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 they’re 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 — that’s 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 won’t 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.