Quake damage spurs calls for stronger designs
Seismic SAN FRANCISCO — safety experts long have warned that brittle concrete-frame buildings pose a particularly deadly risk during a major earthquake.
But a horrifying video taken during last month’s magnitude 7.1 Mexico quake may do more to highlight the risk than years of reports and studies.
In it, sirens blare, utility poles sway. Then in the background, a building wobbles. Concrete starts falling out of a ground-floor column. Then the columns flex, and the upper floors come crashing down, sinking into a cloud of dust.
“Dios mio! Dios mio!” a woman is heard saying. “My God! My God!”
The crumbled Enrique Rebsamen school in Mexico City — a three-story structure where at least 25 died, including 21 students — was made of concrete, as were many other structures that fell to the ground.
While they may be stout and muscular in appearance, concrete buildings without a robust level of steel reinforcement can see their columns peel off in chunks and then explode when exposed to violent side-to-side shaking.
Collapses of concrete buildings have been documented worldwide for decades.
In Los Angeles, dozens died when concrete structures tumbled in the 1971 magnitude 6.1 Sylmar earthquake. Several who perished were on a newly built hospital campus. And when two concrete office towers collapsed in 2011 during a 6.3 temblor in Christchurch, New Zealand, the 133 people who died accounted for more than 70 percent of the final toll.
After the Sylmar quake, officials quickly updated building requirements to add more steel reinforcement to new concrete buildings. But there was no systematic effort by many governments around the world to address the defect in existing concrete buildings.
Concrete buildings dot the California landscape, a popular form of construction during the postwar boom years. But cities are just now beginning to grapple with how to make these buildings safer.
In 2015, Los Angeles Mayor Eric Garcetti pushed through a landmark law mandating retrofits of concrete buildings, hoping to avoid a catastrophe when the next earthquake comes. The city estimates there are about 1,500 such structures.
The law requires that once owners are given an order to evaluate a building, they will have 25 years to retrofit it if a study determines the structure is indeed vulnerable. City officials are in the process of identifying buildings that would be subject to the law.
A couple of other cities have done the same.
Santa Monica earlier this year published a list of vulnerable buildings — concrete, steel and wood-frame apartments — and enacted a new law requiring them to be evaluated and retrofitted if found to be vulnerable. West Hollywood also has enacted retrofit laws for the same classes of buildings.
Garcetti and seismic safety experts say the catastrophic images from Mexico will raise awareness of the dangers.
“Any building owner who thinks they should sit back and relax for the next 20 years should view that video. And let’s figure out a way to get to work now,” Garcetti said in an interview. “What’s more expensive? The loss of your entire property — let alone the loss of lives — or the investment in making sure that no earthquake of that size will destroy your building or kill anyone?”
The collapsed school is a case in point. California-based structural engineers who looked at a photo
Weak versus strong concrete columns
of the school’s remains said the collapse was consistent with the failure of a brittle concrete building.
Structural engineer David Cocke, vice president of the Oakland-based Earthquake Engineering Research Institute, pointed out how a concrete column at the school can be seen broken in half — a clean break. He said there should have been more steel reinforcement in the concrete that would have allowed the column to bend when shaken, not break like a piece of chalk.
“When they break in half like that, then you’ve lost it all,” Cocke said.
Structural engineer Kit Miyamoto, a member of the California Seismic Safety Commission, said the photo “looked like the columns popped out of the building ... there’s no adequate reinforcement. It’s exactly the problem of nonductile (brittle) concrete.”
And the video showing the concrete building collapsing, Miyamoto said, has “such a tremendous impact. Most people think that they are helpless, it’s too expensive to fix. That’s a myth. This video can defeat that myth. Evidence exists, people are dying and we know exactly what to do.”
“Actually being able to physically see the process — I think it’s incredibly effective. It explains what a lot of the issues are,” seismologist Lucy Jones said. “Concrete buildings seem sturdy ... and being able to see directly why that’s not true has got to start.”
To be sure, some buildings in developing nations are not as well-engineered as some buildings in California, Cocke said. But “these buildings are not that dissimilar to some of our worst buildings. We’re going to have failures on some of our older, nonductile concrete buildings that can be catastrophic — when we have intense shaking.”
The video, Cocke said, also shows the threat of buildings with flimsy first stories, where relatively skinny columns hold up heavier upper floors. The so-called “softstory” flaw is found in many California apartments, where the ground floor is built to house carports, garages or storefronts; flimsy supports can snap and collapse in shaking.
Other cities are looking at the issue.
Jones is now working with the Southern California Association of Governments to help cities come up with seismic retrofit legislation to propose to their elected leaders. Jones said Long Beach is looking to hire a consultant to create an inventory of seismically vulnerable buildings. And Ventura has directed its city staff to work with Jones and SCAG to develop an approach for unretrofitted brick buildings and wood apartment buildings with flimsy first stories.