The scenes of tilted, crumbled and collapsed concrete buildings after powerful earthquakes jolted Turkey and Syria last month are sobering. In Turkey alone, the shaking wrecked some 185,000 buildings, and the death toll in both countries has topped 48,000 people.
Could the Bay Area’s concrete buildings suffer a similar fate when The Big One strikes?
Fortunately, experts say California wouldn’t see the level of devastation like that in Turkey and Syria. But there is a danger, and experts say some buildings erected before 1980 using inflexible or “nonductile” concrete construction will fall. 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 question there’s risk,” said Terrence Paret, a senior principal with the engineering firm Wiss, Janney, Elstner Associates in San Francisco. “If the San Andreas fault went off here and the shaking was as intense as what they saw in Turkey — some of which was extraordinary — there’s no question some buildings are going to collapse. But I don’t think it’s plausible they’re all going to collapse.”
California’s 6.6 magnitude Sylmar earthquake in 1971 exposed the vulnerabilities of nonductile concrete construction. 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 toppled.
Code revisions make concrete structures erected after the 1970s sturdier. But there’s no solid inventory of potentially unsafe concrete buildings constructed before then.
The Concrete Coalition, a 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 quake-prone counties predate modern seismic construction codes that came into effect by 1980. Those include 3,200 in San Francisco, 1,300 in Oakland and 363 in San Jose.
The coalition’s reports in 2013 on buildings surveyed in large Bay Area cities listed many well-known landmarks, including San Francisco’s Grace Cathedral and Oakland’s Holy Names University as potential concerns.
But that doesn’t mean those listed are dangerous. David Bonowitz, a San Francisco structural engineer and co-author of the 2011 report, said it was an attempt to develop a “rough estimate” based on the age of the buildings of how many concrete buildings might need more seismic work.
San Francisco has just begun working on assessing the problem and is preparing a retrofitting ordinance. 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 can involve drilling into columns to see how they were constructed. “It can be fairly invasive. That’s the big challenge, and that’s the reason so few cities in California are doing this work.”
Holy Names in December said it will close its 65-year-old campus in May, citing $49 million in debt on the property, declining enrollment and deferred maintenance and compliance upgrade costs that could top $200 million. Spokesman Sam Singer said that while the school currently meets seismic regulations, any changes to the campus and its facilities would require expensive upgrades.
Two of 10 buildings at San Jose’s O’Connor Hospital, now owned by Santa Clara County, are listed in the most vulnerable seismic category. They were granted extensions on seismic work, but that is on pace to be completed this summer.
“There are thousands of concrete buildings in California that would not meet our current standards for new buildings,” said Bonowitz, “but that does not mean they are like those that collapsed in Turkey.”
Engineers blame the multitude of nonductile concrete building failures in Turkey and Syria more on a combination of flawed structure designs, poorer quality concrete mixes, and inadequate regulation and code enforcement.
But after the Turkey and Syria quakes, the Structural Engineers Association of California said “despite our global leadership on 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 with thousands of buildings at risk of collapse,” the association said, calling on public officials to “identify and retrofit our existing vulnerable buildings.”
California has greatly improved building methods over more than a century of experience with earthquakes. The state’s first seismic building law followed the collapse of unreinforced brick-and-mortar schools in the 1933 magnitude 6.4 Long Beach earthquake.
The magnitude 6.7 Northridge and 6.9 Loma Prieta earthquakes exposed the dangers of wood-frame “soft-story” construction, often seen in apartments, where large ground-level openings for garages and windows leave too little support for upper floors.
Because unreinforced masonry and wood-frame soft-story construction are more widespread and prone to failure in the United States and easier to identify and retrofit, public officials have focused their attention there. But with those retrofitting programs now well underway, more attention is turning to nonductile concrete.
Concrete, a mixture of sandy aggregate and cement, is the most widely used building material, dating back thousands of years, and is considered the toughest. The Romans built their empire with it, perfecting a recipe so durable many impressive works like the Pantheon, which dates back to around 126 A.D., still stand.
Modern concrete relies on steel reinforcement for strength, and the extent of that reinforcement and how it is used affects the structure’s earthquake resilience.
Southern California cities have been more aggressive than their Northern California counterparts in pursuing nonductile concrete retrofits. Los Angeles adopted a program in 2015 calling for retrofits over a 25-year time frame. Santa Monica and West Hollywood have also adopted requirements, and Los Angeles County, Beverly Hills, Burbank and Long Beach are exploring programs.
In the Bay Area, only San Francisco has been working on a similar retrofit program. Strong said that with 95% of unreinforced masonry and 90% of wood-frame soft-story retrofitting addressed, “the next step was to address these concrete buildings.”
The city assembled a working group last fall and expects to have recommendations before the board of supervisors by the end of the year.
San Jose is “concentrating on a soft-story retrofit program that we plan to bring to our City Council this June with a recommendation that it be a mandatory program, and that it include exploring incentives and funding mechanisms,” said Cheryl Wessling, an information officer with the city’s planning and building department.
Oakland’s seismic retrofit efforts also are focused on soft-story buildings, predominantly those with multiple dwelling units, spokeswoman Jean Walsh said.
Bonowitz, a structural safety consultant for several cities, said he expects San Francisco’s nascent concrete building retrofit program will prompt others in the Bay Area to follow.
“I, and my colleagues, would of course like things to move faster,” Bonowitz said. “But as long as every community is making a good faith effort to reduce earthquake risk and plan for post-earthquake recovery at the community scale, I’ll take it.”
