Seismic behaviour of isolated bridges with soil-structure-interaction effect under near-fault and far-field earthquakes

Abstract

Seismic base-isolation is recognised as an efficient seismic design strategy for mitigating seismic risk and improving structural performance of bridges. However, some parameters, such as earthquake inputs and soil characteristics, influence the seismic response and may reduce the technology's performance. This research aims to investigate the effects of soilstructure interaction (SSI) with regard to different moderate and strong earthquake ground motions associated with different source distances to the site and frequency content on the seismic responses of conventional and isolated bridges. To this end, groups of moderate and strong Near-fault (NF) records, with and without velocity pulses, and far-field (FF) ground motions are applied to the conventional and isolated bridges with and without considering the underlying soil and soil-structure interaction (SSI) effects. The influence of soil characteristics is investigated by considering three soil properties representing rock, dense and stiff soils. Furthermore, the effect of the modeling approach of SSI is investigated by comparison of results from two modelling approaches: 1-Direct representation of soil; 2-Simplified (substructure) method where soil is represented by equivalent linear springs. The bridge-soil systems are modeled and analysed in the Abaqus general purpose finite element software. Nonlinear time history analyses (NLTHAs) are carried out, and the individual maximum, as well as the average of individual maximums of the structural responses obtained from both approaches in terms of deck acceleration, base shear, and displacement of the deck and within the isolation system, are studied and compared. Results demonstrate that the difference between the two approaches of modeling the soil is significant. Using the simplified (substructure) method should be interpreted alongside careful attention to the validity limits of using the equivalent linear method as many of the records captured on softer soils were not eligible based on the limitation of the shear strain index (generally under 0.03%), and the responses were very scattered, especially for the conventional bridge. Therefore, the simplified method of using linear springs to represent the soil stratum is a rather simple approach to capture all the major mechanisms involved in soil, SSI, and characteristics of each earthquake ground motion. Furthermore, the results of this study reveal that the bridge performance and soil effects are governed by the uncertainty in the ground motions and their frequency contents for both moderate and strong earthquake records. The ratios of the peak ground acceleration to peak ground velocity (PGA/PGV) play a decisive role in dynamic responses. Records with low PGA/PGV presence of the soil diminishes the negative effect of the pulse and decreases the force demand, compared to the response of a fixed-base model, not including soil. Overall, careful attention should be paid to properly incorporate soil structure interaction in order to estimate correctly the displacement demands in the isolation systems, especially on soft soils under pulse-like records. Responses of the different isolation systems demonstrate that the higher characteristic strength (Qd), post-elastic stiffness (Kd), and displacement capacity are needed for strong NF pulse-like records to provide the displacement demand, especially on softer soils.

Date	27 Nov 2023
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Lotfi Guizani (Supervisor) & Nourreddine Ghlamallah (Co-supervisor)