Sur l’effet du retrait et du fluage sur la performance sismique des piles de ponts isolés à la base faiblement armées

Abstract

For the sake of economy and optimization, seismic base isolation of bridges is increasingly used to protect bridges from earthquakes in areas with moderate to high seismicity. Nevertheless, despite the very significant reduction in seismic demand for such bridges’ piers, the longitudinal reinforcement ratio cannot be lowered below a minimum threshold prescribed by the code, mainly to control creep and shrinkage effects. In this context, the present research project aims to evaluate the effect of the lowering of the minimum longitudinal reinforcement ratio on the seismic performance and the level of damage within piers of base-isolated bridges, with the consideration of creep and shrinkage effects. The minimum longitudinal reinforcement ratio in bridge columns specified by the Canadian Highway Bridge Design Code (CSA-S6-14) is 0,8 %. However, several recent research studies support the possibility of relaxing this requirement and lowering this limit below 0,8 % while providing an adequate confining of critical zones. On the other hand, this value was initially mainly intended to protect the columns from the effects of shrinkage and creep, which have not been considered in these previous studies. In this respect, in order to validate the conclusions of these studies, a consideration of the structural effects of shrinkage and creep should be included in the analysis of seismic performance of such bridge piers with low reinforcement. A typical two spans base-isolated bridge, with prefabricated superstructure, and supported by a central column-bent pier, is considered as a case study. Different variants of the case study bridge were designed, and the seismic performances of their central piers evaluated through non-linear time history analysis results. In order to include shrinkage and creep effects, the initial state of the bridge, preceding the earthquake, is determined following non-linear static analysis simulating the shrinkage and creep for different time periods (ages of the bridge). The staged construction method and the CEB-FIP 2010 model were used during this study to predict the effects of shrinkage and creep. Two Canadian site locations, Montreal in the East and Vancouver in the West, were considered. The seismic records were calibrated on the CSA-S6-14 design spectra for these locations. Two reinforcement ratios were considered in this study: (a) The minimum reinforcement ratio of 0,8 % as prescribed by the code and (b) a ratio of 0,5 % as required by the seismic demand of the bridge pier. The rectangular sections of the columns are modelled using fibre elements at the expected zones of plastic hinge formation, in order to track the extent and level of damage under the effect of the earthquake. The results obtained show that all the studied bridge variants behave essentially in the elastic range without significant damage, in accordance with the minimum performance requirements of the CSA-S6-14 code for lifeline bridges. For this case study, it was shown that the minimum reinforcement ratio prescribed by the code could be lowered to 0,5 % without compromising the seismic performance of the bridge piers. However, shrinkage and creep depend on many bridge and pier structural parameters and further studies are required before generalising this finding.

Date	30 Nov 2020
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Amar Khaled (Supervisor) & Lotfi Guizani (Co-supervisor)