Title

Seismic analysis and evaluation of skew highway bridges

Date of Award

5-2000

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

Advisor(s)

Eric M. Lui

Keywords

Skew bridges, Earthquakes, Girders, Seismic analysis, Highway bridges

Subject Categories

Civil Engineering

Abstract

A theoretical and experimental investigation of the seismic response of skew highway bridges is presented. The study begins with the description of a rigid deck bridge model for short span skew bridges. Formulas for computing earthquake response are developed, and parameters that significantly influence the dynamic response of the bridges are identified. The study indicates that the response of a given skew bridge depends not only on its deck aspect ratio, the stiffness eccentricity ratio, the skew angles, and its natural frequencies, but also on its frequency ratio.

For longer span skew bridges, the use of the rigid deck assumption is questionable. To study the effect of deck flexibility, finite element models are developed to study the behavior of a representative medium span skew reinforced concrete box girder bridge. The effects of superstructure flexibility, substructure boundary conditions, structural skewness and stiffness eccentricity on the seismic response of the bridge are studied using response spectrum analyses.

A refined stick model that utilizes a dual-beam representation of the bridge deck is proposed for preliminary dynamic analysis and seismic assessment of medium or long span skew bridges. The validity of the model is established by comparing results with numerical solutions obtained for skew plates and a skew bridge.

When bridges are subjected to moderate-to-strong earthquakes, inelasticity often occurs at incipient failure. The nonlinear analysis of skew highway bridges is performed using nonlinear time history analysis procedure and the AASHTO design approach. Through comparison of the two approaches, the deficiency of the current AASHTO approach is identified.

During an earthquake, both horizontal and vertical ground motions are imparted to the bridge structure. The effects of both these earthquake components are studied. The results show that the vertical earthquake component affects the column axial force noticeably, but its effect on shear and moment is only secondary.

To validate the theoretical and numerical analyses, an experimental study on a model skew bridge is carried out. Tests for static displacements, natural frequencies, mode shapes and damping of the model bridge are conducted. Good correlation is obtained for the theoretical and experimental results.

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