A nine-month road closure will begin Aug. 10 on the Key Peninsula’s South Vaughn Road to allow Pierce County to demolish the Vaughn Bay bridge and build a new bridge in its place.

The road will be closed between 92nd Street KPN and the Key Peninsula Civic Center through April 2010. A 2.2-mile detour will divert traffic around the construction zone via Olsen Road KPN and 92nd Street.

The new bridge will be a 219-foot, single span, concrete girder structure with two 12-foot lanes and 6-foot shoulders. Stormwater treatment embankments will also be installed at several locations to treat road runoff.

The existing 5-span concrete bridge was built in 1966. In 2002, Pierce County Public Works and Utilities engineers determined that the bridge is vulnerable to earthquakes due to its simple spans-and-supports construction. The bridge beams are not restrained against movement, and the beam supports are not big enough to accommodate such movement.

In the event of a major earthquake or even a minor quake in the wrong direction, the bridge beams could slip off their supports, causing the bridge to collapse.

The bridge appeared to be in good condition otherwise, and the engineers’ first thought was to seismically retrofit the structure. But upon closer inspection, they discovered fairly extensive cracks that penetrated the thickness of the concrete bridge deck, as well as reinforcing steel that was also showing signs of degradation.

In 2003 the county began securing funds for bridge replacement. The single span design of the replacement bridge will meet current seismic standards.

Nearly all of $2.86 million project cost will be covered by a Federal Bridge Replacement Fund grant administered by the Federal Highway Administration. The remaining $137,000 will come from the county road fund.

For more information, visit http://www.piercecountywa.org/vaughnbridge .