ASPIRE Summer 08 - Aspire - The Concrete Bridge Magazine

The pontoons are heavily reinforced and posttensionedin all three principle directions.Tight tolerances on placement of formworkand reinforcement are required to maintainthe design height of the pontoon deckabove the water surface.The scene is the epitome of thePacific Northwest: evergreen trees, darkblue water, and majestic mountains. Butthe natural beauty of the Hood Canalhides a beast’s heart. As winter descendson the region, it brings icy rains, galeforce winds, and white-capped wavesthat blow and crash through the areaalmost unimpeded.At the northern end of the waterway,the Hood Canal Bridge spans acrossthe divide to connect the Kitsap andOlympic peninsulas, fluctuating inelevation daily with tidal shifts up to16.5 ft. Its elevated roadway, like bridgeseverywhere, allows drivers a more directroute to their destinations. Yet, thebridge is one of only 11 floating bridgesin the world. With a length of 7869ft—approximately 1.5 miles—the bridgeis the longest of its kind over saltwater.It hasn’t been an easy existence, either.The 1979 StormThe original Hood Canal Bridge’s westhalf sank in 1979 after less than 18years of service. With the wind blowingfrom the south and a very strong currentflowing from the north, the west-halffloating structure overturned at the mostexposed part of the canal. That half wasreplaced in 1982, but now the east half,completed in 1961, is reaching the endof its service life.Why Build a FloatingBridge?At the bridge site, the canal is up to340 ft deep. A concrete floating bridgeprovides a cost-effective solution forcrossing a channel with very deep, softsoils in a high seismic region. While ahigh-level structure was evaluatedduring design, the exorbitant costs forthe site conditions could not be justified.Not only must the bridge float in aharsh marine environment, it must alsopermit marine vessels to navigate thecanal. Essential hydraulic, electrical,and mechanical components housed inkey pontoons allow the bridge to openits 600-ft-wide draw span for marinetraffic. With a naval submarine baseto the south of the structure and themouth of the canal to the north, thisfunction is critical for national security.A total of 14 floatingpontoons are beingconstructed in fourseparate cycles inapproximately2-1/2 years in thegraving dock.Two pontoons incorporated precastsegments and closure pours to facilitateplacement of large, heavy embeddedmechanical components of the drawspan that required maximum tolerancesof 1/16 in. in 500 ft.Construction ProgressThe Hood Canal Bridge West-HalfRetrofit and East-Half ReplacementProject was started in June 2003 andwill be completed by the end of 2010.The new east half is expected to beoperational until 2084. To ensure this75-year lifespan, high-performancePrecast concrete floating bridge / Washington State Department of Transportation, Olympia,Wash., OwnerEpoxy-Coated Strand Supplier: Sumiden Wire Products Corp., Stockton, Calif.Bridge Description: 1.5-mile-long floating bridge with fixed approach spans and movable transition spans between fixed piers on land andstructures floating on tidal saltwaterConcrete Structural Components: Prestressed concrete pontoons; reinforced concrete anchors; two-column reinforced concrete piers;32-in.-deep and 42-in.-deep, precast, prestressed concrete I-girders; and precast, prestressed concrete stay-in-place deck panels topped with a cast-inplaceconcrete roadwayBridge Cost: $471 millionASPIRE, Summer 2008 | 17