Masonry Magazine February 1986 Page. 28
YEAR BUILT
TYPE OF
STRUCTURE
DAMAGE
< 1957
57-76
1976 >
<5
NUMBER OF STORIES
6-10 11-15 15
TOTAL COLLAPSED
OR SEVERELY
DAMAGED
Reinforced Concrete
Frames
Collapse
very severe
35
59
13
36
62
9
0
107
9
19
7
8
23
4
1
36
Steel Frames
Collapse
very severe
5
4
0
4
2
1
2
9
1
0
0
0
0
1
0
1
Flat Slabs
Collapse
very severe
3
35
12
23
23
4
0
50
5
20
11
9
18
8
0
35
Masonry
Collapse
very severe
7
4
1
10
2
0
0
12
2
3
0
4
1
0
0
5
Others
Collapse
0
1
1
1
1
0
0
2
very severe
2
4
2
6
2
0
0
8
Collapse and
Sum
very severe
69
149
47
101
134
27
3
265
TABLE 3.1-Statistics of building damage (UNAM). "Very severe" indicates degree of damage.
very small compared to the total number of buildings in
Mexico City, it is still very substantial. The area of significant structural damage was about as large as that caused by the 1957 earthquake. Of particular importance in the most recent quake, however, was that many of the damaged buildings were relatively new, and many were public or quasi-public buildings, used for federal government offices, public housing, and communications. The 1985 earthquake therefore had a devastating effect on the infrastructure of the entire country that was far out of proportion to the number of buildings affected.
Due to the epicentral distance and the soil conditions peculiar to Mexico City, this earthquake had an unusually severe effect on mid-rise buildings in the range of seven to 20 stories. This was particularly true for flexible, moment-resistant frames. A typical moment-resistant frame with partitions and other non-structural components has a fundamental period of vibration (in seconds) approximately equal to 0.1 times its number of stories.
This would place seven- to 20-story structures in the period range of 0.7 to 2.0 seconds. The lateral flexibility of such structures would be further increased by the soft soil underlying most of the high-damage areas. As a result, the fundamental per period of vibration of such structures would be on the order of 1.5 to 3.5 seconds, very close to the characteristic periods of the recorded ground motions in the lake zone.
These effects, combined with the long duration and near-harmonic nature of the ground motions recorded on soft clay, probably resulted in many cases in a condition of resonance, in which the base motion was greatly amplified at the top of a structure. To develop resonance, a lightly damped structure such as those found in Mexico City requires approximately 20 cycles of harmonic input. The more than 60 seconds of 0.5-Hz motion recorded in Mexico City would certainly have been sufficient to result in resonance.
The damage characteristics of the Mexico City earthquake were also unique. Experience with similar buildings continued on page 38
Reinforced concrete
FIG. 3.2-Locations of buildings severely damaged or
collapsed in the 1985 earthquake in Mexico City (UNAM).
28 MASONRY-JANUARY/FEBRUARY, 1986
Unreinforced masonry
PLAIN INFILL
used in the 1-6 story buildings.
FIG. 3.3-Typical infill used in the 1-6 story buildings.