Construction workers finished opening a large hole under the Transbay Transit Center’s bus deck on Friday as off-site crews began fabricating temporary columns they plan to bring in over the weekend to support two cracked beams.
A team of engineers and fabricators hoped to finalize the plans for a shoring system by early Saturday, so they can begin installing the columns “very soon,” said Dennis Turchon, the transit center’s senior construction manager.
“We will start seeing temporary supports in the next couple days,” Turchon said. “We have started off-site fabrication to minimize the process on-site.”
The columns will support the damaged beams running above the bus deck and under the transit hub’s roof-top park. The beams support the entire section of the building that crosses Fremont Street.
The temporary columns will rise from the street up through the hole to buttress the cracked beams. The design will go through a third-party peer review before workers begin installing the pillars.
Turchon would not estimate how much the work would cost, but he said it will be a “significant amount,” because a large team of designers and fabricators are involved in the project.
Workers rerouted electrical conduits, plumbing lines and other utilities Thursday and Friday to create an opening for the bracing equipment.
“We had many items in the way that had to be taken care of, and those are essentially cleared out at this point, so its clear now to do the installation,” Turchon said.
Officials with the Transbay Joint Powers Authority, which built and operates the facility, hope to reopen Fremont Street by the end of next week. They plan to place the columns in the center of the street so traffic can move around them.
As crews put in the quick fix to avoid a potential collapse, engineers are developing a plan to secure the more-than-60-foot-long beams permanently.
Workers discovered the cracks Tuesday as they were installing ceiling panels. That prompted officials to close the entire facility, along with the stretch of Fremont Street below it, as engineers scrambled to diagnose the problem.
The cracking is isolated to the two beams at the center of the span over Fremont Street, said joint powers authority executive director Mark Zabaneh. Another section of the building over First Street with an identical design to the cracked section has no signs of stress or cracking, he said.
The cracks run along the beams’ 4-inch thick bottom flanges. One crack spans the 2½-foot-wide flange, while the other crack only partially spans the flange. The beams are 5 feet thick at their ends and widen to 8 feet at the center, where they are welded to a column attached to the bus deck below. The cracks are a few inches from the welds.
The cause of the problem remains unknown. Engineers believe the cracking is either the result of a problem with the beams’ manufacture, a mistake with the installation or a failure in the building’s design.
Turchon said the joint powers agency has continued to monitor and test other areas of the structure and “no further issues have be found at all.”
Mayor London Breed has called for an investigation into the cracking. The agency said it will bring in outside engineers to conduct a peer review of the building’s design.
The $2.2 billion, three-block-long building in the center of the city’s new Transbay district opened Aug. 12 after some two decades of planning and construction.