Abstract
The seismic design of a proposed Christchurch bridge is discussed. Initial design was based on supporting the superstructure on elastomeric bearing pads and steel-cantilever dampers. Dynamic analyses showed that the dampers were relatively ineffective, and that the seismic response was dictated by the characteristics of the bearing pads. Final design, based on a conventional ductile approach was found to be more economical without significant increase to seismic risk. It is shown that the decision to opt for a design incorporating mechanical energy dissipators as opposed to a monolithic pier/superstructure design will not necessarily result in a reduction in seismic response.
Highlights
1.1 LocationThe South Brighton Bridge has been designed to replace an existing old timber bridge buiIt in 1926, situated just upstream from the junction of the Avon river with the Christchurch Estuary
Elastomeric bearings support the superstructure at abutments and internal hammerheads, and were chosen for their maintenance-free characteristics in the aggressive salt water environment
It will be noted that the stiffness of the post-yield portion of the loop exceeds 52% of the elastic stiffness, and that the loop appears to be rather thin. This contrasts with the typically fat hysteresis loops published for energy dissipation devices, including the steel cantilever, and results from the inclusion in Fig. 4 of the elastomeric bearings to give total response, rather than isolating the dissipator characteristics
Summary
The South Brighton Bridge has been designed to replace an existing old timber bridge buiIt in 1926, situated just upstream from the junction of the Avon river with the Christchurch Estuary. The seismic shear at each pier was derived by setting the dissipator deflection at 76.2mm, corresponding to a maximum steel strain of 3%, and summing the dissipator and elastomeric bearing shear forces at this displacement. This is a typical level of peak response under a major earthquake. It will be noted that the stiffness of the post-yield portion of the loop exceeds 52% of the elastic stiffness, and that the loop appears to be rather thin This contrasts with the typically fat hysteresis loops published for energy dissipation devices, including the steel cantilever, and results from the inclusion in Fig. 4 of the elastomeric bearings to give total response, rather than isolating the dissipator characteristics. Were carried out using a program 'TWODEE1 developed by Sharped), A time-step o-f
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