May 24, 2013 10:04 pm

Could the I-5 Skagit River Bridge collapse happen in B.C.?


The Skagit River Bridge was one of more than 150 in Washington State that have been deemed deficient in a 2011 report. That does not mean the bridge was unsafe, but it did need repairs to be brought up to code.

The type of bridge is quire rare, with most of the structure above the bridge deck – there is a similar bridge in North Vancouver on Keith Road. However, no main bridges in the Lower Mainland have the characteristics that made the Skagit Bridge susceptible.

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This type of bridge would likely be built on rural roads across the province, and B.C. is also much more proactive in regular check-ups on all our bridges.

Dr. Perry Adebar, professor of structural engineering at UBC, said the I-5 bridge is very different, and a similar event could not happen here.

“The Skagit River is shallow and wide at the location of the crossing and therefore the bridge has numerous closely-spaced piers built into the river,” he said in a statement. “The bridge is really like a number of small short-span bridges one after the other. The distance between the bridge deck and the water level is very small and therefore the structure of the bridge is mostly above the bridge deck.  These two factors, which are important for understanding the collapse, are also the reason that the people that fell with the bridge were not killed – they fell a very short distance into shallow water.”

“If one member in a light steel truss is badly damaged, the entire span has a high change of collapse. In summary, the I-5 Skagit River Bridge, while an important bridge on Highway I-5, is from an engineering perspective, the type of old small bridge we might find on a rural road in BC. None of the important bridges in the Lower Mainland have the same characteristics that made this bridge so susceptible to collapse due to a mistake by the driver of an over-sized vehicle.”

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