High winds damaged windows across four buildings in San … – The Business Journals

San Francisco’s Department of Building Inspection will consider making updates to city building codes after high winds downtown damaged at least four buildings Tuesday. 
“It’s definitely something we’ll be looking at,” spokesperson Patrick Hannan said Wednesday. “We are also discussing the best way to proactively ensure high-rise windows remain safe and stable. We have other programs for seismic safety, accessibility and facades — we haven’t settled on one solution, but are in discussions about what would make sense.”
Winds gusting to 77 miles per hour near the Ferry Building damaged windows at multiple buildings Tuesday:
That damage came exactly a week after a glass pane detached itself from the 43rd floor of 555 California St., one of San Francisco’s premier office buildings, amid a separate, earlier bout of high winds. 
Hannan said Wednesday the department is requiring the owners of impacted buildings to provide an evaluation of all the buildings’ windows by a licensed engineer or architect within 14 days to ensure they are secure. 
Shorenstein owns 50 California, built in 1972, while Kilroy Realty Corp. owns 350 Mission, completed in 2016. Millennium Tower, the city’s notoriously tilting building, was finished in 2009.
The glass design of high-rise buildings in San Francisco is governed by the city’s building code, which regulates both how engineers calculate what’s called “gust value” — what kind of sustained wind speeds buildings might expect to face — and how they secure buildings in the face of that wind, according to Gijs Libourel, who helms the West Coast facade engineering team for engineering firm Thornton Tomasetti.
Building codes, he told me, are based on probabilities: for instance, the probability that winds in California will not sustain speeds greater than, say, 115 miles per hour. But that’s not a guarantee that wind speeds won’t exceed that threshold, he said. 
“If, locally and only for a brief moment, like what happened in San Francisco with the bomb cyclone, the wind speed is much higher than what code anticipated, it is possible for windows to break, as clearly demonstrated by reality,” he told me. 
Asked what might have caused multiple windows in one building to crack or shatter, Libourel said there were three options: the wind load was significantly higher than calculated through building code; the glass was significantly less strong than anticipated; or some combination of both. 
Ron Hamburger, senior principal, director and western region SE head at Simpson Gumpertz & Heger, said under the current code, windows are designed to withstand winds of up to 90 mph — up from a 2009-era standard of 70 mph. The industry standard targets a probability of not more than eight of 1,000 windows failing when subjected to those winds.
“Sounds like a pretty small number, but then think about how many thousands of individual windows there are in all of the tall office buildings in San Francisco,” he said. “It’s not such a small number, right?”
Jeff Corvi, a vice president at facility services company Metro Services Group who has worked in building maintenance in San Francisco for the last 25 years, described Tuesday’s weather as “highly unusual.”
“We just don’t get winds like this,” said Corvi, whose firm is working with some of the impacted buildings to repair the damage. “There’s been instances where windows break, but it’s very infrequent — not like what’s been happening recently.”
Scientists say California — and the globe — should prepare for a world in which extreme weather events become the norm. Libourel said regular updates to California Building Code do take into account changing weather patterns, including wind speeds, but that he’s not yet seen data that would suggest that wind speeds had changed materially enough in California as of late to require radical change.
Still, he said, San Francisco could move to alter local building code on its own. It could usher new construction buildings toward laminated glass, which will crack but not fall to the ground, or ask the owners of existing buildings to reinforce their windows, he said. Current events often present elected officials with motivation to make such changes. But that most often happens after tragedies like the magnitude 7.8 earthquake that roiled Turkey earlier this year, he said, not typically wind events, because wind-inflicted damage to buildings, though often frightening, does not usually result in fatalities. 
“It could,” Libourel said, “I’m not dismissing it. But it’s a different level of life safety than seismic is.”
Libourel said the damage around downtown Tuesday could have been the product of a perfect storm — the same term Allen Nudel, principal at San Francisco-based structural engineering firm Forell | Elsesser, used last week to describe the burst windows at 555 California St. Wind storms often do things like fell trees, for example, that may have given no indication of weakness, he said. 
“You get unusual wind events like this, and you have to hope for the best — unless you want to cut down every tree in your neighborhood,” he told me. Libourel echoed that statement Wednesday, saying the job of engineers, contractors and architects is to limit risk to “an acceptable level” — levels dictated by building codes. 
Reporter Laura Waxmann contributed to this report.
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