Vertical Earthquake Loads on Seismic Isolation Systems in Bridges

Abstract

An important consideration for the design of seismic isolation systems composed of elastomeric and lead–rubber bearings is the safety of individual bearings for maximum considered earthquake shaking. One assessment of bearing safety involves the calculation of the vertical (or axial) earthquake load on the individual seismic isolation bearings. This paper investigates the influence of vertical earthquake excitation on the response of a bridge isolated with low-damping rubber and lead–rubber bearings through earthquake simulation testing. Response data collected from the experimental program are used to determine the vertical load on the isolation system due to the vertical component of excitation. A comparison of the normalized vertical load data to the vertical base acceleration showed significant amplification of the vertical response for each simulation and configuration. Disaggregation of the axial load history showed the summation of maximum values from the vertical earthquake load and overturning moment overestimates the maximum axial load because these maximum values are unlikely to occur simultaneously. Additionally, a spectral analysis procedure using the unreduced vertical stiffness of the bearings was shown to provide a reasonable estimate of the vertical earthquake load.