Seismic performance of steel girder bridge superstructures with ductile end cross frames and seismic isolation

The assumption that the superstructure of a bridge will remain elastic during large earthquake excitation is not necessarily valid for steel plate girder bridges. Past earthquakes have shown considerable damage to end cross frames, bearings, bearing stiffeners and other superstructure components, largely attributed to transverse excitation. These components need to be designed to resist the effects of seismic loading. The potential to use the end cross frames to reduce the seismic demand on a bridge is investigated in this dissertation and compared to the response of a bridge with seismic isolation.; Cyclic and shake table experiments on a ⅖th scale model of a simply supported steel girder bridge superstructure, with two steel plate girders and a reinforced concrete deck slab, are described and demonstrate the effects of transverse seismic loading on steel girder bridge superstructures in high seismic regions. Transverse seismic loads are transferred from the deck slab of a bridge into the ends of the girders through the shear studs located near the girder supports or through a top chord connecting the deck slab to the end cross frames. The transverse loads are then distributed from the top into the base of the girders through the end cross frames and into the substructure through the bearings and transverse bearing restraints. The bearings are subjected not only to transverse seismic loads, but also axial loads due to overturning, and in some cases rotations.; As the end cross frames are critical in the transverse load path they may be designed to deform in a ductile manner to reduce the elastic base shear in a bridge. From experimental results and analytical studies using ductile single angle X-braces, and buckling restrained braces in end cross frames, the base shear in the bridge model was reduced to as low as 40% of the elastic base shear. The buckling restrained braces resulted in 20% to 30% smaller drifts than the X-braces at a given level of base shear, a result that is attributed to better energy dissipation. The buckling restrained cross frames are also less sensitive to the type of input motion, and earthquakes that are larger than the design earthquake. They have the additional advantage of not requiring immediate replacement after the design earthquake. Removing some shear studs near the supports in the girders, and allowing the shear to be transferred into the end cross frames using a top chord, allows the girders to "rock" enabling considerable transverse drifts in the girders. (Abstract shortened by UMI.)...