by R.
Goel and A. Chopra
March 1995, 70 pp.
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Abstract
A procedure is developed to estimate the stiffness of abutmentsoil systems directly from the
earthquake motions of small, wellinstrumented bridges using a simple equilibriumbased approach
without finiteelement modeling of the structure or the abutmentsoil 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 soilstructure
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,
AASHTO83, and ATC6 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 AASHTO83 and ATC6 procedures give an initial
estimate of the abutment stiffness that is too high in both directions.
