Schreyer Award: Lions’ Gate Bridge

Category: Transportation

BUCKLAND & TAYLOR

The Lions’ Gate Suspension Bridge, with a centre span of 472 metres, is a landmark structure at the entrance to Vancouver, one of the busiest harbours on the west coast of North America.

Constructed in 1938 using a light, elegant design, the bridge opened with two traffic lanes. In 1954, the lane markings were changed to allow three narrow 2.9 metre lanes.

Since 1972, Buckland & Taylor has acted as the owner’s engineer for almost every structural engineering aspect relating to the bridge. By the mid-1990s, the suspension bridge deck had deteriorated to the point where maintenance costs were approximately $3 million per year. The provincial government decided to consider options for rehabilitating the crossing and public consultations were held with local groups to determine what to do with the aging structure. After trying to obtain consensus without success, the government chose to use the existing corridor and renovate the structure.

In 1997, the British Columbia Transportation Financing Authority commissioned Buckland & Taylor to design a replacement suspended structure for the existing bridge. Since there are only two crossings from the North Shore to Vancouver, and since Lions’ Gate carries more than 40% of the daily traffic, closing the bridge for any significant length of time for construction would have been a social and economic disaster. Buckland and Taylor’s innovative solution of replacing the suspended structure in pieces at night (and during a few weekends for the deck sections at the towers) virtually eliminated the impact of the construction, and except for brief closure periods maintained the regular daytime traffic of 70,000 vehicles on three lanes of the bridge.

In 1975, the concrete deck on the north approach viaduct had been replaced with steel deck panels, the first time in the world that a deck had been replaced at night while traffic continued to use the full bridge during the day. The method, also designed by Buckland & Taylor, has been copied many times (Golden Gate bridge, George Washington bridge, Angus L Macdonald bridge, etc.). In all of the cases since 1975, however, it was only the deck (the roadway) that was replaced. The main structural system remained. The structural girders or trusses were left in place, and in most cases the existing floorbeams were reused.

On the Lions’ Gate suspension bridge, everything was replaced — deck, stringers, floorbeams, and trusses –such that when a section was being removed there was no connection across the opening. This is the first replacement of its kind on this scale in the world. At the completion of the huge undertaking, only the original towers, main suspension cable, and main cable anchorages remained of the original bridge.

American Bridge/Surespan was awarded the construction contract with a bid of $64 million for the bridge portion of the contract. The final construction cost, including changes and extras, was approximately $74 million, and construction was done over a year, ending September 29, 2001.

The process

Buckland & Taylor had to ensure that the criteria loads used for construction (wind, live, dead, temperature) were conservative enough to ensure public safety, yet not so conservative that construction would be impossible.

The major objectives were:

Replace the seriously corroded original deck;

Widen the traffic lanes from 2.9 metres to 3.6 metres and the sidewalks from 1.3 metres to 2.7 metres (2 metres clear at the hanger locations);

Increase the critical wind speed from 35 m/s to at least 45 m/s;

Install barriers between the traffic and the sidewalks (the original bridge had none);

Only close the bridge for construction at night between 20:00 to 06:00 (i.e. 10 hour night closures), and on a few weekends, from 22:00 Friday to 06:00 Monday;

While the bridge is open to traffic, ensure it is as safe as any other bridge in the province (i.e. the bridge design code had to be satisfied during construction); and

Increase the seismic resistance of the bridge to satisfy modern standards.

The original configuration of the deck had the trusses above the deck. The design of the new wider cross-section positioned the longitudinal stiffening trusses under the deck. This strategy allowed the wider traffic lanes and the traffic barriers to be placed between the two planes of the hangers, and the sidewalks to be placed outside the hangers. It also permitted bracing to be installed between the truss bottom chords, thus increasing the critical wind velocity to approximately 70 m/s, well in excess of the required 45 m/s.

The weight of the new deck had to be close to that of the original deck, which was very light, so that the replacement sections would not induce unacceptably large deformations and stresses in the structural components during construction. An innovative use of the deck plate, which holds most of the mass of the new section, solved the weight factor. In addition to being used as the roadway surface, the deck plate was used structurally as the top flange of the floor beams, and as the top chord of the longitudinal stiffening truss.

Of the original components of the bridge that remained after the construction, only the base of the existing towers and the bracing in the North Cable Bent required strengthening in order to increase the seismic performance to modern standards.

Buckland & Taylor developed in-house computer analysis software to model accurately the behaviour of the cables and hangers. Live loads were about one third of the code values, primarily because large trucks are banned from the bridge. Without the reduction in live loading, the deck replacement would not have been possible. The live load studies done for the project have been used to define the long span bridge loading in both Canadian and American Bridge codes.

Tight site, and tight schedule

After each deck section was replaced, the bridge structure changed. As a result, the design criteria had to be applied to 54 different bridges during the replacement period, each of which had to accommodate the travelling public safely.

The space at the site was very limited due to the presence of Stanley Park on the south side and a Native land reserve, I.R. #5, on the north. The most restrictive part of the site, though, was the bridge deck itself where all the contractor’s erection equipment had to be outside the traffic envelope, either above the deck, or on the narrow sidewalks.

Buckland & Taylor had to ensure that appropriate erection equipment could be designed that would not overload the bridge. All the major components of this equipment were unique, with the contractor doing its final design. The contractor also designed and independently checked all of the erection engineering necessary for the deck replacement.

The design anticipated that the suspended spans would be progressively replaced from north to south in a total of 47 deck sections, typically 20 metres long, but the contractor elected to install the south side span in 10 metre sections, and therefore the actual number of deck sections was 54.

Every detail had to be designed for quick installation. The field splices were located to allow the new deck section to be raised with little interference from the previously erected section. There was not enough time for welding, so most of the splice had to be bolted (the deck plate itself was welded during non-lifting closures). Approximately 800 bolts were required in each splice, almost all of which needed to be installed before opening the bridge to traffic. Using typical fabrication and erection methods, installing this number of bolts would take several days, not several hours as required for this project. Buckland & Taylor therefore required that the fabricator match-assemble the deck sections to the correct geometry in the shop to ensure that the sections would fit together and the bolts would slide into the holes without the customary time consuming pinning or reaming.

Environmental considerations for the project included dealing w
ith nesting birds, and disposing of the old corroded deck sections which had been leaching steel by-products into the environment. Replacing the suspended structure also removed a significant portion of lead paint (the paint on the towers still contains lead). Drainage details were designed for the new deck to minimize the possibility of future corrosion.

The important aesthetic considerations were to maintain the shape of the suspension bridge. Keeping the weight of the new deck the same as that of the old meant there was no need to strengthen the towers and they could keep their slender profile. In addition, the new truss is only half the depth of the old truss, giving the deck a more elegant look than it had originally.cce

Name of project: Lions’ Gate Bridge Deck Replacement

Award-winning firm: Buckland & Taylor, North Vancouver (bridge engineering consultant). Peter G. Buckland, P.Eng.,

Darryl D. Matson, P.Eng., David J. Queen, P.Eng.

Owner: B.C. Ministry of Transportation

Other key players: ND Lea (owners’ engineer for non-bridge

aspects of contract and contract administration); Rowan Williams Davies & Irwin/Danish Maritime Institute (wind studies);

Parsons Brinckerhoff (independent checking for contractor);

Parsons, formerly Steinman (erection analysis for contractor);

PBA Engineering (owner’s electrical engineer)

Contractor: American Bridge/Surespan, Joint Venture