Most stand-alone parking structures across the United States are designed and built using the same approach as the garage that partly collapsed while under construction Wednesday at a Miami Dade College campus in Doral: They are assembled on site from prefabricated concrete pieces hoisted into place like a giant Erector set.
The tried and tested method of precast concrete construction is fast and relatively inexpensive, making it possible to design, permit and build a multi-story garage on a tight budget in a matter of months.
But the assembly of heavy beams, columns and floor slabs must be choreographed and balanced precisely. Because the structure remains potentially unstable until the pieces are permanently connected, typically by welding or bolting them together once theyre all in place, experts say the process requires close supervision from contractors and engineers.
One false move, they say, and the whole thing can come tumbling down in a deadly domino effect, although such catastrophic collapses are uncommon.
Because of the way the floor slabs in one section of the Miami-Dade garage appear to have come down all on one side, while remaining attached to a side wall at the other some structural engineers and construction tradespeople said Thursday that investigators will look first at construction error, as opposed to faulty design, as a likely cause.
Alternatively, some say, the cause could also be design-related or linked to a defect in fabrication of a concrete piece that caused it to fail, undermining the integrity of the still-incomplete structural system holding up the building.
I would look at erection procedures thats probably the one question to ask first, said Mark Santos, a Miami structural engineer who has worked on numerous garage projects for Kimley-Horn and Associates, a major engineering design firm.
William P. Byrne, CEO of garage contractor Ajax Building Corp., which is building the Miami Dade structure, said at a news conference Thursday that there was as yet no indication of any potential cause.
That precise cause wont be known for months.
The federal Occupational Safety and Health Administration will investigate, along with the Florida Department of Business and Professional Regulation, state officials said. Ultimate blame will also likely be sorted out in the courts. The collapse is sure to produce a welter of lawsuits, finger-pointing by contractors and engineers, and possibly even conflicting diagnoses by forensic engineers hired by different parties.
That long list includes Miami Dade College, which under state law has its own building department and, with the aid of consulting engineers, conducts its own permitting, plan review and construction inspections; its general contractor on the $24.5 million garage job, Ajax; and the separate subcontractors responsible for designing the garage, fabricating the precast concrete, erecting the structure, pouring concrete and supervising the work.
Sometimes, similar collapses have been caused by a confluence of factors.
In October 2003, in one of the most notorious of such incidents, part of the Tropicana Casino parking garage in Atlantic City, N.J., collapsed while crews were pouring concrete to attach the structural pieces of the garage together, killing four workers and injuring 20. As in the Miami Dade College collapse, five stories of the Atlantic City garage fell on one side in a pancake pattern while remaining attached on the other, leaving perimeter columns and walls standing.
The photos are hauntingly similar, said Steven J. Smith, the Washington, D.C.-area based vice president of CTLGroup, an engineering firm hired to review the Tropicana collapse as well as the 2007 collapse of the I-35W Mississippi River Bridge in Minneapolis, Minn.
OSHA investigators eventually blamed contractors failure to provide adequate temporary shoring support while the concrete dried. They also concluded that steel reinforcements in the concrete were not properly attached to supporting columns, and said a last-minute design change might also have helped undermine the structural system. OSHA fined contractors and engineering inspectors on the job a total of $119,500.
The courts found the project contractor to be largely at fault. The case produced one of the largest awards in U.S. construction industry, topping $100 million.
In the Miami Dade building, Smith noted, a south-side wall consisting of columns and horizontal girders remained standing and looks reasonably intact, but the garage floors appear to have separated from the wall and dropped away.
That is what you would expect if there is a connection failure. In all structures where there is a failure, one of the first places we look to are the connections, the joints. Thats going to get scrutinized first.
Miami-Dade Countys veteran building director, Charles Danger, concurred. Speaking conjecturally, Danger said a garage floor at or near the top of the building could have fallen, striking and taking down the rest of the structure beneath it. Another possibility, he said, is that a crane operator could have hit a portion of the structure while installing a floor slab.
The whole section went like a domino, said Danger.
It could not be determined how much of the section that collapsed had been permanently tied together. Typically, the structure is held in place by its own weight, though temporarily braced or shored up with wires or cables, as it is assembled. Completed floors or sections are welded or bolted together, with the joints often also covered with concrete, but the bracing is supposed to stay in place until inspections are concluded, Santos said.
The still-standing but separated south wall from which the floor slabs fell away was wrapped in cables suspended from the arm of a large crane on Thursday. Byrne, the contractor, said that was the same position it was in when the collapse occurred.
Another question, Danger said, is how good the checks and balances on the job were. To ensure a building projects integrity, an independent threshold engineering inspector is hired typically not by the contractor but by the design team or the building owner to verify that proper procedures are being followed. Investigators may look at whether the permitting and inspection process carried out by the college was adequate, Smith said.
Byrne, the contractor, said Thursday he did not know what work was going on at the time of the collapse, nor whether the threshold inspector was on site.
Another potential source of trouble could be the plans for the garage, Smith said. Even when the design is solid, plans can sometimes be unclear or incomplete. Its the contractors responsibility to get designers to clarify the plans. But in the Tropicana garage incident in New Jersey, Smith said, the contractor failed to do so, another contributing element to the collapse.
Smith said having the college essentially regulate its own building system is unusual, though Santos said other public institutions across the country do so.
If there is something in the system here that is not functioning as it should, it will come to light as part of the investigation, Smith said.