Mat Chapter 27

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Mat Chapter 27
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  TRUSSES TRUSSES 1.0INTRODUCTION Trusses are triangular frame works in which the members are subjected to essentially axialforces due to externally applied load. They may be plane trusses [ig. ! a #$% wherein theexternal load and the members lie in the same plane or space trusses [ig. ! b #$% in whichmembers are oriented in three dimensions in space and loads may also act in any direction.Trusses are fre&uently used to span long lengths in the place of solid web girders and suchtrusses are also referred to as lattice girders.Steel members subjected to axial forces are generally more efficient than members inflexure since the cross section is nearly uniformly stressed. Trusses% consisting of essentially axially loaded members% thus are 'ery efficient in resisting external loads. Theyare extensi'ely used% especially to span large gaps. Since truss systems consume relati'elyless material and more labour to fabricate% compared to other systems% they are particularlysuited in the (ndian context.) *opyright reser'ed Version II27 - 1 27 (b) Space Truss (  a) Plane Truss(c) Bracings in Multi-storey Building (d) Bracings in Single Storey Buildings Transverse Vertical (wall) Bracings  Rafter (Plan)  Bracings  Longitudinal Vertical (all) Bracings (e) Truss Bridge Fig. 1 Types of Trusses  TRUSSES Trusses are used in roofs of single storey industrial buildings% long span floors and roofs of multistory buildings% to resist gra'ity loads [igs. ! a # and ! b #$. Trusses are also used inmulti+storey buildings and walls and hori,ontal planes of industrial buildings to resistlateral loads and gi'e lateral stability [igs. ! c # and ! d  #$. Trusses are used in long span bridges to carry gra'ity loads and lateral loads [ig. ! e #$.Trusses often ser'e the action of the girder in transferring the gra'ity load o'er larger span% and are referred to also as lattice girders. Such lattice girders are usually deeper and much lighter than regular girders and hence are economical% particularly whenrepetiti'e fabrication is taken ad'antage of. These are used as flooring support systems inmulti+storey buildings% within which depth all the ducts can be easily accommodatedwithout increasing the depth of the ceiling. Steel trusses can also be efficiently used along with concrete slabs in buildings and bridges by mobilising composite action between structural steel and concrete. (n this chapter%initially% the details of structural steel trusses are discussed. Subse&uently% the beha'iour and design of structural steel + concrete composite trusses are discussed. 2.0LOADS The loads on trusses would depend upon the application for which the trusses are used.The loads may be static% as in the case of buildings% or dynamic% as in the case of bridges.These are briefly re'iewed below. 2.1Industrial uildin!s The roof trusses in industrial buildings are subjected to the following loads- 2.1.1Dead load  ead load on the roof trusses in single storey industrial buildings consists of dead load of claddings and dead load of purlins% self weight of the trusses in addition to the weight of  bracings etc. urther% additional special dead loads such as truss supported hoist deadloads% special ducting and 'entilator weight etc. could contribute to roof truss dead loads./s the clear span length column free span length# increases% the self weight of themoment resisting gable frame increases drastically. (n such cases roof trusses are moreeconomical. 2.1.2Live load  The li'e load on roof trusses consist of the gra'itational load due to erection and ser'icingas well as dust load etc. and the intensity is taken as per (S-012+!312. /dditional specialli'e loads such as snow loads in 'ery cold climates% crane li'e loads in trusses supportingmonorails may ha'e to be considered. 2.1.3Wind load  Version II27 - 2  TRUSSES 4ind load on the roof trusses% unless the roof slope is too high% would be usually upliftforce perpendicular to the roof% due to suction effect of the wind blowing o'er the roof.5ence the wind load on roof truss usually acts opposite to the gra'ity load% and itsmagnitude can be larger than gra'ity loads% causing re'ersal of forces in truss members. The hori,ontal and 'ertical bracings employed in single and multi+storey buildings are alsotrusses [ig. ! d  #$% used primarily to resist wind and other lateral loads. These bracingsminimi,e the differential deflection between the different frames due to crane surge inindustrial buildings. They also pro'ide lateral support to columns in small and tall buildings% thus increasing the buckling strength. 2.1.4Earthquake load  Since earth&uake load on a building depends on the mass of the building% earth&uake loadsusually do not go'ern the design of light industrial steel buildings. 4ind loads usuallygo'ern. 5owe'er% in the case of industrial buildings with a large mass located at the roof%the earth&uake load may go'ern the design. These loads are calculated as per (S-!036+!302. 2.2 ulti-Store# uildin!s The lateral load due to wind or earth&uake may be resisted by 'ertical bracings acting astrusses. These bracings% properly designed% make these buildings 'ery stiff in resistinglateral loads. 5ence they are economical in the buildings of intermediate height ranges. (nthe case of earth&uake loading% stiff buildings may attract larger inertia force and hence useof bracings may not be desirable. 2.$rid!e Trusses Trusses are used in bridges to transfer the gra'ity load of mo'ing 'ehicles to supporting piers. epending upon the site conditions and the span length of the bridge% the truss may be either through type or deck type. (n the through type% the carriage way is supported atthe bottom chord of trusses. (n the deck type bridge% the carriage way is supported at thetop chord of trusses. Usually% the structural framing supporting the carriage way isdesigned such that the loads from the carriage way are transferred to the nodal points of the 'ertical bridge trusses. 7ore details of the trusses bridges are discussed in the chapter on bridges. $.0ANAL%SIS O& TRUSSES 8enerally truss members are assumed to be joined together so as to transfer only the axialforces and not moments and shears from one member to the adjacent members they areregarded as being pinned joints#. The loads are assumed to be acting only at the nodes of the trusses. The trusses may be pro'ided o'er a single span% simply supported o'er thetwo end supports% in which case they are usually statically determinate. Such trusses can be analysed manually by the method of joints or by the method of sections. *omputer  programs are also a'ailable for the analysis of trusses. These programs are more useful inthe case of multi+span indeterminate trusses% as well as in the case of trusses in which the Version II27 - $  TRUSSES  joint rigidity has to be considered. The effect of joint rigidity is discussed later in greater detail.rom the analysis based on pinned joint assumption% one obtains only the axial forces inthe different members of the trusses. 5owe'er% in actual design% the members of thetrusses are joined together by more than one bolt or by welding% either directly or throughlarger si,e end gussets. urther% some of the members% particularly chord members% may be continuous o'er many nodes. 8enerally such joints enforce not only compatibility of translation but also compatibility of rotation of members meeting at the joint. /s a result%the members of the trusses experience bending moment in addition to axial force. Thismay not be negligible% particularly at the ea'es points of pitched roof trusses% where thedepth is small and in trusses with members ha'ing a smaller slenderness ratio i.e. stockymembers#. urther% the loads may be applied in between the nodes of the trusses% causing bending of the members. Such stresses are referred to as secondary stresses. Thesecondary bending stresses can be caused also by the eccentric connection of members atthe joints. The analysis of trusses for the secondary moments and hence the secondarystresses can be carried out by an indeterminate structural analysis% usually using acomputer software.The magnitude of the secondary stresses due to joint rigidity depends upon the stiffness of the joint and the stiffness of the members meeting at the joint. 9ormally the secondarystresses in roof trusses may be disregarded% if the slenderness ratio of the chord membersis greater than 2: and that of the web members is greater than !::. The secondarystresses cannot be neglected when they are induced due to application of loads onmembers in between nodes and when the members are joined eccentrically. urther thesecondary stresses due to the rigidity of the joints cannot be disregarded in the case of  bridge trusses due to the higher stiffness of the members and the effect of secondarystresses on fatigue strength of members. (n bridge trusses% often misfit is designed into thefabrication of the joints to create prestress during fabrication opposite in nature to thesecondary stresses and thus help impro'e the fatigue performance of the truss members attheir joints. $.0CON&I'URATION O& TRUSSES$.1(it)*ed Roo+ Trusses 7ost common types of roof trusses are pitched roof trusses wherein the top chord is pro'ided with a slope in order to facilitate natural drainage of rainwater and clearance of dust;snow accumulation. These trusses ha'e a greater depth at the mid+span. ue to thise'en though the o'erall bending effect is larger at mid+span% the chord member and webmember stresses are smaller closer to the mid+span and larger closer to the supports. Thetypical span to maximum depth ratios of pitched roof trusses are in the range of < to 0% thelarger ratio being economical in longer spans. =itched roof trusses may ha'e differentconfigurations. (n =ratt trusses [ig. > a #$ web members are arranged in such a way thatunder gra'ity load the longer diagonal members are under tension and the shorter 'erticalmembers experience compression. This allows for efficient design% since the short membersare under compression. 5owe'er% the wind uplift may cause re'ersal of stresses in thesemembers and nullify this benefit. The con'erse of the =ratt is the 5owe truss [ig. > b #$. Version II27 - ,
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