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

Keywords: Seismic performance.  Steel plate girder bridges.  Superstructures.  Seismic isolation systems.  End cross frames.  Buckling restrained braces (BRB).  Base shear.  Displacement.

Abstract: The end 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.  End cross frames comprising of ductile single angle braces and later buckling restrained braces are studied and the results presented which show reduced base shear of the order of 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, although the displacement capacity of the X-braces was greater.   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 experiments was 7 (percent) of the girder height, with no damage observed in the girders, bearings and minimal distress to the deck slab.