Design of Seismic Joints in Bridges

Abstract

Earthquakes are one of the major actions and sources of collapses for civil engineering structures which makes their study crucial. Bridges, being the most critical civil engineering structures and key elements of transportation systems, make their performance under seismic actions imperative. Boundary conditions greatly affect the response of bridges to seismic actions. Gaps are one of the boundary conditions of bridges and this thesis focuses on the design of gaps to optimize the response of bridges under seismic actions. Appropriate selection of joint gaps is an aspect of seismic design that has hardly got proper attention.

During seismic actions, the bridge deck will move and close the previously open bridge gaps activating the abutment-backfill system (in the longitudinal direction) and shear keys (in the transverse direction). This will reduce the contribution of piers to bridge response as the overall stiffness of the system changes. If the gap is small, it will activate the abutment-backfill system immediately after the seismic action and the abutments will carry more force and the piers will carry less (than they would if the gap didn’t close). If the gaps are large the abutment will carry less force and the piers will carry more (than they would if the gaps were closed) as the gaps will not close during the earthquake which doesn’t allow the transfer of forces to the abutment-backfill system. This makes determining the optimum gap size important to distribute the stiffness of the structures and the forces they carry. Also, gap size and closure affect and determine various damages including backfill deformation and deck unseating. Consequently, varying boundary conditions during an earthquake affect the inelastic behavior of critical bridge components (piers, piles), as well as failure mode (how the damage will occur).

This study aims to design the gap sizes for the end joints on concrete bridges that will optimize the seismic performance of the key components of the bridge i.e., piers and abutments. This is done by selecting the optimum gap size in both directions (longitudinal and transverse) from the gap size results as per Eurocode 8 as well as higher and lower values for different levels of seismic actions. The optimum gaps consider two criteria. The first one is to maintain the functionality of the bridge for moderate earthquakes and the second is to ensure the safety of the bridge under earthquakes that are stronger than the design earthquake.

A non-linear response history analysis program (i.e. SAP2000) will be used to determine the optimum joint gap size for a given bridge as a function of the intensity and frequency content of the ground motion. This will be used for the implementation of a movable shear key (MSK) the idea of a “Dynamic Intelligent Bridge”[1], a device for blocking the movement of the bridge deck, which can slide, hence varying the size of the existing joint gap.

Date of Award	Apr 2023
Original language	American English
Supervisor	Andreas Kappos (Supervisor)