Asynchronous earthquake strong motion and RC bridges response

The dynamic response of long structures (e.g., bridges) is sensitive to the spatial variability of strong ground motion (asynchronous motion). Ground motion differences increase from point to point with increasing foundation distance. This latter is due to two physical phenomena: soil-wave interaction, that causes the loss of coherence and local amplification; wave traveling with finite velocity, that causes signals time lag. This ground motion variability produces a different structural demand compared to the synchronous one, which is the only one considered by designers in the majority of cases. A few codes consider this type of actions, therefore further research efforts are necessary. In this study, asynchronous ground motions are generated by means of a new generation procedure implemented in the software GAS 2.0 using as input the simultaneous strong motion records from the April 6th, 2009, L'Aquila (Italy) at the seismic stations AQA and AQV, located in the Aterno River valley. These records are used to calibrate the generation model and to produce sets of asynchronous earthquake sampling. The asynchronous earthquake sets are applied on a typical highway reinforced concrete bridge to study its dynamic response considering two different configurations: non-isolated with traditional supports and isolated bridge with lead rubber bearings. The bridge is placed in two positions along the wave propagation direction: a position near one recording station and a position between the two stations to consider local soil effects. The response parameters investigated are the maximum relative displacements of soil and deck. The results show that there is an important variation of relative displacement along the direction of wave propagation due to asynchronous motion with effects that designer should consider for the structural details design of isolated and non-isolated bridges.