Live load effects in diaphragms of concrete slab-on-steel girder highway bridges

Abstract

Diaphragms have been shown to have no contribution to lateral distribution of live load as far as design of longitudinal steel girders is concerned. However, they do distribute the transverse moments in a bridge which must otherwise be carried by the deck slab if no diaphragms are present. Inclusion of diaphragm in the design of a highway bridge may increase the required moment capacity for exterior girders if the vehicle-girder distance is large. A method is presented for the prediction of diaphragm member forces under vehicle live load for both the strength and fatigue limit states. These forces are shown to vary significantly among bridges with different diaphragm-deck stiffness ratios and 0 values, where 8 is a property based upon bridge geometry and derived from orthotropic plate theory. These forces also increase for bridges with high skewness, and a correction factor for this is included in the method. Field data is used to confirm that the diaphragm forces predicted by the developed method come close to actual diaphragm forces measured on three different bridges.