1985 Mexico City Earthquake - Geological and Structural Engineering Issues

Geological and Structural Engineering Issues

Most of the earthquake damage was to buildings. Two reasons are the resonance in the lakebed sediments and the long duration of the shaking. The buildings most damaged were from 6 to 15 stories in height. These buildings tended to resonate most with the energetic frequency band of the lakebed motions. One interesting characteristic was that many buildings had their upper floors collapse, leaving the lower floors relatively undamaged. In many damaged buildings, just one floor had collapsed, In some cases, the damage was caused by the top of a lower, adjacent building banging against the walls and the supporting columns of its neighbor. Eventually, the columns gave way. In other cases, the first few floors of buildings were designed as parking garages, open lobbies or large shopping areas. These "soft" stories were particularly flexible and tended to collapse after prolonged shaking. Some types of foundations, particularly those involving piles driven into clay and held in place by friction, turned out to be weak. One 9-story building, for example, overturned. Its piles were pulled entirely out of the ground.

A survey of the damage done by the government found that few buildings from one to five stories suffered serious damage; the same was true for buildings over fifteen stories. When the buildings were built seemed to have an effect as well. Before the 1957 earthquake, there were no building codes with respect to earthquake resistance. Some regulations were passed in that year and more in 1976 after another, stronger earthquake shook the city. However, none of these regulations had an event like 1985’s in mind when passed. Most of the seriously damaged buildings were built between 1957 and 1976, when the city was starting to build upwards, in the six-to-fifteen floor range. In second place were buildings from before 1957, possibly because they were weakened by the earlier earthquakes. Buildings from 1976 to 1985 suffered the least damage.

At the time of the earthquake, Mexico City had one of the most stringent building codes, based on experience gained from earthquakes in 1957 and 1979. However, the codes were not designed for seismic activity of the intensity experienced in 1985. The event was one of the most intense of any recorded in the world, allowing for macroseismic waves to arrive in the Valley of Mexico with unusually high energy content. Prior to the event, estimates about ground movement on the lakebed were generally accepted and a number of buildings were built on these estimates.

Several notable buildings were relatively untouched by the quake. One significant example is the Torre Latinoamericana. Despite being 44 stories tall, it survived the 1985 event almost undamaged. It was constructed with two hundred piles extending down over a hundred feet into the stable earth stratum.