The West Seattle Bridge is fragile enough to need extra support, known as shoring, before contractors this year enter the nearly 600-foot arched span to perform repairs to allow traffic to resume.

But the city’s roadway structures director, Matt Donahue, said the bridge can still support its own weight and he’s not concerned about it sagging or collapsing as the city continues to investigate a sudden acceleration of cracks within its hollow girders.

“We’re taking all measures to protect the integrity of the bridge,” Donahue said in an interview Thursday. “We’re looking at shoring as part of a medium-term repair program.”

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Mayor Jenny Durkan announced the emergency closure Monday, expected to last months. The lower swing bridge is restricted to transit, emergency vehicles and freight. Other traffic must detour through the peninsula’s southern hillsides and the Duwamish industrial area.

Cracks became serious enough last year that engineers started contingency planning to reinforce the main span, an online update by the Seattle Department of Transportation (SDOT) said.

Photos show cracks that lengthened 5 feet since August, including 2 feet of growth just from March 6-23. A few stretch nearly the entire 10 feet from floor to ceiling inside a girder.

The best move, when rapid deterioration shows up, is to remove the vehicles to give structure the best chance to avoid more damage, Donahue said.

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Engineering began Monday to make a shoring plan, said Donahue. He wouldn’t speculate Thursday about possible techniques or equipment — which includes the challenge of bracing the troubled span area that’s above water.

He said the city began yearly inspections in 2013, when four sets of smaller cracks appeared. That’s twice as often as the 24-month federal requirement for major bridges, as reported to the National Bridge Inventory. There were six inspections in the last six months, SDOT says.

The city’s blog post, titled “How we caught the West Seattle high-rise bridge deterioration,” addresses public grumbling that the city should have monitored and fixed the problem sooner.

“Those annual inspections did not indicate a need for repairs that would significantly disrupt standard use of the bridge,” SDOT said. Crews filled cracks with epoxy, a common practice in concrete bridges.

Heather Marx, SDOT mobility director, told the West Seattle Blog that staff last week began to draft a briefing for city leaders that would propose traffic restrictions, before the cracks accelerated.

SDOT Director Sam Zimbabwe told The Seattle Times on Wednesday that uneven weight loads, since a bus lane was added to the then-six-lane crossing last decade, played a role. But including traffic, 80% of the bridge’s weight is the bridge itself, indicating the cracks likely have multiple causes.

The crossing, which opened in 1984, was built for 140 feet clearance at a slope of 6 degrees, in hopes that taller vessels would enter the Duwamish Waterway. The concrete arch replaced a twin drawbridge after a runaway barge destroyed one drawspan in 1978.