Design Principles for Roof Steel Truss

8/19/2019 Design Principles for Roof Steel Truss

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www.sefindia.org/?q=system/files/ROOF+STEEL+TRUSS1.doc

DESIGN PRINCIPLES FOR ROOF STEEL TRUSS

INTRODUCTION:

Steel trusses are being used for both buildings and bridges. But thedesign principles are dierent for dierent uses. Many books arecourse oriented and not with a practical principles. Now an attempt hasbeen made to gather information on the design principles from variousreferences ON T! "#$O%T #N& OT!' &!S()N *'(N+(*"!S.

Steel roof trusses are used for mainly for the (ndustrial buildings wherefree space re,uirement are essential for more working areas. The spanof truss varies from -/01 to 2/01 depending on the type of re,uirement and the available spaces.

The following steps should be considered when designing a truss3-. Select the general layout of truss members and truss spacing.4. !stimate e5ternal loads to be applied including self weight of

truss6 purlins and roof covering together with wind loads.2. &etermine critical 7worst combination8 loading. (t is usual to

consider &ead loads alone and then &ead and (mposed loadscombined.

9. #naly:e the frame work to ;nd forces in all members.or span up to about 4. m6 the spacing of steel trusses is likely tobe about 9.m i.e. -?< of span.# slope of 44@7degree8 is common for corrugated steel and asbestosroo;ng sheets.>or economic spacing of roof trusses6 the cost of truss should be e,ualto twice the cost of purlins Athe cost of roof covering. #s a guide thespacing of the roof trusses can be kept 3

a. of span upto -


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+ON>()%'#T(ONS3

The pitch of roof truss depends on the roo;ng materials.

a8 Min. recommended for )( sheetC-in D.i.e h?lE-?D hEl?D

b8 >or #.+ sheet 0- in -4i.e hEl?-4.

*arallel chord trusses3 The economical span to depth ratio E-4 to 49. Trape:oidal trusses3 The con;guration shown below reduces the a5ial forces in the chordmembers ad=acent to supports.

TENSION CHORD

COMPRESSION CHORD COMPRESSION CHORD

TENSION CHORD

N-GIRDER/PRATT

TRUSS

WARREN GIRDER

!"SPAN

#


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!conomical span to depth ratio is around -. The slope is -?an trusses are used when the 'after members of the roof trusses haveto be subdivided into O&& number of panels.*itch E"?hE9 to -

hE"?9 to "?-

$


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The mass per s,.m of 3

a8 >ink trusses is lowest for spans from -


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>or slope -3-6 E"?2< to "?9 where is the depth at support.>or parallel trusses the relation is appro5imately E"?4. The most advantageous angle between the diagonals and the bottomchord is 9


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Kelded sheeted roof trusses is given appro5imately as3

#. K E-?-7ink type roof truss7>rom Letchum/s structural engineers B8 KE.444*.#."7-A.2D44"?#8in Lg where

*E capacity of truss in Lg?s,.m in hori:ontal pro=ection ofroof7-


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Ties 209 20< I0-<

Total #$%&$ &'%($ '$%)$

4. +olumns with tie Rplatforms

-0-I -I09 H0-4

+rane girder with

bracing beams

0-9 -909


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'educed by 4N?s,.m foreach one

degree increase above 4G.But not less than

9N?s,.m.

T" loads on truss .an / ta0n +1# as pr IS .od )(' . Thisreduction is not for the design of *urlins.

2IND LOADS:

On roof trusses6 unless the roof slope is too high6 would be usuallyuplift force perpendicular to the roof 6 due to suction eect of the windblowing over the roof. ence wind load on roof truss usually actsopposite to the gravity load and its magnitude can be larger thangravity loads6 causing reversal of forces in truss members. 7'ef3

Teaching resources on structural steel design0chapter04H8.>or buildings up to -.m in height6 the intensity of wind pressure maybe reduced by 4


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The spacing of purlins ad=acent to the eaves and the ridge of a roof may be reduced to give a more uniform moment distribution in the roof sheets.>or fully continuous purlin con;gurations the larger B.Ms and the trussloadings in the end span and at the penultimate trusses can bereduced by making the end spans7i.e. at the end bays of thebuilding8smaller than the interior.

(f the purlins are placed at intermediate points i.e. between the =ointsof the top chord6 the chord will be sub=ected to moments.

'!+OMM!N&!& T(+LN!SS O> 'OO> T'%SS )%SS!TS5a67dsign!or.s insupport1diagonals8tons9

%pto4

409< 9


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The width of the members should be kept minimum as far as possiblebecause wide members have greater secondary sresses.

Two angles back to back or a structural tee form the most commonsection for members of a roof truss. Khen the load is light and the

span is short6 a single angel section will often suVce and may be usedin spite of its lack of symmetry. This is true for web members to carryonly nominal stresses

SU;DI,ISION OF THE 5AIN PANELS:

SECONDAR- STRESSES:Normally the secondary stresses in roof trusses may be disregarded if

-. the slenderness ratio of the chord member is greater than


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-. Member under +OM*'!SS(ON under loads otherthan Kind?!Yload

-I

4. T!NS(ON members undergoing reversal due to loadsother than K"

-I

2. Members normally under T!NS(ON but may have to

resist +OM*'!SS(ON under Kind load

4or tension members the type and arrangements of the angles is notso importance since here the determining factor is the net sectionalarea.

!+"2!,

Eu.l .gle se0ti is

re3erre45

12


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Other types of sections than angle are seldom employed and only if there are speci;c re,uirements for design. Thus for e5ample6 chordmade from channels are employed when they are sub=ected not only toan a5ial force but also to a considerable local moments originated by a

load applied between panel points of the trusses.

The verticals of trusses are designed with a T section formed of t,oe,ual leg angles.

EFFECTI,E LENGTH OF CO5PRESSION ELE5ENTS:

The eective length of a compression top chord in the plane of truss ise,ual to its geometrical length7between panel point centres8 "eE".>or diagonals7e5cept for the support one which is considered as a

continuation of chord8 and verticals the eective length in the plane of truss is taken e,ual to "eE.I".

Khen selecting angle sections for compression elements6 the tendencyshould be to use angles of the smallest possible thickness since theirradii of gyration have the relatively greatest value.

"imiting slenderness ratio Z for compression and tension elements

N#M! O> !"!M!NT

+OM*'!S

S(ON!"!M!NTS

T!NS(ON !"!M!NTS

%N&!'&('!+T#+T(ON O>&$N#M(+"O#&S

ST#T(+"O#&

(N B%("&(N)K(T !#J$S!'J(+!+ON&(T(ONS

+hords6 support diagonals Rverticals of trusses6 transmittingsupport reactions

-4 4


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The shape shown is better to take care of the Bending moment.E rise of the truss which is -?I for #+ sheet.

4. #s suggested earlier the top main chord 7rafter8 is divided intomain divisions which in turn subdivided to suite the roof coveringsheets.

2. Nowadays it became obsolete to use the rivets but customary to

make use of welding as well as high tension bolts. There are four

types of bolts available. Bolt )PI means bolt is having a strengthof P Mpa I is U of strength used for calculation.

9. The structural design procedure consists of si6 principal steps.

a. Selection of type and layout of structure.

b. &etermination of loads on the structure.

1$


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c. &etermination of internal forces and moments in the

structural components.

d. Selection of material and proportioning of members and

connections for safety and economy.

e. +hecking the performance of the structure under service

conditions6 and

f. >inial review.

abrication3 !ase of fabrication and erection has an important

inFuence on

the economy of the design.

(n general6 small and medium trusses of symmetrical design are

lifted at the ridge during erection. (n order to prevent buckling of the

bottom chord6 it is necessary to proportion it to carry the

compressive stresses developed during hoisting. #n empirical

relation is given by b?" E-?-4


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TRUSSED ;EA5S:

>O'M%"#! >O' T'%SS!& B!#MS0'!>3 Building engineers hand

book

%sed for long spans and are built up of wooden beams and struts of

steel rods. But the wooden beams may be replaced by steelsections.

1&


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!sE4.-5-\DLg?s,.cm7steel8

Dsign !orula7

Sl.n

o

&escription

Single strut &ouble strut

%niformly distributed load ]in Lg-. Tension in rod .2-4Kh?r Kh?2r4. +ompression in strut .D4

Design Principles for Roof Steel Truss

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