Crossing the Delaware!
The Chesapeake and Delaware (C&D) Canal Bridge is the featured structure of state Route 1, a 74-kilometer (km) limited access highway in central Delaware. Known locally as the "relief route," the facility will address capacity problems existing in this corridor, including traffic volume changes anticipated for the next 20 years. The entire Route 1 will extend from Tybouts Corner on the northern end to the Frederica/Felton area south of Dover. The C&D Canal Bridge will carry the newly constructed roadway over the C&D Canal at Saint Georges.
A unique combination of contractor prequalification, design preparation, structural details, and precast concrete segmental construction was used to build this bridge. Most notably, the precast concrete delta frames and single plane of stay cables made the selected design very economical.
The precast concrete, cable-stayed structure is 1417 meters (m) long with an out-to-out width of 38.8 m. (See figure 1.)
The 45.7-m post-tensioned continuous spans are grouped into five distinct units 183 m (four spans), 228 m (five spans), 503 m (seven spans including the 228-m main span), 274 m (six spans), and 183 m.
The bridge consists of twin parallel roadways for northbound and southbound traffic. Each roadway is formed of precast, single-cell, concrete, segmental box girders. (See figure 2.) The segmental boxes are independent except for the 503-m main-span unit. Here, the two boxes are connected by a 0.25-m-thick transversely post-tensioned median slab and precast delta frames at the cable-stay anchorage locations. (See figure 3.) The initial traffic configuration consists of two 3.05-m shoulders and three 3.66-m traffic lanes in each direction. The structure is designed to accommodate eight lanes of HS-25 loading.
In accordance with the Federal Highway Administration's (FHWA) requirements for projects with estimated construction costs greater than $10 million, alternative designs were prepared for this structure. Both the concrete segmental design and the steel alternative are cable-stayed with a 503-m main span over the canal.
During design, reviews were conducted by the FHWA, Delaware Department of Transportation (DELDOT), and an independent design consultant. These reviews occurred at the 30-percent, 60-percent, and 95-percent plan submittals.
During the design phase, DELDOT conducted a preliminary pile installation and load test program to select the most suitable pile type for the project. Precast prestressed concrete piles, 610-mm square, were selected.
It was recognized during the design process that several areas of the project involved geometrically complex concrete portions that would include both conventional reinforcing and post-tensioning.
To resolve potential conflicts and encourage increased confidence by the bidding contractors, DELDOT had three-dimensional, integrated drawings of selected portions of the structure prepared and included in the contract plans. These drawings aided the contractor in preparing shop drawings for construction of the more difficult portions of the project.
During design, consideration was given to removal of the approach span pier closest to each pylon. However, studies showed that keeping the pier was more cost-effective and offered the additional benefit of decreasing the live load moments in both the main span and the pylons.
Prior to bidding, contractors were prequalified using a thorough prequalification/review process. Only those contractors selected through this process were permitted to submit bids and only for the alternative(s) for which they were prequalified.
The estimates for the concrete and steel alternatives were about $60.5 million and $59.7 million, respectively. Twelve bids were received. Seven were for the steel alternative, ranging from slightly more than $64 million to about $99.5 million. Five bids were for the concrete segmental alternative, ranging from almost $57.9 million to $69.9 million.
On April 1, 1992, the contract was awarded to the lowest bidder. Besides the prime contractor and its partners, DELDOT, FHWA, and the U.S. Army Corps of Engineers were involved in the bridge construction.
Foundations are supported on 610-mm square, precast, prestressed concrete driven piles with a specified design strength of 34.5 megapascals and a minimum ultimate capacity of 680 metric tons (t) three times the allowable load of 227 t. Typical piles are spaced at 1.2 m on centers and are either vertical or battered one to six. The main pylon/pier foundations each contain 130 piles, typically 30.5 m long.
The contract plans allowed the option of conventional cast-in-place construction of the piers. However, the contractor selected the precast option. By precasting the piers, the contractor was able to take advantage of labor rate differences and minimize the total equipment rental costs for construction of the piers. The contractor also decided to use an existing precasting facility and to ferry the segments by barge to the construction site. Starting with a notice to proceed in April, the contractor was able to cast the first segmental unit, a column segment, on July 28, 1992.
Using the precast box-column segments, 30.5 m of column could be built in one day.
One important feature of the project was the selection of the span-by-span construction method for the 45.7-m approach spans. This method has proven to be very efficient on several other projects, with as many as seven spans having been erected in seven consecutive days. Using this method epoxy-coated joints and an overhead erection gantry the contractor consistently erected one span every 3½ working days. This is equivalent to 1168 m2of bridge deck each week.
Approach spans consist of only three element types: the typical segment, the pier segment, and the deviation segment. All segments include identical exterior dimensions and differ only by the shape of the interior void. By maximizing the similarities between the element types, costs for labor and equipment were minimized, and production rates for each element were maximized. Reinforcement of the box girder segments and the transverse post-tensioning are similar for each element.
Precasting allowed many construction operations to occur simultaneously. Typically, tying of reinforcement cages, concrete placement and curing, transverse prestressing/grouting, column erection, approach span erection, final grouting, and cleanup operations were occurring concurrently on various portions of the project. This possibility allowed for rapid construction and project completion in the least amount of time.
Features of the Main-Span Unit
In addition to the precast construction discussed above, the 503-m main-span structural unit has several unique features.
The single plane of stay cables is permitted by incorporation of precast delta frames that allow the otherwise independent box girders to be supported in one central location. The delta frames serve both to connect the roadway box-girder segments and as the anchorage location for the cable stays.
The precast delta frames provide the benefit of allowing the same casting forms to make all portions of the superstructure, including the main span. This minimized both the cost of forms and forming labor. Labor was decreased by avoiding the learning curve that would have accompanied construction of a separate main-span cross-section shape.
The stay cable system incorporates two special features.
First, the stay cables are continuous from one deck level anchorage, up over the pylon, through a curved saddle in the pylon, and down to the other deck anchorage.
Use of the pylon saddle saved both materials and labor costs. The saddle eliminates half of the anchorages, wedges, and bearing plates. As a result, the continuous cable is prefabricated and installed into the sheathing in a single pulling operation. By installing only one cable rather than two independent cables, erection labor requirements in the pylon area are virtually eliminated. (These are the observations of Figg Engineers Inc., the project designer, concerning pylon saddles. These observations are not endorsed by FHWA; the FHWA position on pylon saddles is provided in Technical Advisory T5140.25, dated June 17, 1994, "Cable Stays of Cable-Stayed Bridges.")
Second, the use of a single plane of stay cables similarly decreased materials and labor costs when compared to a system using twin planes of stays along the exterior of the structure. The twin plane system would have twice as many anchorages and installing and stressing operations. Also, twice as many stressing jacks would be required for a twin plane stay arrangement.
It is estimated that a twin plane, stay-cable system would have required 23 percent more cable grout and 52 percent more weight of cable sheathing than the single plane system.
The post-tensioning strands of the cable stays are enclosed in a grouted steel duct with an internal helix spacer to provide long-term protection.
Another feature of the design included the use of the approach span construction method all the way to the main-span pier. This provided several benefits. First, the amount of bridge deck constructed with the efficient span-by-span method was maximized. Also, the resulting one-directional main-span cantilever construction offered more convenient access for manpower and equipment when compared to the traditional balanced cantilever method. In addition, the completed back spans provide a very stable structure to resist wind and other construction loads and to greatly simplify geometry control operations.
By using the unidirectional cantilever construction, the precast roadway, and delta frame elements, one cable stay and 236.5 m2 of main-span cantilever could be completed in one week.
The contractor was able to erect the entire first 114-m main-span cantilever, including the 16 supporting stays, in only 147 days.
Main-Span Construction Method
The erection of the cantilevers was accomplished through the use of two 181-t cranes. The track-mounted cranes erected the twin box-girder segments simultaneously, and the typical erection cycle was accomplished in four working days. The erection cycle consisted of first transporting the box girder segments to the rear of the cranes with segment haulers over completed portions of the bridge. In this way, the erection was done from above without disruption to shipping traffic.
Each crane erected two box girder segments (68 t each) in cantilever and a precast delta frame (43.5 t) prior to installing the next stay. The precast segments were secured with prestressing bars. Two phases of stressing were required for each stay during the erection of the cantilever. A final restressing of the cable stays was accomplished after main-span closure was achieved and the concrete barriers and wearing surface were placed.
Status of the Construction
One hundred percent of precast concrete segments have been completed, shipped to the project site, and erected. On April 3, 1995, the contractor erected the last main-span segments. Geometry control results were excellent with the cantilever tips aligning within 6.35 mm.
The final construction operations will include placement of latex-modified concrete overlay, parapets, and cable-stayed restressing/grouting.
Economy of bridge construction is achieved through the careful balance of efficient details, minimal quantities, and minimizing special equipment costs. All bridges, precast concrete segmental in particular, benefit from attention to these areas.
The C&D Canal Bridge is an example of how alternative designs, competitive materials, and innovative engineering combine to provide major transportation facilities at the least cost.
Mike Britt is a structural engineer in the Office of Structures for FHWA's Region 3 in Baltimore, Md. He is currently on a one-year assignment in Japan. His work on the C&D Canal project included reviewing the final design plan and the PS&E (plans, specifications, and estimates), conducting an in-depth process review of the segment precasting plant, monitoring of all major phases of construction, and presenting a paper at a national professional conference. He received both his bachelor's and master's degrees in civil engineering from Drexel University, and he is a licensed professional engineer in Pennsylvania.
W. Denney Pate, P.E., is the regional bridge engineer for Figg Engineers Inc. He was the lead engineer for the design of the C&D Canal Bridge. He has 14 years of experience in the design and construction of cable-stayed bridges. His previous cable-stayed bridge projects include the Sunshine Skyway Bridge in Tampa, Fla.; Varina Enon Bridge (I-295) near Richmond, Va.; Cochrane Bridge in Mobile, Ala.; and the Neches River Bridge near Port Arthur, Texas. He received his bachelor's and master's degrees in civil engineering from Auburn University.
Lou Triandafilou is the director of FHWA's Region 3 Office of Structures in Baltimore, Md. He has more than 20 years of service to FHWA, including assignments as division bridge engineer for Ohio and as assistant division bridge engineer for Massachusetts. He received two bachelor's degrees civil engineering and business administration from Rutgers University, and he has earned graduate credit for studies in structural engineering at Northeastern University in Boston. He is a licensed professional engineer in Ohio.