preliminary design of steel structures

Ferenc Papp Ph.D. Dr.habil

Steel Buildings DESIGN NOTES

Practice 1 PRELIMINARY DESIGN

Written in the framework of the project TÁMOP 421.B JLK 29 Reviewed by Dr. Béla Verőci honorary lecturer

2012 Budapest

Ferenc Papp Steel Buildings – Preliminary design

1.1 The aim of the design task The objective of the design task is the steel structure of a simple hall. The primary load carrying structural members are the frames made of hot-rolled or welded sections. The distances between the frames are normally equal. The secondary load carrying structural members are the purlins in the roof and the wall beams in the side walls. These structural members are running in perpendicular direction to the plane of the frames. The covering trapezoidal plates are running in the perpendicular direction to the direction of the purlins. The wall beams in the front walls are supported by the wall columns which should be located below the purlins. The gates in the front walls may be framed by secondary columns and wall beams. The spatial stiffness of the building structure is ensured by the wind bracing systems which may be located at the front wall frame units and which are connected by stiffener bars, if it is needed. The described system is illustrated in the Figure 1.1. pulins stiffener bars

wind bracing

double trapezoidal paltes with heat isolation

wall beams

wall bracing

main frames with hot-rolled or welded sections

wall columns secondary beam for gate frame

Fig.1.1 Conceptual system of the structure

1.2 The initial data for the design The work starts with the preliminary design of the structure. It is based on the initial data which are determined and supplied by the architectural engineer which satisfy both the appropriate building regulations and the requirements of the owner. In the case of the present design project the initial data concerns to the outer surfaces of the flanges of the steel main frames (see Figure 1.2): • Base area to be built: A0 [m2]; • Horizontal distance between the flanges of the main frame: b [m]; • Height of the side walls: Hv [m] • Slope of the roof: α [deg]

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b [m]

α [deg] Hv [m] A0 [m2] 0.0 Fig.1.2 Initial data for the design

1.3 The theoretical parameters of the main structure The symmetric frame structure may be fabricated from four structural members (two beams and two columns) and these members are connected to each others at the building site using moment resistance end plated bolted connections at the frame corners and at the ridge point. The column bases are usually connected to the concrete bases by pined joints. Fix column base may be used in special cases since the cost of it may be much more. The beams made of hot-rolled or uniform welded sections may be strengthened by haunches. The haunch should be short (at about 1,5 times the depth of the beam section), if it is used to ensure the construction of the end plated connection. Long haunch (at about 0,4 times the length of the beam) may be used to increase the strength of the beam at about the frame corner where the bending moment has maximum. In case of relatively great span tapered structural members may be used. In this construction haunch is not used. The frames at the front walls might be weaker than the interval ones, but in order to keep the conditions of the extension of the building, these frames should be the as strong as the interval ones. The sizes of the frame sections are determined by the b initial parameter (span of the frame). If the building is relatively low, Hv ≤ 0 .5 b

és

α ≤ 15o

and the dominant design loads are the meteorological loads, the initial depth of the frame sections may be taken as the following: - depth of the beam and column sections: ≈ b / 40 ÷ 50 - width of the flange of welded sections: ≈ b / 80 ÷ 120 If long haunch is used the depth of the beam sections may be reduced (it is suggested). Table 1.1 contains the suggested sizes which are based on practical experiences. The depth of the haunch can net be greater than the depth of the beam section. The width of the flange and the thickness of the web of the haunch may be equal to those used in the beam sections, but the flange should be thicker by 4-6 mm. The symbols of the section parameters used later are shown by the Table 1.2.

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Tab.1.1 Initial size for the cross-sections of the main frames in the function of the span L span L [m]

type of the section

size* [mm]

12÷16 16÷24

hot rolled (IPE/HEA) welded I

24÷32

tapered I

300÷450/200÷260 flange: 200÷300 – 16÷20 web: 400÷600 – 8÷10 flange: 300÷340 – 16÷20 web: 800÷1200 – 6÷8

* in the case of hot rolled sections the values mean depth of the section for the lower and the upper limits of the span L; in the case of welded sections the values mean the width and thickness of the plates for the lower and the upper limits of the span L

Tab. 1.2 Denotations for the geometrical properties of the cross-sections structural member

column

property

bcf tcf hcw tcw bbf tbf hbw hb tbw hh lh

beam

meaning

width of the flange thickness of the flange width of the web thickness of the web width of the flange thickness of the flange width of the web depth of the beam thickness of the web depth of the haunch length of the haunch

(*) used letters in the indexes: column; beam; flange; web; haunch

1.4 Theoretical parameters of the frame The theoretical parameters of the steel frame are needed for the structural analysis (see Figure 1.3 for both the prismatic and the tapered members). The theoretical span of the main frame is equal to the horizontal distance between the central (reference) axes of the columns: L0 = b − hc where hc is the initial height of the column section, b is the outer distance of the columns prescribed by the architectural engineer.

α

hc

hb

hb covering system

Ht

Hw Hc

Hf

Hf

hc b/2

b/2

L0/2

L0/2

Fig.1.3 Theoretical parameters of the main frame 4


The theoretical height of the columns is equal to the distance between the theoretical column base point and the intersectional point of the column and the beam central axis. This parameter may be calculated approximately by the following expression: Hc = Hv −

hb 2 ⋅ cos α

where Hv is the initial height of the side walls, hb is the initial depth of the beam section. The theoretical ridge (top) point of the frame may be calculated by the following expression: H f = Hc +

L0 ⋅ tan( α ) 2

It is noted that the last two parameters may be determined by drawing. The reference axes of the tapered structural members in Figure 1.3 start at the centroid of the lower ends and run parallel to the outer flanges. This is done when the applied design software (for example ConSteel) uses eccentric elements in the mechanical model. Otherwise the reference axes should follow the centroidal axis of the members. 1.5 The number of the main frames and their interval

The architectural concept has prescribed the basic area of the building (A), from which the theoretical length of the steel structure may be calculated, dn =

A0 b

where the parameters are defined in the Section 1.2. The required number of the main frames may be determined as following:

nn =

dn +1 cf

In the expression cf denotes the interval between the main frames, where the optimal value is c=5÷7 meters. Different distance may be used in special circumstances only. The applied number na of the main frames should be an integer, which is determined on the base of the required number of frames nn. The real theoretical length between the final frames is the following (see Figure 1.4): d a = (na − 1) ⋅ c f 1

cf

2

cf

na

cf

cf

da

Fig.1.4 The applied number of main frames and the real theoretical length of the structure 5


Since the distance between the main frames is normally uniform, therefore the initial basic area (A0) of the building may be kept only approximately. The real basic area can be calculated by the main parameters of the structures which were determined previously: Aa = b ⋅ (d a + bbf + 2 ⋅ hcsw )

where bbf [m] is the flange width of the beam section, hcsw is the depth of the column section in the end wall system (see Figure 1.5). It should be noted that the previous expression is valid for the structural solution illustrated in the Figure 1.5. purlin

beam of the frame

wall beam

wall column

hcsw

bbf da

Fig.1.5 Structural system of the end wall 1.6 The initial grade of material The main structural elements are normally made from S235 or S355 steel. Unless there is any previous reason to use S355 steel grade, the grade of S235 is suggested using. If it is reasonable, the initial grade of steel may be changed during the analysis and design of the structure. At the and of the design the quality of steel material should be selected with great care (see the course of Steel Structures II).

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1.7 Application 1. PRELIMINARY DESIGN 1.1 Initial parameters 2

- area to be built

A 0 := 725 ⋅ m

- width of the building

b := 20.0 ⋅ m

- heigth of the side walls

Hw := 7.5 ⋅ m

- slope of the roof

α := 10 ⋅ deg

1.2 Initial data for the main structural members - main frames (welded I section) column flange

b cf := 240 ⋅ mm

t cf := 16 ⋅ mm

web

h cw := 468 ⋅ mm

t cw := 8 ⋅ mm

depth

h c := h cw + 2⋅ t cf = 500⋅ mm

beam flange

b bf := 240 ⋅ mm

t bf := 16 ⋅ mm

web

h bw := 368 ⋅ mm

t bw := 6 ⋅ mm

depth

h b := h bw + 2⋅ t bf = 400 ⋅ mm

- columns in side walls

HEA160

h csw := 150 ⋅ mm

- purlin

Lindab Z 200

h p := 200 ⋅ mm

- beams in walls

Lindab C 200

h bsw := 200 ⋅ mm

1.3 Theoretical properties of the structural model - span of the frames

L0 := b − h c = 19.5⋅ m

- height of the columns

Hc := Hw − 

- heigth of the frame

Hf := Hc +

 hb   = 7.3⋅ m  2

L0 2

⋅ tan ( α) = 9.019⋅ m

1.4 Number of the main frames d 0 :=

- prescribed length of the building

A0 b

= 36.25⋅ m

- interval of the frames

cf := 6.0 ⋅ m

- required number of the frames

n n :=

d0 cf

+ 1 = 7.042

n a := 7

- applied number of the frames

The building consists of 7 frames!

1.5 Area of the bulding - length of the building

d a := ( n a − 1) ⋅ cf = 36⋅ m

- actual area of the building

A a := b ⋅ ( d a + b bf + 2⋅ h csw) = 730.8⋅ m

- deviation

δ :=

2

Aa

− 100% = 0.8⋅ % A0 The actual area of the building satisfies the official plan!

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1.8 Arrangement of the purlin system The wind bracing system may be in the following relationship with the purlin system: • Concept A: Purlin system is independent to the wind bracing system • Concept B: Purlin system and wind bracing system form a unified static system In case of Concept A it is assumed that the purlin system carry the loads and effects which act directly to the roof, and it does not take part in the bracing of the building. In this case the wind bracing system is a spatial trussed structure, which consists of the two neighboring main frames, the diagonals and the longitudinal bars which are placed under the purlins independently to them. In the case of Concept B the longitudinal bracing bars are replaced by the purlins. Which concept to be followed in the design may be supported by the following rules and comments: • Application of the Concept A may be suggested in the case greater span (more than 20m) and/or for considerable design loads (e.g crane load) since the solution is not economical for relatively small spans with relatively low design loads and effects. • Application of the Concept B may not be suggested for relatively small span (less than 20m) where besides the dead load and the meteorological loads the seismic effect is not dominant. More details can be available in the material of the Practice 4. In this design project the Lindab Z purlin is suggested for the roof system. It is a practical experience that the optimal distance between two neighboring purlins is e=1,5÷3,0 meters. The depth of the purlin may change form 200 mm to 300 mm, while the thickness from 1,5 mm to 2,5 mm. The distance is determined also by the rule that the optimal value of the angle of the bracing diagonals to the axis of the frame beam is about 45 degrees, but it is not greater than 60 degrees and not lower than 30 degrees. The suggested numbers for intermediate units are 4, 6 or 8, since the application of a half-bracing unit can be avoided by this way (see Figure 1.6). b≈12÷24m Ls b≈18-36m Ls b≈24-48m Ls Ls - distance between the ridge point of the roof and the outer point of the edge purlin in the plane of the roof system (see Figure 1.7)

Fig.1.6 Optimal arrangement of purlin system The practical arrangement shown in Figure 1.7 may differ from the theoretical arrangement shown in Figure 1.6: (i) at ridge double purlins are used (Figure 1.7a); (ii) at edge of the roof special edge shape is used (Figure 1.7b).

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The distances denoted in Figure 1.6 may be calculated by the following expressions: hc + hbsw + hcov + le L + f where f = 2 Ls = 2 ⋅ cos α cos α where hbsw is the depth of the wall beams, hcov is the thickness of the covering and le is the extension (about 150mm). The distance between the purlin and the ridge point may be g≈150÷200 mm.

Ls

ea g

ea ea f

(a) (b)

Fig.1.7 The scheme of the practical purlin arrangement: (a) double purlins at the ridge; (b) C shaped edge purlin The two suggested constructions for the covering system are shown in Figure 1.8. In any case the external loads and effects are carried by the external trapezoidal sheet. (a) (b) external trapezoidal sheet vapour permeable leaf heat insulation (150 mm) vapour proof leaf internal trapezoidal sheet

external trapezoidal sheet vapour permeable leaf heat insulation (150 mm) vapour proof leaf internal trapezoidal sheet

spacer members

Fig.1.8 Covering system with heat insulation and double trapezoidal sheets: (a) insulation is placed between the purlins (b) insulation is placed on the purlins

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1.9 Application 1.6 Arrangement of the purlin system The covering is constructed with insulation layer placed onto the purlins (wall beams): - thickness of the covaring h cov := 200 ⋅ mm - extension le := 150 ⋅ mm - distance between the edge purlin and the theoretical point of the frame corner hc + h bsw + h cov + le 2 f := = 812 ⋅ mm cos ( α) - distance between the edge purlin and the ridge point Ls :=

L0 2⋅ cos ( α)

+ f = 10.713⋅ m

- interval of purlins case of four spans case of six spans applied spans

e4 := e6 :=

Ls 4 Ls 6

= 2678 ⋅ mm = 1785 ⋅ mm

ea := 2640 ⋅ mm g a := Ls − 4⋅ ea = 153 ⋅ mm The e=2640 mm distance is choosen for the arrangement of the purlin system (except the last distance at the ridge) !

1.10 Wall system The rules of the arrangement of purlins are valid for the arrangement of the wall beams (see Section 1.8). The arrangement is governed by the dimension of the openings (gates and windows). It is important that the wall beams in side and front walls are located at the same levels (see Figure 1.1). The wall columns in the front walls should be located below the purlins. Figure 1.9a shows the situation where the gate is framed by two neighboring wall columns and a wall beam. Figure 1.9b shows the situation where the gate is wider than the distance between two wall columns and therefore the frame of the gate is ensured by secondary columns. wall beam

(a)

wall column

secondary columns

(b)

Fig.1.9 Wall columns and beams in the front wall (a) gate framed by wall columns and beam (b) gate framed by secondary columns

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1.11 Wind bracing system The spatial stiffness of the building structure is ensured by the wind bracing system. As it is mentioned in Section 1.8, the wind bracing system may be design on two different concepts. The so called ‘conservative” concept (Concept A) uses longitudinal bracing members between the braced units and the purlins and the wall beams are not the parts of the bracing system. The so called “economic’ concept (Concept B) assumes that the purlins and the wall beams can replace the longitudinal bracing members, therefore they may be neglected (partially or totally). In the practice the mixed construction is also used, where longitudinal bracing members are used only at the frame corners and the ridge point. Theoretically, using the modern computational tools the optimum wind bracing system may be determined by advanced numerical methods. Practically, these methods are time and cost consuming. In this design project the conservative design method is discussed. In Figure 1.10 thick lines denote the frames, dashed lines denote the members of the wind bracing system, while thin lines show the purlins and the wall beams. Here it is assumed that the planes of the bracing structures are located in the reference (centroid) planes of the main (walls and roof) structures. Later it is allowed to move these planes. (a) bracing members

wind bracing (in roof) purlins

wall beams

(b)

wind bracing (in wall)

Fig.1.10 Wind bracing system designed by Concept A (dashed lines denote the bracing members) 1.12 Preliminary drawings The aim of the preliminary drawing is to establish the initial parameters of the design in drawings. The preliminary drawings are the basic documents for the structural analysis and design. Therefore, these drawings should contain all the initial parameters of the building used in the procedure of the analysis and design. These drawings should not be confused with the architectural plans and the scenario of the building. In this design project the following three drawings should be prepared (the format of the drawings is A4 or A3): • top view of the foundation and the roof structure • side views of the building

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•

side views of the main frame.

1.12.1 Foundation and roof view (M 1:200) The building is symmetric, therefore the one half of the drawing may show the top view of the foundation, while the other half of it may show the top view of the roof. If the wind bracing system follows the Concept A (the bracing system is independent to the purlin system), the top view side of the drawing may be divided into two symmetrical parts: the upper quarter of the drawing shows the arrangement of the purlin system, while the lower quarter of the drawing shows the bars of the bracing system. The view of the foundation and the roof system is projected to the horizontal plane. The drawing gives exact answer to the following parameters: • top view of the foundation (right side of the drawing): - theoretical span - number of the frames - distance between the frames - arrangement and initial parameters of the columns in the side walls - scheme of the foundation • top view of the roof structure (left side of the drawing): - arrangement and initial parameters of the purlins - arrangement and parameters of the wind bracing system. The drawing of the top view of the foundation and the roof structure which satisfies the Sections 1.7 and 1.9 (Applications) is shown in the Figure 1.11. It can be seen that the bracing system follows the design Concept A. Furthermore, it can be seen that the column foundations are tied up by beams, and this system works together with the concrete slab of the industrial floor. 1.12.2 Side views of the building (M 1:200) The aim of the side view drawings of the building is to give direct information about the arrangement of the wall beams and about the area and place of the openings as well. The building is symmetrical, therefore the right hand side of the drawing may show the arrangement of the openings, while the left hand side may show the arrangement of the wall beams and the bracing system. The drawing should give exact answer for the following parameters: - places and initial section of the wall beams - arrangement and initial sections of the bracing system - place and area of the openings. The drawing of the side view does not contain architectural sceneries, it concentrates to the above parameters. The drawing which satisfies the Sections 1.7 and 1.9 (Applications) is shown in the Figure 1.12. It can be seen that the wind bracing system is an ‘independent’ structure, the wall beams are not the part of it. 1.12.3 Side view of the frame (cross section of the building) (M 1:100) The aim of the side view drawing of the frame is to give direct information to take the structural and load model for analysis and design. The frame is symmetrical, therefore the

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right hand side of the drawing may contain the general parameters, while the left hand side of it may show the theoretical parameters and the arrangement of the structural members: • general parameters (right hand side) - distance between the outer flanges of the columns (b) - height of the facade (Hv); - slope of the roof (α α); - height of the structure (Ht); - parameters of the column section (bcf;tcf.hcw;tcw); - parameters of the beam section (bbf;tbf.hbw;tbw); - parameters of the haunch (bhf;thf.hhw;thw); - layers of the covering system; • arrangement of members and theoretical parameters (left hand side) - theoretical height of the columns (Hc); - theoretical height of the frame (Hf); - arrangement and initial section of the purlins; - arrangement and initial section of the wall beams; - type of the joints; - type of the column base; - length of the haunch; - quality of materials - standards are used; The drawing which satisfies the Sections 1.7 and 1.9 (Applications) is shown in the Figure 1.13. It can be seen that the column foundation, the beams between the concrete blocks and the concrete slab of the industrial floor form a unified structural system.

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Top view (projected to horizontal plane) 2600

wall columns (HEA or IPE)

Top view of the foundation

purlin (Lindab Z200)

4400 2600

Purlin system

10550 2600 5350 2750 19500 5350

5350

bracing members (L or rod section) bracing member, if it is needed (CHS)

9750 4400

Bracing system

bracing members in wall (L or

Fig. 1.11 Top view drawing of the roof and the foundation

4400

6000

6000

6000

Department of Structural Engineering BUTE Steel Buildings Draw No. 1: Preliminary drawing/Top view M 1:200 Designer Clever Student (XYZVW) Supervisor Clever Teacher assistant professor


Arrangement of wall beams and bracing system

7700

3000

Lindab wall beam (C200)

1200

Bracing bars (CHS), if it is needed

7,860

Arrangement of openings 4,600 3,600

Bracing diagonals (L or rod section

2900

0,0

600 6000

6000

6000

length of the window: 11600 18 400

18 000

9,700 7,860

7700

3000

4,600

1200

3,600

2900 0,0

600 4400

5350

gate: 5000

3600

Department of Structural Engineering BUTE Steel Buildings Draw No. 002: Preliminary drawing/Side view M 1:200 Designer Clever Student (XYZVW) Supervisor Clever Teacher assistant professor

Fig.1.12 Side view drawing of the building

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10713

2640

153

2640

Welded I section - flanges: 240-16 - web: 368-6

2640 2640

Covering system: - external trapezoidal sheet - vapour permeable leaf - heat insulation (150 mm) - vapour proof leaf - internal trapezoidal sheet - purlin

Slope of roof: 100 330 2850

Moment resistant endplated bolted connections

3500

9019 1200

Haunch: - flange: 240-20 - web: 330-6

9219

Purlins (Lindab Z200) Wall beams (Lindab C200)

7300

Welded I section - flanges: 240-16 - web: 468-8

CHS bracing members CHS bracing members, if it is needed 2900

19500/2

7500

20000/2

Grade of steel: S235 Standard: Eurocode 3 600

Fix column base

Department of Structural Engineering BUTE Steel Buildings Draw No. 003: Preliminary drawing /Side view of the frame M 1:200 Designer Clever Student (XYZVW) Supervisor Clever Teacher assistant professor

Fig.1.13 Side view drawing of the structural frame

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preliminary design of steel structures

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