Effect of vertical ground motions on shear demand and capacity in bridge columns.
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2012-03-01
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Abstract:The objective of this project was to examine the effects of axial force variation in bridge columns due to strong vertical
ground motions and the influence of these axial force fluctuations on shear strength degradation.
Two quarter scale specimens (SP1 and SP2) with different transverse reinforcement ratios were constructed and tested on
the UC-Berkeley shaking table at the Richmond Field Station. As a result of an extensive numerical investigation and
preliminary fidelity tests, the 1994 Northridge earthquake acceleration recorded at the Pacoima Dam was selected as an
input motion for the shaking table study. The chosen ground motion was applied to the test specimens at various intensity
levels ranging from 5% to 125% of the actual recorded motion.
As part of the computational modeling, a new shear spring model which incorporates shear strength estimations based on
ACI and Caltrans SDC equations is developed and implemented in the computational platform, OpenSees [28]. The models
are reasonably successful in capturing the shear-force and displacement histories measured during the tests.
The study indicates that strong vertical excitations can induce axial tension in the column which causes marginal degradation
of the shear strength (mainly due to the degradation of the concrete contribution to shear strength). The degradation did not
adversely affect the performance of the column. Since the vertical frequency of bridge columns is much higher than the
transverse or longitudinal frequency, the imposed axial tension is sustained for very short durations.
The present study of a single column bent suggests that ignoring the concrete contribution to shear capacity when the
column is in tension is conservative. Given the fact that the shear capacity expression in SDC is specified to deal with cases
of sustained axial tension, attention should be paid to the use of SDC expression in cases where axial tension occurs only
due to vertical excitation.
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