Inelastic dynamic response of RC bridges subjected to spatial non-synchronous earthquake motion

The assumption that earthquake response of extended structures, of which bridges is one example, may be studied ignoring the possibility of out-of-synch motion of various supports is examined in this paper. The purpose is to assess whether the reduction of dynamic response is sufficient to offset the increase in relative displacements, due to independent motion of different supports. The paper starts with a review of possible damage patterns due to asynchronous motion. Thereafter, a brief literature review is undertaken, followed by an assessment of the status of seismic design codes concerning this issue. Advanced inelastic analysis, using sophisticated material models and analysis techniques, is then employed to explore characteristics of input motion and structural configuration which may lead to unconservative results from conventional (synchronous) analysis. Natural and artificial earthquake records are applied, representing travelling wave as well as geometric incoherence effects. Two model structures of medium span RC bridges are studied, subjected to different boundary conditions that influence the mode contributions. This is undertaken under transverse, longitudinal and vertical earthquake motion. The large volume of results, represented as displacement and force time-histories as well as Fourier amplitude spectra of the acceleration response, are distilled and used to assess the balance between dynamic de-tuning and static relative displacements. It is concluded that the conventional synchronous case provides conservative results for vertical vibrations. However, unconservative results of up to 30% are obtained for transverse and longitudinal response of short periods of structural vibrations, as well as cases where the higher modes of response are likely to be excited.