The Transbay Transit Center is the pride and joy of San Francisco’s transit system. An innovative train/bus station featuring an organic façade and a rooftop garden, the center is also closed after being open to the public for less than a month, because of significant cracks in its support beams.

The Transbay Joint Powers Authority—the organization responsible for overseeing the center’s construction—is not yet sure why the support beams cracked. It will be working with its primary contractor and the structural engineering firm that helped create the design to try to determine exactly what went wrong.

Until they return their verdict, here are some of the facts surrounding the building and the beams, and some of the reasons they might have cracked.

The Building

The Center is a $2.2 billion bus and train station, informally called “the Grand Central of the West.” The terminal is meant to serve 100,000 customers per weekday, in a facility that includes a bus deck, a train station and a 5.4-acre rooftop park. The terminal opened this past August after being under construction for nearly a decade, with workers breaking ground back in 2010.

The center stretches three blocks and passes over two major roads, with the spans supported by four 60-foot-long I-beams. The beams are tallest at the middle of the roadway, where they are 8 feet tall, and narrow to five feet tall at both ends. In the middle, where the beams are thickest, they are each welded to a column that supports the bus deck below it.

On September 25, workers installing roof tiles discovered a crack through one of the supporting beams. The crack, 2½ feet long and 4 inches deep, went right through the bottom flange of the I-beam (equivalent to the bottom stroke on a capital “I”). It occurred at the beam’s tallest point, where it connects to the column.

After the company realized the extent of the cracking, it shut down the station, at 5 p.m. local time. An overnight safety inspection revealed a smaller crack in the structure’s parallel beam, but no cracks in the beams stretching over the other roadway.

The Steel

The structural steel work for the center was handled by contractor Skanska USA Civil West as part of a $189 million contract. The company oversaw the creation of approximately 22,000 tons of structural steel by over a dozen manufacturers across the Western U.S., including Herrick Corp. Both beams were made from U.S. steel manufactured at Stockton-based Herrick Corp at one of its California locations.

According to a 2016 Transbay Joint Powers Authority memo released by NBC Bay Area, Herrick had difficulty maintaining constant quality assurance testing on all the sites where the steel was being manufactured. Turner Construction, the project’s oversight firm, had come into the project expecting work shifts at two different plants, both with two eight-hour work shifts per day. Instead, the company was faced with work sites spread out over the Western U.S., many with two 10-hour shifts per day.

Because of the project’s federally mandated “buy American” regulations and the large amount of steel required, the project needed to use suppliers and fabricators from 19 different states. The distance between the manufacturing sites presented a particular challenge to the project, as the memo explained, “the locations were geographically spread apart and therefore, special inspection staff could not monitor two or more locations within the same work shift or area.”

The memo also indicated that the weight of the beams made inspection more difficult. “All of the structural steel members and connections for the new Transbay Transit Center are heavier and thicker than used in comparable conventional building design for steel due to the enhanced seismic as well as risk and vulnerability criteria,” the memo noted. “The increased steel member sizes and thickness require more welding passes (time) to join members and produce steel fabrication components. Therefore, the built-up steel sections require additional inspections than originally assumed during the construction planning and additional inspections than were contemplated.”

The unusual size of the beams may have meant that internal imperfections went unnoticed, or the shifting requirements and far-flung production locations may have meant that the steel wasn’t all at the required quality level.

But the project’s potential problems don’t end with its materials.

The Design

The center’s design, cocreated by Thornton Tomassetti structural engineers and Pelli Clark Pelli Architects, is unique in several ways.

First of all, it’s a hybrid structure, combining building structures with bridge-like spans over the roads, and community spaces with more industrial transit areas. Second, the building’s organic façade, “leaning” diagonal columns around the outside of the building, and conical skylight, mean that it’s both visually striking and structurally unique. Finally, the building has stronger-than-average earthquake protections in place, since the freeway that previously stood on the land had been rendered unusable by the 1989 Loma Prieta earthquake that struck the region.

All of these factors mean that the beams, braces and other architectural elements are larger and more custom-fabricated than with an average structure. “That’s not a cookbook approach,” said Greg Deierlein, faculty professor of structural engineering at Stanford University, in interview with the San Francisco Chronicle. “It requires more thinking and more analysis.”This unique design also had to undergo unforeseen changes as a result of material unavailability. The type of steel that was planned for the original design wasn’t available domestically, and the project—like all federally funded American infrastructure projects—had a “Made in America” requirement. Both the complexity of the structure and the quick turnaround time on design changes could have added room for error.

One disturbing potential error is that the building design didn’t properly account for or distribute the building’s weight. Speaking to ABC News, engineer David Friedman said that once the center’s other structures were loaded onto the beam, "new stresses may have exacerbated the possible fabrication flaws."

Friedman was troubled by the crack’s orientation. According to him, a horizontal crack running the length of a beam can mean there’s too much weight on it—and its placement. “The fact that the (main) crack is so close to the support column is very alarming,” he added. “From a designer’s perspective, you need to carefully figure out all the loads so that there isn’t too much stress” over time.

The Construction

There’s also the possibility that the construction process could have caused the cracks. The beams both cracked near the columns they were welded to, and improper or difficult welding could have caused or contributed to the cracks.

Welding can cause or contribute to cracks in several ways. Sometimes, the weld contains defects, and sharp defects can generate stress at their tips. But more often, it’s a question of temperature. Welding is a process that uses extreme heat followed by rapid cooling. If the cooling occurs too quickly, the metal can develop a hard and brittle microstructure. Likewise, if the cooling occurs too slowly, the grain size might be too large, reducing the material’s toughness. Finally, one part of the metal cooling faster than a neighboring part can leave residual stress, which can become cracks later on in the process.

All these problems could have been exacerbated by the fact that the column the beam with the larger crack was welded to was load-bearing, and the fact that photos show multiple beams welded to the steel around the crack (see the image below).

This would not be the first newsworthy construction failure for Skanska in the past few months. Earlier in September, it was reported that the company or one of its subcontractors installed used/noncompliant communications equipment during work on a transit extension. As a result, all the electronic components will need to be replaced, and the extension’s opening has been delayed. The mistake is still under investigation.

The Answer?

Over the weekend, crews brought six shoring jacks to temporarily support the structure. The jacks are in the process of being replaced by steel-pipe towers that will sit in the middle of the street below, closing the road's middle lane. Like the beams, the pipes are also made by Herrick.

Transbay spokesperson Christine Falvey said that the organization is attempting to open the street crossing by October 12, but that the transit park itself would probably be closed for at least three more weeks. Before the center reopens, the search team must perform metallurgical tests on the steel of the beams, taking off small slivers to test their properties.

The TJPA hopes that these tests will determine the cause of the beams' failure—not just so it can ensure that the building is safe to use, but so that it can also determine who should be responsible for the cost of these repairs. "The builder is responsible for it," Zabenah said in a recent public appearance. Once we know what the result is, what the cause is, responsibility and allocation [of cost] will be distributed to the responsible parties.” San Francisco Mayor London Breed echoed the statement, stating that, “someone needs to be held accountable once the cause is determined.”