Workers have completed the most complex phase of fixing the cracked West Seattle Bridge by installing and tightening about 45 miles of new steel cables from end to end.

This milestone keeps the bridge on pace to reopen in mid-September, after a final round of carbon-fiber wrapping, the closure of worker-access holes in the bridge deck and load testing.

The steel tendons, strung west to east within the bridge’s hollow concrete girders, help all three spans better resist the downward forces of traffic and more importantly, the leviathan concrete structure’s own dead weight of 220,000 tons.

The Seattle Department of Transportation announced Monday that Kraemer North America, the repair contractors, used hydraulic equipment to pull the cables from 20% of full tension to 100% over the past few days. They’re attached to new concrete anchors, which were delayed by last winter’s concrete-truck driver strike.

The steel strands, known as post-tensioning cables, are common in concrete bridges. They’re especially crucial in West Seattle, where the arch shape is too slight for the 590-foot-long main span to be self-supported by gravity.

When it was built in 1984, engineers did provide post-tension cables and anchors, but those reinforced only the midsection of the central span, because calculations showed the structure would be robust near the huge vertical columns. Instead, a combination of time, force and aging concrete led to diagonal shear cracks, discovered in 2013. Those accelerated at a dangerous pace until it risked collapsing.

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SDOT declared an emergency closure March 23, 2020, forcing drivers on the peninsula to detour as far as six miles through Duwamish River Valley neighborhoods. Before the pandemic and the closure, the high bridge carried about 100,000 vehicles and 20,000 transit riders per weekday.

Seattle considered a replacement bridge, but former Mayor Jenny Durkan chose repairs which are supposed to last 40 years.

SDOT is paying Kraemer about $58 million, within a broader Reconnect West Seattle budget of $175 million that includes engineering, surface-street safety and repaving jobs and reinforcement of the lower Spokane Street swing bridge.