360 to 500 buildings and homes are in danger of collapsing days after a magnitude 7.1 earthquake rocked Mexico City. The earthquake rattled the city on Tuesday, the 19th of September 2017. As many as 38 structures suffered catastrophic failure when the earthquake hit, but the concern for civil engineers is that other structures may yet suffer a delayed collapse. The earthquake has already caused up to 355 fatalities.
Engineers on popular social media site Reddit have been mulling over this shocking video of a building simply crumbling after the earthquake had hit the city. The overarching question was why there was “no resistance” to collapse.
The consensus among the engineers - the building appeared to have old masonry. Some pointed out that the sides of the building could have been brick; brick generally performs poorly (if not reinforced) during events such as earthquakes.
Engineering teams are now descending on Mexico City to generate data on the brittle buildings and to deliver reports to the Earthquake Engineering Research Institute (EERI).
A Matter of Geography and Geology
To make matters worse Mexico’s capital is built on an ancient lake and therefore on damp sand and clay. Its situation results in the prolonging of the earthquakes and increasing the damage to buildings, particularly to those which are five stories or more.
Virtual representation of the 19 September 2017 earthquake. Source: New York Times
When an earthquake strikes, the sediments the city is built upon slow the shock waves, leading to them strengthening and becoming more violent. To make matters worse the area is prone to earthquakes as it is near to the meeting of renowned tectonic plates.
This geological knowledge is not new so the question remains: why are Mexico’s buildings not better prepared for earthquakes?
According to the New York Times, building inspectors and private engineers have said that the strict building codes are not being enforced. A 2016 study revealed that many buildings failed to meet city standards - a staggering 71 percent of the 150 buildings studied, failed to meet standards.
A school that collapsed during the 19 September earthquake was marked safe by city inspectors after the earthquake 12 days earlier, on the 7th of September.
The Vice President of the Mexican Society of Civil Engineers, Sergio Alcocer said:
“We are concerned if we have a huge earthquake like the one in 1985 we may have problems in buildings. It’s a wake up call.”
Even the most recently constructed blocks of apartments in Mexico City tumbled down during the earthquake.
Buildings using concrete are said to be lacking vital reinforcing bars with concrete columns facing catastrophic failure during an earthquake if not properly reinforced. This apparently was the case in the collapsed Enrique Rebsamen school. 26 children lost their lives.
There is an urgent need for the review of civil engineering practices and standards in Mexico. More importantly, however, is the determination to enforce them.
White, Derek Watkins And Jeremy. “Mexico City Was Built on an Ancient Lake Bed. That Makes Earthquakes Much Worse.” The New York Times, The New York Times, 2017, www.nytimes.com/interactive/2017/09/22/world/americas/mexico-city-earthquake-lake-bed-geology.html?mcubz=1.
“A Building Suddenly Collapsing after a 7.1 Earthquake Strikes Mexico City. - Can Someone Explain Why There Is No Resistance as It Came down. • r/Engineering.” Reddit, www.reddit.com/r/engineering/comments/72armp/a_building_suddenly_collapsing_after_a_71/.