Seismic Isolation of Bridge Footings


September 2, 2014

Category: Structural Bearings, Seismic Isolation  -  Published by: Parinaz Pakniat

In 2013, Canam-Bridges supplied structural steel components for the reconstruction of an overpass for the Highway 20/73 Interchange rehabilitation project in Lévis, Quebec. This 1,040-ft. (317-meter) curved bridge is supported by five piers that straddle the two highways; each of the deck spans is comprised of six steel girders. The structure was designed by a consortium formed by CIMA+ and Dessau, and the project was executed by Roxboro Excavation Inc. for the project customer, the Ministère des Transports du Québec. Canam-Bridges also supplied 24 Goodco Z-Tech pot bearings that were installed at each abutment and all exterior piers as well as 18 seismic isolators fabricated by Goodco Z-Tech in collaboration with mageba, a supplier specialized in seismic protection components. The seismic isolators, which were installed on the central and two intermediate piers, act to decouple the movements of the superstructure from ground and pier movements in the event of an earthquake.

The seismic isolators installed on the Highway 20/73 Interchange overpass are lead rubber bearings that allow for unidirectional horizontal movement only. Transversal movement is impeded by guiding bars that are welded to the upper and lower bearing plates. These guiding bars also feature a sliding system with a low coefficient of friction to allow longitudinal movement.

The seismic isolators were fabricated by Goodco Z-Tech in collaboration with mageba.
Bridge construction
Formwork used to pour the concrete deck of the new Highway 20/73 Interchange ramp


Under service loads, vertical loads are transferred from the superstructure to the piers through compression of the seismic isolators; horizontal loads and movements are transferred by the isolators’ shear deformation. The lead core increases the characteristic strength of the system (Qd: horizontal force that must be applied to the isolator to produce displacement). In the event of an earthquake, for the same ground movement, the horizontal forces transferred to the superstructure are reduced, thus allowing for energy dissipation, or damping, and consequently, decoupling of the piers and superstructure.

Force displacement curve during prototype tests of a lead-rubber bearing performed at room temperature, as specified in CAN/CSA-S6-06, section c) ii).

During winter, the rubber and lead become more rigid. The rubber’s shear modulus (G) and the lead core’s ultimate tensile strength (Fu) increase. For the same displacement, the horizontal load transferred to the structure increases as the temperature decreases. The strict limit imposed on prototype test results at low temperatures (−30⁰C) was a particular technical requirement for this project. Tests were therefore performed at room temperature (between 15 and 25⁰C) and at low temperatures. Preliminary testing showed that the increased rigidity of the seismic isolators at low temperatures was greater than the specified limit. The tests performed at room temperature, however, all produced satisfactory results.

To mitigate the excessive increase in rigidity of the isolator in cold weather, several rubber recipes were elaborated. As with different types of construction steel, modifications in the proportions of the ingredients used to manufacture the rubber will impact its performance. New mixtures were tested to determine which combination would best fulfill the design criteria. Repeated tests were conducted on full-size samples at room temperature and low temperatures, and finally proved satisfactory. With prototype and quality tests within acceptable limits, the seismic isolators were installed on the main and intermediate piers on schedule for pouring of the concrete deck.

Prototype Tests

View a video of a prototype test performed in compliance with CAN/CSA-S6-06, section Two 100% displacement tests are shown, followed by cyclic tests of 25%, 50%, 75%, 100% and 125% of the maximum displacement. A cycle of 100% of the maximum displacement is presented at the end.

The new Highway 20/73 Interchange overpass opened to traffic in September 2013.

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