STUDY OF SECONDARY STRESSES IN STEEL TRUSS BRIDGES

Abstract

Conventional analysis of bridge trusses is carried out by assuming that all the members are straight and free to rotate at the joints and the joints are truely frictionless. All loads including self weight of members are applied at the joints only. Thus members of the bridge truss are subjected to axial force (compressive or tensile)only. Resulting stresses are defined as primary stresses. But in actual practice, these joints are not truely pinned joints and all the loads do not act at joints only. Thus,certain additional stresses develop mainly due to rigidity of joints and self weight of members acting as UDL over the length of members. These stresses are normally defined as secondary stresses. IRC code of practice permits the design of bridge trusses without any consideration of secondary stresses. It also allows an increase_in the given permissable stresses by 16 percent,if the secondary stresses are considered in the design. It has been observed that the secondary stresses become quite significant in the some of the members such as in chords members in the end panels,end posts and in vertical web members The design of bridge may become unsafe or uneconomical on the basis of conventional design approaches. Hence a detailed investigation of secondary stresses becomes essential for getting a safe and economical design. Three steel truss bridge of span 42m,64m,and 95m have been analysed by using space frame programme. Results of 3-D analysis have been compared with those obtained by conventional analysis. In all the three bridges secondary stresses_are found higher in chord - members in end panels,vertical members and in end posts. Detail study of the three bridges has i7V-14.44( that all the members designed on the basis of conventional design approach remain well within the safe limit mainly because in the conventional method, stresses for wind loads are estimated on the conservative side giving higher values of moment in the end posts. A considerbale saving in the design can be achieved, if the design is based on analysis: Variation of various primary as well as secondary stresses was studied by varrying the L/C ratio(where L is member length and C is half the depth of member). This study indicates that the secondary stresses are minimisedif L/C ratio is kept in vicinity of 20 to 25.