Engineering experts tell NBC Bay Area the cracks found in the Transbay Terminal superstructure appear to have started at the sharp corners of holes cut into vital steel beams during construction – edges long understood to heighten stresses in any steel structure.
“It’s not a good structural element,” says mechanical engineer Bernard Cuzzillo, referring to rectangular notches clearly cut in the four-inch thick steel at the bottom of the 85-foot long I-beam used to support the terminal deck across Fremont Street.
Cuzzillo referred to one of the holes visible in the 2.5 foot wide strip, or flange, where a vertical steel plate is attached to the I-beam.
“To have a hole right there creates a weak spot right where the force is greatest,” says Cuzzillo, who studies why such massive structures fail.
“It looks a little ominous,” says David Williams, an engineer with four decades of experience building marine foundations and steel structures. He says the rectangular shape of the hole is what poses the danger.
“The sharp edge, the sharp corner, concentrates stresses,” he explained. “It’s just been left in a condition that really invites fracture.”
In fact, enhanced photos appear to show at least one of the cracks started at one of those sharp-edged corners. Normally, such edges would be rounded to prevent stresses from being concentrated at the corners, the engineers say. It is not clear why the corners were not rounded in the Transbay project or what was their intended purpose.
San Francisco Supervisor Aaron Peskin, who oversees the city’s transportation funding authority, says the latest problems have convinced him it is time to change management at the Transbay governing authority. He even suggested reconstituting entirely to create a new agency to manage outstanding construction to serve high speed rail.
“This has been a decade in the making,” Peskin said. “It’s cost $2.2 billion and we’ve lost public faith in the government and we’ve really got to show people that San Francisco can be the city that knows how.”
Shoring up the structure is ongoing and is expected to be completed by next week, with the authority relying on the same company that made the beams to provide the steel.