Experimental Behavior of Steel Fixed Bearings and Implications for Seismic Bridge Response

Steel fixed bearings are commonplace structural elements for transmitting loads from superstructures to substructures, and they have typically occupied a role of elastic force transfer elements within the overall scheme of an earthquake resisting system (ERS). Recent revisions to design and guide specifications have acknowledged the possibility of bearings acting as fuses, but there is little research available to characterize bearing behavior for such design roles or the associated bridge response to be expected when bearings have fused. One design approach, adopted by the Illinois DOT (IDOT), applies capacity design principles and permits the bearings and superstructure to slide on the substructure. The intent of this design approach is to capture some of the beneficial aspects of conventional isolated systems, such as period elongation, reduction of force demands, and protection of substructures from large inelastic displacement demands, without incurring the additional design provisions and fabrication costs to satisfy the requirements for seismic isolation systems. To achieve this quasi-isolated bridge response, steel fixed bearings are used as fusing elements, where the steel pintles or anchor rods rupture, and the fixed bearing plates become free to slide on the supporting pier cap. A properly proportioned bearing will fuse prior to superstructure/substructure elements experiencing inelastic demands. The University of Illinois has been collaborating with IDOT to investigate the behavior of quasi-isolated bridge systems and to calibrate and refine IDOT’s ERS design and construction methodology. The research is composed of experimental testing to characterize fundamental bearing behavior, coupled with nonlinear global bridge modeling to evaluate limit state progression and estimate maximum displacement demands of the superstructure relative to the substructure. The cyclic response of full-scale steel low-profile fixed bearings demonstrates predictable sliding behavior, but based on current design procedures, these bearings are often overdesigned for use as fuses in quasi-isolated bridges. Consequently, a bridge, which in other respects may exhibit satisfactory quasi-isolated response, might also incur significant damage to the substructure unit where fixed bearings are provided. A parametric study of global bridge response demonstrates that the anchorage of fixed bearings to substructures could be reduced to limit the damage to the supporting substructure unit while incurring only a nominal increase in superstructure displacement demands.