Work platforms are up for repairs to the West Seattle Bridge; Carbon fiber wraps and post tensioned steel to be added
Crews from Kraemer North America and the Seattle Department of Transportation worked together in the early morning on Monday, July 20 to get two work platforms hoisted into place beneath the West Seattle Bridge.
The platforms will allow workers to have direct access to the exterior of the concrete girders that over time have developed shear cracks forcing the bridge to close on March 23. The tasks ahead include applying carbon fiber wrapping, with high strength epoxy to the outside plus the addition of post tension steel cables, both in an effort to secure the bridge and halt the cracking.
If their efforts are successful they hope to be able to make a decision this fall as to whether or not to go further with shoring and reinforcement efforts to potentially open the bridge to at least some traffic.
Also up for resolution is the locked bearing at Pier 18. This neoprene bearing that allows the bridge to flex in wind and weather has locked and must be replaced. Kraemer North America will use precision concrete removal techniques to effect repairs.
Crews will use a core drill to install new holes to secure the steel beams that the post-tensioning strands will be anchored to. Once all of the holes have been drilled, they will begin to install the carbon fiber wrapping around the bottom of the bridge in areas where strengthening is required and inside some of the girders most affected by cracking. Once the carbon fiber wrap is set, they can begin installing the external post-tensioning strands. There will be some overlap of carbon fiber wrapping work and external post-tensioning installation.
The steel tendons will actually be inside the box girders, as noted in the diagram below. From the outside of the bridge, you will not see the steel tendons except for where they anchor to the bottom of the bridge.
All of the steel tendons will run along the inside of the bridge girders and will be anchored to beams that we will secure using previously drilled holes. Once the strands are in place, they will begin to tighten (post-tension) them to achieve the required tension that will support the bridge and, with the carbon fiber wrap, help slow cracking.
The initial carbon fiber wrapping will take approximately 10 weeks, with initial post-tensioning strand installation beginning a bit after the carbon fiber wrapping and completing a week or two later. Only after these initial stabilization measures are complete can the release of Pier 18 take place. Once Pier 18 has been released, we will go back and complete carbon fiber wrapping and post-tensioning.