The images of toppled, collapsed and collapsed concrete buildings after powerful earthquakes rocked Turkey and Syria last month are sobering. In Turkey alone, the shaking destroyed some 185,000 buildings, and the death toll in both countries has surpassed 48,000 people.
Could the Bay Area’s concrete buildings suffer the same fate when The Big One strikes?
Fortunately, experts say that California would not experience the level of devastation seen in Turkey and Syria. But there is a danger, and experts say some buildings erected before 1980 using inflexible or “non-deformable” concrete construction will collapse. The trick is figuring out which ones — and what to do about them — before it’s too late.
“I don’t think there’s any doubt there are risks,” he said Terrence Parette, a senior director at the engineering firm Wiss, Janney, Elstner Associates in San Francisco. “If the San Andreas Fault originated here and the shaking is as intense as what they’ve seen in Turkey – some of which has been extraordinary – then there’s no doubt some buildings will collapse. But I don’t think it is likely that they will all collapse.”
from California Sylmar earthquake with a magnitude of 6.6 on the Richter scale in 1971 exposed the vulnerabilities of non-deformable concrete structures. The roof collapsed and two buildings were destroyed at the San Fernando Veterans Administration Hospital, where 49 people died. At Olive View Hospital, three wings of the main building pulled away and collapsed.
Code revisions make concrete structures built after the 1970s sturdier. But there is no solid inventory of potentially unsafe concrete buildings built before that time.
The concrete coalitiona project of researchers, engineers, industry and government officials, reported in 2011 that an estimated 16,000-17,000 concrete buildings in California’s 23 most earthquake-prone counties predate modern seismic building codes that went into effect in 1980. That’s 3,200 in San Francisco, 1,300 in Oakland, and 363 in San Jose.
Coalition’s reports in 2013 on surveyed buildings in major Bay Area cities cited many well-known landmarks, including Grace Cathedral in San Francisco and Holy Names University in Oakland, as potential concerns.
But that doesn’t mean the ones mentioned are dangerous. David Bonowitz, a San Francisco structural engineer and co-author of the 2011 report, said it was an attempt, based on the age of the buildings, to make a “rough estimate” of how many concrete buildings would need more seismic work. can have.
San Francisco has just begun assessing the problem and is preparing an amendment regulation. But identifying vulnerable concrete buildings is difficult and not just a matter of their age, said San Francisco Chief Resilience Officer Brian Strong.
“Just because it’s a concrete building doesn’t mean it’s vulnerable,” Strong said, adding that determining that could mean drilling into columns to see how they’re built. “It can be quite drastic. That’s the big challenge, and that’s why so few cities in California are doing this job.”
Holy Names said in December it will close its 65-year-old campus in May, citing $49 million in debt on the property, declining enrollments and deferred maintenance and compliance upgrade costs that could reach $200 million. Spokesperson Sam Singer said that while the school is currently compliant with seismic regulations, any changes to the campus and facilities will require costly upgrades.
Two of the 10 buildings at O’Connor Hospital in San Jose, now owned by Santa Clara County, are in the most seismic-vulnerable category. They were deferred for seismic work, but that is on track to be completed this summer.
“There are thousands of concrete buildings in California that wouldn’t meet our current standards for new buildings,” Bonowitz said, “but that doesn’t mean they’re like the buildings that collapsed in Turkey.”
Engineers blame the multitude of non-deformable concrete construction failures in Turkey and Syria more on a combination of flawed structure designs, lower quality concrete mixes, and inadequate regulation and code enforcement.
But after the earthquakes in Turkey and Syria, the said the Structural Engineers Association of California “despite our global leadership in seismic safety” and commendable work in Los Angeles, Santa Monica, San Francisco, Berkeley, Oakland and San Jose to identify vulnerable structures, “these measures are not enough.”
“California cities are plagued by thousands of buildings at risk of collapse,” the association said, calling on public officials to “identify and adapt our existing vulnerable buildings.”
California has significantly improved construction methods over more than a century of earthquake experience. The state’s first seismic construction law followed the collapse of unreinforced physical schools in the 1933 magnitude 6.4 Long Beach earthquake.
The magnitude 6.7 Northridge and 6.9 Loma Prieta earthquakes have exposed the dangers of “soft storey” timber frame constructions, often seen in apartments, where large ground floor openings for garages and windows can be used. provide little support for the upper floors.
Because unreinforced masonry and soft-storey timber frame constructions are more prevalent and prone to defects in the United States and easier to identify and retrofit, government officials have turned their attention to them. But now that these adjustment programs are in full swing, more attention is being paid to non-deformable concrete.
Concrete, a mixture of sandy aggregate and cement, is the most commonly used building material, thousands of years old, and considered the strongest. The Romans used it to build their empire and perfect a recipe so enduring that many impressive works, such as the Pantheon, which dates from about AD 126, are still standing today.
Modern concrete relies on steel rebar for strength, and the size of that rebar and how it is used affects the earthquake resistance of the structure.
Cities in Southern California have been more aggressive than their Northern California counterparts in pursuing non-deformable concrete retrofits. Los Angeles passed a program in 2015 calling for retrofits over a 25-year period. Santa Monica and West Hollywood have also passed demands, and Los Angeles County, Beverly Hills, Burbank and Long Beach are exploring programs.
In the Bay Area, only San Francisco is working on a similar retrofit program. Strong said that with 95% unreinforced masonry and 90% wood-framed retrofit, “the next step was to address these concrete buildings.”
The municipality put together a working group last autumn and expects to have submitted its advice to the Supervisory Board by the end of this year.
San Jose “focuses on a soft-story retrofit program that we plan to present to our city council in June with a recommendation that it should be a mandatory program and that it includes research into incentives and funding mechanisms,” said Cheryl Wessling , an information society officer in the city’s planning and construction department.
Oakland’s seismic renovation efforts are also targeting soft-story buildings, primarily multi-unit buildings, spokeswoman Jean Walsh said.
Bonowitz, a structural safety consultant for several cities, said he expects San Francisco’s burgeoning concrete building renovation program to prompt others in the Bay Area to follow suit.
“I, and my colleagues, would of course like to see things move faster,” Bonowitz said. “But as long as each community makes a good faith effort to reduce earthquake risk and plan for post-earthquake recovery at the community level, I will accept it.”