricities

Beam end reactions are applied at the offset specified from the column face.


Step

Dialog

15

Attribute Set


Page Size

Instructions (Continued) Choose the steel grade and if in Check Column Mode choose the section size

*In order to speed the design process a distinction is made between those combinations consisting of gravity loads only and those which contain some components acting laterally (e.g. NHFs and wind loads). Setting columns to be designed for gravity loads only can significantly reduce the design time.

•



Eurocode Member Design Handbook page 128

Chapter 8

Chapter 8 : Braces

Braces

Introduction BRACE - “steel member with pinned ends designed for axial loads only” This chapter describes Fastrak’s Brace design application. This allows you to analyse and design a member with pinned end connections for axial compression and tension. Unless explicitly stated all brace calculations are in accordance with the relevant sections of BS 5950-1:2000(Ref. 2).

Scope The scope of the Brace design application is as follows:

Steel sections The design of braces is carried out for rolled I-sections, C, T, RHS, SHS, CHS, A, Double As, and Flat. Where: I rolled = UKB, UKC, UB, UC, RSJ, IPE, HE, HD, IPN C = RSC, PFC, UAP, UPN T = STB, STC RHS = RHS, Euro RHS SHS = SHS, Euro SHS CHS = CHS, Euro CHS A = RSA (equal and unequal), Euro equal angles and unequal angles Double As = 2xRSA (equal, unequal long and short leg back to back), Euro double angles (equal, unequal long and short leg back to back)

End Connections Braces can only be connected to supports or to the supporting structure via pinned connections. A torsional release can be applied at one end if required. If the brace connects into a beam (e.g. an A brace) an axial end release can be specified at one end to prevent vertical load from the beam being carried by the brace.

Applied loading The following points should be noted: • Loads for the brace are derived from the building model.

• Element loads can not be applied directly to the brace itself. • Imposed load reductions are not applied. • Moments due to self weight loading are ignored.

Chapter 8 : Braces


Design Forces The design forces for strength checks are obtained from an analysis of the entire structure. Braces can be subject to axial compression or tension, but will not be subject to major and minor axis bending.

Design checks The brace can be set to automatic design or check design. Axial capacity and buckling checks are carried out as required, details of the checks performed are given in the Theory and Assumptions section that follows.

Theory and Assumptions This section describes the theory and the major assumptions that have been made for brace design, particularly with respect to interpretation of BS 5950-1:2000. A basic knowledge of the design methods for braces in accordance with the design code is assumed.

Analysis method An elastic analysis is used to determine the forces and moments to be resisted by the brace.

Design method The design methods employed to determine the adequacy of the section for each condition are those consistent with BS 5950-1:2000, unless specifically noted otherwise.

Classification No classification is required for braces in tension. Braces in compression are classified according to Clause 3.5 as either: Class 1, Class 2, Class 3 or Class 4. Class 4 sections are not allowed. Hollow sections — The classification rules for SHS and RHS relate to “hot-finished hollow sections” only (“cold-formed hollow sections” are not included in this release).

Axial Tension An axial tension capacity check is performed according to Clause 4.6.

Axial Compression An axial compression capacity check is performed according Clause 4.7.

Compression Buckling If axial compression exists, the member is also assessed according to Clause 4.7 with all relevant sub-clauses.


Chapter 8 : Braces

The default effective length in each axis is 1.0L.

Brace Input In order to create a brace within Fastrak Building Designer, you will need to define an appropriate set of attributes. Listed below is the typical procedure for defining these attributes. Items in brackets [] are optional.

Step

Dialog

Page

Instructions

1

none

none

Create a new Brace Attribute Set.

2

Attribute Set

General


3

Attribute Set

Design

Check the Automatic Design box if Design Mode is required, else leave it unchecked to work in Check Mode

4

Attribute Set

Design


5

Brace Design Properties

Size Constraints

[Define the Size Constraints: • max and min size]

6

Brace Design Properties

Sections for Study

If in Design Mode choose the Order File

7

Attribute Set

Alignment

[Change the rotation/alignment of the brace as required]

8

Attribute Set

Size

Choose the steel grade and, if in Check Mode choose the section size

9

Attribute Set

Releases

[Define any vertical or torsional releases as required]

10

Attribute Set

Compression

[Adjust effective length factors as required]

Chapter 9 : Refining Member Designs

Chapter 9


Refining Member Designs

Introduction Having carried out an initial design of your model you may then require to investigate key members in more detail. This is possible for any member that has been analysed by using Building Designer’s design refinement capabilities.

Why would you want to refine the original design? Using Building Designer interactively provides you with still greater control over the design of an individual member: • to enable multiple order files to be considered at the same time to determine a list of alternative sections, all of which can withstand the applied loading.

• to determine a suitable alternative to the initially designed section size without having to re-design the whole building. The new section size (and steel grade if required) can then be passed back to the building model, only the individual member will be updated. If the changes are to be applied to other members also, you would need to update the building model separately and then re-design it.

• to consider the possible effects on the member of more far reaching changes. For example the loading could be adjusted, or the restraint information modified. The resulting design can then be output, however, these wider changes can not be passed back to the building model.

Interaction Effects Because simple beams and composite beams are by definition pin ended in Building Designer, interaction effects are of no concern. This is not the case for general beams and general columns - these member types can also be investigated interactively, however, they always interact with other members in the structure. This interaction is demonstrated in two examples in the Building Designer Handbook: • “Continuous Beam Example”, and

• “Braces Carry Gravity Loads Example” When you export elements to General Beam and General Column you lock in the interaction effects. If you do not change anything you can check the same beam or column and see the same results. However, you can also run an automatic design in which case the locked in interaction effects do not change. What this can mean in practice is that you may select a different section (larger or smaller), interactively. This section may seem to work satisfactorily when designed in isolation. You can then return this amended section size to the main model (where you will have to re-analyse and check your model). It is quite possible that the section which appeared to work when



designed in isolation will fail when checked after re-analysis of the full model. It is also entirely possible that other members in your model may fail (or have more capacity in hand) since the distribution of forces will be affected by the different section size which you picked.

How to Access Design Refinement The following sections illustrate how to use each of the Building Designer design modules in either Check or Design Mode to refine the design.

Simple Beam - Check Mode. Listed below is the typical procedure for checking a simple beam. Caution

Step

Shaded items in the below table can not be returned to the building model. You are advised not to make these changes interactively if your intention is to make the change permanent. Instead you should make the required change(s) directly to the building model.

Icon

Instructions

1

Right click on the beam to be investigated and launch Simple Beam,

2

Set Simple Beam into Check Beam Mode,

3

Modify the properties for the beam as required: • section size, • grade,

4

Modify the beam span,

5

Modify the details of the beam restraints.

6

Modify the loadcases that apply to the simple beam.

7

Modify the design combinations,

8

Make any Design Wizard settings that you want to use to control the design,

9

Perform the check,

10


11

Return the section size and grade to the building model.



Simple Beam - Design Mode Listed below is the typical procedure for interactively designing a simple beam. Shaded items in the below table can not be returned to the building model. You are advised not to make these changes interactively if your intention is to make the change permanent. Instead you should make the required change(s) directly to the building model.

Step

Icon

Instructions

1

Right click on the beam to be investigated and launch Simple Beam,

2

Set Simple Beam into Design Beam Mode,

3

Modify the steel grade if required,

4


5

Modify the details of the beam restraints.

6

Modify the loadcases that apply to the simple beam.

7


8


9

Perform the design

10


11


12




Composite Beam - Check Mode Listed below is the typical procedure for interactively checking a composite beam. Caution

Step


Icon

Instructions

1

Right click on the beam to be investigated and launch Composite Beam,

2

Set Composite Beam into Check Beam Mode,

3

Modify the properties for the beam as required: • section size, • grade,

4


5

Modify the details of the floor construction: • steel deck • slab details • reinforcement details (continuity and any other reinforcement present in the slab, • shear stud size, layout, spacing • construction stage restraint details (if applicable)

6

Modify the loadcases that apply to the beam.

7


8


9

Perform the check,

10


11




Composite Beam - Design Mode Listed below is the typical procedure for interactively designing a composite beam. Shaded items in the below table can not be returned to the building model. You are advised not to make these changes interactively if your intention is to make the change permanent. Instead you should make the required change(s) directly to the building model.

Step

Icon

Instructions

1

Right click on the beam to be investigated and launch Composite Beam,

2

Set Composite Beam into Design Beam Mode,

3


4


5

Modify the details of the floor construction: • steel deck • slab details • reinforcement details (continuity and any other reinforcement present in the slab, • shear stud size, layout, spacing • construction stage restraint details (if applicable)

6


7


8


9

Perform the design

10


11


12




General Beam - Check Mode Listed below is the typical procedure for interactively checking a general beam. Caution

Step


Icon

Instructions

1

Right click on the beam to be investigated and launch General Beam,

2

Set General Beam into Check Beam Mode,

3

Modify the properties for the beam as required: • section size; • grade,

4

Modify the beam spans: • add additional spans if required; • supports,

5

Modify restraint details,

6


7


8


9

Perform the check,

10


11




General Beam - Design Mode Listed below is the typical procedure for interactively designing a general beam. Shaded items in the below table can not be returned to the building model. You are advised not to make these changes interactively if your intention is to make the change permanent. Instead you should make the required change(s) directly to the building model.

Step

Icon

Instructions

1

Right click on the beam to be investigated and launch General Beam,

2

Set General Beam into Design Beam Mode,

3


4

Modify the beam spans: • add additional spans if required; • supports,

5


6


7


8


9

Perform the design

10


11


12




General Column - Check Mode Listed below is the typical procedure for interactively checking a general column. Caution

Step


Icon

Instructions

1

Right click on the column to be investigated and launch General Column,

2

Set General Column into Check Column Mode,

3

Modify the size and grade for the column as required,

4

Modify the other column properties: • add additional floors if required; • releases; • eccentricities,

5


6

Modify the loadcases that apply to the column.

7


8


9

Perform the check,

10


11




General Column - Design Mode Listed below is the typical procedure for interactively designing a general column. Shaded items in the below table can not be returned to the building model. You are advised not to make these changes interactively if your intention is to make the change permanent. Instead you should make the required change(s) directly to the building model.

Step

Icon

Instructions

1

Right click on the column to be investigated and launch General Column,

2

Set General Column into Design Column Mode,

3


4

Modify the other column properties: • add additional floors if required; • releases; • eccentricities,

5


6


7


8


9

Perform the design

10


11


12


Effective Use of Order Files in Refined Design This section aims to explain how intelligent use of the Design Wizard can reduce design time and make the design process more effective. The following situations are investigated: • limiting the range of Sections for Study to reduce design time and prevent selection of non preferred sections,



• applying Size Constraints to further reduce design time, To investigate the above an example model can be defined in Building Designer and a beam then extracted into Simple Beam. Let’s take a simple example of a 9 m span spine beam with 6 m span secondary beams at third points.


Condition

Value


Dry Slab

2.0 kN/m2

36kN

Services

1.0 kN/m2

18kN

Live load

5.0 kN/m2

90kN

Design Pass 1 If you input this model and then run a design you will find that Simple Beam shows a dialog of acceptable sections. If no one has tailored the sections that Simple Beam investigates, then the list will appear as below.

If you move down the list of Available files, you will see all the Section Designations that can carry the applied loading. These are only the ones that pass the design, Simple Beam has tried all the sections in each of the Available files, to determine the acceptable ones. You may have



noticed the different section designations in the progress bar as the design ran. However checking all these sections comes at a price, the more sections there are to investigate, the longer the design takes. Click Cancel to return to the Beam Definition window. Note

Clicking Cancel leaves the program in Design Beam Mode. Clicking OK would have flipped the program in to Check Beam Mode with the highlighted acceptable section becoming the check section.

Simple Beam allows you to choose just the sections you want to include for the design through its Design Wizard.

Design Pass 2 Sections for Study Remove the tick against all the Available files whose section types you don’t want to investigate, and Simple Beam won’t look at any of these sections during the design process. If you remove the tick against all the Available files other than UBBeamOrder.Eur, and then re-perform the design you will find a significant increase in speed as Simple Beam only investigates the universal beams.



Furthermore Simple Beam investigates the sections in the order that they appear in the Section Designation list. If you scroll down many of the lists, you will find that there is a point at which larger sections give way to smaller ones again.

We have ordered the Section Designation list based on our many years experience of the industry, the sections at the top of the list are the ones we know you prefer to use, whilst those at the bottom are those which you use less frequently if at all. By default all the Section Designations are ticked, but you might want to remove the ticks against some or all of the non-preferred sections. In doing so, you are controlling the design, making Simple Beam look at just the section designations you are likely to accept, and in the process speeding up the design itself. Simple Beam maintains the Sections for Study settings that you make, until you choose to change them again. It is therefore worthwhile taking the time to tailor the list so that Simple Beam picks sections of which you are likely to approve during its designs. Note


Design Pass 3 You may also have other constraints specific to this particular project, for instance you may need to restrict the use sections with flanges less than a certain width for erection purposes, or you may need to set a limit on the maximum beam depth due to height constraints.

Size Constraints Such restrictions can be set via the Size Constraints tab, Simple Beam will then only consider section designations that fall within these limits (once again speeding up the design).


Note


Size Constraints settings are only applied to the current project, unlike changes made to the Sections for Study which are applied to all projects.

For the third design pass the minimum width of beam will be set to 150mm. With this constraint applied all narrower sections are excluded, Simple Beam has to check far fewer sections and the design is almost instantaneous.

By default the acceptable sections are listed in a descending order of the capacity ratio. Full design results are available for each section on the list. To see the results, highlight a section and click Preview. This will take you to the Design Summary for the chosen section. Closing the Design Summary allows you to preview the results for further sections if required. When you have decided on the most appropriate section, highlight it from the list and click OK.

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Chapter 10

Chapter 10 : References

References 1. British Standards Institution. BS 5950-3.1:1990+A1:2010: Structural use of steelwork in building . Design in composite construction. Code of practice for design of simple and continuous composite beams. BSI 2010. 2. British Standards Institution. BS 5950 : Structural use of steelwork in building; Part 1. Code of practice for design in simple and continuous construction: hot rolled sections. BSI 2000. 3. The Steel Construction Institute. Publication 078. Commentary on BS 5950 : Part 3 : Section 3.1 : 1990. SCI 1989. 4. The Steel Construction Institute. Publication 055. Design of Composite Slabs and Beams with Steel Decking. SCI 1989. 5. The Steel Construction Institute. Publication 068. Design for openings in the webs of composite beams. SCI 1987. 6. The Steel Construction Institute. Publication 076. Design Guide on the Vibration of Floors. SCI 1989. 7. The Steel Construction Institute. Publication 100. Design of Composite and Non-Composite Cellular Beams. SCI 1990. 8. The Steel Construction Institute. Joints in Steel Construction. Simple Connections. SCI/ BCSA 2002. Publication P212. 9. The Steel Construction Institute. Joints in Steel Construction. Moment Connections. SCI/ BCSA 1995. Publication P207. 10. The Steel Construction Institute. Publication P355. Design of Composite Beams with Large Web Openings. SCI 2011.

Chapter 10 : References


BS5950 - Member Design Handbook

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