Author : A.Sharma 1
Date of Publication :9th November 2017
Abstract: The most common cause of structural failure in steel bridges is buckling of a compression member. Buckling failure is a sudden failure and offers no warning before collapse. Recently, in the year 2012, a 190m span steel truss bridge over river Alaknanda, in Uttrakhand, India, suddenly collapsed during casting of the deck slab due to buckling of one of its top cord compression members. Its failure during construction stage raised doubts on current design practices where, the factor of safety provided by the codes do not guarantee performance of the bridge in overload conditions. In the case of composite under slung bridges, premature buckling of top cord compression member is prevented by the RCC deck connected with the steel truss with the help of shear studs. Further, this allows the steel truss members to take stress up to their ultimate strength. A 30 m span bridge is analysed with composite action of RCC deck for service and overload conditions. The maximum flexural strain due to live load alone in the RCC deck slab is found to be 0.00026. Shrinkage strain for M30 concrete deck slab is taken as 0.0003. Hence, even during service condition, composite action between the steel girder and RCC deck slab may not take place. For the analysed deck type bridge, total load on the bridge in terms of uniformly distributed load in service conditions for (DL+LL) case is 153.7kN/m, and in overload condition for 1.5x(DL+LL) case it is 230.55kN/m. At plastic stage the bridge can carry an equivalent udl of 635.98kN/m. Thus, for the plastic collapse, apart from warning due to excessive deflection, there is a factor of safety of 4.1 in comparison to service load. For prestressed concrete bridges, load combination at ultimate strength for severe condition, as per Cl. 12 of IRC: 18-2000, is prescribed as 1.5G+2SG+2.5Q. Whereas, in case of steel truss bridges, as per IRC: 24-2010 and IRC: 6-2010, load combinations and permissible stresses are given in Table 1, Cl. 202.3 for service condition only. IRC codes haven’t provided any specific provision for ultimate strength of steel truss bridges. Therefore, a parallel clause for ultimate strength of steel truss bridges also may be added, for which composite under slung bridges may be found to be suitable.
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