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Date of Award

12-1993

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Civil Engineering

Supervisor

John C. Wilson

Abstract

Cable-stayed bridges have been gaining popularity in the last few decades as a viable, efficient, economical and aesthetically appealing design for spanning medium to long crossings. The construction of these bridges in seismic regions has created a need to more fully understand their dynamic behaviour and seismic response.

The objectives of this research work are to: (1) define a dynamic modal analysis procedure suitable for cable-stayed bridges, and (2) use the developed procedure to study seismic response characteristics of cable-stayed bridges.

The research program was divided into four main stages: (1) development of analytic models for estimating frequencies of two of the main structural systems of cable-stayed bridges (i.e. deck and tower); (2) a 3-D model of Quincy Bayview Bridge was used as a prototype structure to study the modal characteristics of cable-stayed bridges and investigate the effect of major geometric parameters on the modal characteristics of cable-stayed bridges; (3) eigenvectors and load dependent Ritz vectors were examined to compare their appropriateness as bases for modal transformation of the equation of motion of cable-stayed bridges. A seismic response study was conducted of the Quincy Bayview Bridge in each vector basis to judge which is more appropriate for dynamic and seismic analysis of cable-stayed bridges. Frequency cut-off criteria were proposed that would ensure the inclusion of all important modes in a dynamic analysis; and (4) the proposed criteria were applied in a seismic response study that used a 3-D finite element model of the Quincy Bayview Bridge. The study investigated the effect of frequency content of ground motion on the seismic response of cable-stayed bridges with different deck supports.

The results of the study showed that modal characteristics of cable-stayed bridges are most affected by the cable arrangement, the tower shapes, and deck supports. Almost the same number of eigenvectors or Ritz vectors are required to ensure the inclusion of all important modes in a modal analysis. To ensure the inclusion of all important modes, it is proposed to calculate an upper limit frequency using the developed analytic models, generate modal vectors up to the set limit and then check the effective modal mass of these vectors to meet a pre-set percentage of the total mass. The study showed that seismic response of a cable-stayed bridge is strongly dependent upon the deck support condition, and the frequency content of input motion. It also showed that towers are less sensitive to changes in frequency content of the ground motion than the deck.

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