Bridge Grade Steel

The grade of Steel Bridge recommends method to allow for the effect of fatigue in design of parts of steel bridges, which are subjected to repeated fluctuations of stress. These fluctuations may cause fatigue failure of members or connections at lower stresses than those at which they would fail under static load.  Such failures are primarily due to stress concentrations introduced by the constructional details.     Thus all the details are designed to avoid as far as possible the stress concentrations likely to result in excessive reductions of the fatigue strength of members or connections.  Care is also taken to avoid a sudden reduction of the section of a member or part of a member, especially where bending occurs.
To allow for the effect of fatigue the allowable working stress is determined from the Appendix ‘G’ of IRS Steel Bridge Code for wide range of constructional details.  The code covers mild and high tensile steel fabricated or connected by welding, riveting or bolting.  The allowable stress ‘P’ depends on the ratio of minimum stress σmin to maximum stress σmax, number of repetitions of stress cycle ‘N’, the method of fabrication and the type of connection.  In determining the ratio σmin/σmax gross area is considered. The code classifies the constructional details into seven classes i.e. Class A to Class G according to type of steel, type of fabrication and connection.  All the details are designed such that the stress induced under design loads are within the allowable limits.
The allowable stresses are the principal stress at the point under consideration.  Thus in the design of girder the combined effect of both bending and co-existent shear stress is considered and the bridge members are generally designed for 10 million cycles of stresses produced under the design load. Concept of EUDL (Equivalent Uniformly Distributed Load) is used to determine the maximum bending moment and maximum shear force for the type of IRS loading.  The EUDL for maximum bending moment and maximum shear force depends upon the span and the dynamic augment increases with speed. IRS Bridge Rules gives tables for determination of EUDL for maximum bending moments and shear forces alongwith the co-efficient of dynamic augment for maximum speed of 160 kmph.