An initial reconnaissance of the damage from the Sept. 19 Puebla-Morelos earthquake that killed over 300 people and toppled over 40 buildings in Mexico City found that seismic building codes—adopted after a deadly quake in 1985—were effective, according to a Stanford University professor.
Eduardo Miranda, professor of civil and environmental engineering at Stanford, and a student team flew to the city the day of the magnitude-7.1 quake. “We were able to create a database of 46 structures” that had totally or partially collapsed, he notes. Only three built with the post-1985 codes were on the list. “Of those, I’d say only one was very problematic,” he says.
One of the three was a pedestrian bridge connecting buildings. “There was sliding support for the bridge on one side, but it was not large enough,” Miranda says. As for a new supermarket that partially collapsed, “I think there were other details that contributed to that collapse,” he says.
The problematic one is a new apartment building, Miranda says, suspecting a possible design error. At ENR press time, his team was still preparing an official report on its findings.
“Other new buildings we went through had no damage at all,” Miranda notes.
In a Sept. 27 blog from Mexico City, structural engineer Kit Miyamoto stated, “This earthquake was different from the 1985 one. The Mexico City earthquake in 1985 was a long-distance earthquake with long, swaying motion. This affected taller structures, six stories and higher. But this earthquake had a much sharper and faster shaking motion, which impacted three- to six-story structures more. It’s what we called ‘resonance effect.’ ”
Other U.S. engineering researchers are preparing to send teams to Mexico City. They hope to learn more about how the post-1985 building codes fared after the current earthquake.
Sergio Manuel Alcocer, a research professor at the National Autonomous University of Mexico and a member of the Earthquake Engineering Research Institute, told ENR on Sept. 26 that the local authorities have begun recovery efforts to look for bodies. “Unfortunately, there are still 40 to 50 people buried in a 10-story structure,” he told ENR by phone. “It will take some time to remove the floors piece by piece.”
As of ENR press time, the death toll was nearing 400 and recovery operations were still in effect. Alcocer says his team has seen about 150 buildings with significant structural damage. But most of them were built before the post-1985 codes or may have been built with questionable construction quality, he adds.
A Miyamoto International report, released on Sept. 23, noted that the Sept. 19 quake epicenter was located 120 kilometers from Mexico City, while the 1985 quake was located 300 km away. When the epicenter is farther away, ground acceleration causes more severe shaking.
Buildings that had not previously been damaged were now, partly due to the different effects of movement on soils within versus outside the ancient lake bed upon which Mexico City sits, says Alcocer. “Buildings at the edges of the lake bed got more damage. Within the lake bed, building designs are typically stronger and stiffer” to compensate for the weaker soils.
The post-1985 codes include ductile detailing in buildings—for example, “stirrups closely spaced within concrete elements to form a cage so that the concrete doesn’t crumble,” says Alcocer.
This article was amended on Oct. 12 to include the correct date and the proper name of the earthquake.