Nasty surprise for anniversary

A major earthquake struck Mexico in September 2017 on the anniversary of the 1985 disaster. Changes in building codes since then have had an impact: nearly 95 percent of the buildings that have now collapsed or been severely damaged were built more than 30 years ago.

Only a short warning time

After the early warning system kicked in, people had only 15 seconds to get to safety before the earthquake waves reached the city. The system is activated early for subduction earthquakes on the Pacific coast, but not for an intraplate earthquake like the one that occurred this afternoon. Still, this short warning time was enough for many people to leave their homes or take suitable shelter inside.

The earthquakes damaged buildings and infrastructure in Mexico City and other states. Total death toll was 369. Many sectors such as commerce, industry, schools, roads, water, and wastewater infrastructure suffered significant damage, but the housing sector was hit the hardest. In states such as Morelos and Puebla, homes with unreinforced masonry and adobe buildings were badly affected. Many older cultural buildings, such as churches and monasteries, also partially collapsed or sustained structural damage. Total losses were six billion U.S. dollars, of which two billion was insured.

© Ronaldo Schemidt / Getty Images

Not since the 1985 quake had so many buildings collapsed in Mexico City and such a large number of people been trapped or died in the rubble. Most deaths (two-thirds) were counted in the capital city. There, the severe damage was concentrated in the so-called transition zone and at the lake according to the seismic microzonation of the city (see figure on page 41) – a pattern that had already been observed during the violent earthquakes in the 20. The magnitude of the failure occurred in the twentieth century with magnitudes of 7.6 (1957 and 1979) and 8.0 (1985). This shows the enormous influence soil conditions have on the extent of damage.

Nearly 95 percent of the buildings that collapsed or were hit hard had been built before 1985 and had withstood the stresses of the time. To what extent these buildings were retrofitted is unclear. The damage pattern shows that newer buildings (built after 1987) withstood this earthquake well.

Problematic soft storeys

About half of the buildings that collapsed in whole or in part had at least one of the following three characteristics: (1) they had a "soft storey," meaning that the first floors were too flexible and did not damp the movements; (2) they were mainly located in corner locations and were thus mainly affected by torsion – a rotational movement that the structure has little to resist; or (3) their building structure consisted of reinforced concrete columns with flat slabs, where the floor slabs rest directly on the columns without beam supports.

This design with flat slabs is regulated in only a few earthquake zones worldwide, for example in New Zealand’s building codes. The American Concrete Institute’s ACI-318 rule restricts the use of flat slabs only in high seismic risk areas such as California. Because many buildings with flat ceilings collapsed in the 1985 quake, Mexico’s current building code limits displacement of adjacent floors to a maximum. The maximum permissible displacement depends on the floor height.

Retrofitting possible, but costly

Buildings with soft storeys that make room for parking or commercial space have often attracted negative attention in earthquake zones. Insufficient ductility of piers (lack of deformability due to weak longitudinal and transverse reinforcement) has also led to spectacular building collapses. Both problems can be solved by retrofitting, but this is costly. For example, soft storeys can have additional reinforced concrete walls or steel frames added to improve stability. Or encase the pillars with steel plates to increase their ductility.

Buildings in corner locations that partially or completely collapsed were apparently not sufficiently designed for the torsional forces that occurred. Already in 1985, it had proved to be a serious mistake to pay too little attention to the torsional stiffness and the required resistance of supporting piers or walls to shear stresses. But it is not only planning errors that drive up the damage: Also conceivable in the case of collapses or severe building damage is that the construction was poorly executed (lack of supervision of the construction, poor quality of building materials). In addition, it is possible that the original structure was altered years later – for example, by the removal of partition walls during a change of use, without any additional retrofit.

Local authorities in Mexico City have responded: An initiative launched in November 2017 aims to include new building retrofit provisions in building codes.

© Munich Re

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