The effect of local scour of a single pier on the vibration parameters of a multi-span bridge under seismic excitation

Abstract

Several regions in Europe are prone to both seismic and scour hazards thus bridges can suffer from the joint actions of these two phenomena. The two types of hazards are actually independent as to the generation process but the loss of surrounding soil due to scour may significantly modify the dynamic parameters of the structure thus changing its response to a subsequent earthquake. Permanent monitoring through vibration-based methods can be an effective support in bridge assessment procedures providing updated information about the structural state and performance. Information from the monitoring system allow both the prompt detection of a possible damage state after an extreme event and also support for long term assessment of the structural conditions. For bridges under multiple hazard the possibility of using the same network of sensors for different hazards can increase the benefit/cost ratio related to the use of a monitoring system. In this paper, some preliminary results of a numerical investigation carried out on the finite element model of a bridge under flood and seismic actions are reported. The numerical model of a multispan bridge, developed using OpenSees, is subjected to a seismic base input considering several defined scour conditions. Scour has been modelled as the incremental increase in length of one of the piers and the effect of scour on the seismic response is investigated. The evolution of modal periods and of deck peak accelerations under seismic excitation have been studied in order to investigate the feasibility of these parameters as performance indicators able to detect the presence of scour in one piers for a bridge under seismic excitation. The first part of the paper reports a brief description of the main damage scenarios connected to scour and to seismic actions. The second part of the paper reports the results of a non-linear dynamic analyses of a scoured bridge under seismic input. Several scour depths have been considered in order to study the sensitivity of several vibration-based parameters to the intensity of the scouring.