As investigators pull twisted bridge parts from the Skagit River north of Seattle, the probable cause of the collapse is theorized to be this: A trucker with a state-issued permit to haul an oversized load down Interstate 5 nicked a few beams overhead, causing the structure to crumple and fall into the waterway.
What’s not theoretical is that the bridge, built in 1955 in the steel through truss-style, was “fracture-critical,” meaning all it takes is one good hit for it to collapse.
Oregon has at least 60 steel through truss bridges.
The I-5 bridge over the Skagit River had a clearance of 14.6 feet, exactly 1 foot less than the clearance of the northbound lanes of the 1917-issue I-5 bridge over the Columbia River, another steel through truss structure that is rated fracture-critical and the subject of fierce debate owing to its proposed $3.5 billion replacement. But the issue with both bridges is less about structural fitness than modern conveyance: The Portland-Vancouver bridge’s 1-foot margin against potential calamity would look crazy if the wrong big load were allowed to cross or if a hauler were to err in judgment and trigger a collapse.
Earlier this year, a hauler with a permit overturned on the I-5’s Marquam Bridge in downtown Portland in the middle of a busy weekday afternoon, spilling a 168-foot-long, 75,000-pound I-beam that crushed one car but miraculously failed to penetrate a guard rail and take with it cars to the Willamette River below.
The I-5 Skagit hauler was plain unlucky, it appears.
Only precise permitting, timed to low-traffic volumes and allowing a good margin for error, will help to rule luck out.