Project Overview
Outside of Charlotte, N.C., the majestic Swift Island Arch Bridge spans the Pee Dee River. Its unique history harkens back to events that occurred before it was built. Just a few years earlier, in 1922, a three-span, open-spandrel concrete arch bridge had been erected to cross the river. Then, to make way for a dam and hydroelectric plant, the crossing was flooded to create Lake Tillery. The new, higher-elevation Swift Island Arch Bridge was constructed to replace the original bridge, and the older bridge was destroyed. The original bridge’s demise provided valuable information on bridge construction and demolition. In what would become known as the “Battle of Swift Island Bridge,” the U.S. Army tried various techniques to demolish it. First, the bridge was overloaded with weight, then it was bombed from the air, and, finally, explosives were used to bring it down.
The new Swift Island Bridge opened in 1927, and a more modern parallel span was built down river in the 1970s. In 2005, the Swift Island Bridge crossing was closed temporarily, so the second span had to carry traffic in both directions. Limited to only one lane of traffic, the 1927 bridge was slated to be replaced in 2003 when the second battle of Swift Island Bridge began in an effort to save its historic properties.
To address structural deterioration and weight restrictions, the bridge needed to be widened and upgraded. The grand old structure was eligible for the Historic Register, and residents were keen on preserving it. Rather than replace the bridge and convert it into a bike and pedestrian facility, the project team opted to replace and widen the superstructure, using precast concrete girders, deck slabs, and fascia panels that replicate the original design. “The use of precast concrete helped to preserve the original arches rather than having to demolish the bridge and build a new conventional structure. This preserved an historic resource while providing significant cost savings for the North Carolina Department of Transportation (NCDOT),” says John Sloan, PE, North Carolina bridge program manager for AECOM. “To preserve the existing arches and piers without overstressing them, precast concrete added redundancy to the structural system and reduced bending moments in the original members in order to carry vehicular loads in a safe manner,” he adds.
The design featured precast and prestressed concrete box girders and deck panels, as well as mild reinforced precast concrete fascia panels. The spread box girders simplified construction, and a simple closure pour allowed the box girders to be continuous from pier to pier over the spandrel bents. This design established full flexural continuity of the girders before the precast concrete deck panels were placed on the bridge, which relieved loading and flexural demands from the arch ribs. The precast and prestressed concrete deck panels facilitated construction by serving as a stay-in-place form for the cast-in-place deck. Placing the precast concrete deck panels on the bridge ahead of the deck placement also helped relieve and balance the load on the arch ribs.
Deconstruction
“The bridge was first deconstructed down to its arches and rebuilt with a wider bridge deck, so its architectural character and detail remained intact,” says Kevin Fischer, NCDOT assistant state structures engineer, field operations. This process involved creating a four-dimensional model to accurately predict structure demands throughout construction and developing a construction sequence that would be beneficial for the original arches and piers. Those components had limited capacity, so a detailed construction sequence and structure articulation were provided to prevent overstress.
Further complicating schedule demands were the location of the structure and access limitations. “The remote location of Lake Tillery and the complex construction sequencing needed to avoid overstressing the arches warranted the use of precast concrete. Precast concrete beams reduced the construction timeline and improved constructability. The unique continuity details of the precast concrete beams allowed the elimination of several joints in the bridge to reduce future maintenance needs,” says Fischer.
The project required a delicate balance to meet all the project goals, which were to complete the project as quickly as possible to minimize the inconvenience to those traveling by boat or car; keep the bridge in balance to avoid damage to the arches or foundation; tread lightly across the beautiful Piedmont area and Lake Tillery; and preserve the historic character of the existing bridge.
The team made every effort to preserve the architectural character by using precast concrete fascia panels. These panels were specified to facilitate construction, provide a durable solution, and be aesthetically pleasing. “The historic preservation team was pleased that the bridge could be preserved, and they were very satisfied that the aesthetics captured the architectural character of the original bridge,” notes Sloan.
The original concrete arch foundation lives on through the efforts of the project team. “Not only did we save the department of transportation considerable cost and accelerate the completion of construction, but we also preserved the history of this structure,” Fischer says. ”It’s gratifying to see the results, which provide a safer passage for local residents as well as an historic aesthetic appearance.” |