A parking-deck structure under construction partially collapsed Feb. 14 in San Antonio, injuring two construction workers at the University Health System’s University Hospital site.

OSHA investigators yesterday began examining the site, and UHS is in the process of hiring a engineering forensics team to help determine the cause of the accident, says Mark Webb, president of facilities development and project management for UHS.

Footage from a nearby security camera caught the entire collapse as it occurred. It appeared to happen as two events, one immediately after the other, in a matter of seconds.

But it is too early to speculate on the root cause or causes, Webb says. “Anything else is purely speculation and rumor. We’d rather wait. And we want to be very transparent in all of this. We have to ensure confidence in our staff, patients the county.”

The 10-story, 3,000-plus-car garage is being built in three phases by a joint venture of Zachry Construction, San Antonio; Vaughn Construction, Houston; and Layton of Phoenix, as part of a $900-million system-wide expansion project by the public health system owned by Bexar County.

Just before the collapse, workers reported hearing a rumbling sound, and construction supervisors evacuated the site, according to Melissa Sparks with the San Antonio Fire Dept. “Thank God they paid attention,” she says. Some 120 employees were working on the site. Some had left the structure for a lunch break, she says.

“Excellent safety planning and quick response absolutely saved lives,” says Leni Kirkman, a spokeswoman for UHS. She agrees it is too early to speculate about the cause of the collapse.

The accident occurred at approximately 11:40 a.m. according to Sparks. Two workers were injured. One is stable and the other remains in critical condition at University Hospital, according to Julie Wiley, a spokesperson for the University Hospital. The fire department completed two searches, found no one trapped, and the construction firms were able to account for all of their workers.

“We’re concerned about the why and how,” says Vicky Waddy, Zachry spokeswoman. “But we are so pleased that the safety program limited the number of those injured.”

She says the contractors don’t know what caused the accident, and adds that Zachry is hiring its own engineering forensics team as are some of the subcontracors.

The collapse brought down concrete columns, which fell atop vehicles, construction equipment and storage units leaving behind approximately 1,500 tons of fallen debris, according to Sparks. The area is adjacent to UHS’ existing hospital as well as The University of Texas Health Science Center at San Antonio.

“Unintentional errors can occur when structures are under construction,” says Steven J. Smith, Ph.D., P.E., vice president of CTLGroup in Washington, D.C., which is not involved with the project. CTLGroup offers investigation and analysis of structures and materials. He adds that it is this time when structures are most vulnerable, because they are not self-supporting, and cast-in-place concrete will need temporary shoring and bracing.

The garage broke ground in March 2010 and was scheduled for completion this summer, with a portion of it open to vehicles. Hospital officials say a new section of the garage that opened last month was inspected, and aside from a small section that will require additional shoring, was found to be “structurally sound.” The fire department assessed the connecting parking deck to make sure the vehicles parked in it could be safely removed and then cordoned off the area.

Smith says the hospital’s faith in the existing sections of the garage indicates the owner had confidence in the final structure.

“That means there will be attention on the construction done and if the temporary configuration for the support was appropriate or if the temporary support was sufficient during erection,” Smith says.

Sparks says all work has been suspended, pending the investigation.

Webb says work will not begin on the garage again “until we are completely satisfied that we know about the cause” but some other, smaller projects will resume, he adds.

The Zachry-Vaughn-Layton team broke ground last month on the nearly 1-million-sq-ft hospital tower directly in front of the existing University Hospital.

Waddy says work will resume on other, multiple projects the contractor is building at UHS, work will resume. Zachry management has reconfirmed with its crews where “muster areas” are. “That’s where the work crews go so we can do a head count quickly in the event an incident.”

“That’s what happened Monday, and how we were able to quickly confirm that everyone was accounted for,” she says.