New bridge links Eastern Seaboard route

Dec. 28, 2000
The Chesapeake and Delaware (C&D) Canal Bridge, St. Georges, Del., is a major link for an essential north-south corridor on the Eastern Seaboard, known as Delaware State Route 1. The C&D Canal Bridge demonstrates that precast segmental cable-stayed bridges can be an economical and aesthetically pleasing solution in an area of the country where conventional steel bridges are common.
The Chesapeake and Delaware (C&D) Canal Bridge, St. Georges, Del., is a major link for an essential north-south corridor on the Eastern Seaboard, known as Delaware State Route 1. The C&D Canal Bridge demonstrates that precast segmental cable-stayed bridges can be an economical and aesthetically pleasing solution in an area of the country where conventional steel bridges are common. The bridge is the first major concrete segmental cable-stayed bridge structure completed in the Northeast, where it holds the record for the longest concrete span at 750 ft.

Two complete bid documents were prepared for the Delaware DOT (DelDOT)-a structural steel alternate and a precast concrete segmental alternate. The low bid was awarded to Recchi America, Inc. in December 1991 for a precast concrete alternate priced at $57,869,004. It was designed by the Figg Engineering Group who also provided the construction engineering inspection for the bridge. The use of two bid alternatives saved DelDOT $6.2 million with the second lowest bid being $64,021,455 on the structural steel alternate.

In designing the bridge, there were many challenges and considerations that had to be overcome. Construction required a minimal disruption of the navigational traffic or existing traffic on adjacent roadways while posing no damaging effects on the environment. It also had to be built quickly and economically, and be aesthetically pleasing.

The solution to these challenges was the design of a precast concrete segmental bridge 4,650-ft in length with a 750-ft cable-stayed center span. Twin parallel precast box girders-each 58 ft 8 in. wide-were designed to provide six lanes of travel with three 12-ft lanes and two 10 ft shoulders. This gave the bridge a total width of 127 ft. The main span unit is 1,600 ft long from expansion joint to expansion joint with a center span 750 ft in length, supported by a single plane of 16 cable-stays at each pylon. The main span provides navigational clearances of 138 ft vertically and 450 ft horizontally.

The 150-ft typical approach spans combine to make four, five and six continuous span units that comprise the approach bridge structure. The bridge has no horizontal curvature and only a slight 1,900-ft vertical curve with 3% tangent grades. The superstructure has a 2% transverse slope with no super elevation transitions.

The substructure is composed of precast box pier segments that were match cast in typical 10-ft lengths and weigh approximately 37 tons. The box pier segments are a voided rectangular shape and provide strong structural characteristics that lessen the dead load of the piers and therefore reduce the number of piles required to support the structure. Each box pier was erected using a land-based crane.

Precast elements allowed the approach superstructure to be constructed from above, thus protecting the environment while eliminating disruption of traffic on existing roads. The approach spans were constructed using the span-by-span method with a self-launching overhead gantry.

The spans are made up of 14 segments each 10-ft long, with two 6-in cast-in-place closure pours between them and the two 4.5-ft pier segments. Once all 14 segments were erected and the closure had obtained adequate strength, a system of external longitudinal post-tensioning tendons were stressed. The span was then self supporting and the gantry could be launched ahead over the completed span for the erection of the next span.

For ease of construction and no disruption to navigational traffic, the center-span was designed and built from above. The 1600-ft main span erection was completed using two schemes-the overhead erection truss and twin 200-ton crawler cranes.

The erection began at the pylons with the main span portion leading up to each pylon being completed with the use of an overhead erection truss or gantry. This scheme allowed a portion of the main span to be completed using the span-by-span erection method.

Erection of the center-span began on the south side of the canal with the twin crawler cranes simultaneously erecting the northbound and southbound girders. The south half or south cantilever of the center-span was built first, cantilevering out 375 ft over the canal. Each northbound and southbound girder of each cantilever has 37 9.5­p;ft and 10-ft segments with 6-in. cast-in-place closure joints at four locations to facilitate geometric corrections. On April 3, 1995, the last two segments were erected, and the cranes mobilized off the north cantilever bringing the two cantilever tips into perfect alignment. A cast-in-place concrete closure was placed and main span longitudinal tendons stressed to combine the two cantilevers.

A unique design feature of the bridge is the precast concrete delta frame. The delta frame serves two important purposes-first, it is a part of the cable-stay anchorage system that supports the main span, and secondly, it allowed the same size box girder to be used throughout the length of the bridge. This eliminated the need for expensive and time-consuming form changes for casting of the main span superstructure box girders.

A single pylon supports a single plane of cable stays, down the center of the structure. This design allows for the use of only half the number of cable stays and anchorages normally required by a bridge this long. It also reduced the design requirement for the main span foundation, making them smaller and more economical. The arrangement allows the cable stays, which are anchored in the precast concrete delta frames, to run continuously through a steel saddle pipe within the pylon. It also allowed the cable-stay stressing during construction to be performed from the delta frames at deck level.

The bridge deck has a 1.5-in. latex-modified concrete overlay that will provide an extra barrier of protection against the deicing salts used during the winter months. The use of long continuous units reduces the number of expansion joints required and thus reduces the future maintenance costs. A transversely post-tensioned deck, reduces the number of expansion joints and increases the durability of the bridge deck. In addition, provisions for future deck replacement were included in the design.

The bridge was completed on time with opening dedication ceremonies held on Dec. 9, 1995.

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