Sacrificial bearing components for quasi-isolated response of bridges subject to high-magnitude, low-probability seismic hazard

Bridges in low- to moderate-seismic zones, such as Mid-America, commonly employ elastomeric expansion bearings to accommodate service-level thermal deformations in the superstructure. Any potentially beneficial effect of these bearings during seismic loading is typically ignored, and the surrounding substructure or superstructure is designed to accommodate the seismic demands through traditional means of ductile plastic response. This scenario presents an opportunity for bridge designers, particularly in these geographical regions (and potentially for all seismic zones). If the elastomeric bearings can reliably be depended upon to provide reduced transmissibility of ground motion excitation to the superstructure, then the structure can be said to be partially isolated, or "quasi-isolated". This study explores the potential benefits and applicability of sacrificial bearing components (i.e., "fuses") for bridge structures subjected to seismic effects. Fundamentally, a design paradigm embracing such components requires that two disparate objectives are met. First, the bearing fuses must perform with predictable, reliable response for service loading conditions, including small earthquakes. Second, the bearing fuses must permit a clear and reliable transition to an effectively isolated response when subjected to the seismic effects of a major earthquake. In this study, three general types of bearings were considered: low-profile steel fixed bearings, steel-reinforced elastomeric bearings, and a compound bearing composed of both a steel-reinforced elastomer and a stainless steel-on-Teflon sliding surface. The elastomeric bearings are further restrained in the transverse bridge direction with stiffened, L-shaped retainer brackets. Full-size bearing specimens were subjected to compression loads of up to 100 kips, cyclic displacements of up to 12-1/2 inches, and velocities ranging from quasi-static up to 4 inches per second. Bearings were tested -- to simulate both longitudinal and transverse bridge motions -- on replaceable concrete pads, which enabled new pads to be exchanged for those used to evaluate the response of fusing bearing assemblies with embedded anchors into the concrete. The experimental program examined in detail the full nonlinear, cyclic response of the various bearing types and their components. Mechanical characteristics that permit a transition to an effectively isolated response include combined shear- and tension-induced fracture of anchor bolts and sliding at elastomer-on-concrete, steel-on-Teflon, steel-on-steel, and steel-on-concrete surfaces. Elastomeric bearings that were designed only for service level peak deformations up to 50% shear strain exhibited stable, resilient response with shear strains from about 100% to 250% transitioning to sliding, depending on applied load and contact surface roughness. Total elastomer-on-concrete sliding displacement accumulations up to 140 to 160 inches for individual bearings resulted in minor surface abrasion of the elastomer, but the bearings maintained their structural integrity throughout testing despite surficial damage. Friction coefficients for elastomeric bearings on concrete ranged from about 0.55 to 0.2, depending on compression stress at the contact surface and contact surface roughness. Retainers exhibited complex interactions with the concrete surfaces depending primarily on the width of the retainers on the concrete. Shear resistance at the retainer toes contributed significantly to the overall resistance of the bearing and the maximum load that would be transmitted into substructures prior to fusing during major earthquakes. Bearings with steel-on-Teflon sliding surfaces performed well with up to 20% of the Teflon exposed at peak displacements, but exhibited damage to the Teflon and potentially unstable response at larger displacements. The sliding friction at the steel-on-Teflon interface increased by about 100% when subjected to increased strain rates. Anchor bolt-controlled fusing mechanisms were found to be preferable for low-profile steel fixed bearings. Such bearings exhibited a peak force capacity determined by the shear capacity of anchors into the concrete combined with friction equal to about 25% to 30% of the applied compression load. The data produced by the experiments is suitable for calibration of design guidelines to ensure elastic response where appropriate and also illuminates the process of the transition from the elastic range to an effectively isolated seismic response. The findings from observations at the experimental component level have been extrapolated to the global system level by incorporation into computational models of complete bridges developed by others. The culmination of the experiments and computational models provides insight into appropriate design methodology approaches and limitations for use with bridges when a quasi-isolated response is sought to address hazards comprising infrequent, large-magnitude earthquakes.