Masonry Magazine February 1986 Page. 38
MEXICO EARTHQUAKE
continued from page 28
In California earthquakes indicated that structural damage usually tends to lengthen the period of vibration, thereby moving a structure away from the high-energy region of the response spectrum. In Mexico City, however, the opposite occurred: structural damage, and the corresponding lengthening in period, tended to move short-period structures even closer to the predominant period of the ground motion, resulting in still higher response, and in some cases, collapse. For these reasons, medium-rise buildings in Mexico City were particularly vulnerable to the earthquake ground motions.
The effects of resonance were clearly evident: many buildings suffered upper-story collapse, some of which probably led to subsequent collapse of the entire structure. Many buildings showed evidence of damage in the upper stories resulting from one building hammering neighboring buildings.
No building failures were observed which seemed immediately attributable to soil failure, although several examples of foundation settlement were noted. Overturning failures seemed due to the presence of lateral forces far in excess of design values.
Building Codes Used in Mexico City
Before 1951, there were no formal earthquake provisions in the Mexico City building code. By the time of the strong earthquake of 1957, some seismic provisions had been included. As a result of the 1957 quake, revision of the then-current code was begun. A new code was adopted in 1966. Buildings designed between 1957 and 1966 conform to a mix of standards, generally somewhere between the requirements of the 1957 code and the 1966 code. The present lateral force code [3.1] was adopted in 1976. It prescribes design lateral forces which are consistent with a response spectrum with high ordinates in the period range of 1.0 to 3.5 seconds. This implies that when the code was drafted, its authors certainly anticipated the possibility of resonance such as evidently occurred in the recent earthquake.
The relatively proor performance of some buildings in that earthquake could also have been due to other factors, such as detailing, construction and materials quality, or non-compliance with the code. These factors certainly will require further study. In any event, it is clear that the current seismic code will probably be revised once again.
Part II of this report will be featured in the March/April, 1986 issue of Masonry.
The International Masonry Institute is a labor/management trust fund organized through agreement between the Mason Contractors Association of America and the International Union of Bricklayers & Allied Craftsmen to advance the masonry industry in the United States and Canada. The Institute's programs include Apprenticeship and Training: Research and Development; Labor Management Relations, and Market Development. Part II of the preceding document on the Mexican earthquake will appear in the March/April, 1986 issue of Masonry.
Three feet out of plumb, the 22-story tower at Pino Suarez leans menacingly in a southerly direction. The columns on the south side of the third floor level were buckled.
38 MASONRY-JANUARY/FEBRUARY, 1986
Videotape Available from NBS on Mexican Eqrthquake Findings
The National Bureau of Standards has released a videotape of the findings of a team of experts from the NBS Center for Building Technology and the U.S. Geological Survey that assessed the structural damage following the Mexican earthquake. The engineers and seismologists arrived in Mexico the week after the earthquake to provide technical assistance.
The 20-minute videotape includes extensive footage of the damage as well as pertinent slides, charts and computer simulation which shows how some of the damage probably occurred. The videotape is available for $25 from the National Bureau of Standards by writing: Ms. Wanda Eader, Structures Division, B268 Building Research, Gaithersburg, MD 20899. Checks should be made payable to the National Bureau of Standards.