Students: Terry Gryting, Kelinda Larson, Steven Lemmel, Gabriel Pierce, Sean Baski
Faculty Advisor or Community Project Lead: Osama A. Abaza
Client Organization: Alaska Department of Transportation & Public Facilities
Summary:
Twenty Mile Bridge No. 634 is located near Milepost 80 of the Seward Highway, and is classified as structurally deficient. The bridge deck is in poor condition and the load rating is less than the original design load. Pier walls are cracked and vertical rebar is exposed. The existing bridge was constructed in 1967 using steel girders and concrete pier walls and replaced the previous bridge, which was destroyed in the 1964 Great Alaskan Earthquake. The bridge underwent a Phase 1 seismic retrofit in 1999; however, it does not meet standards for a design seismic event.The existing roadway alignment contains horizontal curves on approaches to the bridge from the north and south (a “broken-back” curve). The Average Daily Traffic (ADT)is currently 5,760 vehicles per day through this portion of the Seward Highway, which increases during the summer months.
The Seawolf Engineering team researched design specifications and consulted with experts in the field, and then evaluated various design alternatives. The preferred alternative is a four-span bridge, with precast concrete decked bulb-tee girders and steel pipe pile extensions. The bridge design is 576 feet long and 54 feet wide, featuring accommodations for a shared use path along the east side of the bridge. The horizontal alignment features a single 4900-foot radius curve with a 3.2% superelevation, and a 0.5% longitudinal grade to provide drainage on the bridge. On the southern end, the minimum low-chord elevation is 33 feet, based on the high tide line elevation; while the northern end has a minimum low-chord elevation of 35 feet, to accommodate a shared use path passing beneath the bridge.The design improves safety by replacing the broken-back curve with a single horizontal curve, expanding parking facilities in the area, and adding accommodations for shared use pathways crossing both over the river and beneath the highway. The engineer’s estimate indicates a total cost of $18.4 million, which includes the bridge, roadway, and non-material costs. The project team produced a Design Study Report and a 35% plan set. The design meets American Association of State Highway and Transportation Officials (AASHTO) specifications, is cost effective, and minimizes environmental impacts.
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