Highway 99 will close in both directions this weekend south of the West Seattle Bridge.
The closure begins Saturday morning at 4:30 a.m. and stretches southbound from the Battery Street tunnel to south of the West Seattle bridge. That's where a 450-foot elevated section of roadway is supported by aging crossbeams and timbers.
"It was built in 1958 as a temporary structure and has held on for quite a while but it's due for replacement," said Broch Bender with the state Transportation Department.
The northbound detour begins at South Michigan Street but drivers crossing the West Seattle Bridge can still access northbound 99. A map provided by WSDOT shows the work area and detours.
During the weekend closure, Bender said crews will re-stripe the pavement near the old wooden structure for lane reconfiguration and a slower speed limit of 40 mph that takes effect Monday at 5 a.m.
To keep traffic moving, crews will replace the structure in two sections, with the project wrapping by the summer of 2014. The work will require periodic weekend closures.