STRUCTURES: North Viaduct to Lions Gate Bridge

The refurbishment of Vancouver’s Lions Gate Bridge suspension span has had lots of media attention, including for winning the Schreyer Award in the 2002 Canadian Consulting Engineering Awards (CCE Oct-Nov., p. 23-25). However, the renewal of this critical urban crossing involved other important components, one of which was to retrofit the bridge’s North Approach Viaduct so that it could withstand a major earthquake.

Consulting engineers Klohn Crippen were engaged by American Bridge/Surespan to provide structural and geotechnical services for the 762-metre viaduct, which links the bridge’s suspension span to the North Shore. The viaduct consists of 25 steel plate girder spans supported on 24 bents, including four H-bents for longitudinal stability.

As a design-build seismic retrofit the project was highly unusual. The engineers had to both identify the deficiencies in the existing bridge and develop a solution during the short three-month tender period. Traditionally work of this kind could take more than a year.

Klohn Crippen recognized that by taking the nuts off the bridge’s anchor bolts, and installing baseplate uplift guides, the bents would be able to tip up on one column leg briefly during an earthquake. This “rocking” mechanism would limit the loads so that only relative minor structural strengthening and connection upgrades were required. In effect, a 5,000 ton steel structure was going to be allowed to bounce up and down. Although there are precedents for allowing foundation to rock, this is the first retrofit of a major structure to allow the guided uplift of columns off their footings. The solution cost much less than other retrofit techniques.

To capture accurately how the bridge would behave during an earthquake, the engineers did non-linear time-history analyses using the ADINA program. Liquefaction assessment techniques recently developed by Klohn Crippen’s Dr. Sy showed that only seven of the bent foundations required mitigation. These vulnerable spread footings were underpinned by 40 metre piles and connected to the existing footings with concrete pile caps. Other strengthening measures included adding new bracing that matched the existing bridge, and adding reinforcing to the built-up sections. The project was completed in December 2001 at a cost of approximately $3 million.

Client/design-builder: American Bridge/ SureSpan Joint Venture

Independent checking: Parsons Brinckerhoff