Developing a Soil-bridge Interaction Model for Studying the Effects of Long-duration Earthquake Motions

Earthquake engineering analyses are often performed using shallow, crustal earthquake motions (e.g., 1940 El Centro). However, large areas of the world are subject to subduction zone earthquake motions (e.g., the Pacific Northwest). A subduction zone earthquake motion is characterized by its long duration (e.g., strong shaking lasts for more than a minute). Observations of unexpected bridge damage following the recent subduction zone earthquakes in Chile and Japan highlight the importance of understanding soil-foundation-bridge (SFB) interaction during long-duration earthquake motions. Accordingly, the main objective of this paper is to report the seismic response of a SFB system during long-duration earthquake motions. The SFB system was created within the finite-element framework OpenSees. The pile foundation was modeled using fiber-section elements (representing a reinforced concrete pile), and the pile was attached to a soil continuum, which was specified as a dense, non-liquefiable sand, using nonlinear soil springs. The bridge column was modeled using force-based fiber-section elements attached to the linear elastic bridge deck. A double span bridge was considered herein. Gap elements were used at the ends of the bridge deck to represent backfill response. The soil-bridge system was subjected to seven selected subduction zone earthquake motions and seven selected shallow, crustal earthquake motions. For each earthquake motion, the number of inelastic excursions, defined by the number of exceedances of a reference yield rotation, θy, was plotted against five earthquake intensity measures: peak ground acceleration (PGA), cumulative absolute velocity (CAV), significant duration (D5-95), Arias intensity (IA), and spectral acceleration (Sa). Although the results illustrate similar peak seismic response (displacements and forces) of the SFB system due to the two earthquake types, the number of inelastic excursions shows a strong trend correlating the measures of duration (CAV, D5-95, and IA) and accumulation of damage. This indicates the importance of considering the effects of duration in design and analysis of bridge structures.