Francisco Galvis, Eduardo Miranda, Pablo Heresi, Héctor Dávalos and José Ramón Silos
John A. Blume Earthquake Engineering Center
Department of Civil and Environmental Engineering
Stanford University
Abstract
On September 19th, 2017 a Mw7.1 intermediate-depth normal-fault earthquake occurred approximately 120 km away from Mexico City. The earthquake produced the collapse of 46 structures in Mexico City which resulted in 219 deaths. This was the most intense earthquake to hit Mexico City since the Ms 8.1 September 19th, 1985 earthquake. A preliminary study of the characteristics of collapsed buildings is presented. It is shown that most collapsed buildings had less than 10 stories and were primarily located in zones IIIa and IIIb of the microzonation of the 1987 Mexico City building code which are characterized by having total thickness of soft clay deposits between 25 and 40m and by predominant periods of vibration between 1s and 2s. It is shown that, although regions and building heights that were hit hardest were somewhat different than in 1985, there are many similarities in the characteristics of buildings that collapsed. Similarly to the 1985 earthquake, most of the collapse buildings consisted of reinforced concrete buildings whose lateral resisting system consisted of flat slabs supported by reinforced concrete columns. Collapses occurred primarily in buildings constructed prior to 1985, Furthermore, 57% of the collapsed buildings had a soft story. Both the flat slab and column lateral resisting system and soft stories have long been identified as building vulnerabilities. This highlights the need to possibly issue a mandate to evaluate and possibly seismically upgrade existing buildings located in the former lakebed of Mexico City that have these characteristics as they may be in danger of collapse in future earthquakes.
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