preliminary design of steel structures

7/30/2019 preliminary design of steel structures

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Ferenc PappPh.D. Dr.habil

Steel BuildingsDESIGN NOTES

Practice 1

PRELIMINARY DESIGN

Written in the framework of the project TMOP 421.B JLK 29

Reviewed byDr. Bla Verci

honorary lecturer

2012 Budapest


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Ferenc PappSteel Buildings Preliminary design

2

main frames with hot-rolled orwelded sections

wall beams wall bracing

stiffener bars

wall columns

double trapezoidal paltes withheat isolation

secondary beam forgate frame

pulins

wind bracing

1.1 The aim of the design task

The objective of the design task is the steel structure of a simple hall. The primary loadcarrying structural members are the frames made of hot-rolled or welded sections. Thedistances 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 structuralmembers are running in perpendicular direction to the plane of the frames. The coveringtrapezoidal 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 locatedbelow the purlins. The gates in the front walls may be framed by secondary columns and wallbeams. The spatial stiffness of the building structure is ensured by the wind bracing systemswhich 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.

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 datawhich are determined and supplied by the architectural engineer which satisfy both theappropriate building regulations and the requirements of the owner. In the case of the presentdesign project the initial data concerns to the outer surfaces of the flanges of the steel mainframes (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: [deg]


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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 beamsand two columns) and these members are connected to each others at the building site usingmoment 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 columnbase may be used in special cases since the cost of it may be much more. The beams made ofhot-rolled or uniform welded sections may be strengthened by haunches. The haunch shouldbe short (at about 1,5 times the depth of the beam section), if it is used to ensure theconstruction of the end plated connection. Long haunch (at about 0,4 times the length of thebeam) may be used to increase the strength of the beam at about the frame corner where thebending moment has maximum. In case of relatively great span tapered structural membersmay be used. In this construction haunch is not used. The frames at the front walls might beweaker than the interval ones, but in order to keep the conditions of the extension of thebuilding, 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,

ov 15s5.0b

H

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: 5040/b - width of the flange of welded sections: 12080/b

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 ofthe haunch can net be greater than the depth of the beam section. The width of the flange andthe thickness of the web of the haunch may be equal to those used in the beam sections, butthe flange should be thicker by 4-6 mm. The symbols of the section parameters used later areshown by the Table 1.2.

A0 [m2]

0.0

Hv [m]

b [m] [deg]


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Tab.1.1 Initial size for the cross-sections of the main frames in the function of the span LspanL [m] type of the section size* [mm]

1216 hot rolled (IPE/HEA) 300450/2002601624 welded I flange: 200300 1620

web: 400600 810

2432 tapered I flange: 300340 1620web: 8001200 68* 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 andthe upper limits of the span L

Tab. 1.2 Denotations for the geometrical properties of the cross-sectionsstructural member property meaning

bcf width of the flangetcf thickness of the flangehcw width of the web

column

tcw thickness of the webbbf width of the flangetbf thickness of the flangehbwhb

width of the webdepth of the beam

tbw thickness of the webhh depth of the haunch

beam

lh 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 Figure1.3for both the prismatic and the tapered members). The theoretical span of the main frameis equal to the horizontal distance between the central (reference) axes of the columns:

c0 hL = b

where hc is the initial height of the column section, b is the outer distance of the columnsprescribed by the architectural engineer.

Fig.1.3 Theoretical parameters of the main frame

HcHw

L0/2

b/2

hc

Ht

hb

covering system

hb

hc

Hf

b/2

L0/2

Hf


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The theoretical height of the columns is equal to the distance between the theoretical columnbase point and the intersectional point of the column and the beam central axis. Thisparameter may be calculated approximately by the following expression:

cos2

hH bc

= vH

where Hv is the initial height of the side walls, hb is the initial depth of the beam section. Thetheoretical ridge (top) point of the frame may be calculated by the following expression:

)tan(2

LHH 0cf +=

It is noted that the last two parameters may be determined by drawing. The reference axes ofthe tapered structural members in Figure 1.3 start at the centroid of the lower ends and runparallel to the outer flanges. This is done when the applied design software (for exampleConSteel) uses eccentric elements in the mechanical model. Otherwise the reference axesshould 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 thetheoretical length of the steel structure may be calculated,

b

A0=nd

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

1c

dn

f

nn +=

In the expression cf denotes the interval between the main frames, where the optimal value isc=57 meters. Different distance may be used in special circumstances only. The appliednumber na of the main frames should be an integer, which is determined on the base of therequired number of frames nn. The real theoretical length between the final frames is thefollowing (seeFigure 1.4):

( )faa c1nd =

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

da

cf

1 2 na

cf cf cf


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Since the distance between the main frames is normally uniform, therefore the initial basicarea (A0) of the building may be kept only approximately. The real basic area can becalculated by the main parameters of the structures which were determined previously:

cswbfaa h2bdbA ++=

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

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 anyprevious reason to use S355 steel grade, the grade of S235 is suggested using. If it isreasonable, the initial grade of steel may be changed during the analysis and design of thestructure. At the and of the design the quality of steel material should be selected with greatcare (see the course of Steel Structures II).

purlin

beam of the frame

wall column

bbfhcsw

wall beam

da


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1.7 Application

1. PRELIMINARY DESIGN

1.1 Initial parameters

- area to be builtA0 725 m

2:=

- width of the building b 20.0 m:=

- heigth of the side walls Hw 7.5 m:=

- slope of the roof 10 deg:=

1.2 Initial data for the main structural members

- main frames (welded I section)

column

flange bcf 240 mm:= tcf 16 mm:=

web hcw 468 mm:= tcw 8 mm:=

depth hc hcw 2 tcf+ 500mm=:=

beam

flange bbf 240 mm:= tbf 16 mm:=

web hbw 368 mm:= tbw 6 mm:=

depth hb hbw 2 tbf+ 400 mm=:=

- columns in side walls HEA160 hcsw 150 mm:=

- purlin Lindab Z 200 hp 200 mm:=- beams in walls Lindab C 200 hbsw 200 mm:=

1.3 Theoretical properties of the structural model

- span of the frames L0

b hc

19.5m=:=

- height of the columns Hc Hwhb

2

7.3m=:=

- heigth of the frame

Hf Hc

L0

2tan ( )+ 9.019m=:=

1.4 Number of the main frames

- prescribed length of the building

d0

A0

b36.25m=:=

- interval of the frames cf 6.0 m:=

- required number of the frames nnd0

cf

1+ 7.042=:=

- applied number of the frames

na 7:=

The building consists of 7

frames!1.5 Area of the bulding

- length of the building

da na 1( ) cf 36 m=:=

- actual area of the building Aa b da bbf+ 2 hcsw+( ) 730.8m2

=:=

- deviation Aa

A0100% 0.8%=:=

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 ofConcept A it is assumed that the purlin system carry the loads and effects whichact directly to the roof, and it does not take part in the bracing of the building. In this case thewind bracing system is a spatial trussed structure, which consists of the two neighboring mainframes, the diagonals and the longitudinal bars which are placed under the purlinsindependently to them. In the case ofConcept B the longitudinal bracing bars are replaced bythe purlins. Which concept to be followed in the design may be supported by the followingrules and comments:

Application of the Concept A may be suggested in the case greater span (more than20m) and/or for considerable design loads (e.g crane load) since the solution is noteconomical for relatively small spans with relatively low design loads and effects.

Application of the Concept B may not be suggested for relatively small span (lessthan 20m) where besides the dead load and the meteorological loads the seismic effectis not dominant.

More details can be available in the material of the Practice 4. In this design project theLindab Z purlin is suggested for the roof system. It is a practical experience that the optimaldistance between two neighboring purlins is e=1,53,0 meters. The depth of the purlin maychange form 200 mm to 300 mm, while the thickness from 1,5 mm to 2,5 mm. The distance isdetermined also by the rule that the optimal value of the angle of the bracing diagonals to theaxis of the frame beam is about 45 degrees, but it is not greater than 60 degrees and not lowerthan 30 degrees. The suggested numbers for intermediate units are 4, 6 or 8, since theapplication of a half-bracing unit can be avoided by this way (seeFigure 1.6).

Fig.1.6Optimal arrangement of purlin system

The practical arrangement shown in Figure 1.7 may differ from the theoretical arrangementshown inFigure 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).

b1224m

b18-36m

b24-48m

Ls

Ls

Ls

Ls - distance between the ridge point of the roof and the outer point of the edge purlin inthe plane of the roof system (seeFigure 1.7)


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The distances denoted inFigure 1.6may be calculated by the following expressions:

cos

lhh2

h

fwherefcos2

LL

ecovbswc

s

+++

=+

=

where hbsw is the depth of the wall beams, hcov is the thickness of the covering and le is theextension (about 150mm). The distance between the purlin and the ridge point may beg150200 mm.

Fig.1.7The 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 anycase the external loads and effects are carried by the external trapezoidal sheet.

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

eaea

g

(b)

(a)

f

Ls ea

external trapezoidal sheetvapour permeable leafheat insulation (150 mm)vapour proof leafinternal tra ezoidal sheet

external trapezoidal sheetvapour permeable leafheat insulation (150 mm)vapour proof leafinternal tra ezoidal sheet

spacer members

(a) (b)


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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

hcov 200 mm:=

- extension le 150 mm:=- distance between the edge purlin and the theoretical point of the frame corner

f

hc

2hbsw+ hcov+ le+

cos ( )812 mm=:=

- distance between the edge purlin and the ridge point

LsL0

2 cos ( )f+ 10.713 m=:=

- interval of purlins

case of four spans e4

Ls

4 2678 mm=:=

case of six spans e6Ls

61785 mm=:=

applied spans ea 2640 mm:=

ga 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 (seeSection 1.8). The arrangement is governed by the dimension of the openings (gates andwindows). It is important that the wall beams in side and front walls are located at the samelevels (see Figure 1.1). The wall columns in the front walls should be located below thepurlins. Figure 1.9a shows the situation where the gate is framed by two neighboring wallcolumns and a wall beam. Figure 1.9b shows the situation where the gate is wider than thedistance between two wall columns and therefore the frame of the gate is ensured bysecondary columns.

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

wall columnwall beam secondary columns

(a) (b)


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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 ismentioned 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 membersbetween the braced units and the purlins and the wall beams are not the parts of the bracingsystem. The so called economic concept (Concept B) assumes that the purlins and the wallbeams can replace the longitudinal bracing members, therefore they may be neglected(partially or totally). In the practice the mixed construction is also used, where longitudinalbracing members are used only at the frame corners and the ridge point. Theoretically, usingthe modern computational tools the optimum wind bracing system may be determined byadvanced numerical methods. Practically, these methods are time and cost consuming. In thisdesign project the conservative design method is discussed. In Figure 1.10 thick lines denotethe frames, dashed lines denote the members of the wind bracing system, while thin linesshow 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.

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 indrawings. The preliminary drawings are the basic documents for the structural analysis anddesign. Therefore, these drawings should contain all the initial parameters of the buildingused in the procedure of the analysis and design. These drawings should not be confused withthe architectural plans and the scenario of the building. In this design project the followingthree 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

(a)

(b)

wind bracing(in roof)

bracing members

purlins

wind bracing(in wall)

wall beams


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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 bracingsystem follows the Concept A (the bracing system is independent to the purlin system), thetop view side of the drawing may be divided into two symmetrical parts: the upper quarter ofthe drawing shows the arrangement of the purlin system, while the lower quarter of thedrawing shows the bars of the bracing system. The view of the foundation and the roofsystem is projected to the horizontal plane. The drawing gives exact answer to the followingparameters: 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 theSections 1.7 and 1.9 (Applications) is shown in the Figure 1.11. It can be seen that thebracing system follows the design Concept A. Furthermore, it can be seen that the columnfoundations are tied up by beams, and this system works together with the concrete slab of theindustrial 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 thearrangement of the wall beams and about the area and place of the openings as well. Thebuilding is symmetrical, therefore the right hand side of the drawing may show thearrangement of the openings, while the left hand side may show the arrangement of the wallbeams and the bracing system. The drawing should give exact answer for the followingparameters:- 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 theabove parameters. The drawing which satisfies the Sections 1.7 and 1.9 (Applications) isshown in the Figure 1.12. It can be seen that the wind bracing system is an independentstructure, 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 ofit 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 ();- 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 theFigure 1.13. It can be seen that the column foundation, the beams between the concreteblocks and the concrete slab of the industrial floor form a unified structural system.


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Department of Struc

Steel Buildings

Draw No. 1: Prelim

Designer

Supervisor

Top view

(projected to horizontal plane)Top view of the found

Purlin system

Bracing system

10550

2600

2750

2600

2600

9750

5350

4400

bracing member, if it is needed (CHS)

bracing members (L or rod section)

purlin (Lindab Z200)

wall columns

(HEA or IPE)

bracing members in wall (L or6000 6000

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


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Ferenc Papp

Steel Buildings Preliminary design

15

Department of Str

Steel Buildings

Draw No. 002: Pr

Designer

Supervisor

6000 6000 6000 length of the window: 11600

18 400

3000

1200

2900

600

18 000

Arrangement of wall beams and bracing system Arrangement of open

3000

1200

2900

600

3,60

4,60

9,700

4400 5350 gate: 5000 3600

7700

7700

Bracing bars (CHS), if it is needed

Bracing diagonals (L or rod section

Lindab wall beam (C200)

Fig.1.12 Side view drawing of the building


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Ferenc Papp

Steel Buildings Preliminary design

16

Department of Structural Engin

Steel Buildings

Draw No. 003: Preliminary d

Designer Cl

Supervisor Cl

19500/2 20000/2

Grade of steel: S235

Standard: Eurocode 3

Fix column base

Welded I section- flanges: 240-16

- web: 368-6

Moment

resistant end-

plated bolted

connections9019

10713

2640

2640

2640

2640153

330

35002850

2900

600

1200

7300

9219Purlins (Lindab Z200)

Wall beams (Lindab C200)

CHS bracing members

Slope of roof: 100

CHS bracing members, if it is needed

Fig.1.13 Side view drawing of the structural frame


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preliminary design of steel structures