Nearly two-thirds of the buildings that collapsed in Mexico City’s monstrous earthquake last month were built using a construction method that is now forbidden in seismic hot spots in the US, Chile and New Zealand, data compiled by a team of structural engineers at Stanford University showed.
The suspect building technique called flat slab — in which floors are supported only by concrete columns — caused 61 percent of the building collapses in last month’s magnitude 7.1 earthquake, which killed 369 people and blanketed tree-lined avenues in rubble.
Now, several prominent engineers have said some of those structural failures could have been prevented and lives could have been saved had Mexico City officials only gone forward with a proposal to forbid that type of construction when they toughened building codes after a 1985 earthquake in Mexico’s capital.
“We have known for 30 years that this system killed lots of people, so why are we still using it?” asked Eduardo Miranda, a professor of civil and environmental engineering at Stanford and global expert on earthquake-resistant design who compiled the data. “The right decision after ’85 would have been to completely ban this kind of construction. We could have saved lives.”
Concrete slabs used to build floors and ceilings can be cast to include some rebar for reinforcement and give builders greater flexibility in room layout and allow for higher ceilings.
However, in an earthquake, without reinforced concrete walls or lateral bracing to resist forces pushing structures sideways, buildings with that design can move too much. The columns, and connections between the slabs and columns, can easily break, prompting collapse, as was the case at a school where 26 people died, most of them children.
Experts have concurred that the devastation caused by last month’s earthquake in the city of 8.9 million people could have been much worse had building codes not been so strong, but it has also forced an uncomfortable conversation about their shortcomings.
Now, as experts race to toughen standards to retrofit hundreds of damaged buildings, they are grappling with the reality that corruption has allowed hundreds of structures to be built outside the rules atop the soft soils of Mexico City’s ancient lake bed.
In the crisis following the 1985 earthquake, a group of academics, building officials and engineers drafted emergency recommendations to strengthen Mexico City’s seismic codes, which were swiftly passed into law.
Some architects and builders were opposed to an outright ban on flat slab construction, said Miranda, who wrote reports that informed the committee.
The new codes allowed flat slab construction if developers designed the building to be seismically stronger than structures with beams or concrete walls.
Authorities did not pay enough attention to evaluating if existing flat slab structures needed a seismic retrofit, Miranda said.
The new codes gave more responsibility to a network of private engineers who are hired and paid by developers, and who submit structural plans to borough authorities.
In practice, that means private engineers — not government experts — vet projects’ structural safety, and corruption can intervene.
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