Engineers follow formula for safety
Though many area bridges rate poorly on national standards of structural integrity, safety precautions make a collapse unlikely, city and county engineers said Friday.
“One of the reasons this is huge news is that the state and the bridge engineers around the country do such an excellent job that it doesn’t happen too often,” said Spokane County Assistant Engineer Chad Coles.
The Federal Highway Administration compiles an inventory of the nation’s bridges, including the roughly 7,000 in Washington state. Those that have suffered deterioration to road-surface and foundation integrity are labeled “structurally deficient,” while those with traffic flow exceeding their original design are classified as “functionally obsolete.” These measures are thrown into a formula that determines each bridge’s score on a 100-point “sufficiency rating” scale.
In Spokane County, 22 bridges have been deemed structurally deficient, including downtown’s Post Street span over the Spokane River. But Andy Schenk, operations engineer for the city, said that the bridge is posted with a tested weight capacity and the classification doesn’t mean a collapse is imminent.
“We always err on the side of caution,” Schenk said. “If the bridge is going to fall down, we’ll close it.”
The Skagit River bridge that collapsed Thursday night rated a 57 on the 100-point scale and earned functionally obsolete status. However, no weight restrictions were posted.
Sixty bridges in Spokane County qualify as functionally obsolete. But Schenk said that number illustrates the country’s aging infrastructure and greater demands from heavier commercial vehicles, not a major safety threat.
The classifications do help local lawmakers and engineers prioritize upgrades or replacements. A handful of bridges within Spokane County have been identified for updates based on safety tests conducted at least every two years.
“It’s quite an intense inspection and analysis, and it’s a national inspection,” Coles said.
The Bruce Road bridge over Peone Creek, slated to reopen after repairs this fall, rated 25 on the sufficiency scale before construction began. The city has studied options to rehab the Latah Bridge connecting Spokane to the West Plains. A structurally deficient bridge, it rates a 36 on the sufficiency scale.
Both Coles and Schenk said Thursday’s accidental collapse in Western Washington likely had nothing to do with the structural integrity of the bridge or a lapse in safety oversight. There’s only so much you can plan for in designing a bridge, Schenk said.
“I truly believe our bridges are safe for the loads that are posted,” Coles said. “Those guys in the bridge department do an excellent job.”