Paul Kinderman, Washington State DOT
The Manette Bridge 303/4A replacement project replaced an 80-year-old bridge consisting of five truss main spans and eight plate girder approach spans across the Port Washington Narrows in Bremerton, Washington. The replacement bridge is 1,550 feet long, carrying two traffic lanes, two five-foot-wide shoulders, and one 12-foot-wide bike/pedestrian path.
The new bridge is a seven-span, continuous, prestressed/post-tensioned splice girder design, supported by two-column bents founded on drilled shaft foundations. The superstructure consists of five typical 250-foot spans, with end spans of 140 feet and 160 feet. The superstructure design included a unique post-tensioning layout and sequence, which is span-by-span, and staged to allow roadway deck slab placement in some spans, resulting in unique opposing tendon anchorages in the center of the hammerhead segments at the intermediate piers. In addition, the design of the prestressed segments included development of custom I-girder sections that included a parabolic haunch that varied in depth from 6 feet to 12.5 feet, with some segments weighing up to 306,000 lbs (heaviest ever produced by Conc. Technology Corp.). Other significant design features include precast shaft caps founded on 12-foot diameter drilled shafts.
The bridge is set in a small town with an historic United States Navy ship yard. The look of the new bridge was driven by architectural details provided after lengthy public input. The main architectural feature is the parabolically haunched precast girders that make up the bridge superstructure. Other details include compass rose motifs on the piers which reference seafaring navigation. The deep green colored railing is chosen to recall the old replaced steel truss. The columns are classic forms rising outboard of the superstructure to embrace pedestrian overlooks. Girder closures are highly detailed with nautical themes and traditional looking brackets.
Key Design Challenges
Bridge aesthetics were very important for this bridge because the surrounding neighborhood had a strong sense that the existing bridge defined their community. They were adamant they did not want a typical “highway” bridge and strongly resisted chorded haunches like a nearby bridge. The budget for the replacement bridge did not allow for a truly signature bridge. The spliced parabolically haunched precast girders provided an esthetically pleasing structure for a reasonable cost.
The new bridge was constructed approximately 3.0 feet from the existing bridge while the existing bridge was kept open to traffic. The exception was on the west end, where the new bridge overlapped the existing bridge. This overlap meant the existing bridge had to be closed to traffic before the new bridge was open. The use of precast girders allowed this closure to be minimized.
Innovations/Accomplishments
The precast spliced girders utilize a truly parabolic shape for the girder soffit. The top of the bottom flange also has a parabolic shape. This results in a varying web and bottom flange thickness over the entire length which adds to aesthetics of the bridge.
Each girder segment type is identical to each other. For example all of the 24 hammerhead girders are the same. Each of the drop-in girders are also the same. This allowed economy in forming the girders.
The hammerhead girders are the largest single product produced by Concrete Technology Corporation (by weight).
The hammerhead girders, have opposing P.T. anchorages at each pier. This allowed each span to be post-tension individually which reduced losses and allowed each girder to be the same. This also allowed most of the bridge to be post-tensioned and deck placed before constructing the last span which overlapped with the existing bridge. This reduced the bridge closure time for the community.