Concrete California style: high-rise construction in a seismic region.

Speed is often one of the defining factors in today's construction. In the June 2005 issue of CONCRETE CONSTRUCTION, we reported that New York City concrete contractors had taken this to the extreme--building high-rise concrete structures on a two-day per floor cycle. By contrast, California

concrete contractors building flat plate high-rise concrete buildings are able to achieve only about an eight-day cycle. What causes such a big difference? Are California contractors that much slower? Is it that laid-back California style in contrast to the fast-paced life in New York? Let's take a closer look at concrete construction in the high-seismic regions of California to see what's different.

Design in seismic areas

Watching a high-rise go up in Southern California, you immediately notice one big difference: reinforcing steel. "If you look at a Chicago concrete column versus a San Francisco column, the number of longitudinal bars may not be that different, but the ties are much, much closer," says S.K. Ghosh, one of the nation's leading experts on seismic design of concrete structures and president of S.K. Ghosh Associates, Palatine, Ill. "The reinforcement is tied up a whole lot more in high seismic areas."

Design of concrete buildings in the United States is dictated by the ACI 318 "Building Code Requirements for Structural Concrete." Chapter 21, Special Provisions for Seismic Design, "contains provisions considered to be the minimum requirements for a cast-in-place or precast concrete structure capable of sustaining a series of oscillations into the inelastic range of response without critical deterioration in strength." In other words, when an earthquake shakes the building, and the building begins to deform from the motion, it will continue to stand up. It may seem odd to think about it this way, but in an earthquake, concrete buildings are designed to become sort of flexible. According to Chapter 21, "As a properly detailed cast-in-place or precast concrete structure responds to strong ground motion, its effective stiffness decreases and its energy dissipation increases."

This flexibility is primarily accomplished with reinforcing steel, which Ghosh says "gives the concrete structure the ability to deform beyond the elastic stage while retaining gravity load-carrying capacity." But he cautions that this is not the place for novices. "The designer has to be aware of the requirements but also be experienced in using the requirements. In other words, in concrete frame construction, the detailing of the beam-column joints becomes critical. It can get so congested that the concrete can't get in so you have to think of it beforehand. In experienced hands, a concrete building can be built to 30 stories without problems, but in inexperienced hands even a 12-story building can get severe congestion problems at the beam-column joints. The contractor has to work hand in hand with the engineer because whatever the engineer has in mind has to be implemented at the jobsite and quite often the contractor would have a better idea how to do that. Or the contractor may want to speed up construction, and the engineer has to take that into consideration and still make sure the building meets code requirements."

Building for seismic forces

Placing and working with the reinforcing steel dominates construction in high seismic zones. "The detailing starts to add various levels of complexity," says Chris Forster, Morley Construction, Santa Monica, Calif. "Things like the lengths and locations of splices, or staggered