June 30, 2020

Kraemer starts stabilization work on West Seattle Bridge this week

SDOT image [enlarge]

New post-tension cables will be installed inside the box girders for additional support.

Crews with Kraemer North America are starting work this week to stabilize the cracking high-rise portion of the West Seattle Bridge.

Stabilizing the bridge is needed for either repairing or replacing the bridge, which the Seattle Department of Transportation is studying options for.

Kraemer last week moved staging equipment onto the bridge, including an under-bridge inspection platform that will be used for an operation to open up over 100 holes that were used by form traveler machines during the construction of the original bridge. These holes were filled after the bridge's box girder segments were connected decades ago.

Workers will use hydro-demolition — like a giant pressure washer — to erode the concrete from the holes. The inspection platform will reach under the bridge deck to gather runoff water from this operation in drums. That water will then be processed and sent into the city's sewer.

The resulting holes will serve as guides for cables fed by electric winches on the road deck to pick up work platforms from a barge below in the river. When the work platforms are secure, crews will be able to work from both the top and underside of the bridge.

SDOT said it could have core drilled these holes, but didn't want to risk cutting rebar or post-tension cables in the girders. The agency estimated that the hydro-demolition will take at least two weeks.

Crews on the movable work platforms will install carbon fiber wrap around the bottom of the bridge and inside some of the girders most affected by cracking. Initial wrapping work will begin as soon as late July and take about 10 weeks.

After the wrap is in place, crews will install steel tendons inside the box girders, tightening them afterwards to create additional support for the bridge. This operation is expected to take one to two weeks and will require core drilling new holes into the box girders to secure steel beams that will anchor the new cables.

Once these initial fortifications are made, crews will release bearings on Pier 18 that were stuck and then finish up wrapping and post-tensioning.

Inspectors earlier this year found lateral bearings on the east side of the bridge were compressed and bulging outward at Pier 18. Crews will have to jack up the bridge slightly to replace the bad bearings. Bearings allow the bridge to move slightly due to thermal expansion and traffic loads.

SDOT said it is moving forward with its plan to either repair or replace the bridge while the stabilization work is underway. It said recent analysis indicates that repairing the bridge may be possible, but officials still don't know how much repairs would cost, how long they would take, how many lanes could be restored, and whether they would last long enough to be worthwhile.

SDOT picked Kraemer in April to stabilize the bridge, which was closed on March 23 after inspectors found expanded cracking.