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CSMIP 95-01

"Seismic Response Study of the Hwy 101/Painter Street Overpass Near Eureka Using Strong-Motion Records"

by R. Goel and A. Chopra

March 1995, 70 pp.

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A procedure is developed to estimate the stiffness of abutment-soil systems directly from the earthquake motions of small, well-instrumented bridges using a simple equilibrium-based approach without finite-element modeling of the structure or the abutment-soil systems. This procedure is used to estimate abutment stiffnesses of the US 101/Painter Street Overpass during past earthquakes. The calculated abutment stiffnesses, which include the effects of soil-structure interaction and nonlinear behavior of the soil, are used to investigate effects of abutment deformation on the abutment stiffness during an earthquake and to explain the torsional motions of the road deck. It is demonstrated that the abutment stiffness decreases significantly at larger deformations. The torsional motions of the road deck resulted, in part, because of the differences in transverse stiffnesses of the two abutments. Also evaluated are the CALTRANS, AASHTO-83, and ATC-6 procedures for estimating the abutment stiffness. The CALTRANS procedure leads to a good estimate of the stiffness along the abutment (transverse to the deck) provided the deformation assumed in computing the stiffness is close to the actual deformation during an earthquake. However, this procedure leads to overestimation of the stiffness normal to the abutment (along the length of the deck). The AASHTO-83 and ATC-6 procedures give an initial estimate of the abutment stiffness that is too high in both directions.