Masonry Magazine April 1986 Page. 40
5.0 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS
Summary
Following the destructive Mexican earthquakes of September 19 and 20, 1985, the International Masonry Institute sent a team of structural engineers to investigate the damage in Mexico City, and also in the coastal area nearer the epicenter. The team's specific objectives were:
1) to examine the earthquake performance of a variety of different buildings in the Mexico City and epicentral areas;
2) to evaluate the influence of prevalent design, detailing and construction practices on that earthquake performance; and
3) to evaluate the role played by masonry in the earthquake performance of various buildings.
The team found that ground accelerations and structural response were significant in the firm-soil areas of Mexico City. However, the resonant response of the deep, soft clay deposits underlying the central part of the city caused the lake zone to experience 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 type of ground motion severely affected buildings whose fundamental periods of vibration were in that same range.
In the lake zone of Mexico City, the most seriously affected buildings were those which responded in a resonant fashion to this long duration, nearly harmonic ground motion. Almost two-thirds of the collapsed or severely damaged buildings were over six stories in height, and most of these were in the seven- to 20-story range. Any factors which increased lateral flexibility also tended to increase the severity of response; framed buildings were damaged far more than similarly tall buildings having structural walls or masonry infilled frames.
The coastal region closer to the epicenter probably did not experience significant soil filtering or amplification. Maximum accelerations there were probably higher than those experienced on rock or firm soil in Mexico City, but
FIG. 14: An unreinforced four-story brick apartment building. The structure had significant cracks but remained standing. Located on Calle Abraham Gonzales, the building is 73 years old.
FIG. 15: A two-story unreinforced brick housing development built in 1913. Located on Calle Abraham Gonzales, a section of delicate balustrade was lost but the structure was in use with no other discernible damage.
FIG. 17: A building on Berlin Street near the corner of Hamburgo. This structure, although basically open in front, had enough shear walls to withstand rotational and displacement forces. The building experienced only minor cracking and remained functional.
FIG. 16: This building, at the corner of Berlin and Hamburgo, is a masonry structure with an elaborate balustrade. There was no damage to the parapet balustrade.