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San Francisco's new Transit Center has a clean, unified look, with an undulating white shape that rests on clusters of diagonal columns. In fact, these are three buildings on one that cover 1,500 feet and twice on major roads.
These spans are supported only by four massive beams.
Two of them have cracked.
The cracks were discovered Tuesday and resulted in the closure of the $ 2.16 billion structure for security reasons. Although it is too early to determine the cause of the cracks, their existence is reminiscent of how complex this simple looking structure is – and how much can potentially go wrong.
"It's a bit like a building and a bridge," said Leo Panian, director of Tipping Structural Engineers in Berkeley. "When you have a hybrid structure, it can lead to uncertain effects and unforeseen constraints in design."
The cracks are located near the bottom of the two beams that span Fremont Street and do a double job: reinforce the roof with its 5.4-acre park and support the bus bridge below. Although hidden above the ceiling panels of the bridge-bus, they are immense: they measure between 5 feet high and far ends to a point in the middle, about 8 feet deep.
Think of a very large V.
At this median point, where each beam meets a single column that in turn supports the bus bridge located below, is where each of the cracks occurred.
The most important workers were spotted while they were installing ceiling panels nearby: a clean crack crossing the bottom of the closest Mission Street beam, of a length of 2.5 meters and from a height of 4 inches. The beam to the south also has cracks, but not as severe; they were discovered during an inspection Tuesday night by engineers involved in the original design.
No cracks were found on the two beams, similar but still longer, that cover the first street. Additional testing is underway to determine the cause of stress and work has already begun to strengthen the Fremont Street Transit Center. The center is closed at least until Friday.
What makes the situation so worrying is that cracked beams are the primary means of support for the transit center, which runs through Fremont. The only things under the bus bridge are sidewalks and asphalt.
"You have a bus bridge hanging on a column, which in turn is held back by two cracked beams," said Joe Maffei, founder of Maffei Structural Engineering of San Francisco.
Like all the engineers interviewed for this story, Maffei pointed out that he had not conducted a personal inspection of the site. Still, he said, "I think we're lucky that there was no collapse."
Panian agrees that the cracks are in a particularly vulnerable place.
"These two beams work hard, cover a good length and carry a heavy load," Panian said. "And the place where he should bear the most load is where he is cracked."
The Joint Powers Authority Transbay, which oversees the transit center, did not respond to calls for comment.
What may have helped to prevent a disaster is that the two beams do not hold the span in place alone. They are parallel to smaller and more conventional beams on each side that connect to the transit center structure to the east and west.
Unusual diagonal columns that surround the transit center and light up to support the structure are also useful. They add support at each end of the term.
Finally, the thick girders and concrete on the roofline must provide horizontal bracing at the top of the transit center, just below the park.
If the litters are not orthodox, the same goes for the general design of the transit center.
The 1,000-meter-tall structure, which extends from Beale Street near Second Street on three floors, is longer than the adjacent Salesforce tower is high. It is as deep as it is high: under the two underground levels reserved for a future station, there is a concrete foundation 5 feet deep and 1,800 micropiles. At two points along the route, one meter diameter seismic seals provide room for maneuver – literally – in case a major earthquake occurs.
However, engineers say the unique design does not defy structural standards. It's aggressive, not reckless.
"It's a one-of-a-kind structure, but the elements at play – we (the profession) use them all the time," said David Friedman, senior director at Forell / Elsesser Engineers. "There is no reason why this approach is not wise."
And then, what happened?
The most troubling scenario is that the design engineers of Thornton Tomassetti and architects Pelli Clark Pelli did not sufficiently take into account the double pressure of a bus bridge and a roof. Especially a roof that houses a sumptuous park as well as lifts and mechanical structures, a cumulative load that Engineering News-Record magazine in 2015 compared to that of a four-story building.
With regard to the bus bridge, the vibrations exert a different pressure on the beams than the more static park located above.
"The fact that the (main) crack is so close to the support column is very alarming," Friedman said. "From the designer's point of view, you have to be careful about all the loads to avoid stress" over time.
But the problem could also be more limited.
The welding of the beam to the column, for example, could have made the steel next to it too fragile. Steel is supposed to have some flexibility. If it becomes too rigid and where tension comes from several directions, cracks can result.
The beam itself could have been made in such a way as to introduce small defects that have only appeared now. Although the steel used in the project is American-made, the size of the beam is such that no American steel manufacturer could manufacture it in one piece. Instead, layers of steel plates were welded together.
The good thing about steel is that, if the cracks are the result of localized defects, it is not difficult to add resistance. Maybe more plates will be added, or there will be a replacement of small amounts of steel and another solder cycle.
"It's a problem that can be solved," Maffei said. "And the pressures are confined" to two beams. "It's good news."
Greg Deierlein, professor of structural engineering at Stanford University, said it was not yet possible to tell the cause of the crack. But the size and complexity of the structure increased the risk of error.
The transit center was built to withstand a powerful earthquake, beyond what is required for most urban buildings. This is part of the reason beams, braces and larger columns, often made in a unique way.
"This is not a cookbook approach," said Deierlein. "It requires more thought and more analysis."
And with that, he said, there is a small but greater risk of problems.
"This is not necessarily an obvious mistake," he said.
Kurtis Alexander, editor of Chronicle, contributed to this report.
John King is the urban design critic for the San Francisco Chronicle. Email: [email protected] Twitter: @johnkingsfchron
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