The story behind British Columbia's bridge and wooden trestle
Source of Article:http://www.journalofcommerce.com/article/id32508
The Pattullo Bridge was opened to traffic in November 1937 by Premier Duff Pattullo, who wielded a welder’s torch and cut a metal chain across the roadway.
“It is a thing of beauty,” said Pattullo at the ceremonial opening.
The bridge was the brain child of consulting engineer Major W.G. Swan, who played a significant role in the design and construction of bridges, railways and harbours for more than six decades in BC.
Born in Kincardine, Ontario, Swan graduated from the University of Toronto in 1906 and moved to B.C., after he was hired as an engineer on the Canadian National Railway.
After serving overseas during First World War, Swan returned to B.C. and established his own consulting firm, which was retained to work on the Pattullo Bridge and the Lion’s Gate Bridge in 1939.
Swan used his overseas experience to overcome the challenges of linking Surrey to Greater Vancouver.
To begin, Swan decided to build a compression arch suspended-deck bridge.
This type of bridge is made from modern materials such as steel and reinforced concrete, in which a compression arch rises above the deck.
Cables connect the deck to the arch.
The Pattullo Bridge was constructed in 1936 and 1937 by Dominion Bridge Co., and Northern Construction & J.W. Stewart Ltd.
The total cost of the bridge was $4 million, including the main contract, cement, purchase of right of way, realignment, and resurfacing of highways.
To build the north and south approaches, 365,000 tons of earth was needed for fill, as well as 130,000 tons of sand and rock.
Construction also required two million board-feet of timber and seven thousand tons of steel.
One of its unique design features, and the most recent topic of discussion, was a wooden trestle.
“The unique part of the design is an area that has deep soft soil,” said Susan Holingshead, TransLink’s manager of roads and bridges.
“In geotechnical terms, soft soils are weak and compressible. When you put weight on they will settle considerably over a long period of time.”
According to Hollingshead, Swan’s original plan was to put in a granular embankment that led to the Surrey end of the bridge or approach ramp.
But, during construction more settlement than anticipated was encountered and the risk of failure in the underlying soils increased.
“So they did not complete the granular embankment fill,” she explained.
“Instead, they filled in the gaps with a wooden span because timber was readily accessible, light weight and they could use a shallow foundation.”
A concrete section would have required a deep foundation. The short wooden section with a shallow foundation and timber structure was engineered into the design, so it could be shimmed up periodically to accommodate the sinking.
TransLink was planning to replace the wooden trestle in the summer of 2009, but before these plans could be implemented, the trestle caught fire.
“The preservative they used for the wooden beams is creosote, which is a petroleum-based product that is very flammable,” explained TransLink spokesman Ken Hardie.
TransLink engineers and their design consultant, Buckland and Taylor, assessed the situation and found the wooden structure was not sound. The wooden section was not repairable and was still burning, so it had to be demolished.
Surespan Construction repaired the fire-damaged section of the old bridge in record time, with a piece of bridging deck that was used during the construction of the Canada Line rapid-transit project.
Despite the repairs, the 72-year-old Pattullo Bridge has withstood the test of time. It has serious design flaws by modern standards, which include extremely narrow lanes and ridiculous curves on each end of the bridge, but the aging structure still functions as a key transportation link.
It handles an average of 67,000 cars and 3400 trucks daily, about 20 per cent of vehicle traffic crossing the Fraser River.