Cooper River Bridge:
The Cooper river Bridge, also known as the Arthur Ravenel JR. Bridge, is a cable-stayed bridge over the Cooper River in South Carolina USA, connecting downtown Charleston to Mount Pleasant. The eight lane bridge has satisfied the capacity of U.S. Highway 17 since its opening to replace two obsolete cantilever truss bridges. The bridge was built as a design-build project, meaning that one contract was signed to both design and construct the bridge. This meant that construction could begin even while the design was not yet finalised.
The bridge was built by a joint venture of two major construction firms operating under the name Palmetto Bridge Constructors. The joint venture partners were Tidewater Skanska of Norfolk, Virginia and Flatiron Constructors of Longmont, Colorado.
The construction joint venture hired Parsons Brinckerhoff to complete the design. For the sake of simplifying labour and equipment resources, Palmetto Bridge Constructors actually managed the building of the bridge as five separate projects (the two highway interchanges at either end of the bridge, the two approach spans, and the cable-stayed span) going on simultaneously.
The bridge superstructure is designed to withstand shipping accidents and the natural disasters that have plagued Charleston’s history. The span is designed to endure wind gusts in excess of 300mph (480 km/h), far stronger than those of the worst storm in Charleston's history, Hurricane Hugo in 1989.
Raising financial support for a new eight-lane bridge over the Cooper River was a struggle 20 years in the making, prolonged by the state's insistence that it could not afford such a bridge and by Charleston's reluctance to provide any funds for the project. Several proposals were made for a toll bridge, but the mayors of Charleston and Mount Pleasant objected. When officials revealed in 1995 that the Grace Bridge scored 4 out of 100 for safety and integrity, retired U.S. Congressman Arthur Ravenel, Jr. ran for the South Carolina Senate with a goal of solving the funding problem. He helped to establish the S.C. Infrastructure Bank and worked with local, state, and federal officials to create partnerships that helped to materialize the final funding.
The State Infrastructure Bank (SIB) budgeted US$325 million to accompany US$96.6 million from the Federal Highway Administration. The project did not become a reality, however, until the SIB agreed to commit to a US$215 million federal loan, provided that Charleston County would contribute US$3 million a year for 25 years, including an 8.33percent sales tax increase, to the federal loan, as well as yearly payments from the South Carolina Department of Transport (SCDOT) and State Ports Authority. The overall price of the bridge totaled around US$700 million.
The Design
Cooper River Bridge has two diamond-shaped towers, each soaring to a height of 175 metres. The total length of the structure is 4.0 kilometres with the main span stretching 471 metres between the towers making it the longest among cable-stayed bridges in the Western Hemisphere.128 individual cables anchored to the inside of the diamond towers suspend the deck 57 metres above the river. The roadway consists of eight 12-foot (3.7 metre) lanes, four in each direction. A campaign by locals eventually led to the addition of a 12-foot bicycle and pedestrian path to the design, which runs along the entire south edge of the bridge overlooking Charleston Harbor and the Atlantic Ocean.
Engineers also had to be mindful of the 1886 earthquake that nearly leveled Charleston. The Ravenel Bridge is designed to withstand an earthquake to approximately 7.4 on the Richter scale without total failure. To protect the bridge from uncontrolled ships, the towers are flanked by one-acre rock islands. Any ship will run aground on the islands before it can collide with the towers.
Construction
By the summer of 2004 the foundations for the towers and most of the piers were in place and the rock islands were completed. The steel and concrete towers began to ascend from the islands soon after. Originally, each of the towers was to be topped with a 50 foot multicolored LED "beacon," but public opinion caused this plan to be scrapped.
The fast-paced construction schedule led the contractor to use a self-climbing form system to build the towers. The formwork, supplied by PERI, provided a solution to meet the tight construction tolerances and provide safe access for workers labouring hundreds of feet in the air. The self-climbing system meant that the tower cranes did not have to spend time raising the forms after each segment of concrete hardened, and instead could be better used to haul material from barges below.
The first cables were hung from the towers in 2006-as a time-saving measure, this was done before the towers were wholly completed. Sections of the deck were built outward from each of the towers as more cables were hung.
The decks of the approaches were taking shape as well. Construction of part of the roadway actually occurred over the top of the old cantilever bridges, which remained open to traffic without interruption.
This was the critical path as PBC crews worked six days a week, with two 12-hour shifts, during the erection of both the towers and the main-span deck. The towers were built with cast-in-place concrete, while the main-span deck consists of structural steel framing and precast concrete panels constructed and barged in from Savannah, Ga. A latex concrete overlay was then applied over the precast panels.
The cable stays on the main span are state-of-the-art and have been studied by several other state Districts of Transport ( DOTs). Instead of a grouted stay, Cooper River's stays are wax-coated and sheathed in a polyethylene material. Maintenance issues were minimised because individual cables or wires can be replaced if any corrosion or other problem occurs. A single cable - which can hold more than 1 million lbs. - is composed of as many as 90 seven-wire strands twisted together. The protective white polyethylene pipes range in diameter from 12 to 20 inches.
Conclusion
This award-winning bridge replaced the two aging truss bridges over Cooper River and Town Creek, and increased the capacity from five to eight lanes of traffic. The project provides US$20,000 in college scholarships - two 2,000 USF scholarships per year for five years - to students from the impacted area pursuing transportation-related degrees.
PROFESSIONAL TEAM
Design: Parsons Brinckerhoff
Client: Parsons Brinckerhoff Quade & Douglas Inc.
Owner: State of south Carolina
Constructors: Palmetto bridge constructors
Supplier: Peri
Cablestayed portion: buckland & Taylor
