Probabilistic seismic response and uncertainty analysis of continuous bridges under near-fault ground motions

Performance-based seismic design can generate predictable structure damage result with given seismic hazard.However,there are multiple sources of uncertainties in the seismic design process that can affect desired performance predictability.This paper mainly focuses on the effects of near-fault pulse-like ground motions and the uncertainties in bridge modeling on the seismic demands of regular continuous highway bridges.By modeling a regular continuous bridge with OpenSees software,a series of nonlinear dynamic time-history analysis of the bridge at three different site conditions under near-fault pulse-like ground motions are carried out.The relationships between different Intensity Measure(IM)parameters and the Engineering Demand Parameter(EDP)are discussed.After selecting the peak ground acceleration as the most correlated IM parameter and the drift ratio of the bridge column as the EDP parameter,a probabilistic seismic demand model is developed for near-fault earthquake ground motions for 3 different site conditions.On this basis,the uncertainty analysis is conducted with the key sources of uncertainty during the finite element modeling.All the results are quantified by the"swing"base on the specific distribution range of each uncertainty parameter both in near-fault and far-fault cases.All the ground motions are selected from PEER database,while the bridge case study is a typical regular highway bridge designed in accordance with the Chinese Guidelines for Seismic Design of Highway Bridges.The results show that PGA is a proper IM parameter for setting up a linear probabilistic seismic demand model;damping ratio,pier diameter and concrete strength are the main uncertainty parameters during bridge modeling,which should be considered both in near-fault and far-fault ground motion cases.

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 6 th, 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 animportant 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.