Concrete Beam and Slab Design Using Prokon

Continuous Beam and Slab Design

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Continuous Beam and Slab Design

The Continuous Beam and Slab Design module is used to design and detail reinforced concrete beams and slabs as encountered encountered in typical building projects. The design incorporates automated pattern loading and moment redistribution. Complete bending schedules schedules can be generated for editing and printing using Padds. Padds.

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Theory and application

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Theory and application The following text gives an overview of the theory and application of the design codes.

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Theory and application

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Theory and application The following text gives an overview of the theory and application of the design codes.


Design scope

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Design scope The program designs and details continuous concrete beams and slabs. You can design structures ranging from simply supported single span to twenty-span continuous beams and slabs. Cross-sections can include a mixture rectangular, I, T and L-sections. Spans can have constant or tapered sections. Entered dead and live loads are automatically applied as pattern loads during the analysis. At ultimate limit state, moments and shears are redistributed to a specified percentage. Reinforcement Reinforcement can be generated for various types of beams and slabs, edited and saved as Padds compatible bending schedules.


Design codes

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Design codes The following codes are supported:

x

BS 8110 - 1985.

x

BS 8110 - 1997.

x

SABS 0100 - 1992.

Reinforcement bending schedules are generated in acc ordance to the guidelines given by the following publications:

x

General principles: BS 4466 and SABS 082.

x

Guidelines for detailing: 'Standard Method of Detailing Structural Concrete' published by the British Institute of Structural Engineers.

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Sub-frame analysis

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Sub-frame analysis A two-dimensional frame model is constructed from the input data. Section properties are based on the gross uncracked concrete sections. Columns can optionally be specified below and above the beam/slab and can be made pinned or fixed at their remote ends. Note: No checks are made for the slenderness limits of columns or beam flanges.


Pattern loading

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Pattern loading At ultimate limit state, the dead and live loads are multiplied by the specified ULS load factors (see page 14). Unity load factors are used at serviceability limit state. The following load cases are considered (the sketch uses the load factors applicable to BS8110):

x

All spans are loaded with the maximum design load.

x

Equal spans are loaded with the maximum design ultimate load and unequal spans with the minimum design dead load.

x

Unequal spans are loaded with the maximum design load and equal spans loaded with the minimum design dead load.

Note: The case where any two adjacent spans are loaded with maximum load and all other spans with minimum load, as was the case with CP 110 - 1972

and SABS 0100 - 1980, is not considered. The following are special considerations with pertaining to design using SABS 0100 - 1992:

x

x

SABS 0100 - 1992 suggests a constant ULS dead load factor of 1.2 for all pattern load cases. In contrast, the BS 8110 codes suggest a minimum ULS dead load factor of 1.0 for calculating the minimum ultimate dead load. The program uses the more approach given by the BS 8110 codes at all times, i.e. a ULS load factor of 1.0 for minimum dead load and the maximum load factor specified for maximum dead load. The South African loading code, SABS 0162 - 1989, prescribes an additional load case of 1.5×DL. This load case is not considered during the analysis – if required, you should adjustment the applied loads manually. In cases where the dead load is large in comparison with the live load, e.g. lightly loaded roof slabs, this load case can be incorporated by increasing the entered dead load or increasing the ULS dead load factor. This adjustment applies to cases where 1.5×DL > 1.2×DL + 1.6×LL or, in other words, LL < 19%. Using an increased dead load factor of 1.4 instead of the normal 1.2 will satisfy all cases except where 1.5×DL > 1.4×DL + 1.6×LL or, in other words, LL < 6%×DL.

Not needed for bs 8110 design


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Moment redistribution

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Moment redistribution Ultimate limit state bending moments are redistributed for each s pan by adjusting the support moments downward with the specified percentage. If the method of moment redistribution is set to 'optimised', the design moments are further minimised by redistributing span moments upward as we ll. Note: No moment redistribution is done for serviceability limit state calculations.

The moment envelopes are calculated for pattern loading and then redistributed using the procedures explained in the following text. Downwards redistribution

The downward distribution method aims to reduce the hogging moments at the columns without increasing the sagging moments at midspan. The redistribution of moments and shear forces procedure is performed as follows: 1.

The maximum hogging moment at each column or internal support is adjusted downward by the specified maximum percentage.

2.

The corresponding span moments are adjusted downward to maintain static equilibrium. The downward adjustment of hogging moments above is limited to prevent any increase in the maximum span moments of end spans.

3.

The shear forces for the same load cases are adjusted to maintain static equilibrium. Optimised redistribution:

The optimised distribution procedure takes the above procedure a step further by upward distribution of the span moments. The envelopes for the three pattern load cases are redistributed as follows: 1.

2.

The maximum hogging moment at each internal support is adjusted downward by the specified percentage. This adjustment affects the moment diagram for the load case where the maximum design load is applied to all spans. The relevant span moments are adjusted accordingly to maintain static equilibrium.

3.

The minimum hogging moment at each internal support is subsequently adjusted upward to as close as possible to the reduced maximum support moment, whilst remaining in the permissible redistribution range. A second load case is thus affected for each span.

4.

The relevant span moments are adjusted in line with this redistribution of the column moments to maintain static equilibrium.

5.

For each span, the moment diagram for the remaining third load case is adjusted to as near as possible to the span moments obtained in the previous step. The adjustment is made in such a way that it remains within the permissible redistribution range.

6.

Finally, the shear force envelope is adjusted to maintain static equilibrium.

7.

The following general principles are applied when redistributing moments:

8.

Equilibrium is maintained between internal and external forces for all relevant combinations of design ultimate load.

9.

The neutral axis depth is checked at all cross sections where moments are redistributed. If, for the specified percentage of moment redistribution, the neutral axis depth is greater than the limiting value of (ßb0.4)d, compression reinforcement is added to the section to sufficiently reduce the neutral axis depth.

10. The amount of moment redistribution is limited to the specified percentage. The maximum amount of redistribution allowed by the codes is 30%. Note: The exact amount of moment redistribution specified is always applied, irrespective of the degree of ductility of the relevant sections. Where necessary, ductility is improved by limiting the neutral axis depth. This is achieved by adding additional compression reinforcement.


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Deflection calculation

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Deflection calculation Both short-term and long-term deflections are calculated. N o moment redistribution is done at serviceability limit state. Elastic deflections Short-term elastic deflections are calculated using unfactored SLS pattern loading. Gross uncracked concrete sections are used. Long-term deflections Long-term deflections are determined by first calculating the cracked transformed sections: 1. 2.

The full SLS design load is applied to all spans to obtain the elastic moment diagram. The cracked transformed sections are then calculated at 250 mm intervals along the length of the beam. The results of these calculations are tabled in the Crack files on the View output pages. Note: The calculation of the cracked transformed section properties is initially based on the amount of reinforcement required at ULS. However, once reinforcement is generated for beams, the actual entered reinforcement is used instead. You can thus control deflections by manipulating reinforcement quantities.

Next, the long-term deflection components are calculated by numerically integrating the curvature diagrams: 1.

Shrinkage deflection is calculated by applying the specified shrinkage strain. Unsymmetrical beams and unsymmetrical reinforcement layouts will cause a curvature in the beam.

2.

The creep deflection is calculated by applying the total dead load and the permanent portion of the live load on the beam. The modulus of elasticity of the concrete is reduced in accordance with the relevant design code.

3.

The instantaneous deflection is calculated by applying the transient portion of the live load on the transformed crack section.

4.

The long-term deflection components are summed to yield the total long-term deflection. Note: When calculating the curvatures for integration, elastic moments are used together with cracked transformed sections, which implies plastic behaviour. Although this procedure is performed in accordance with the design codes, the use of elastic moments together with cracked sections in the same calculation is a contradiction of principles. As a result of this, long-term deflection diagrams may show slight slope discontinuities at supports, especially in cases of severe cracking.


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Calculation of flexural reinforcement

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Calculation of flexural reinforcement

The normal code formulae apply when calculating flexural reinforcement for rectangular sections and for flanged sections where the neutral axis falls inside the flange.

If the neutral axis falls outside the flange, the section is designed by considering it as two sub-sections. The first sub -section consists of the flange without the central web part of the section and the remaining central portion defines the second sub-section. The reinforcement calculation is then performed as follows: 1.

Considering the total section, the moment required to put the flange portion in compression can be calculated using the normal code formulae. This moment is then applied to the flange sub-section and the required reinforcement calculated using the effective depth of the total section.

2.

The same moment is then subtracted from the total applied moment. The resulting moment is then applied to the central sub-section and the reinforcement calculated.

3.

The tension reinforcement for the actual section is then taken as the sum of the calculated reinforcement for the two sub-sections. If compression reinforcement is required for the central sub-section, it is used as the required compression reinforcement for the actual section.


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Design and detailing of flat slabs

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Design and detailing of flat slabs When entering the input data for a flat slab, you should use its whole width, i.e . the transverse column spacing (half the spacing to the left plus half the spacing to the right). The program will then calculate bending moments and shear forces for the whole panel width. When generating reinforcement, however, the program considers the column and middle strips separately. The program does the column and middle strip subdivision as suggested by the design codes. The procedure is taken a step further by narrowing the column strip and widening the middle strip to achieve a simpler reinforcement layout – a procedure allowed by the codes. Initial column and middle strip subdivision The flat slab panel is divided into a column strip and middle strip of equal w idths and then adjusted to simplify reinforcement detailing: 1.

The width of the column strip is initially taken as half the panel width. The total design moment is then distributed between the column and middle strips as follows: Moment position

2.

Column strip

Middle strip

Moment over columns

75%

25%

Moments at midspan

55%

45%

Reinforcement is calculated for each of the column and middle strips.

Adjusted column and middle strip subdivision The design codes require that two-thirds of the column strip reinforcement be concentrated in its middle half. The codes also state that a column strip may not be taken wider than half the panel width, thereby implying that it would be acceptable to make the column strip narrower than the half the panel width. To simplify the reinforcement layout and still comply with the code provisions, the program narrows the column strip and widens the middle strip. The widening of the middle strip is done as follows: 1.

The middle strip is widened by fifty percent from half the panel width to three-quarters of the panel width.

2.

The reinforcement in the middle strip is accordingly increased by fifty percent. Reinforcement added to the middle strip is taken from the column strip.

The column strip is subsequently narrowed as follows: 1.

The column strip is narrowed to a quarter of the panel width.

2.

As explained above, reinforcement is taken from the column strip and put into the widened middle strip.

3.

The remaining reinforcement is checked and additional reinforcement added where necessary. This is done to ensure that the amount of reinforcement resisting hogging moment is greater than or equal to two-thirds of the reinforcement required for the original column strip. Designing the slab for shear The program considers the column strip like a normal beam when doing shear calculations. A possible approach to the shear design of the slab is:

x

Consider the column strip like a beam and provide stirrups equal to or exceeding the calculated required shear steel.

x

In addition to the above, perform a punching shear check at all columns. Implications of modifying the column and middle strips In applying the above modifications, the moment capacity is not reduced. The generated reinforcement will be equal to, or slightly greater, than the amount that would be calculated using the normal middle and column strip layout.

The above technique gives simplified reinforcement details:

x

A narrower column strip is obtained with a uniform transverse distribution of main bars and a narrow zone of shear links.

x

Detailing of the adjoining middle strips is also simplified by the usage of uniform reinforcement distributions.

The design procedures for flat slabs and coffer slabs are described in more detail on page 36.


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Input

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Input The beam/slab definition has several input components:

x

Parameters: Material properties, load factors and general design parameters.

x

Sections: Enter cross-sectional dimensions.

x

Spans: Define spans and span segments.

x

Supports: Define columns, simple supports and cantilevers.

x

Loads: Enter dead and live loads.

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Parameters input

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Parameters input Enter the following design parameters:

f cu : Characteristic strength of concrete (MPa). f y : Characteristic strength of main reinforcement (MPa). f yv : Characteristic strength of shear reinforcement (MPa). Redistr : Percentage of moment redistribution to be applied. Method : Method of moment redistribution, i.e. downward or optimised. Refer to page 8 for detail. Cover top : Distance from the top surface of the concrete to the centre of the top steel. Cover bottom: Distance from the soffit to the centre of the bottom steel. DL factor : Maximum ULS dead load factor. LL factor : Maximum ULS live load factor. Note: The ULS dead and live load factors are used to calculate the ULS design loads. The ULS dead and live loads are then automatically patterned during analysis. Refer to page 7 for more information.

Density : Concrete density used for calculation of own weight. If the density filed is left blank, the self-weight of the beam/slab should be included in the entered dead loads. LL perm : Portion of live load to be considered as permanent when calculating the creep components of the long-term deflection. I H

cs

: The thirty-year creep factor used for calculating the final concrete creep strain. : Thirty-year drying shrinkage of plain concrete.

The graphs displayed on-screen give typical values for the creep factor and drying shrinkage strain. In both graphs, the effective section thickness is defined for uniform sections as twice the cross-sectional area divided by the exposed perimeter. If drying is prevented by immersion in water or by sealing, the effective section thickness may be taken as 600 mm.

Note: Creep and shrinkage of plain concrete are primarily dependent on the relative humidity of the air surrounding the concrete. Where detailed calculations are being made, stresses and relative humidity may vary considerably during the lifetime of the structure and appropriate judgements should be made.

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Sections input

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Sections input You can define rectangular, I, T, L and inverted T and L-sections. Every section comprises a basic rectangular web area with optional top and bottom flanges. The top levels of all sections are aligned vertically by default and they are placed with their webs symmetrically around the vertical beam/slab centre line. The web and/or flanges can be move horizontally to obtain eccentric sections, for example L-sections. Whole sections can also be moved up or down to obtain vertical eccentricity. Note: In the sub-frame analysis, the centroids all beam segments are assumed to be on a straight line. Vertical and horizontal offsets of sections are use used for presentation and detailing purposes only and has no effect on the design results. Section definitions are displayed graphically as they are entered. Section cross-sections are displayed as seen from the left end of the beam/slab.

The following dimensions should be defined for each s ection:

Sec no : The section number is used on the Spans input page to identify specific sections. Bw : Width of the web (mm). D : Overall section depth, including any flanges (mm). Bf top : Width of optional top flange (mm). Hf top : Depth of optional top flange (mm). Bf bot : Width of optional bottom flange (mm). Hf bot : Depth of optional bottom flange (mm). Y-offset : Vertical offset the section (mm). If zero or left blank, the top surface is aligned with the datum line. A positive value means the section is moved up. Web offset : Horizontal offset of the web portion (mm). If zero or left blank, the web is taken symmetrical about the beam/slab centre line. A positive value means the web is moved to the right. Flange offset : Horizontal offset of both the top and bottom flanges (mm). If zero or left blank, the flanges are taken symmetrical about the beam/slab centre line. A positive value means the flanges are moved to the right. Note: There is more than one way of entering a T-section. The recommended method is to enter a thin web with a wide top flange. You can also enter wide web (actual top flange) with a thin bottom flange (actual web). The shear steel design procedure works with the entered web area, i.e. B w × D, as the effective shear area. Although the two methods produce similar pictures, their shear modelling is vastly different.

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Spans input

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Spans input Sections specified on the Sections input page are used here with segment lengths to define spans of constant or varying sections.

Spans are defined by specifying one or more span segments, each with a unique set of section properties. The following data should be input for each span:

Span no : Span number between 1 and 20. If left blank, the span number as was applicable to the previous row is used, i.e. another segment for the current span. Section length : Length of span or span segment (m). Sec No Left : Section number to use at the left end of the span segment. Sec No Right : Section number to use at the right end of the span segment. If left blank, the section number at the left end is used, i.e. a prismatic section is assumed. If the entered section number differs from the one at the left end, the section dimensions are varied linearly along the length of the segment. Tip: When using varying cross sections on a span segment, the section definitions are interpreted literally. If a rectangular section should taper to an Lsection, for example, the flange will taper from zero thickness at the rectangular section to the actual thickness at the L-section. If the flange thickness should remain constant, a dummy flange should be defined for the rectangular section. The flange should be defined marginally wider, say 0.1mm, than the web and its depth made equal to the desired flange depth.

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Supportsinput

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Supports input You can specify simple supports, columns below and above, fixed ends and cantilever ends. To allow a complete sub -frame analysis, columns can be specified below and above the beam/slab. If no column data is entered, simple supports are assumed. The following input is required:

Sup no : Support number, between 1 to 2'. Support 1 is the left-most support. C,F : The left-most and right-most supports can be freed, i.e. cantilevered, or made fixed by entering 'C' or 'F' respectively. By fixing a support, full rotational fixity is assumed, e.g. the bea m/slab frames into a very stiff shaft or column. D : Depth/diameter of a rectangular/circular column (mm). The depth is measured in the span direction of the beam/slab. B : Width of the column (mm). If zero or left blank, a circular column is assumed. H : Height of the column (m). Tip: For the sake of accurate reinforcement detailing, you can specify a width for simple supports at the ends of the beam/slab. Simply enter a value for D and leave B and H blank. In the analysis, the support will still be considered as a normal simple support. However, when generating reinforcement bars, the program will extend the bars a distance equal to half the support depth past the support centre line.

Code : A column can be pinned at its remote end by specifying 'P'. If you enter 'F' or leave this field blank, the column is assumed to be fixed at the remote end.

Tip: You may leave the Support input table blank if all supports are simple supports.

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Loads input

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Loads input Dead and live loads are entered separately. The entered loads are automatically patterned during analysis. For more detail on the pattern loading technique, refer to page 7. Distributed loads, point loads and moments can be entered on the same line. Use as many lines as necessary to define each load case. Defined loads as follows:

Case D,L : Enter 'D' or 'L' for dead load or live load respectively. If left blank, the previous load type is assumed. Use as many lines as necessary to define a load case. Span : Span number on which the load is applied. If left blank, the previous span number is assumed, i.e. a continuation of the load on the current span. Wleft : Distributed load intensity (kN/m) applied at the left-hand starting position of the load. If you do not enter a value, the program will use a value of zero. Wright : Distributed load intensity (kN/m) applied on the right-hand ending position of the load. If you leave this field blank, the value is made equal to Wleft, i.e. a uniformly distributed load is assumed. P : Point load (kN). M : Moment (kNm). a : The start position of the distributed load, position of the point load or position of the moment (m). The distance is measured from the left-hand edge of the beam. If you leave this field blank, a value of zero is used, i.e. the load is taken to start at the left-hand edge of the beam. b : The end position of the distributed load, measured from the start position of the load (m). Leave this field blank if you want the load to extend up to the right-hand edge of the beam.

Note: A portion of the live load can be considered as permanent for deflection calculation. For more detail, refer to the explanation of the Parameters input on page 14.

Note: If you enter a concrete density on the Parameters input page, the own weight of the beam/slab is automatically calculated and included with the dead load.

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Design

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Design The analysis is performed automatically when you access the Design pages.

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Analysis procedure

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Analysis procedure Two separate analyses are performed for SLS and ULS calculations. Serviceability limit state analysis Elastic deflections are calculated by analysing the beam/slab under pattern loading using the gross uncracked sections. When determining long-term deflections, however, the all spans of the beam/slab are subjected to the maximum design SLS load. Sections are then evaluated for cracking at 250 mm intervals, assuming the reinforcement required at ultimate limit state. The long-term deflections are then calculated by integrating the curvature diagrams. Tip: After having generated reinforcement for a beam, the long-term deflections will be recalculated using the actual reinforcement. Refer to page 10 for more detail on calculation of long-term deflections. Ultimate limit state analysis At ultimate limit state, the beam/slab is subjected to pattern loading as described on page 7. The resultant bending moment and shear force envelopes are then redistributed. Finally, the required reinforcement is calculated.

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Fixing errors that occurred during the analysis

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Fixing errors that occurred during the analysis The Input pages incorporate extensive error checking. However, serious errors sometime still slip through and cause problems during the analysis. Common input errors include:

x

Using incorrect units of measurement. For example, span lengths should be entered in metre and not millimetre.

x

Entering too large reinforcement cover values on the Parameters input screen, gives incorrect reinforcement. Cover values should not be wrongly set to a value larger than half the overall section depth.

x

Not entering section numbers when defining spans on the Spans input screens causes numeric instability. Consequently, the program uses zero section properties.

Long-term deflection problems

The cause of unexpected large long-term deflections can normally be determined by careful examination of the analysis output. View the long-term deflection diagrams and determine which component has the greatest effect: x

The likely cause of large shrinkage deflection is vastly unsymmetrical top and bottom reinforcement. Adding bottom reinforcement over supports and top reinforcement at in the middle of spans generally induces negative shrinkage deflection, i.e. uplift.

x

Large creep deflections (long-term deflection under permanent load) are often caused by excessive cra cking, especially over the supports. Compare the span to depth ratios with the recommended values in the relevant design code.

x

Reduced stiffness due to cracking also has a direct impact on the instantaneous deflection component.

To verify the extent of cracking along the length of the beam/slab, you can study the contents of the Crack file. Check the cracked status and stiffness of the relevant sections. The extent of cracking along the length of the beam/slab is usually a good indication of its serviceability.

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Viewing output graphics

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Viewing output graphics The analysis results can be viewed graphically or in tabular format. Output data, including graphics and tabled values, can be selectively appended to the Calcsheets using the Add to Calcsheets function on each output page.

Diagrams can be displayed for deflection, member forces and stress and shell reinforcement of any load case. Deflections The elastic deflection envelope represents the deflections due to SLS pattern loading. The long-term deflection diagram represents the behaviour of the beam/slab under full SLS loading, taking into account the effects of shrinkage and creep:

x

The green line represents the total long-term deflection.

x

The shrinkage deflection is shown in red.

x

The creep deflection (long-term deflection due to permanent loads) is given by the distance between the red and blue lines.

x

The distance between the blue and green lines represents instantaneous deflection due to transient loads. Note: Long-term deflections in beams are influenced by reinforcement layout. Initial long-term deflection values are based on the reinforcement required at ultimate limit state. Once reinforcement has been generated for a beam, the long-term deflections will be based on the actual reinforcement instead.

Moments and shear forces

The bending moment and shear force diagrams show the envelopes due to ULS pattern loading. Steel diagrams Bending and shear reinforcement envelopes are given for ULS pattern loading. The bending reinforcement diagram sows required top steel above the zero line and bottom steel below.

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Viewing output graphics

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Viewing output tables

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Viewing output tables Open the Output file page for a tabular display of the beam/slab design results. Results include moments and reinforcement, shear forces and reinforcement, column reactions and moments and deflections. The Crack file gives details of the cracked status, effective stiffness and concrete stresses in the beam/slab at regular intervals. You should find the information useful when trying to identify zones of excessive cracking.


Reinforcing

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Reinforcing Reinforcement can be generated for the most types of continuous beam and slabs using the automatic bar generation feature. Reinforcement is generated in accordance to the entered detailing parameters after which you can edit the bars to suit your requirements. To create a bending schedule, use each detailing function in turn:

x

Detailing parameters: Select the detailing mode, enter you preferences and generate the reinforcement.

x

Main reinforcement: Review the main bars and adjust as necessary.

x

Stirrups: Enter one or more stirrup configurations.

x

Shear reinforcement: Distribute stirrups over the length of the beam.

x

Sections: Specify positions where of cross-sections details should be generated.

x

Bending schedule: Create the Padds file.

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Detailing parameters

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Detailing parameters The detailing parameters set the rules to be used by the program when generating reinforcement: Beam/slab type: Different detailing rules apply to different types of beams and slabs:

x

Type

Description

Main reinforcement

Shear reinforcement

1

Normal beam

Nominal reinforcement as for beams

Beam shear reinforcement

2

One way spanning flat slab

Nominal reinforcement as for slabs.

No shear reinforcement.

3

Column strip portion of flat slab on columns

4

Middle strip portion of flat slab on columns

Main reinforcement in accordance with moment distribution between column and middle strips. Nominal reinforcement as for slabs.

No shear reinforcement. Separate punching shear checks should be performed.

5

Rib

Nominal reinforcement as for slabs.

Shear reinforcement as for beams.

x

Maximum bar length: Absolute maximum main bar length to be used, e.g. 13 m.

x

Minimum diameter for top bars, bottom bars a nd stirrups: The minimum bar diameter to be used in each if the indicated positions.

x

Maximum diameter for top bars, bottom bars a nd stirrups: The maximum main bar diameter to be used in each if the indicated positions. Tip: To force the program to use a specific bar diameter, you can enter the same value for both the minimum and maximum diameters.

Note: The default bar types used for main bars and stirrups, e.g. mild steel or high tensile, are determined by the yield strength values entered on the Parameters input page – refer to page 14 for detail. High tensile steel markings, e.g. 'T' or 'Y', will be used for specified values of f y and f yv exceeding 350MPa. Stirrup shape code: Preferred shape code to use for stirrups. Valid shape codes include:

x

x

x

BS 4466: 55, 61, 77, 78 and 79.

x

SABS 082: 55, 60, 72, 73 and 74. First bar mark top: The mark of the first bar in the top of the beam/slab. Any alphanumerical string of up to five characters may be specified. The rightmost numerical or alpha portion of the bar mark is incremented for subsequent bars. Examples of valid marks include: x

'001' will increment to 002, 003 etc .

x

'A' will increments to B, C, etc.

x

'B002' will increment to B003, B004 etc.

x

First bar mark middle: The mark of the first bar in the middle of the beam/slab. If you do not enter a mark, the bar marks continue from those used for the top reinforcement. Middle bars are generated for all beams with effective depth of 650 mm or greater.

x

First bar mark bottom: The mark of the first bar in the bottom the beam/slab. If you leave this field blank, the bar marks will continue from those used for the top or middle reinforcement.

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Detailing parameters

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Cover to stirrups: Concrete cover to use at the top, bottom and sides of all stirrups.

x

Minimum stirrup percentage: Nominal shear reinforcement is calculated according to the code provisions for beams and slabs. In some cases, it may be acceptable to provide less than the nominal amount stirrups, e.g. for fixing top bars in a flat slab. The minimum amount of stirrups to be generated can be entered as a percentage of the nominal shear reinforcement.

x

Note: For beams and ribs, the minimum stirrup percentage should not be taken less than 100% of nominal shear reinforcement. Loose method of detailing: The envisaged construction technique can be taken into account when detailing reinforcement:

x x

With the 'loose method' of detailing, also referred to as the 'splice-bar method', span reinforcement and link hangers are stopped short about 100 mm inside each column face. This is done at all internal columns were congestion of column and beam reinforcement is likely to occur. The span bars and stirrups are often made into a cage, lifted and lowered between supports. For continuity, separate splice bars are provided through the vertical bars of each internal column to extend a lap length plus 100 mm into each span. Top bars will extend over supports for the required distance and lapped with nominal top bars or link hangers. Allowance is made for a lap length of 40· I and a 100 mm tolerance for the bottom splice bars that are acting in compression.

x

Alternatively, where accessibility during construction allows, the 'normal' method of detailing usually yields a more economical reinforcement layout. This method allows bottom bars to be lapped at support centre lines. Top bars will extend over supports for the required distance and lapped with link hangers. Where more practical, top bars over adjacent supports may be joined. Adjacent spans are sometimes detailed together.

Note: The 'normal' method of detail may give rise to congested reinforcement layouts at beam-column junctions, especially on the bottom beam/slab layer. Reinforcement layout details at such points should be checked. Generating reinforcement Use the Generate reinforcing to have the program generate bars according the detailing parameters. Note: The aim of the automatic reinforcement generation function is to achieve a reasonable optimised reinforcement layout for any typical beam or slab layout. More complicated layouts will likely require editing of the generated reinforcement as described in the text that follows. Very complicated layouts may require more detailed editing using Padds. Editing reinforcement You can modify the generated reinforcement to suite your requirements by editing the information on the Main reinforcing, Stirrups, Shear reinforcing and Sections pages.

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Main reinforcing

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Main reinforcing The main reinforcement bars are defined as follows:

x

Bars: The quantity, type and diameter of the bar, example '2T20' or '2Y16'. The bar defined at the cursor position is highlighted in the elevation.

x

Mark: An alphanumerical string of up to five characters in length, example 'A', '01' or 'A001'.

x

Shape code: Standard bar shape code. Valid shape codes for main bars include 20, 32, 33, 34, 35, 36, 37, 38, 39 and 51.

x

Span: The beam/slab span number.

x

Offset: Distance from the left end of the span to the start point of the bar (m). A negative value makes the bar start to the left of the beginning of the span, i.e. in the previous span.

x

Length: Length of the bar as seen in elevation (m).

x

Hook: If a bar has a hook or bend, enter 'L' or 'R' to it on the left or right side. If this field is left blank, an 'L' is assumed.

x

Layer: Position the bar in the top, middle or bottom layer. Use the letters 'T', 'M' or 'B' with an optional number, e.g. 'T' or 'T1' and 'T2'.

The bending reinforcement diagram is shown on the lower half of the screen. The diagrams for required (red) and entered (blue) reinforcement are superimposed for easy comparison. Bond stress development is taken into consideration in the diagram for entered reinforcement.

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Stirrups

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Stirrups Define stirrup layouts as follows:

x

Stirrup number: Enter a stirrup configuration number. Configuration numbers are used on the Shear reinforcing input page (see page 31) to reference specific configuration. If left blank, the number applicable to the previous row is assumed, i.e. an extended definition of the current configuration.

x

Section number: Concrete cross section number as defined on the Sections input page (see page 32). If left blank, the number applicable to previous row in the table is used.

x

Bars: Type and diameter of bar, example 'R10'. Note: Mild steel bars are normally used for shear reinforcement. However, in zones where much shear reinforcement is required, you may prefer using high yield stirrups. You can do this by entering 'T' or 'Y' bars instead of 'R' bars. In such a case, the yield strength ratio of the main and shear reinforcement, i.e. f y /f YV as entered, will be used to transpose the entered s tirrup areas to equivalent mild steel areas.

x

Mark: Any alphanumerical string of up to five characters in length, e.g. 'SA1', '01' or 'S001'.

x

Shape code: Standard double-leg bar shape code. The following shape codes can be used:

x

BS4466: 55, 61, 77, 78 and 74.

x

SABS082: 55, 60, 72, 73 and 74.

Bars are automatically sized to fit the section web. The first stirrup entered is put against the web sides. Subsequent stirrups are positioned in such a way that vertical legs are spaced equally. Tip: Open stirrups, e.g. shape code 55, can be closed by entering a shape code 35.

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Shear reinforcing

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Shear reinforcing Stirrup layouts defined on the Stirrups input page (see page 29) are distributed over the length of the beam/slab: x

Stirrup number: The stirrup configuration number to distribute.

x

Spacing: Link spacing (mm).

x

Span: The beam/slab span number.

x

Offset: Distance from the left of the span to the start point of the distribution zone (m). A negative value makes the zone start to the left of the beginning of the span, i.e. in the previous span.

x

Length: Length of the stirrup distribution zone (m).

The diagrams for required and entered shear reinforcement are superimposed. The required steel diagram takes into account shear enhancements at the supports. It may sometimes be acceptable to enter less shear steel than the calculated amount of nominal sheer steel, e.g. when the stirrups are only used as hangers to aid the fixing main steel in slabs. This option can be set as default on the Detailing parameters input screen – see page 25 for detail.

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Sections

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Sections Cross-sections can be generated anywhere along the length of the beam/slab to show the main and shear steel layout:

x

Label: The cross-section designation, e.g. 'A'.

x

Span no: The beam/slab span number.

x

Offset: The position of the section, given as a distance from the left end of the span (m).

Sections are displayed on the screen and can be used to check the validity of steel entered at the different positions. Stirrup layouts defined on the Stirrups input (see page 29) rely on appropriate section positions specified. All specified sections will be included in the final bending schedule.

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Bending schedule

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Bending schedule The Bending schedule input page is used generate a complete Padds compatible bending schedule. The parameters allow flexibility in the bending schedule creation, e.g. you can have the details of a beam/slab on a single bending schedule or split it onto more than one schedule to improve clarity. Each bending schedule can then be given a unique name and the associated spans entered. The following information should be entered:

x

File name: The name of the Padds drawing and bending schedule file

x

First span: For clarity, a beam/slab with many spans can be scheduled put on more than one bending schedule. Enter the first span number to be included in the bending schedule.

x

Last span: Enter the last span number to be included in the bending schedule.

x

Grid lines: Optionally display grid lines and numbers appear on the bending schedule drawing.

x

Columns: Optionally display column faces on the bending schedule drawing.

x

First grid: The name or number of the first grid. Use one or two le tters and/or numbers.

x

Number up or down: Specify whether grids must be numbered in ascending or descending order, i.e. 'A', 'B' and 'C' or 'C', 'B' and 'A'

x

Drawing size: Select A4 or A5 drawing size. If A4 is selected, the drawing is scaled to fit on a full page and the accompanying schedule on a separate page. The A5 selection will scale the drawing to fit on the same page with the schedule. Typically, a maximum of three to four spans can be shown with enough clarity in A5 format and four to six spans in A4 format. Note: When combining a drawing and schedule on the same page, the number of schedule lines is limited to a maximum of twenty-four in Padds. Using more lines will result in the drawing and schedule being printed on separate pages.

Use the Generate schedule function to create and display the Padds bending schedule. Editing and printing of bending schedules Detailed editing and printing of bending schedules are done with Padds. For this, following the steps below:

x

Exit the program and launch Padds.

x

Choose Open on the File menu and double-click the relevant file name. The file will be opened and displayed in two casc aded widows. The active windows will contain the drawing of the beam and the second window the bar schedule.

x

Make any necessary changes to the drawing, e.g. editing or adding bars and adding construction notes.

x

Click on any visible part of the window containing the cutting list to bring it to the front. Enter the following information at the relevant positions:

x Member description: Use as many lines of the member column to enter a member description, e.g. '450x300 BEAM'. x General schedule information: Press PgDn to move to the bottom of the bending schedule page and enter the detailers name, reference drawing number etc. x Bending schedule title: Enter the project name and bending schedule title in the centre block at the bottom of the bending schedule. x Bending schedule number: The schedule number in the bottom right corner defaults to the file name, e.g. 'BEAM.PAD'. The schedule number can be edited as required to suite your company's schedule numbering system, e.g. 'P12346-BS001'. Note: The bottom left block is reserved for your company logo and should be set up as described in the Padds User's Guide. Finally, combine the beam drawing and schedule onto one or more A4 pages using the Make BS Print Files command on the File menu. Use Alt+P to print the schedule immediately or Alt+F to save it as a print file for later batch printing.

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Calcsheets

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Calcsheets The beam/slab design output can be grouped on a calcsheet for printing or sending to Calcpad. Various settings are available to include input and design diagram and tabular result.

Tip: You can embed the Data File in the calcsheet for easy recalling from Calcpad.

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Appendix: Suggested design procedures for slabs

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Appendix: Suggested design procedures for slabs Some suggestions are made below with regards the design and detailing of solid slabs and coffer slabs.

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Suggested design procedure for solid slabs

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Suggested design procedure for solid slabs The suggestions are explained by way of an example. A flat slab with a regular rectangular column layout of 6.0 m by 5.5 m is considered. Typical strip over a row of internal columns (Strip A) The strip is modelled as a 6000 mm wide panel, i.e. 3000 mm either side of the columns. The program calculates moments and shear forces for the whole panel width. It then details a column strip, 1500 mm wide, and middle strip, 4500 mm wide. For an explanation of the division into column and middle strips, see page 11. External strip (Strip B) The external strip, strip B, is defined as the portion over the external columns that extending halfway to the first row of internal columns. Strip C is the first internal strip and it extends to midspan on both sides. Consider the end panel, i.e. the portion between edge columns and the first row of internal columns or, in other word, strip B together with half of strip C. The portion over the internal columns (portion of strip C) will tend to attract more moment than the portion over the external columns (strip B). Using a rule of thumb, a reasonable moment distribution ratio would be about 62.5% to 37.5%. The external strip (strip B) can thus be conservatively modelled as a panel with width equal to half the transverse column spacing, i.e. 3000 mm, carrying the full load for that area. The program will analyse the strip and the generate reinforcement for a column strip, 750 mm wide, and a middle strip, 2250 mm wide. First internal strip (Strip C) The first internal strip can subsequently be modelled using the same width as a typical internal panel, i.e. 6000 mm. Because of the moment distribution explained above, the loading is increased to 50% + 62.5% = 112.5% of the typical panel loading. The small overlap in loading between the edge and first internal panels should take care of any adverse effects due to pattern loading. Note: If the own weight is modelled using a density, you should account for the increased loading by either increase the density value by 12.5% or increasing the applied dead load. The program will analyse the panel and generate a column strip, 1500 mm wide, and a middle strip 4500 mm wide. Reinforcement layout Careful combination of the column and middle strips generated above, should yield a reasonably economical reinforcement layout:

x

For typical internal strips (strip A), use the generated column strip (CA) and middle strip (MA).

x

For the column strip over the external row of columns, use no less than the column strip reinforcement (CB) generated for the external strip (strip B).

x

For the column strip over the first row of internal columns, use no less than the column strip reinforcement (CC) generated for the first internal strip (strip C).

x

The first middle strip from the edge (MC/MB) can be conservatively taken as the worst of middle strip generated for the first internal strip (MC) and twice that generated for the external strip (MB).

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Concrete Beam and Slab Design Using Prokon

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