OTHER OPINION: Old bridges, new lessons

By The Oregonian

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.