Abstract:
Bridges are essential to the delivery of emergency services and relief supplies during an earthquake, but they also represent the portion of the transportation infrastructure most susceptible to seismic induced damage. The assessment of these existing bridges is difficult due to uncertainties regarding the influence of the foundation and the surrounding soil on the seismic response of the bridge. Currently there exists little guidance as to which of the many analytical tools should be used when accounting for this effect. Additionally, there is a lack of experimental work investigating the interactions between the bridge structure, foundation, and surrounding soil at the system level. This lack of validation established the need for a systematic determination of the in-service system response of typical New Zealand bridges with realistic boundary conditions and to establish if the aggregations of existing bridge component models are capable of capturing this response. The characteristics of the existing New Zealand bridge stock were categorised based upon likely seismic performance into a series of bridge typologies. This categorisation determined that the majority of the New Zealand bridge stock consisted of short cast in situ reinforced concrete or precast concrete bridges built prior to the mid-1970’s. From the typologies developed during categorisation test bridges were selected for field testing. Forced vibration testing and system identification were used to investigate the in situ dynamic response of three bridge components and two in-service bridges, and modal properties of these test structures were determined. Field testing results allowed the contribution of structural and foundation subcomponents to the dynamic behaviour of the bridge to be ascertained through consistent observed behaviour at multiple bridge sites. Abutments were found to contribute significant stiffness in both the longitudinal and transverse axes of the bridge. Computational models of the test bridges were developed using existing modelling techniques to represent the structure, foundation, and surrounding soil by comparing the predicted response of the model to the bridge response identified from field testing. The influence of neglecting the contribution of various subcomponents, and alterations of element material properties on the overall bridge response was investigated. The modelling study determined that the dynamic bridge response was dominated by the structural layout and stiffness, and confirmed field testing observations that the passive resistance of the abutment backfill and buried friction slab contributed significantly to the stiffness in the longitudinal and transverse directions respectively. The results of the modelling study were used to develop a series of guidelines for the development of integrated structure-foundation models of bridges at the initial conditions of seismic loading.