Seismic Performance of Steel Girder Bridge Superstructures with Ductile End Cross Frames and Seismic Isolators

摘要

The end cross frames of steel plate girder bridges are critical elements in the transverse seismic load path of the superstructure. It has been suggested that if these cross frames were to yield in a ductile manner, the elastic base shear in the bridge would be substantially reduced. This report describes experimental and analytical studies on a two girder bridge superstructure to investigate the validity of this proposition. Two types of end cross frames are studied, one comprising ductile single angle braces, and the other buckling restrained braces. The results show a reduced base shear that is about 40 percent of the elastic base shear. The buckling restrained braces resulted in 20 percent to 30 percent smaller drifts in the superstructure than with the X-braces at the same level of base shear, a result that is attributed to better energy dissipation. Removing some shear studs near the supports of the girders, and allowing the transverse shear to be transferred into the end cross frames using the top chord, allows the girders to 'rock', enabling considerable transverse drifts in the girders without distress to the slab-girder connection. The maximum drift measured in the girders during the experiments was 7 percent of the girder height, with no damage observed in the girders or bearings, and minimal distress to the deck slab.