Teamwork and planning are the two key elements to any successful project. No company knows this better than Concrete Supply Co. of Charlotte, and Batson-Cook Co. of Atlanta, the general contractor for Wachovia's new Corporate Center in uptown Charlotte.
From early in the planning stages, a partnership was developed between Batson-Cook, Concrete Supply Co., and the structural engineering firm, TRC International, LTD, of Sarasota, Fla. During the first meeting between the groups, Don Davis, a consultant for Batson-Cook, asked, “What is the highest strength concrete you can produce?” Initial discussions focused on 15,000 psi concrete, but quickly changed to 16,000 psi and eventually 18,000 psi concrete as the building's design evolved.
The 52-story Wachovia Corporate Center is the focal point of the new mixed-use Wachovia Cultural Campus, which will include a 42-story condominium tower, art museums, and 1200-seat performing arts theater. The two towers, along with a nine-story building annex, sit atop an eight-story parking structure that extends 95 feet below street level. Wachovia Bank is moving its headquarters to the 1.3 million-square-foot building. Duke Energy and the Wake Forest University Babcock Graduate School of Management also will occupy parts of the building. It truly is our project of a lifetime.
Tommy Hagood, structural engineer with TRC International, explains that the building design is a punched tube system, with an exterior punched with windows that form columns 10 feet on-center and deep beams at the floorlines. This system is very rigid and acts as a shear wall and rigid frame combination. This eliminates the need for shear walls in the core area of the building and creates a more open design.
The core is designed only for gravity loads and it houses the vertical transportation for the tower. Both seismic and wind loads controlled the lateral design for the exterior columns.
Upping the ante
Planners selected a 16,000 psi reinforced concrete to minimize the size and number of columns in the tower. High-strength concrete was also required to achieve a high modulus of elasticity (MOE) in the exterior columns, greatly reducing the lateral movement of the tower. An 18,000 psi mix design was added to the project a few months after the owner requested a design change to the upper floors of the tower.
By increasing the concrete strength from 16,000 psi to 18,000 psi, the amount of reinforcing steel was reduced in many of the columns, while the column dimensions remained unchanged. This saved money and time, compared to using mechanical connections and increased column sizes. Batson-Cook also chose precast double-tees to accelerate the construction schedule. Double-tees topped with lightweight concrete are used through the 44th floor. This is believed to be the tallest structure using precast double tees.
Construction of the parking structure began in March 2007 and, as of January, more than 16,000 yards of 16,000 psi concrete and close to 1000 yards of 18,000 psi concrete had been successfully placed. It took months of research, several trial batches, and in-depth training to move us from the initial conversations that took place in 2005 to completing a 500-yard beam pour with 16,000 psi concrete in spring 2007.