By TIM COOPER, P.E.
The new Sarah Mildred Long Bridge, which connects the cities of Kittery, Maine, and Portsmouth, New Hampshire, features precast, posttensioned concrete, segmental box girder approaches in a stacked configuration with the vehicle spans on top and the railroad spans below. This type of approach span meant fewer piers were required, a move that affords expansive vistas, lowered construction costs, and less environmental impact.
The roadway of the 2,434 ft long vehicular bridge comprises two 12 ft lanes (one in each direction) and two approximately 5 ft wide shoulders that accommodate bicyclists. The Portsmouth roadway approach comprises a 1,552 ft, six-span bridge unit, and the Kittery roadway approach comprises an 882 ft, four-span bridge unit. Span lengths vary from a 132 ft end span to a 320 ft interior span. The bridge is supported by cast-in-place, reinforced-concrete piers founded on a combination of spread footings and drilled-shaft foundations.
The 1,437 ft precast segmental railroad bridge provides a heavy rail line that serves the Portsmouth Naval Shipyard. The Portsmouth approach comprises a 786 ft, six-span bridge unit, and the Kittery approach comprises a 651 ft, five-span bridge unit. Span lengths vary from 69 ft end spans to 160 ft typical interior spans. The low profile of the 19 ft wide railroad bridge makes the additional piers barely noticeable.
A total of 355 precast concrete superstructure segments were cast off-site and transported 174 mi by truck to the site. This design allowed segments to be erected at multiple locations simultaneously with land-based cranes and a barge-mounted ringer crane. Once on-site, the segments were erected by the balanced cantilever construction method.
The substructure comprises cast-in-place, reinforced-concrete piers supported by spread footings (on or near land) and 10 ft diameter drilled shafts (in the river). Single drilled shafts were used to support the typical railroad bridge piers, and dual drilled shafts were used to support the vehicular bridge piers. There are three shared piers at the locations where the vehicular bridge lies directly above the railroad bridge. At these locations the railroad bridge is supported at the footing between two vertical columns that are integrated into the vehicular bridge superstructure above.
Permanent precast footing tubs were constructed off-site and floated into position over the drilled shafts for the vehicular and shared piers. Once the seal slabs were placed in these precast tubs, the water was removed to provide dry conditions for construction of the reinforced-concrete footings. Precasting the tubs reduced construction time and provided a safe working environment for construction of the footings. Typical cofferdam and seal slab construction were employed to build the spread footings located close to shore.
-Tim Cooper, P.E., is a project manager for FIGG Bridge Engineers Inc.
Civil Engineering, November, 2019, © American Society of Civil Engineers. All Rights Reserved