Stairs especially offer benefits, particularly in speeding construction. “Prefabricated stairs can give you immediate use of upper levels with no delays, so it aids the crews,” says Becker.
Bart Wallis, a partner in the structural engineering firm of Bliss & Nyitray Inc. in Miami which has worked on a variety of stadiums, finds other benefits. “Making forms and casting pieces in the yard is more economical and also provides more control than casting in the field,” he says. “It's a big advantage to have the seating bowl prefabricated and to bring it in after the superstructure is erected.”
In many cases, the lower seating bowl is left until later, so cranes have better access to the upper levels. Then the lower bowl is finished. “It can help get the project finished quicker,” Wallis says.The need for speed
Speed has become a critical factor, says CEG's D'Arcy, pointing to the company's work on the Cowboys stadium. “As soon as we do the engineering, they're out there erecting pieces,” he says. The goal is to have the project ready for the 2009 football season, which created a tight schedule. At its peak, the project had 11 tower cranes working. “Speed is a key ingredient, because owners want to start generating revenue quickly.”
Precast concrete walls and seats also provide durability, which is becoming more important, says David Landis, senior principal with the consulting-engineer firm Walter P. Moore in Kansas City, Mo. “With thousands of fans and concession carts bumping into walls, they want a durable material,” Landis says. “Owners also are becoming more sensitive to long-term maintenance costsas part of the expenses of a new stadium.” Concrete walls also help give a look of permanence and classical design, D'Arcy adds.
The dominant use of concrete and the shift of entrance stairs to ramps for Americans With Disabilities Act requirements provide sufficient safety and quick egress. Making ramps wider to speed access would cut into seating capacity, creating a balancing act. “Most of the security changes have occurred in procedures, not in design,” says Moore's Landis.Using new mixes
Although high-performance concrete (HPC) aids both speed and durability, precast producers don't typically use HPC for stadiums—at least as they define it today. But that's mostly a matter of the definition changing as precasters have increased their capabilities.
“We do 6000- to 8000-psi concrete that looks like HPC, but we don't consider that to be high-performance concrete anymore,” says Spancrete's Becker. HPC's definition has moved into the 10,000- to 14,000-psi range, which is not required for most stadium projects.
Landis considers any concrete over 6000-psi, with a water-cement ratio less than 0.4, as high-performance, so he sees more HPC being used on projects. D'Arcy notes that most designers consider concrete in the 7000- to 8000-psi range as HPC, and that is being used on projects. So whether HPC is used depends more on the definition than on the concrete itself.
Self-consolidating concrete (SCC) also is being used by more precasters, but not typically for seating units. “We have been using SCC for some time, but risers are difficult to make from it,” says Becker. “SCC flows so well, that it wants to run down out of the vertical element into the horizontal part.” But for other components, the smooth finish and lack of bug holes make it a compelling choice.