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 splices over two or three floors, create problems in that as you set reinforcing steel cages they may be up to 60 feet tall. The numbers of bars, the size of the bars, if they are using couplers, if there are welded splices—all that adds time to the process. And the engineers don't want all the splices in the columns to occur at one floor, so they start to stagger them. If that happens over two floors, it isn't too bad but when you get to three floors, you're setting column cages that can be 40 to 60 feet high. So you have to try to support them in the air with bracing and cabling, which then creates other problems like blockouts through the intermediate floors just for those braces or cables.”
“The main factor in determining the required crane size on a project is the weight of the rebar cage,” says Hal Long, president of Largo Concrete, Tustin, Calif. Largo has just completed its work on the Segerstrom Concert Hall, part of the Orange County Performing Arts Center. On that project, and on the many others Largo has completed throughout California, the key to achieving speed equivalent to an eight-day cycle is organization. “We really try to maximize use of the crane,” says Largo's Ted Rebelowski. On one job we were able to lift eight column cages at once. We had to find ways to minimize our picks, because crane time was so valuable to everyone on the job-site.” Forster notes that sometimes a rebar cage gets so heavy that it can't be lifted, so “we end up setting a skeleton cage and then individually charging bars into that skeleton cage. All those things affect the cycle time.”
Getting everything into the forms is always an issue. “The formwork looks the same in California but trying to stuff a tie in there can be nearly impossible,” says Bob McCracken, with form supplier EFCO. “Many times it has to be engineered into the rebar system. Sometimes they put the ties in place and put the rebar around them.” Add to this the difficulties when ties need to go in a specific spot. “Sometimes to get the ties in you have to start moving bars slightly,” says Forster. “A lot of what we do is exposed architectural concrete, and they want tie patterns to be repetitious, so tie patterns end up being a driving force.”
Concrete for such congested construction is not particularly unique, but it must be fluid. Few contractors in California are using a true self-consolidating mix, though. “We've not used SCC much yet,” says Forster. “I'd like to, but the demand for concrete is so high that the ready-mix guys are already selling as much as they can make, so they don't want to fool around with it.” Mixes typically are proportioned for 5000 psi compressive strength (higher sometimes in columns), incorporate high-range water reducer, and use a maximum aggregate size of 3/8 inch to allow it to get between the bars.
One of the ways Largo handles all of the complicated bar assemblies is by converting the various sets of plans on a job into their own CAD system. “We focus heavily on CAD,” says Largo vice president, Ken Long. “We create a CAD drawing for anything we can think of that we might need. It serves as a pre-planning guide to show where the tower cranes will be located and where the pours will be placed around the cranes. On jobs with 80 to 100 men, finding the problems ahead of time saves us lots of money.”
Largo's CAD experts create 2-D and 3-D drawings. “For example,” says Ken Long, “on a foundation, every type of foundation is marked, site elevations are marked, step locations are marked. And we start by making sure the dimensions close out. We jokingly tell people that we scare developers into giving us projects since we are able to show where things don't fit. One time a developer told me he didn't need that stuff and asked how much he could save without it. I told him if you don't let us do the CAD drawings, then we would have to raise our price.”