Impact of diaphragms on seismic response of straight slab-on-girder steel bridges

Many steel bridges have suffered diaphragm (cross frame) damage during recent earthquakes. Diaphragms provide an important load path for the seismically induced loads acting on slab-on-girder steel bridges, but their impact on seismic response is still unclear in many ways. The relative role played by intermediate and end diaphragms in providing lateral load resistance, along with the consequences of diaphragm damage on bridge seismic response, has not been studied. This paper quantitatively investigates the impact of diaphragms on the seismic response of straight slab-on-girder steel bridges. Typical 20 to 60 m span slab-on-girder bridges with and without diaphragms are considered and studied through elastic and inelastic static pushover analyses. Two hand-calculation analytical models are proposed to evaluate their period, elastic response, and pseudospectral acceleration at first yielding. It is shown that a small end-diaphragm stiffness is sufficient to make the entire superstructure behave as a unit in the elastic range. However, a dramatic shift in seismic behavior occurs once an end diaphragm fractures, with a sizable period elongation, considerably larger lateral displacements, and higher propensity to damage owing to P-Delta effects. It is also found that the presence of intermediate diaphragms does not significantly influence the seismic performance of these types of bridges, in either the elastic or the inelastic range.