On the afternoon of Tuesday, September 25, 2018, Marc Benioff, founder and co-CEO of Salesforce, stepped on stage at the Moscone Center in San Francisco to deliver the keynote speech at Dreamforce, his company's annual conference. The event—a combined business meeting, marketing rally, and New Age retreat—attracted more than 100,000 people from around the world, closing off an entire city block.

Benioff had built Salesforce and its core product of cloud-based customer management software from a Telegraph Hill apartment into a $13 billion-revenue-​ a-year juggernaut employing 30,000 people worldwide, with 8,500 in San Francisco. Just a few days before Dreamforce, he'd sealed a deal to purchase the struggling Time magazine, prompting an admiring profile in The New York Times. Completing his apotheosis, September 25, 2018, was Benioff's 54th birthday. After his speech, he could return to his office in the 1,070-foot-high Salesforce Tower, the second-tallest structure west of the Mississippi, whose naming rights he'd purchased in 2017, and look down upon the Salesforce Transit Center and Park, his native city's new crown jewel.

Conventional wisdom warned against Benioff buying naming rights to the transit center. What if there was a wreck or derailment, chaining your brand's name to a disaster? But to Benioff, the potential payoff seemed to outweigh the risk.

Built at a cost of $2.2 billion, the Salesforce Transit Center and Park formed the cornerstone of the Bay Area's ambitious regional transportation plan: a vast, clean, efficient web of trains, buses, and streetcars, running through a hub acclaimed as the Grand Central Station of the West. Naming this structure—the embodiment of a transformative idea—could yield marketing gold for Salesforce. It also could make Benioff a household name on the level of Bezos, Gates, or Zuckerberg.

Benioff took the gamble in 2017, pledging $110 million over 25 years, with $9.1 million up front and the rest committed to supporting operations when the trains started running. For now, the train box sat vacant on the bottom level, awaiting a 1.3-mile tunnel connection.

The rest of the complex had been open for six weeks. Bus traffic was running through the terminal, cutting commute times to the East Bay by up to 20 minutes thanks to its direct ramp to the Bay Bridge. Visitors flocked to the sumptuously landscaped rooftop park, compared by many to Manhattan's famous High Line. The entire four-block-long, million-plus-square-foot structure formed a modernistic gem, environmentally sustainable, covered in an undulating white aluminum exoskeleton patterned by physicist Sir Roger Penrose. Suffused with natural light, the building featured striking, playful art everywhere you turned.

As he took the stage on his birthday at the Moscone Center, Marc Benioff must have been confident his gamble on naming rights had paid off. He couldn't imagine that at that moment, less than a mile away, the ambassadors trained to welcome the public to the STC were now frantically waving commuters away. Rather than Grand Central Station or the High Line, the Salesforce Transit Center and Park suddenly resembled the Titanic.

Earlier that day, workers installing panels in the STC's ceiling beneath the rooftop park un­covered a jagged crack in a steel beam supporting the park and bus deck. "Out of an abundance of caution," officials said, they closed the transit center, rerouting buses to a temporary terminal. Inspectors were summoned. They found a similar fracture in a second beam.

Structural steel is exceptionally strong, but given certain conditions—low temperatures, defects incurred during fabrication, heavy-load stress—it remains vulnerable to cracking. Two types of cracks occur in steel: ductile fractures, which occur after the steel has yielded and deformed, and brittle fractures, which generally happen before the steel yields. Ductile fractures develop over time, as the steel stretches during use, explains Michael Engelhardt, Ph.D., a professor of civil engineering at the University of Texas at Austin and chair of the peer-​review committee overseeing the STC's response to the cracked-beam crisis.

"Engineers can predict ductile fracture and make adjustments during design, such as redistributing the load among various parts of the structure," Engelhardt says. "Brittle fractures, by contrast, happen suddenly and release a great deal of energy. They're concerning. They aren't supposed to happen."

The cracks discovered beneath the rooftop park were classic brittle fractures. The tapered 4-inch-thick steel beams—2.5 feet wide and 60 feet long, with a horizontal flange on the bottom—undergirded the 5.4-acre park on the building's fourth level, and buttressed the roof of the bus deck on the second level. By themselves, the cracks formed a point of weakness with potentially hazardous consequences. But they also suggested the possibility of a larger crisis.

If two brittle fractures had appeared in the building's 23,000 tons of structural steel, couldn't there be others?

At the peak of the evening rush hour, the transit center that normally teemed with buses was summarily closed. Mass confusion, an epic traffic jam, and a stampede toward BART trains and Ubers ensued. TV crews reported live outside the STC, interviewing angry and bewildered citizens.

Engineers and officials at the Transbay Joint Powers Authority (TJPA), the agency managing the transit center, were trained to deal with emergencies, but this was especially shocking. The project had been built by some of the most respected firms in the industry. Pelli Clarke Pelli Architects conceived the design. Thornton Tomasetti, Pelli's collaborators on Malaysia's iconic Petronas Twin Towers in Kuala Lumpur, served as the designer and engineer of record. The Bay Area's preeminent contractor, Webcor/Obayashi, led the construction. Skanska, the construction firm behind New York's World Trade Center Transportation Hub and Oculus, won the $189 million subcontract to furnish the structural steel. And the Herrick Corporation, another California construction heavyweight, had shop-fabricated the girders in question, using steel flange plates supplied by two subcontractors.

There had been layers of inspection and code verification, including certifications of quality for the steel in the beams that fractured. In 2011, a year after workers broke ground on the STC, the TJPA had ordered a comprehensive review of its seismic design, which halted progress for 18 months. For a massive construction project in the heart of earthquake country, however, the time seemed well spent. After the reworking of the seismic plan, Fred Clarke, the project's lead architect, had declared the STC as "probably one of the safest buildings in the world."

At first, events moved swiftly after the cracks were discovered. To ensure safety and stability, 20-foot-high hydraulic jacks were installed to shore up the affected Fremont Street overpass. Crews stripped the fireproofing from the steel so engineers could begin inspection. Reporters arrived from CBS, The Wall Street Journal, and The New York Times. An Associated Press story cited the transit hub as the "...latest example of problems in a city brimming with homelessness and poor infrastructure."

Engineers conferred, contractors scrambled to dig out blueprints proving the problem wasn't their fault, and attorneys braced for lawsuits. Then the pace slowed, as officials realized that the two central questions raised by the fractures—what went wrong? and was the problem localized?—would take months rather than days to resolve. On October 4, the mayors of San Francisco and Oakland sent a letter directing the Metropolitan Transportation Commission to assemble an elite peer-review committee to oversee the investigation and repair. The agency selected Engelhardt to lead the effort.

Professionals in steel-fracture mechanics tend to learn from catastrophes. For Michael Engelhardt and many of his peers, the defining disasters included the 1994 Northridge earthquake in Southern California and the 1995 Kobe earthquake that devastated Japan. While Engelhardt was earning his doctorate in metallurgy at University of California, Berkeley, the 1989 Loma Prieta earthquake knocked down a section of the Bay Bridge, destabilized the Embarcadero Freeway in San Francisco, and disabled the city's aging Transbay Bus Terminal. These events precipitated the demolition of the freeway and terminal, and the eventual construction of the Salesforce Transit Center and Park in their place. Now, to square the circle, Engelhardt had been summoned back to the Bay Area to help rescue the project.

"Our job wasn't to decide who was going to get sued," he says. "Our job was to find out what went wrong, determine the scope of the problem, approve the fixes, and make recommendations moving forward." In assembling the review committee, Engelhardt made a point of including a welding expert. "In the world of structural steel," he says, "it's usually the connections and joints that tell the tale."

These points in a steel-formed building are also the potential weak spots, places where art and error come into play.

"For function and economy in large-scale construction, steel is quite possibly the best choice," says Amit Kanvinde, professor and chair of civil and environ­mental engineering at University of California–Davis. "The tricky part is making connections in steel construction, accounting for the various geometries and the changes brought about by welding during fabrication."

On December 13, 2018, Robert Vecchio, CEO of LPI, Inc., a New York City firm that provides forensic metallurgy services, rose to speak at a TJPA board meeting. The gallery was packed with city and state officials, reporters from local and national media outlets, construction and civil engineering professionals, and members of the public, all hungry for news about the Salesforce Transit Center, whose ignominious closure now stretched into its fourth month.

An internationally recognized expert in steel-fracture analytics who had worked on the breakage in the hull of the Exxon Valdez and the collapse of the Twin Towers during the 9/11 attack, Vecchio had been hired to determine the "root cause" of the STC's fractured beams. He was about to announce his preliminary findings to the board, and along the way provide a crash course in steel-fracture analytics. Seven weeks earlier, shortly after the transit center shut down, Vecchio's team had traveled to San Francisco to supervise the removal of core samples from the damaged beams and bring them back to the New York lab for testing. LPI technicians performed scanning electronic microscopy, Charpy V-notch testing, Rockwell hardness testing, tensile testing, and fractographic analysis, with representatives from the project's key stakeholders looking over their shoulders.

Now, in a concise PowerPoint presentation, Vecchio explained to the board that the cracks were due to a "perfect storm" of the three factors that Engelhardt says characterize brittle fractures: weakness in the metal, damage during fabrication, and the stress of load during use.