Masonry Magazine February 1986 Page. 27
The IMI investigative team is pictured at the intersection of Reforma and Avenida Juarez in Mexico City, with the Old National Lottery Building in the background. This building exhibited a general lack of structural integrity-columns were pulled away from slabs. The IMI team consisted of (from left) team leader Robert J. Beiner, P.E.; Richard E. Klingner, Ph.D.; James C. Anderson, Ph.D.; Victor M. Robles, and James E. Amrhein, S.E. (Photo by Andrew Lautman)
distance to the deep deposits corresponds to the depth of the bowl.
Most medium- and high-rise buildings in Mexico City are founded on piles resting on the first hard layer. The depth to that layer is therefore an important factor in determining the vibrational characteristics of the building foundations, and hence of the buildings themselves. Investigators at UNAM have found that there is some correlation between the geographical distribution of structural damage and the depth to the deep deposits. However, they have found an even stronger correlation with the depth to the first hard layer.
Characteristics of Ground Shaking in Mexico City
The Engineering Institute of UNAM operates a strong-motion network in Mexico City. At the time of this report, free-field records were available from at least five stations: 1) on the UNAM campus; 2) near the Secretariat of Communication and Transport (SCT); 3) near the Central Produce Market; 4) at Viveros de Coyoacan, and 5) at the Seismological Observatory in Tacubaya.
Details of the instruments themselves and the motions they recorded were available from UNAM soon after the earthquake. Free-field ground motions in Mexico City all displayed strong motion durations of at least 60 seconds. This was probably due to the far epicentral distance. At 400 km, most earthquake energy would reach Mexico City in the form of surface waves, which attenuate more slowly than do body waves. Because surface waves are dispersive (having different velocities corresponding to different frequencies), an earthquake 400 km away would produce a long-duration train of surface waves in Mexico City.
As might be expected considering the large epicentral distance, the maximum accelerations on rock were small, about three or four percent of gravity. However, they lasted sufficiently long to excite the bowl of soft clay on which Mexico City is founded. The resonant response of that bowl produced surface accelerations at some sites (SCT and the Central Produce Market) which were much stronger in magnitude, and much more harmonic in nature.
STRUCTURAL PERFORMANCE IN MEXICO CITY
Characteristics of Structural Response in Mexico City
As noted previously, significant structural response in Mexico City was not likely, nor was it observed, in the hill area nor in the transition zone. In the lake zone, however, the resonant response of the deep clay deposits created near-sinusoidal ground motions of more than 60 seconds in duration, with maximum accelerations near 20 percent g, and characteristic periods between 1.5 and 3.5 seconds. That kind of ground motion could be expected to have a severe effect on buildings with fundamental periods in the same range.
General Characteristics of Damage in Mexico City
Mexico City is one of the world's largest metropolitan areas, containing about 1 million buildings. As shown in Fig. 3.2, the earthquake severely damaged many buildings in the lake zone. A preliminary estimate of structural damage, prepared by UNAM, is shown in Table 3.1. While the number of collapsed or severely damaged buildings was