The New Cooper River Bridge

Job Facts
Contractor: Tidewater Skanska Inc.
Subcontractor: B&B Welding Company, Inc.

Built 600 feet tall, made to carry eight lanes of traffic, and spanning some 1546 feet, the New Cooper River Bridge will be the longest cable-stayed span in North America. With a 100-year design life, the new bridge is designed to withstand an earthquake as powerful as the Great Charleston Quake of 1886, a tropical storm, the likes of Hurricane Hugo, and even to withstand a direct hit by an ongoing ship. In the words of its builder Tidewater Skanska, the bridge is quite simply “one of the most technologically advanced and challenging design/build projects in the world”.

Not to mention one of the most expensive. The total price tag-nearly one billion dollars.

To construct the base of the two main cable support towers — it’s very foundation — Tidewater Skanska called on B&B Welding Company. Our mission: to build four rebar template towers reaching 60ft above the bridge foundations. Each tower would set the correct alignment of the massive rebar lattice work for the remainder of the 400ft cast-in-place concrete towers. We went straight to work fabricating the towers, each measuring 26ft by 25ft at their bases, using wide flange and channel shapes.

To complicate matters, each template sloped 19 degrees away from the roadway. This slope created what would be the road’s opening between the diamond profiles of the 400ft tower while also tapering in width from their base dimensions. To visualize the geometry, imagine a tapering hollow concrete tower 2ft thick that demanded rebar near the inner and outer concrete surfaces of all four sides of the hollow tower. The template tower we created would not only maintain proper alignment of the vertical rebar, but would become an integral part of each tower when the concrete was cast. Each tower consisted of four 8” wide flange columns, compound mitered at each base plate and bolted together at three different heights by back to back 8” channel/compound beams that would either have an end plate and bolt to the column flange or “swallow” and bolt to the column flange depending upon their position in the tower.

“Getting everything at the right slope and converging at the right point was a real geometric puzzle,” says B&B’s Vice President Dennis R. McCartney. “With tremendous weight from the slope, the towers had to be rigid enough to keep from falling over.”

But fabricating the steel is one thing. Verifying the math is quite another. As always, B&B built smart. To verify the geometry, they devised a false work dunnage system in their shop yard to simulate the riverbed in the field. They hired a crane service to erect the towers on the premises. Once assembled, the towering structure stood twice as tall as the B&B building. The false work allowed Tidewater engineer, Jim Zhang, to inspect the slope and dimension in the yard (rather than on the jobsite). After careful inspection, Mr. Zhang gave his approval. The tower was then unbolted, knocked down and shipped in pieces to the Cooper River work site, where, on a barge, the pieces were set, by crane, on anchor bolts and bolted to the footer in each caisson of the bridge. Tidewater, then, poured 2-foot thick concrete walls.

Looking back on the job, Dennis takes pride in the shop’s work. “At B&B.” he says, “we started the main support towers for one of the largest construction jobs in the country, this 900 million dollar bridge, off on the right foundation.”