It looks simple, but it’s not. Complexity of Transbay...

San Francisco’s new transit center has a clean, unified look, with a rippling white form that rests atop clusters of diagonal columns. In reality, it’s three buildings in one that stretch for 1,500 feet and twice span major roadways.

Those spans are supported by just four massive beams.

Two of them have cracked.

The cracks were discovered Tuesday and prompted the closure of the $2.16 billion structure for safety reasons. While it is too early to determine the cause of the fissures, the fact that they exist is a reminder of how complex this simple-looking structure actually is — and how many things, potentially, can go wrong.

“This is kind of like a building and a bridge all in one,” said Leo Panian, principal of Tipping Structural Engineers in Berkeley. “When you have a hybrid structure, that can lead to uncertain effects and stresses that may not be anticipated during design.”

The cracks are near the bottom of the two girders that span Fremont Street and do double-duty work — bracing the rooftop with its 5.4-acre park, and supporting the bus deck below. Though tucked out of sight above the bus deck’s ceiling panels, they’re immense, measuring from 5 feet tall on the far ends to a point in the middle that’s roughly 8 feet from bottom to top.

Think of a very broad V.

At that midway point, where each girder meets a single column that in turn supports the bus deck below, is where each of the cracks occurred.

The largest was spotted by workers as they happened to be installing ceiling panels nearby: a crack running clean through the bottom of the girder closest to Mission Street, 2½ feet long and 4 inches high. The girder to the south also has cracks, though not as severe; they were discovered during a Tuesday night inspection by engineers involved with the original design.

No cracks were found on the two girders, similar but even longer, that span First Street. More testing continues in an attempt to determine the cause of the stress, and work has begun to shore up the transit center at Fremont Street. The center is closed until at least Friday.

What makes the situation so worrisome is that the cracked girders are the primary means of support for the transit center as it spans Fremont. The only things below the bus deck are sidewalks and asphalt.

“You’ve got a bus deck hanging from a column that in turn is held up by two cracked beams,” said Joe Maffei, founder of San Francisco’s Maffei Structural Engineering.

As with all the engineers interviewed for this story, Maffei stressed that he has not made a personal inspection of the site. Still, he said, “I think we are lucky that there was not a collapse.”

Panian agrees the cracks are in a particularly vulnerable spot.

“Those two girders are working hard, spanning a good length and bearing a heavy load,” Panian said. “And the place where it’s expected to carry the most load is where it is cracked.”

The Transbay Joint Powers Authority, which oversees the transit center, did not return calls for comment.

What may have helped prevent disaster is that the two girders don’t hold the span in place all by themselves. They’re paralleled by smaller, more conventional beams on either side that connect to the transit center structure on the east and west.

Also helping out are the unusual diagonal columns that ring the transit center and flare out to cradle the structure. They add support at either end of the span.

Finally, thick beams and concrete at the roofline should provide horizontal bracing at the top of the transit center, immediately below the park.

If the spans are unorthodox, so is the transit center’s overall design.

The 1,500-foot-long, three-story-high structure that runs from Beale Street nearly to Second Street is longer than the adjacent Salesforce Tower is tall. It’s also as deep as it is high: Beneath the two subterranean levels reserved for a future train station there’s a 5-foot-deep concrete foundation attached to 1,800 micropiles. At two points along the way, yard-wide seismic joints provide wiggle room — literally — in case a major earthquake hits.

Yet engineers say the unique design does not defy structural norms. It’s aggressive, not reckless.

Status of repairs Work is to begin Sunday on stabilizing two cracked beams at the Transbay Transit Center in San Francisco, officials said. Crews have been working for the past several days to design, acquire and build materials and steel beams before the initial stabilization can begin, Christine Falvey, a spokeswoman for the Transbay Joint Powers Authority, said Saturday. The work comes after two cracks were discovered last week in two large steel beams over Fremont Street, resulting in the closure of the transit center and part of the street. Temporary steel columns will be placed under the cracked beams and will run through the bus deck onto Fremont Street. Engineers are still working on plans to brace the structure before designing a permanent fix. The temporary beams will be removed once the new design is installed. The transit center and Fremont Street will remain closed at least through this week. — Sarah Ravani, sravani@sfchronicle.com

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“It is a one-of-a-kind structure, but the elements involved — we (the profession) use them all the time,” said David Friedman, a senior principal at Forell/Elsesser Engineers. “There’s no reason why this approach wouldn’t be sound.”

Then what happened?

The most troubling scenario is that the design by Thornton Tomassetti structural engineers and Pelli Clark Pelli Architects didn’t adequately account for the twin pressures of a bus deck and rooftop. Especially a rooftop that holds a lavish park as well as elevators and mechanical structures, a cumulative load that the magazine Engineering News-Record in 2015 likened to that of a four-story building.

As for the bus deck, the vibrations put a different sort of pressure on the girders than does the more static park above.

“The fact that the (main) crack is so close to the support column is very alarming,” Friedman said. “From a designer’s perspective, you need to carefully figure out all the loads so that there isn’t too much stress” over time.

But the problem might also be more limited.

The welding of the girder to the column, for instance, could have caused the steel alongside it to become too brittle. Steel is supposed to have some flexibility. If it gets too rigid, and at a location where stress is coming from several directions, cracks can be the result.

The girder itself could have been fabricated in a way that introduced small flaws that only now have emerged. While the steel in the project is American made, the size of the girder is such that no American steelmaker could forge it in a single piece. Instead, layers of steel plates were welded together.

The good thing about steel is that if the cracks are the result of localized flaws, adding extra strength shouldn’t be too difficult. Maybe more plates will be added, or there will be a replacement of small amounts of steel and another round of welding.

“This is a solvable problem,” Maffei said. “And the pressures are confined” to two girders. “That’s good news.”

Greg Deierlein, faculty professor of structural engineering at Stanford University, said there’s no telling at this point what caused the cracking. But the size and complexity of the structure increased the opportunity for error.

The transit center was built to stand up to a powerful earthquake, beyond what is mandated for most city buildings — ironically, given the current closure, it is designed to remain operational in such a catastrophic event. This is part of the reason for the bigger and often uniquely fabricated beams, braces and columns.

“That’s not a cookbook approach,” Deierlein said. “It requires more thinking and more analysis.”

And with that, he said, is a small but increased chance of problems.

“It’s not necessarily an obvious mistake,” he said.

Chronicle staff writer Kurtis Alexander contributed to this report.

John King is The San Francisco Chronicle’s urban design critic. Email: jking@sfchronicle.com Twitter: @johnkingsfchron