BRIDGE RESCUE: Valiant and buoyant

WSDOT’s extra effort keeps bridges—some floating—in shape

Bridges Article May 09, 2014
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The Washington State Department of Transportation (WSDOT) manages roughly 3,200 bridges and culverts longer than 20 ft in length. Washington also is home to four of the world’s floating bridges, 17 movable bridges and the double-decker Alaskan Way Viaduct along the Seattle waterfront. All told, 139 of these bridges are considered structurally deficient.

Safety is at the forefront of WSDOT’s bridge-management and inspection program. Most bridges are inspected every two years as required by the Federal Highway Administration. Some are inspected every six months. WSDOT has more than 40 trained engineers and technicians, including specialized dive teams to perform underwater work.

WSDOT’s program to remove bridges from the structurally deficient list utilizes a varied approach that emphasizes cost-effective methods ranging from painting, concrete-deck rehabilitation, bridge repairs and rehabilitation and scour mitigation to total bridge replacement. Contracts have been completed on 15 bridges, with another 10 contracts in progress.

Two of the largest structurally deficient bridges are part of active replacement projects. The S.R. 99 Alaskan Way Viaduct, built in the 1950s, developed significant damage following the 2001 Nisqually earthquake and was classified structurally deficient that year. 

Construction is under way to replace the bridge with a new bored tunnel at a cost of nearly $3.1 billion. The new highway also will feature surface streets and bridges. Much of the construction will be complete in 2015.

Another large project is the replacement of the 52-year-old S.R. 520 Evergreen Point Floating Bridge in Seattle. It is the world’s longest floating bridge. A severe storm in 1993 damaged it. Longitudinal post-tensioning rehabilitated it, but the bridge is still vulnerable to severe windstorms and earthquakes. The bridge is being replaced at a total cost of
$4.65 billion.

The state’s three other floating bridges also are classified as structurally deficient due to the condition of the anchor cables that keep them in place. These anchor cables are inspected by underwater divers and a remotely operated vehicle. In 2015, WSDOT will replace 42 cables on the three bridges.

Seventy-five of the 139 bridges are classified as structurally deficient due to the concrete deck condition. To address these concerns, WSDOT uses bridge management deck elements to measure the deterioration and maintenance patches. Once a bridge has more than 2% deterioration (patching or spalls) in the concrete deck, it is classified as structurally deficient. 

WSDOT developed a proactive concrete-bridge-deck program in the mid-1980s to rehabilitate and overlay selected concrete bridge decks before a complete deck replacement is required. As part of a repair, WSDOT typically specifies the use of a hydromilling machine to remove a minimum of 1?2 in. of the existing deck and deteriorated concrete. The entire bridge deck is then overlaid with a 11?2-in. modified concrete overlay. WSDOT has developed three different overlay mix designs with either latex, fly ash or microsilica. A contractor can select between these three options as part of a rehabilitation contract. 

WSDOT reports annually on the condition of bridges in the state. The June 30, 2013, report is available on WSDOT’s web page at www.wsdot.wa.gov/accountability/. R&B

About the author: 
<p>Wilson is a bridge engineer for WSDOT.</p>
Overlay Init