Thursday, October 8, 2020

Repairs mean closures, lane reductions on SR 99 Duwamish River Bridge

By Tom Pearce

If you follow the work that our crews and contractors do, you will notice that most of it is preservation – repaving highways, maintaining facilities, repairing structures, etc. This work protects the investments the people of Washington have made to build our highway system.

That's what's happening right now on the southbound State Route 99 Duwamish River Bridge – also known as the First Avenue South bridge – in Seattle. We started this preservation work after our crews noticed wear on the bearings of two piers during a regular inspection of the bridge. These bearings are critical as they allow the bridge deck to move up and down a little when traffic goes over the pier.
A worn bearing under a support beam for the SR 99 southbound Duwamish River Bridge
has created a gap, as the pen inserted in the opening shows.

Our bridge maintenance crews began implementing a temporary fix for these bridge bearings this past Wednesday, Oct. 7. We're now developing a project for a permanent repair.

Bridges 101
When you build a bridge, it needs to be strong to support the weight of whatever will cross it, but it also has to be flexible. Heavy loads add stress to the bridge. Hot or cold weather causes a bridge to expand or contract. We're not talking large movements, only fractions of an inch, but being able to move a little as conditions change means less stress than a rigid structure may endure.

Bridge bearings are the support for the bridge deck and floor system. The bearings sit on top of the bridge piers. Some bearings are fixed or pinned; others tilt or slide. Whatever the bearing, it allows the bridge to move while maintaining support.
In this case, the bridge bearings have worn to the point that the bridge settles a little when heavy loads go over it. The video above starts with a heavy maintenance vehicle parked over the bearing. As the truck moves off the bearing, you can see the deck rise slightly. In immediate terms, this isn't too big a deal, but it should be fixed as soon as possible. Left as it is, eventually it will become a big deal.

What's happening now
We have coordinated with the Seattle Department of Transportation on brief daytime closures of the bridge that began Wednesday, Oct. 7. These closures, which occur several times during the day, are about the same length as an opening for marine traffic. They allow crews to repair these bearings. During each closure, crews jack up the deck a little, put in shims – in this case a piece of metal to close the gap – to counter the settlement, then lower the deck onto the shims.

This sort of repair is good for several months, but it's not a permanent answer. Remember, the bridge moves. That could eventually cause the shims to move, which means we'd need to go back every so often to replace them. The bridge remains safe for travel, and we'll continue to monitor the temporary repairs until we are ready for the permanent fix.

A permanent repair in 2021
In early 2021, we'll have a contractor crew replace the worn bearings atop the piers. We're still designing how this work will take place, so we don't have all the details yet. Right now we're looking at a project that will require us to reduce the bridge to two lanes for about four weeks to replace cement and grout. This will eliminate the settlement on that side of the bridge. When one side is finished, it will take another roughly two weeks to do the same thing on the other side. This could change as plans are finalized.
Drivers have several alternatives if they want to avoid the brief closures
on the southbound Duwamish River Bridge.

The SR 99 southbound Duwamish River Bridge carries about 50,000 cars a day. Reducing its capacity by half is going to mean heavier traffic. Drivers can help by using alternate routes like Interstate 5, Tukwila International Boulevard, East Marginal Way and the South Park Bridge.

We understand that reducing highway capacity temporarily is a challenge for the people who rely on these roads. But regular maintenance and repairs help keep our highways in good condition, reducing the need for costly major projects later. We appreciate your patience!