Defect found in new SR 520 Bridge pontoons

Updated:

ABERDEEN, Washington —

Damaged concrete in one of the first pontoons built for the new State Route 520 Bridge will mean a delay in construction and an unknown cost while workers fix the problem, the Washington State Department of Transportation said Friday.

The damage was spotted on the pontoons being cast in Aberdeen. Fixing the damage will delay towing the pontoons to Lake Washington until July or August.

“We have a plan, and by making these repairs and modifications before the pontoons leave Aberdeen, we can assure that the new SR 520 floating bridge will safely carry traffic for 75 years or more,” said Julie Meredith, WSDOT’s SR 520 program director, in a news release.

Each pontoon costs between $8 million and $9 million to build, but WSDOT said it’s too early to tell how much the repair work will cost.

Here’s the specific problem with the pontoons, as described by WSDOT:  

The damage was discovered May 11 after construction crews completed post-tensioning on a longitudinal pontoon measuring 360 feet long, 75 feet wide and more than 28 feet tall. Post-tensioning is a process whereby steel tendons are stretched through the top and bottom slabs of a pontoon to better strengthen the concrete box against the forces expected on Lake Washington once the bridge is complete and carrying traffic.

Inspections of the first pontoon after post-tensioning showed damaged concrete where there was insufficient steel rebar reinforcement to hold the tendons in place. Crews plan to detension the steel tendons, remove concrete and add steel rebar reinforcement needed to contain the steel tendons. New concrete will be poured in the damaged area and allowed to cure before the steel tendons are retensioned.

Three other pontoons – two longitudinal and one cross pontoon that measures 240 feet long, 75 feet wide and about 35 feet tall – face the same issue though no post-tensioning work has yet occurred. Those pontoons will undergo preemptive modifications before steel tendons are installed and stretched into position, said Dave Ziegler, principal engineer for WSDOT’s pontoon project.

More information on the SR 520 pontoon project can be found on WSDOT's website.