Keep it moving

Aug. 20, 2002

As anyone with a backyard clothes line knows, when you remove one piece of laundry, the line springs up and reshapes itself. Cutting a section of deck, including stiffening truss, out of a suspension bridge produces a similar rebound, but with a suspension bridge the rebound can be catastrophic. The weight of the bridge pulls and twists in unusual directions.

That was just one of the engineering challenges to reconstructing the Lions' Gate Suspension Bridge in Vancouver, British Columbia.

As anyone with a backyard clothes line knows, when you remove one piece of laundry, the line springs up and reshapes itself. Cutting a section of deck, including stiffening truss, out of a suspension bridge produces a similar rebound, but with a suspension bridge the rebound can be catastrophic. The weight of the bridge pulls and twists in unusual directions.

That was just one of the engineering challenges to reconstructing the Lions' Gate Suspension Bridge in Vancouver, British Columbia.

One of the other challenges was the fact that the bridge had to be open to traffic every day by 6 a.m. The workers had to cut out a section of the old bridge, lower it to a barge waiting on the water below, haul up a new section and bolt it in place in a 10-hour window starting at 8 p.m. The reconstruction work was done overnight and on a few weekends between Friday at 10 p.m. and Monday at 6 a.m.

Lions' Gate is a vital link between Vancouver and the North Shore of Burrard Inlet. Built in 1938 as a two-lane bridge, Lions' Gate was later rearranged to create a third--reversibl--lane down the middle. The bridge now carries 70,000 cars a day. Trucks are not allowed.

The Lions' Gate reconstruction was the first time the entire deck of a suspension bridge, plus suspenders, sidewalks and stiffening trusses, was replaced while the bridge was kept open to traffic. The only components of the original structure that remain are the towers, the main suspension cables and the main cable anchorages.

The Engineers' Society of Western Pennsylvania, in association with Roads & Bridges, awarded the Lions' Gate reconstruction the George S. Richardson Medal, presented at the International Bridge Conference in Pittsburgh on June 10. The Richardson Medal is given for a single, recent, outstanding achievement in bridge engineering. The winning project must have a significant improvement or advancement in bridge technology.

Accepting the award was Geoff Freer, regional director of the northern region for the British Columbia Ministry of Transportation.

The original structure was built as a toll bridge by the Guinness family, known for its beer, and it was built for economy, not longevity. The family owned about 4,000 acres of land across from Vancouver that they wanted to develop, and they had to build a bridge to get there.

"The bridge had to finance itself," Darryl Matson told Roads & Bridges. Matson was the project manager for the design, developed by Buckland & Taylor Ltd., North Vancouver, and the owner's bridge engineer and representative during construction. "As a result of that, everything to do with the bridge was driven by the fact that it had to be as cheap as it could be."

Going to rust

The old structure lacked a means for channeling water safely off the bridge: "The water just runs right off the deck of the bridge, and it falls right on the structural elements below," Ronald W. Crockett, contractor's representative for American Bridge Co., Pittsburgh, told Roads & Bridges. "Over the years, it's caused a tremendous amount of corrosion. It mainly has to do with the drainage systems were not designed to modern standards."

Corrosion was the main reason the bridge had to be replaced.

In 1955, the private owners had recovered their investment, and they sold the bridge to the government of British Columbia.

The reconstruction cost $105.6 million. The total price, with engineering studies, administration and extras, came to $125 million.

The owner is the British Columbia Transportation Financing Authority. The designer of the replacement structure was Buckland & Taylor. The contractor was American Bridge/Surespan, a joint venture of the Canadian subsidiary of American Bridge and Surespan General Contractors Corp., West Vancouver.

Engineering for night owls

Because most of the reconstruction had to take place during 10-hour shifts at night, the constructors had to make sure the operation went smoothly.

Every move of the erection procedure was planned and tested in computer engineering simulations. Because each section of the deck of a suspension bridge is subject to different forces, each section required its own, unique simulation.

"For each of 54 sections that were removed, we had to do a completely different set of analysis, come up with completely different sets of adjustments and different instructions for the contractor to follow," Seth Condell, an engineer for Parsons, told Roads & Bridges. "In the end, it took us well over 2,000 computer models to complete this project."

Parsons, New York, was hired to work with American Bridge to produce a plan for the reconstruction and to perform an erection analysis. Parsons Brinckerhoff Inc., New York, an unrelated company, was hired to perform an independent check of the erection analysis.

"We worked very closely with American Bridge to develop the method," John Clenance, a senior engineer at Parsons, told Roads & Bridges. "We worked on the analytical end. They worked on the field procedure end."

The solution for getting one section out and another in was to build what the constructors called a "jacking traveler." The jacking traveler was a platform that attached directly to the suspenders of the bridge and acted as a crane for lowering the old section and raising the new one into place. Then it detached, rolled along the bridge to the next section and reattached. The jacking traveler was equipped with four strand jacks, with a capacity of 60 tons each, to provide the lifting force.

The main span of Lions' Gate is 472 m, with a total length of 847 m, including the north and south side spans.

The first deck section was replaced during the weekend of Sept. 9-10, 2000, and the last section during the weekend of Sept. 29-30, 2001.

American Bridge/Surespan replaced 47 sections of the bridge in order from north to south, each about 20 m long and weighing 106 tonnes (1 tonne equals 1,000 kg). The south side span was replaced in 10-m pieces for a total of 54 sections.

Balancing act

The original deck was a very light, steel T-grid.

To keep the weight of the new bridge approximately the same as that of the old bridge, the engineers decided to use a steel orthotropic deck, a steel plate with longitudinal troughs, transverse floor beams and longitudinal stiffening trusses.

"An orthotropic deck is typically very light, so the use of an orthotropic deck helps immensely in terms of the weight," Matson said. "The new trusses couldn't be what you would normally make them out of, which is built-up plates or I-sections. To get the capacity out of them, we had to go with tubular sections."

Also to conserve weight, the engineers decided to move the trusses from sitting on top of the deck to underneath. By moving the trusses, they could make the deck serve triple duty--as the deck itself, as the top chord of the trusses and as the top flange of the floor beams.

The weight of the new bridge sections was almost the same as the weight of the old sections. The new sections were actually a little lighter because they lacked the permanent pavement wearing course, which could only be placed after the entire bridge was reconstructed.

"If you're replacing something on a suspension bridge with something that's heavier, the shape of the bridge will change," explained Matson. "If I replace one side span with something that is heavier, that will pull up the main span and lower the side span I just replaced. As you progress into the main span what'll happen is half of the bridge wants to go down, half of it wants to come up, and you can't pull the two of them together without inducing a lot of stresses in the truss."

To make the old bridge line up with the new bridge during construction so traffic could run on it, the engineers came up with several ideas.

One was to attach adjustable extensions to the suspenders.

The extensions were necessary. The suspenders attached to the top of the old stiffening trusses. But the new stiffening trusses were underneath the deck, so extensions had to be attached to the suspenders to reach to the new deck. Later, the suspenders and adjustable extenders were replaced with new suspenders.

Putting the trusses under the deck was one reason it was possible to widen the traffic lanes and sidewalks, put the sidewalks outside the suspenders and give the bridge a more comfortable feel.

Claustrophobia-free zone

The driving lanes on Lions' Gate are now 3.6 m each instead of 2.9 m.

The reconstructed bridge is 35% wider and more open than the original.

The sidewalks are now 2 m wide instead of 1.3 m and separated from the traffic by a barrier and the suspenders. Pedestrians have better views from the bridge. They do not have the cars whizzing past right next to them. And they do not feel boxed in by the trusses.

Another advantage of placing the trusses underneath and making them integral with the deck is that the bridge is now much more stable in a wind.

The original trusses and deck were separate elements, and they lacked torsional stiffness. On the new bridge, the trusses and deck--with a layer of bracing between the bottom chords--form a closed box, a stiffer structure.

The original bridge had a critical wind speed of 35 m/s. The new bridge's critical speed is 70 m/s.

The reconstruction is almost complete. In July, all the work was done except for the epoxy asphalt wearing surface.

To prevent future corrosion, Lions' Gate now has a modern water runoff system, with channels that lead to drain pipes that drop the water below the level of the bottom of the bridge.

"We took a lot of care to funnel off the water and to get rid of it and not have it splash on the structural steel," said Matson.

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