EFFECT OF VERTICAL MOTION ON THE RESPONSE OF HIGHWAY BRIDGES: A CASE STUDY FROM THE NORTHRIDGE EARTHQUAKE

Measurement of high ground accelerations in the vertical direction during the Northridge Earthquake of January ۱۷, ۱۹۹۴ point to the possible contribution of this component of the ground motion to the sustained damage by many structures including the l-۵/SR-۱۴ Separation and Overhead bridge. The ۱-۵/SR-۱۴ Separation and Overhead sustained significant damage during the Northridge earthquake including total failure of one of its frames. Results of a detailed analytical evaluation, including nonlinear time history analysis, show that the seismic forces due to vertical component of ground motion could have caused plastic moment hinges in the superstructure. However, the shear demand associated with hinging of the superstructure exceeds the shear capacity of the superstructure in the upward direction resulting in brittle shear failure. The postulated failure scenario based on this mode of failure is remarkably similar to the observed damage. A brief summary of the effect of vertical motion on the response of bridge columns is also presented.