68 SF high-rises have potential flaws prone to crack in quake, report says

Construction cranes join the Transamerica Pyramid and 555 California Street (R) jutting through the fog at sunrise in San Francisco on October 20, 2013. San Francisco is seeing a boom in new housing and commercial construction alter the skyline.. UPI/Terry Schmitt less Construction cranes join the Transamerica Pyramid and 555 California Street (R) jutting through the fog at sunrise in San Francisco on October 20, 2013. San Francisco is seeing a boom in new housing and ... more Photo: Terry Schmitt / United Press International 2013 Photo: Terry Schmitt / United Press International 2013 Image 1 of / 18 Caption Close 68 SF high-rises have potential flaws prone to crack in quake, report says 1 / 18 Back to Gallery

A landmark report released recently that evaluates the seismic resiliency of San Francisco’s tallest buildings revealed a disquieting fact.

Dozens of high-rises dotting the city’s skyline — 68 in all — share a set of features that could render them particularly vulnerable when the next major earthquake strikes.

The buildings were all completed between 1964 and 1989, and all have steel skeletons. Beneath the buildings’ skin, the joints connecting the vertical columns and horizontal beams are fused together with a welding technique that experts now know is particularly susceptible to fracture during an earthquake.

The weakness of the welds wasn’t widely known when the Loma Prieta quake struck San Francisco in 1989. It wasn’t until January 1994 — after the Northridge quake in Los Angeles — that developers realized the welds were prone to crack in an earthquake.

Since San Francisco inspectors never checked for the same kinds of fissures after the magnitude 6.9 Loma Prieta, damage to the buildings’ frames could have gone undetected for decades.

That doesn’t necessarily mean the high-rises are unsafe, but city officials won’t know for sure until inspections are completed. City officials are figuring out how to implement new requirements to ensure that buildings that might have sustained unseen damage from the Loma Prieta quake get the scrutiny they need.

“We don’t want to cause panic that these are very dangerous buildings,” said Danielle Hutchings Mieler, a principal analyst in the city’s Office of Resilience and Capital Planning. Some of the buildings might have been privately inspected for insurance appraisals or when the building goes up for sale.

Plus, Hutchings Mieler said, it’s “unlikely that there is much significant damage — the shaking levels from Loma Prieta weren’t very strong in most parts of downtown.”

Still, top city officials, including City Administrator Naomi Kelly, say they want the buildings inspected using evaluation methods and repair standards developed by the Federal Emergency Management Agency. That was one of the many recommendations laid out by the tall-buildings report issued in October.

The report was written by experts from the Applied Technology Council, a nonprofit California research organization. It focused on buildings at least 240 feet tall, or around 18 stories.

The report was commissioned by former Mayor Ed Lee after revelations that the Millennium Tower on Mission Street was sinking and tilting to one side. The building’s troubles sparked fears about what could happen to it and similar structures in a major earthquake.

“San Francisco must continue to lead on seismic safety, especially in our tall buildings. Experts agree that if we inspect and repair buildings now, we improve safety and reduce damage tomorrow,” Kelly said in a statement sent by a spokesman. “The city should take the advice of professional engineers to ensure that tall buildings are safe for residents, workers and visitors.”

The U.S. Geological Survey estimates that San Francisco has a 72 percent chance of getting hit by at least a 6.7-magnitude earthquake by 2043.

The so-called steel moment-frame welds were in vogue by the mid-1960s, said Gregory Deierlein, a professor of civil and environmental engineering at Stanford who consults with the city and worked on the tall-buildings report. The welds were more efficient than using bolts and rivets, but, as the Northridge quake would later show, they weren’t always reliable when subjected to the stresses of an earthquake.

“These welds ... fractured in a brittle way,” Deierlein said. “The Transbay Terminal is a good example of a brittle fracture.” The $2.2 billion transit hub has been closed since late September, when cracks were discovered in two support beams. It’s not clear when the transit center will reopen.

The weaknesses in the moment-frame welds weren’t fully exposed until the 6.7-magnitude Northridge quake in Los Angeles. No steel-frame buildings collapsed in that earthquake, Deierlein said, but subsequent inspections revealed that those particular welds were prone to crack when the earth heaved.

“Frankly, Northridge pointed out that the whole profession was a little blindsided by this,” Deierlein said.

Currently, San Francisco’s building codes automatically require seismic upgrades when construction alterations affect two-thirds of a building’s floors, but that’s a rarity for high-rises. As a result, “even the most collapse-prone buildings almost never receive the scrutiny intended by the code,” the report said.

Exactly how city officials will go about altering building codes to require inspections for moment-frame weld damage in high-rises completed before the Loma Prieta quake in 1989 remains in question. Certain changes might be more effective as legislation, others may only require administrative changes, Hutchings Mieler said.

The city could, for example, require a moment-frame weld inspection when building owners get permits to perform renovations or other work at their property. And beginning next year, officials will begin amending the city’s building code as part of a process that happens every three years to ensure that local regulations comply with changes to state law.

“It’s obvious what the next step is for us — addressing the potential damage in these buildings,” Hutchings Mieler said.

“What we don’t have right now is a way to relay, ‘If you found X number of damaged welds, how does that translate into a trigger that would require an upgrade.’”

Dominic Fracassa is a San Francisco Chronicle staff writer. Email: dfracassa@sfchronicle.com Twitter: @dominicfracassa