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PUBLICATIONS

AlllDE bbmi

NATL INST OF STANDARDS & TECH

A11 102661991

NBS BUILDING SCIENCE SERIES 165

Engineering Aspects

of the September 19, 1985

Mexico Earthquake

TA

EPARTMENT OF COMMERCE • NATIONAL BUREAU OF STANDARDS

ubo

NO. 165

1987

C.2
Tm he National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall J/L goal is to strengthen and advance the nation's science and technology and facilitate their effective application for pubUc benefit. To this end, the Bureau conducts research to assure international competitiveness and leadership of U.S.

industry, science arid technology. NBS work involves development and transfer of measurements, standards and related science and technology, in support of continually improving U.S. productivity, product quality and reUabihty, innovation

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Laboratory, the National Engineering Laboratory, the Institute for Computer Sciences and Technology, and the Institute for Materials Science and Engineering.

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'Lx)cated at Boulder, CO, with some elements at Gaithersburg, MD
NBS BUILDING SCIENCE SERIES 165

Engineering Aspects

of the September 19, 1985

Mexico Earthquake

Research informalioji Center

JNational bureau of Slantiarda

Gailhersburg, Marykud 20899

M 21

William C. Stone

Felix Y. Yokel

U.S. Department of Commerce

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

and

Mehmet Celebi

Thomas Hanks

Edgar V. Leyendecker

U.S. Department of the Interior

Geological Survey

Office of Earthquakes, Volcanoes,

and Engineering

Menlo Park, CA 94025

Golden, CO 80401

Issued May 1987

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
Library of Congress Catalog Card Number: 87-619819

National Bureau of Standards Building Science Series 165

Natl. Bur. Stand. (U.S.), Bldg. Sci. Ser. 165, 215 pages (May 1987)

CODEN: BSSNBV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1987

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402
Abstract

Following the September 19, 1985 Mexico earthquake, a team

consisting of four engineers and one seismologist from the

National Bureau of Standards (NBS) and the United States

Geological Survey (USGS) was dispatched to Mexico City to provide

technical advice to the US rescue effort and to assess structural

damage. This report is primarily based on data gathered by the

team, but it also contains a compilation of other available

information. The report addresses the origin and characteristics

of the observed ground motion, the ability of buildings designed

in accordance with present and proposed seismic design provisions

to resist this type of ground motion, and observed data on

structural and foundation failures.

Key words: building codes; earthquake; foundations; geology;

geotechnical engineering; ground motion; response spectra;

seismology; seismic design; standards; structural engineering

iii
Acknowl edgement

The NBS/USGS team is indebted to several groups who helped

substantially during the conduct of the field investigation. We

would particularly like to thank the Office of Foreign Disaster

Assistance for their help in making arrangements for the team to

enter Mexico. Special thanks are also due to past U.S.

Ambassador to Mexico John Gavin and his staff at the embassy;

U.S. Embassy Science Attache Dr. Reynaldo Morales; and the A.I.D.

office at the Embassy during the weeks following the earthquake

for their assistance in arranging for ground transportation and

access permission to structural collapse sites.

Numerous researchers at the University of Mexico's (UNAM)

Institute de Ingenieria freely provided timely and extremely

useful data on the earthquaKe within days of the tremor and

assisted us with our search for background material on subsurface

conditions, building codes, and structural performance. We are

particularly grateful to Professors Jorge Prince, Roberto Meli,

Miguel P. Romo and Neftali Rodriguez Cuevas for their

cooperation during our visits to Mexico City, and to Professor

Romo for reviewing an advanced draft of some of the material

presented in the report. Furthermore, we wish to extend our

thanks to Mexico City consulting engineers Francisco Serna and

Enrique Martinez Romero for providing further statistics on the

extent of structural damage, photos of critical structural

details not available at the time of visit, and their

illuminating comments on structural behavior and construction

practices in Mexico City.

iv
Preface

This report conveys the findings of a team of engineers and

seismologists from the National Bureau of Standards (NBS) and the

United States Geological Survey (USGS) , sent to Mexico by the

U.S. government shortly after the September 19, 1985 earthquake

which struck Mexico City. The mission of this team was to

provide technical advice through the office of Foreign Disaster

Assistance, and to make an assessment of structural performance.

Team members, whose specialty areas are shown in parentheses,

were: Dr. Mehmet Celebi, USGS, (earthquake engineering) *

Dr. Thomas Hanks, USGS, (seismology and strong ground motion)

Dr. Edgar V. Leyendecker, USGS, (structural engineering),' Dr.

William C. Stone, NBS, (structural engineering) ; and Dr. Felix

Y. Yokel, NBS, (geotechnical and structural engineering).

The team advised U.S. rescue workers and Mexican officials on

means to more rapidly remove debris from collapsed buildings in

order to speed the search for survivors, and worked with Mexican

experts to identify fruitful opportunitites for in depth studies

of structural performance. Team representatives also briefed the

U.S. Ambassador to Mexico and his staff on the earthquake.

The dispatch of the above team to Mexico City represented the

first effort conducted as part of the Interagency Committee on

Seismic Safety in Construction (ICSSC) post-earthquake reconnaissance activity. The mission of the ICSSC Subcommittee on

Post-Earthquake Response Activities, jointly chaired by NBS and

USGS, is to:

" Be aware of existing Federal procedures and representative

State, local and private sector procedures with respect to

v
post-earthquake response for damage assessment and analysis, and

to establish guidelines, operating procedures and criteria for

the conduct of post-earthquake response activities in the event

of a damaging earthquake. Activities include: general assessments, deployment of instruments and certain in-depth studies of

structural performance, technical assistance to affected Federal

agencies, and technical support for those Federal officials who

have the responsibillity for carrying out the National Plan in

the event of a damaging earthquake"

The ICSSC plans to conduct reconnaissance activities under the

auspices of the Federal Emergency Management Agency (FEMA)

following earthquakes in the U.S.

vi
TABLE OF CONTENTS

ABSTRACT iii

ACKNOWLEDGEMENT iv

PREFACE V

1. INTRODUCTION 1

2. EARTHQUAKE ORIGIN AND CHARACTERISTICS 5

3. GEOLOGIC SETTING AND GROUND MOTION IN MEXICO CITY 13

3 . 1 General 13

3.2 Subsurface Conditions 13

3.2.1 Geological Profile and History 13

3.2.2 Stratigraphy of Soil Deposits 19

3.2.3 Engineering Characteristics of Soil Deposits . 25

3.3 Ground Motions in the Mexico City Area 34

3.3.1 Ground Motion Characteristics 34

3.3.2 Effect of Subsurface Conditions on Ground Motion 43

3.3.3 Comparison of Response Spectra With Design

Spectra Used in Mexico City and the U.S 52

4. FOUNDATION FAILURES 63

4.1 General 63

4.2 Loss of Shear Resistance of Piles 65

4.3 Tilting or Overturning 71

4.4 Warping Effects 82

4.5 Other Observed Soil Disturbances 82

4.6 Discussion of Foundation Failure Mechanisms 87

5. STRUCTURAL FAILURES 89

5.1 History of Building Regulations 89

5.2 Failure Patterns 93

5.3 Performance of Different Structural Systems 95

5.4 Reinforced Concrete Frame Structures 95

5.4.1 General 95

5.4.2 Hospitals 99

5.4.3 Apartments Ill

5.5 Flat Plate and Waffle Slab Buildings 122

5.5.1 General 122

5.5.2 Column Failures 12 6

5.5.3 Punching Shear Failures 13 7

5.6 Waffle Slabs 142

5.6.1 General 14 2

5.6.2 Column Failures 142

5.6.3 Porous Pan Formed Waffle Slabs 14 3 vii
5.7 Steel Structures 150

5.7.1 General 150

5.7.2 Steel Frame Structures That Failed 151

5.7.3 Tall Moment Resisting Steel Structures That

Performed Well 167

5.8 Building Configuration and Design Problems 169

5.8.1 Building Separation (Hammering) 169

5.8.2 Torsional Effects 175

5.8.3 Upper Story Collapses 186

5.9 Masonry Structures 190

6. FINDINGS 195

7. REFERENCES 204

viii
1. INTRODUCTION

As a result of the September 19, 1985 Mexico City Earthquake

4,287 persons lost their lives, 2,000 are unaccounted for, 14,268

were injured and 150,000 lost their jobs. The above figures are

from an official count by Mexico City authorities [1].

Unconfirmed statistics of casualties were estimated to be much

higher [2] and will probably never be known precisely. The

official count also lists damage estimates as 5,728 damaged

buildings of which 15% suffered total or partial collapse, 38%

suffered major damage (approximately half will be demolished) and

47% suffered minor damage. Pavement damage occurred along 310

streets, there were 5285 breaks in water mains, and 14,500

interrupted phones. In addition, all long distance phone service

was disrupted for an extended period of time.

According to a block by block survey of the damaged area

conducted by a team from the National Autonomous University of

Mexico (UNAM-Universidad Nacional Autonoma De Mexico) [3], about

330 buildings collapsed or were heavily damaged (a higher damage

estimate was made on the basis of aerial photographs) . The total

damage incurred by the city, not including the long term cost of

rebuilding, was estimated by the Economic Commission for Latin

America at $ 4 billion [1].

1
Even though this was a disaster of major dimensions, it is

important to realize that Mexico City has a population of 18

million and that there are over 800,000 buildings in the affected

area. The earthquake (Mg = 8.1) struck at 7:18 AM Mexico time

[4]. The epicenter was located in Michoacan Province near the

Pacific Coast of Mexico, approximately 3 0 km from the small town

of Lazaro Cardenas, about 400 km to the west of Mexico City. The

main shock reached Mexico City at about 7:19 AM. At that early

hour many people were commuting to work and thus office buildings

were largely unoccupied. Had the earthquake struck during working

hours the number of fatalities could have been much greater,

since many of the collapsed buildings were office buildings.

The damage pattern observed was highly selective. Most of the

severe damage was confined to buildings in the height range of 7

to 18 stories. Many other buildings performed well, even though

they were located in the same area as those which suffered

structural failure. Subway tunnels sustained very little damage.

However, many underground utility lines had to be repaired or

replaced. The zone of damage was principally confined to the

"Lake Region" of Mexico City, which is underlain by deep layers

of soft clays and silts. Other areas of Mexico City sustained

little damage.

Most of the observed building collapses were attributed to

structural failure; relatively few foundation failures were

observed. However, subsurface conditions had a profound effect on

the characteristics of the ground motion, and thus on structural

performance.

One unfortunate aspect of the September 19, 1985 Mexico City

earthquake (Richter Scale magnitude Mg = 8.1) is that in many

essential respects it was a repeat of the July 28, 1957 earthquake (Mg = 7.5), differing mainly in the number of destroyed

buildings and confirmed dead and missing people. The following
statements are quotations from the August 1, 1957 issue of

"Engineering News Record", describing the effects of the 19 57

earthquake in Mexico City; all but the first were written under

the by-line of senior editor Frederick S. Meritt.

"The problem of enough separation between buildings with

different modes of vibrations to keep them from banging

against one another also appears as a result of the quake

damage .

"The earthquake. . .affected principally the taller

buildings. Few under five stories high were damaged if they

had good foundations and were not pounded by adjoining

buildings."

"It appears highly probable that due to peculiar subsurface

conditions the shock may have been many times as severe on

some buildings as on structures only a few hundred feet

away .

"There is much evidence that peak vibrations of the ground

in this quake coincided with the natural frequency of many

of the tall buildings that were damaged severely."

"Emilio Rosenblueth, consulting engineer, Mexico City,

believes the magnification factor [of incoming seismic

amplitudes relative to hard rock at the resonant frequencies] may reach 12 or more."

It will be seen in the following chapters that every one of these

statements could be applied to the 1985 earthquake. In 1985, of

course, the magnitude of the tragedy was considerably greater

than in 1957.

3
Damage of the sort sustained in Mexico City only rarely occurs at

4 00 km distance from an earthquake of any magnitude. The special

geologic conditions of the sedimentary basin underlying Mexico

City are the cause of this phenomenon. Many aspects of past and

present design and construction practices contributed to the

observed building failures, and much can probably be learned from

the more than 800,000 buildings that survived the earthquake with

only minor damage.

Chapter 2 discusses the origin and characteristics of the

earthquake. In Chapter 3, the character of the Mexico City ground

motion, the unique subsurface conditions which caused the great

amplification of the ground motion, and the ability of present

and proposed design provisions to protect structures against this

type of ground motion are discussed. Foundation and structural

failures are described in Chapters 4 and 5, respectively. Conclusions and recommendations are presented in Chapter 6.

4
2 . EARTHQUAKE ORIGIN AND CHARACTERISTICS

The September 1985 Michoacan, Mexico earthquake occurred as a result of the subduction of the Cocos Plate along the Middle

American Trench beneath the North American and Carribean plates,

a process that has been ongoing for millions of years. The

location and relative motion of the plates is shown in figure

2.1. The earthquake initiated at 18.2°N, 102. 6°W, with a focal

depth of approximately 18 km [4], and propagated approximately

170 km to the southeast (length of slip area ; 170 km. width z 50

km [4]). This event ruptured much but not all of the "Michoacan

seismic gap" [5] an area along the subduction zone which did not

generate a major earthquake in recent history while adjacent

areas generated major earthquakes. Because of the unrelieved

accumulated strains caused by the slip movement (about 57

mm/year) , the area was believed to have the potential for a major

earthquake. The magnitude of the earthquake (Mg) was originally

given as 7.8 on the Richter scale, but later revised to 8.1 by

the National Earthquake Information Service (NEIS) on September

26, 1985. Similarly, Mg of the major aftershock occurring 36

hours later was upgraded from 7.3 to 7.5. Otherwise, the

aftershock sequence of an event that large was surprisingly

lean. In the first month following the main shock, only two other

earthquakes above Mg = 5 occurred. A preliminary estimate of

the seismic moment (the product of the shear modulus of the

rock, the average slip along the area of rupture, and the

5
September 19, 1985

Earthquake Epicenter

PLATE

Figure 2.1 Location and relative motion of major tectonic plates

in southwestern Mexico, (from Ref. [2])

6
area of rupture) for the main shock is 0.9-1.5xlo28 dyne-cm

(0.9-1. 5xlo21 N-m) [4], yielding a moment magnitude of 7.97 to

8.12 for the main shock.

Such earthquakes are no strangers along and near the Middle

American trench, as can be seen in figure 2.2, which shows the

location of epicenters of past earthquakes, and a space-time

diagram, in which the epicenters are projected down and plotted

at the year of occurrence [4]. Dozens of Mg >7 earthquakes have

occurred at comparable epicentral distances from Mexico City

since the turn of the century, and more are sure to come. Two

noteworthy features of the main shock are that: (1) it is as

large (to 0.1 magnitude units) as any of its companion earthquakes known in the historic record; and (2) this event and its

aftershock occurred within a longstanding gap for Mg > 7.5, as

can be clearly seen from the space-time plot ([5], [6]).

An important achievement was the installation of the Guerrero

strong-motion accelerograph array (figure 2.3) prior to the

occurrence of the recent earthquake, a cooperative venture of the

Institute of Geophysics and Planetary Physics, U.C. San Diego

(J.N. Brune and J.G. Anderson) and the Institute de Ingenieria,

UNAM (J. Prince and J.G. Singh) . The array provided extensive

information from stations in the general area near the epicenter

and to a distance of 230 km in the direction of Mexico City.

Data on the ground motion characteristics of the main shock are

given in table 2.1 and figure 2.4 [4]. The data in figure 2.4 are

for the east-west component of the ground motion for three

sections in the Guerrero array. These stations were chosen

because they are located in the general direction from the

epicenter to Mexico City: La Villita, which has an epicentral

distance of 44 km. La Union, which has an epicentral distance of

84 km, and Teacalco which has an epicentral distance of 333 km

7
2000

1980

1950

1900

1850

1800

100 200

I I I I I I km

Middle America TrencVi

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7.9.7.4 m

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7.4

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TYPICAL RUPTURE

DIMENSION

Ms = 7 .0 I I Ms= 7.5 I I Ms= 8.0 I I

—> Convergence direction 48 Rate (mm/yr)

O Epicenter, year (depth) • Projection of epicenter

7 8 Magnitude

Figure 2.2. Historical seismicity of soutliwest IVIexico. Bottom half

of figure is a space-time diagram for the purpose of

recognizing seismic gaps. September 1985 events are

drawn in. (from Ref. [4])

8
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Table 2.1 Strong Motion Data from the Guerrero Array

station

1 Epicentral

1 Distance

1 Km

1 Subsurface Condition 1 Direction Acceleration

(peak, gal**)

La Villita 1 44 1 Gabbro 1 N

1 W

1 DN

125

122

58

La Union 1 84 1 Metavolcanic 1 N

1 w

1 DN

166

148

120

Teacalco 1 333 1 Volcanic Tuff 1 N

1 W

1 DN

49

24

27

Zacatula* 1 17 1 Argillaceous Lime 1 N

1 W

1 DN

271

181

I 144

*not part of the Guerrero Array but provided here for comparison

and to show effect of subsurface condition

*1 gal = 10 mm/Sec2

and is about 100 km from Mexico City. All of these stations were

placed on "the most competent rock outcrop consistent with the

target location" [4]. It is therefore reasonable to assume that

these accelerograms were not substantially modified by the

effects of wave propagation through unconsolidated deposits. None

of these stations recorded peak accelerations in excess of the 168

gal peak acceleration in the east-west direction (W) recorded in

Mexico City. For comparison, figure 2.5 shows acceleration records

from a fourth station at a distance 17 km from the epicenter. This

latter station, Zacatula is not part of the Guerrero array. The

records presented by UNAM [7] show peak accelerations of 271 gal

in the north-south direction. The station is located on a compact

argillaceous lime formation and it is reasoned that the ground

motion was amplified by that formation [4]. More extreme ground

motion amplifications were encountered in Mexico City.

10
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12
3 . GEOLOGIC SETTING AND GROUND MOTION IN MEXICO CITY

3 . 1 General

The now well known unusual ground motion characteristics of

Mexico City have been known at least since 1957, and site

resonances within the city have been documented with instrumental

recordings of two earthquakes in May 1962 at two sites: Torre

Latino Americana and Alameda Park, several hundred meters to the

west [8]. As proven once more during the September 19, 19 8 5

earthquake, the unusual characteristics and the regional

variation of the ground motions, as well as the response of

structures to the ground motion, are directly related to the

subsurface conditions in the Mexico City area. In this chapter

information on the subsurface conditions and their effect on the

ground motion in the Mexico City area is provided.

3 . 2 Subsurface Conditions

3 . 2 . 1. Geological Profile and History

Mexico City lies in the southwestern quadrant of a broad basin

which was originally formed by block faulting of an uplifted

plateau, approximately 30 million years ago.

13
Figure 3.1, which was published by Marsal (1975) [9], shows a

schematic geological cross section through the Mexico City area

in the west to east direction. The figure was developed from

information from one 2065m deep boring (PPl in the figure) , many

shallower borings, geophysical explorations and surface studies.

The age of the various formations encountered in boring PPl, as

determined by C^^ and K-Ar dating, is shown in table 3.1, which

is also taken from Ref.[9]. Figure 3.2 shows the surficial

geology of the area. The geology of the area is discussed in

references [9] and [10].

The basin that was originally formed by faulting was subsequently

blocked by successive lava flows that formed a dam across the

valley just south of Mexico City. This dam resulted in the

formation of Lake Texcoco, which slowly began to fill with silt,

clay, and ash from nearby volcanoes. Changes in the climate

during this period led to oscillations in the level of the

lake. Remnants of ancient beaches indicate that about 7000 years

ago the level of the Lake was about 160 m above the main level of

the present city. Indian civilizations developed around the lake,

and the Aztecs built their capital on an island in Lake Texcoco,

connected to the shore by causeways.

The Spaniards, after conquering the Aztecs, destroyed the

causeways. However, they soon learned that the causeways played

an important role in flood control for the Aztec city. To solve

the flood problem, canals were built to drain the lake to the

north. Originally, drainage was provided by the Nechistongo Cut

which was completed in 1789. At the beginning of this century

the two Tesquisquiac Tunnels and the Grand Drainage Canal were

constructed, followed by a modern 45 km drainage tunnel. The

maximum water level of Lake Texcoco at the end of the 19th

century is outlined in figure 3.3.

14
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15
TABLE 3.1 STRATIGRAPHIC SEQUENCE AT LAKE TEXOCCO

(Taken from Marsal (1975) [9])

Depth in m Period 1 Name of formation or equivalent | Approximate

millions of

age, ii years

0 0 - 53 0 1 Lacustrine clay sediments | 0. 0 0 .008

53 0 - 59 5 ARY 1 Becerra Formation (?)* | 0. 008 - 0 .012

59 5

- 64 5 TERN

1 Caliche Morales Formation (?) | 0. 012 - 0 .013

S - 1 80 0

<co- 1 J. L4.L/CL y d X 1^ J- llLd X U Ll \ • / 1 0. 013 - 0 04fi

180 0 - 505 0 1 Tarango Formation | 0. 046 - 8 .00

505 0 - 814 0 1 Equivalent to Tlalyecac Formation | 8. 0 13 .0

814 0 - 1 030 0 ARY 1 Huatepec Rocks | 13. 0 21 .0

1 030 0 - 1 125 0 ERTI

1 Equivalent to Tepoztlan Formation | 21. 0 24 .0

1 125 0 - 1 437 0 1 Equivalent to Xochitepec Formation! 24. 0 29 .0

1 437 0 - 2 065 0 1 Balsas Formation | 29).0 - ( ?)

^Question marks indicate uncertain identification

16
Figure 3.2 Surficial geology of the Mexico City area (taken from

IVIarsal (1975)[9]).

17
Figure 3.3. The Lake Texcoco area (taken from Marsal (1975)[9].

18
As the water level dropped, several small lakes were formed from

the single lake and portions of the old lake bed were exposed

around the ancient Aztec capital. This lake bed has been used for

the expansion of Mexico City. Today much of the city rests on

lake deposits, which overlay older sedimentary sequences.

3.2.2. Stratigraphy of Soil Deposits

Soil deposits generally consist of three consecutive layers of

clays, separated by thinner layers of alluvial deposits, and

underlain by a thick layer of dense deposits. Near the center of

Lake Texcoco (boring BNP 1 in figure 3.3) the upper clay layer is

approximately 3 5m thick and is underlain by an approximately 3m

thick dense sandy clayey layer. The second clay layer is about

15m thick and rests on a second sandy layer between 55 and 6 0m

below surface. A third clay layer extends to a depth of 82m

below surface and is underlain by another 100m of compact strata

of sands, silts, and clays, intermixed with some gravel. Figure

3.4 shows the soil profile at BNP 1 together with a plot of the

water content Wj^ = (weight of water/weight of solids) x 100 which

is a measure of the density of the deposits. Note that the water

content of the clay layers varies from 150 to 500 percent and

tends to diminish with depth. The water content of the first

sandy layer is about 100 percent, and that of the deep alluvial

deposits is low. For comparison, figure 3.5 shows a boring log

taken near the center of the city. Near the surface is a 5m

layer of man-made fill and desiccated clay. The upper clay layer

is about 30m thick and has water contents between 100 and 400

percent. The second clay layer is about 10m thick with a 2 00

percent water content and the third clay layer between 62 and 7 0m

below surface is quite dense. The difference between the

stratigraphic profiles near the center of Lake Texcoco and the

profile shown in figure 3.5 is attributable to two factors:

(1) the deposits near the center of the city have been compressed

by the weight of man-made fill and buildings and by groundwater

19
ure 3.4. Soil profile near center of Lake Texcoco (Boring BNP1,

taken from Marsal (1975)[9]).

20
Figure 3.5. Typical boring log from the lake region of Mexico

City (Boring PC 28, taken from Marsal (1975)[9]).

21
withdrawal resulting in an increase in density and decrease in

thickness; (2) the thickness of the deposits in general tends to

decrease toward the edge of the basin, where there is a transition to shallow soil.

The effects of groundwater withdrawal on the consolidation of the

deposits can be explained by their low permeability. Any volume

change requires displacement of water from the pores of the

soil. This process is very slow and may take decades or even

centuries. In the meantime part of the load acting on the soil is

supported by excess pore water pressures. When water is pumped

out from intermediate sand layers, excess porewater pressures are

relieved and negative pressure gradients are often created which

further increase overburden pressures causing consolidation

settlements, which can be of the order of several meters.

Figures 3.6 and 3.7 give an overview of the soil conditions in

the part of Mexico City which is underlain by deep deposits (the

"Lake Zone"). Figure 3.6 shows equal depth contours for the

bottom of the upper clay layer that is the top of the first

compact sandy layer which is called "capa dura" or "hard cap."

This layer, which is from 0 to 6m thick and averages about 3m

supports many of the pile foundations for the taller structures.

Figure 3.7 shows equal depth contours for the top of the "deep

deposits," which may also include the third clay layer. Overall

data on the depth of bedrock are not available, since most

borings stop either at the first sand layer or near the top of

the deep deposits. The depth of the phreatic surface is approximately 2m over most of the "Lake Zone" of Mexico City.

In accordance with the different types of foundation conditions

encountered, Mexico City has been subdivided into three principal

zones: the "Lake Zone" which is underlain by deep deposits; the

"Transition Zone" where the soil deposits become progressively

shallower and consist of erratically interspersed alluvial and

22
(•) Strong motion

seismographs

r.7.-.- Lake zone

ffrrrm-Foothill zone

TRANSITION

ZONE

U.S. Embassy

../

..I .•1

•

• 4

• 'l V // e — < - 0/ <-, /

(b / cc ^ /

LAKE ZONE

OLD TOWN

MOST CONSOLIDATED

LAKE DEPOSITS

/^-Ve Juare; •34

Airport

Tacubaya

FOOTHILL ZONE

4 km

SHALLOW BASALT ROCK

LESS CONSOLIDATED

LAKE DEPOSITS

VIRGIN LAKE DEPOSITS

Of icina 0 CENTRAL MARKET

Wprigorifico

Cerro de la Estrella

Figure 3.6. Depth contours for bottom of upper clay layer (after

Resendiz et al. {1970)[11].

23
(•) Strong motion

seismographs

r-.-- Lake zone

-w;7TTTj-Foothill zone

50

/ LAKE ZONE

OLD TOWN

MOST CONSOLIDATED

LAKE DEPOSITS

Airport

LESS CONSOLIDATED

5gLAKE DEPOSITS

VIRGIN LAKE DEPOSITS

Of icina

(•) CENTRAL MARKET

v—'Frlgorif ico

SHALLOW BASALT ROCK Cerro de la Estrella

Figure 3.7. Depth contours for top of deep deposits (after

Resendiz et al. (1970)[11].

24
lake deposits, and the "Foothill Zone" where shallow, compact

deposits of lava rock are encountered. The three zones are shown

in figure 3.8. Only the Lake Zone will be discussed herein,

since earthquake damage in the other zones has been minor. Note

that within the Lake Zone the limits of the old city are delineated, since in that area the consolidation of the clay layers,

resulting from man-made fill, construction, and groundwater

withdrawal was the greatest. The Lake Zone is further subdivided

into "consolidated" (often overconsolidated) lake deposits (by

loading and groundwater withdrawal in the zone of the city

center) and "virgin" (normally consolidated) lake deposits

(Central de Abastos, airport and toward the east)

.

3.2.3. Engineering Characteristics of Soil Deposits

(1) Mechanical Properties

The average properties of the three clay layers encountered in

the borings in figures 3.4 and 3.5 are shown in table 3.2. Note

that the initial void ratio is lower in the soils within the

city. Figure 3.9 shows a plot of the average mechanical properties of clays, derived from many tests, as a function of the

water content. As expected, strength increases and compressibility decreases with decreasing water content. Note that there

is a large difference between the undisturbed unconfined compressive strength (Qui) "^^^ remolded strength (qu^) • This

means that the clays are sensitive and will lose much of their

shear strength when subjected to large shear deformations.

Another important property when evaluating earthquake effects is

the dynamic shear modulus. Some determinations have been made by

Marsal, et al (1959) [10] by torsional dynamic tests. Typical

values of the dynamic shear modulus (G) reported by Marsal are

1.2 MPa for w^ > 250 percent, and 2.5 MPa for 150 percent < w-^ < 250 percent. More recent information obtained in UNAM indicates

that the maximum dynamic shear modulus for the clay deposits

25
Table 3.2 Average Values of Index Properties in Borings BNP-1 and Pc-28

BNP- 1 Pc-28

Property- FAS CD FAI FAS CD FAI

Natural water content, w, in % 290 78 203 270 58 191

Liquid limit wl, in % 299 85 254 300 59 288

Plastic limit, Wp , in % 72 34 66 86 45 68

Specific gravity, Sg 2.55 2.60 2.61 2.30 2.58 2.31

Initial void ratio, e^^ 7.89 1.40 6.59 6.17 1.36 4.53

Unconfined compressive

strength, q^, in kg/cm^ 0.72 >2.0 1.0 0.85 2.4 1.6

FAS = upper clay layer.

FAI = lower (second) clay layer.

CD = capa dura (first sandy layer)

26
® Strong motion

seismographs

Lake zone

f»nTmFoothill zone

TRANSITION

ZONE

U.S. Embassy

Tacubayj

FOOTHILL ZONE

4 km

SHALLOW BASALT ROCK

LAKE ZONE

OLD TOWN

MOST CONSOLIDATED

LAKE DEPOSITS

Airport

LESS CONSOLIDATED

LAKE DEPOSITS

VIRGIN LAKE DEPOSITS

Of icina

(5) _ CENTRAL MARKET

vS'Frigorif ico

Cerro de la Estrella

Figure 3.8. Zoning of Mexico City in accordance with subsurface

conditions.

27
Mean value = 212.7 %

Number of tests = 1 1 745

0 200 400 600

30

0 200 400 600

251—

20

15 —

10

T 1

\

1

I ^—Standard deviation

•Mean value —

200 400 600

W;

List of S ymbo I s :

Wp,

natural water content. In

percent

Initial void ratio

spec I f I c gravity

liquid limit. In percent

plastic limit. In percent

plasticity Index = W|_-Wp

^ul'

^ur'

compressive strength

(undisturbed). In kPa

compressive strength

( remo I ded ) In kPa

friction angle, conso I I dated-undra I ned tests

coef f I c I ent of consolidation. In mm^/sec x 10"'

Figure 3.9. Mechanical properties of Mexico City clays as a

function of water content (taken from Resendiz, et

al (1970)[11]).

28
ranges from 3 to 6-7 MPa and that the Damping Ratio for strains

in the order of 10"^% is approximately 6 percent. It is also

important to note that the water content decreases with a decreasing thickness of the clay layer. This tendency is

illustrated in figure 3.10 which was presented by Resendiz, et

al. (1970) [11]. The phenomenon is attributed to the increase in

the rate of consolidation with decreasing layer thickness.

The properties of the first firm layer (capa dura) were also well

defined. Figure 3.11 shows histograms of layer thickness, grain

size characteristics, and Standard Penetration Test (SPT) blow

count

.

(2) Subsidence

The clay deposits in the Mexico City area are slowly consolidating under the effect of their own weight, the weight of

superimposed fill and structures, and the effect of groundwater

withdrawal. Thus, the Lake Region of Mexico City is subsiding.

Figure 3.12 illustrates the problem. The Monument of the Revolution, which is supported by end bearing timber piles, slowly

emerges as the surrounding ground subsides. As the elevation

difference increases, steps have to be added around the periphery

of the monument.

The rate of subsidence is not uniform and depends on the amount

of water withdrawal and surface loading. Since the turn of the

century, subsidence in the center of the city ranged from 3 to

8.5m [9]. The rate of settlement was greatest in the 1950's

(about 0.5m/year) and has since decreased to less than O.lm/year

as the rate of groundwater pumping was reduced. There is evidence

that in many locations the pattern of settlement was irregular

resulting in erratic subsurface conditions [11]. Data in Reference 11 indicate that in one case about 60 percent of the settlement was attributed to the compression of the upper clay layer.

29
o

CO

o

CO

oo 1—

Q_

LU

Q

o

C\J

LO

o

o

LO

o

o

o

o

CO

(%) 1N31N00 a31VAA NVIOaiAl

cn

CD

o

c

0-t—

c

o

o

I_(D

-»—

CO D

c

(0

c

o

c0

0

*—

0

JD

Q.

c

o

0

cco

CO

0

O)

30
Number of borings: 230

(D 100

ccO

m

LLO

cc

LU

CO 50

0 4 10 30 50

SPT BLOW COUNT (N)

70

O C/3 Num ber of b orings: 1 10

0 1 2 3 4 5 6 THICKNESS OF CAPA DURA (m)

140

c/5 2 100

^ 50

0

Number of borings:

329

GP GWSW SPSM SC ML MH CL CH SOIL PROPERTIES

ure 3.11. Physical characteristics of the Capa Dura.

31
Figure 3.12. Emergence of the Monument of the Revolution from

the surrounding terrain.

32
(3) Foundation Characteristics

Foundation design in Mexico City has always been dominated by

settlement control. Even many of the antique colonial structures

were supported by 5 to 15m long timber piles (pile driving is

relatively easy in the sensitive clays) . The most common

foundation types used are: spread footings for very low

structures (1 to 2 stories) , mat foundations which sometimes use

inverted arches or box foundations to spread the load, friction

pile foundations within the upper clay layer and pile foundations

resting on the "capa dura" or below the capa dura. The pile

foundations are usually combined with mats, box foundations or

heavy gradebeams. Excavation is widely used to compensate for all

or part of the building weight, however, the use of this

technique is limited by stability, heave, and settlement problems

associated with deeper excavations. Sometimes excavations are

executed in stages to minimize vertical and horizontal

displacements of adjacent structures, caused by ground movement

associated with bottom heave in the excavation. Foundation piles

are subjected to large negative friction forces (downdrag forces

exerted on the pile by the surrounding soil) as the penetrated

clay layer is compressed. The ratio of negative friction forces

to tip resistance of piles is sometimes artificially increased by

reduced sections for piles penetrating to the capa dura or by

design configurations of pile groups, in order to facilitate

downward displacement as negative friction forces become large,

and thereby to minimize the emergence of the supported

buildings. Sometimes special piles are installed to provide the

reaction forces for correcting excessive tilting caused by

differential settlement.

33
Under these conditions, it is reasonable to assume that:

1) many building foundations are subject to stresses

introduced by differential settlements caused by nonuniform

soil conditions and adjacent construction

2) many buildings were out of plumb before the earthquake

struck, causing a " P - delta" effect (eccentricity of

gravity loads which exerts moments on columns)

3) pile foundations are subjected to negative friction

forces

These conditions contributed to some of the structural failures

caused by the earthquake.

3 . 3 Ground Motions in the Mexico City Area

3.3.1 Ground Motion Characteristics

Peak accelerations for stations in the Mexico City area are

summarized in table 3.3, which also lists subsurface

characteristics at these stations (refer to figure 3.8 for

locations). For comparison, the Teacalco record from table 2.1 is

also shown in table 3.3. Figures 3.13, through 3.16 show

acceleration records obtained from stations at UNAM, the

Communications Center (SCT) , and the Central Produce Market,

respectively [12-14]. Figure 3.17 shows a comparison of the

east-west acceleration records for four stations in the Mexico

City area. Figures 3.18 and 3.19 show strong motion records in

the east-west direction for the SCT station [13] and the Central

Produce Market site [14], respectively, to provide information on

the magnitude of ground displacements and velocities.

34
Table 3.3 Peak accelerations in the Mexico City Area

Station

UNAM MV

Subsurface Condition

3-5in basalt over softer

strata - free field

Direction

N-S

E-W

DN

Acceleration

(peak, gals)

37

39

20

UNAM IP 3-5in basalt over softer

strata - free field

N-S

E-W

DN

32

35

22

UNAM 01

SCT

3-5in basalt over softer

strata - Main Building,

Hilly Zone

Lake Zone, "less

consolidated"

N-S

E-W

DN

N-S

E-W

DN

28

33

22

98

168

36

Cent. Prod.

Market , Frig.

Lake Zone, "Virgin"

free field

N-S

E-W

DN

81

95

27

Cent. Prod.

Market, Of.

Lake Zone, "Virgin"

1 Sto. Bdg.

N-S

E-W

DN

69

80

36

Viveros Soft Soil, Transition

Zone - Free Field

N-S

E-W

DN

44

42

18

Tacubaya Hard Soil, Hilly Zone

Free Field

N-S

E-W

DN

34

33

19

Teacalco Volcanic Tuff N-S

E-W

DN

49

24

27

35

37
38
39
EAST-WEST ACCELERATION

UNAM

2

10 20 30 40 50 60

TIME, s

Figure 3.17. Comparison of acceleration records in the east-west

direction from the Mexico City area (from Ref. [4]).

40
41
(STVO) (s/ujo) (tuoj

N0llVa31300V A1I0013A lN3W30VldSm

42
It can be seen from figure 3.4 that the strong motion records

vary considerably from site to site. The variation is not only in

amplitude and frequency content of the ground motion, but also in

the duration of the strong motion, indicating that some sites

continued to vibrate even after cessation of large amplitude

ground motions in the underlying bedrock. This phenomenon is

particularly pronounced at the Central Produce Market site record

shown in figure 3.19, where high amplitude motions continued for

about 100 seconds, as compared to about 60 seconds at the UNAM

site (figure 3.13). Note that, of the strong motion records

shown, only the UNAM stations are located on rock

deposits. However even these stations are located on a basaltic

deposit of recent origin, which rests on weaker deposits of

unknown depth. Data on the frequency content of the strong motion

of the bedrock underlying the various soil deposits would help

explain the amplification of the ground motion associated with

these deposits.

3.3.2 Effect of Subsurface Conditions on Ground Motion

A comparison of the records from the various stations in the

Mexico City area gives an indication of the effect of the

subsurface conditions on the ground motion. The UNAM site is in

the Foothill Zone. It is underlain by a 3-5m thick basaltic rock

layer of recent origin. However, this layer is on top of softer

strata of unknown thickness. The Tacubaya site is also in the

foothill zone, but the instrument is located on shallow deposits

of compact soil, rather than bedrock. The Viveros station is on

shallow deposits of soft soils in the transition zone, and the

SCT and Central Market (CDA) stations are located in the Lake

Zone of Mexico City.

The comparison of the ground motion observed at the two stations

in the lake zone (SCT and CDA) with that of the other stations in

the Mexico City area (see Table 3.3) gives an indication of the

43
effect of the lake deposits on the ground motion at their

surface. It can be seen from these records that the ground

motion is affected by the deep deposits in two ways: (1) the

ground motion is amplified; and (2) the dominant period of the

ground motion is modified by the dynamic characteristics of the

soil column. These two observed effects are discussed below:

Amplification: As previously noted, none of the records

in Mexico City presented herein was taken on competent

bedrock. The nearest strong motion record on bedrock is

from Teacalco, about 100 km west, in the direction of

the epicenter. In Teacalco, as well as in most of the

other stations of the Guerrero array, the north-south

component of the acceleration record tended to have a

larger amplitude than the east-west component. The

reverse is true for the records taken in the Lake Zone

of Mexico City. This is probably related to the

frequency content of the strong motion. One would

expect that the amplitude of the rock motion in

Teacalco was somewhat attenuated over the 100 km

distance to Mexico City. The north-south components of

the ground acceleration at the UNAM and Tacubaya

stations are somewhat smaller than the acceleration at

Teacalco and also consistent with each other. The

East-west components, on the other hand, are greater

than that at Teacalco.

From table 3.3 it can be seen that the peak

accelerations at the SCT station were approximately

500% of those at the UNAM stations in the east-west

direction and 325% in the north-south direction and at

the Central Market station (f rigorif ico) they were

approximately 290% of those at the UNAM stations in the

east-west direction and 2 50% in the north south

direction.
To further illustrate the amplification of ground

motions, the spectral ratios derived from the vertical

and horizontal components of motions at the SCT station

with respect to the UNAM station are shown in figure

3.20 (taken from unpublished USGS open file

report: Celebi M. et al., Mexico City Site Response

Studies) . These spectral ratios are frequency dependent

amplification ratios and depict that at the 2 second

period the horizontal accelerations at the SCT site

were 7-10 times those at the UNAM site.

Dominant Period: The dominant period of the ground

motion at the SCT station was 2 seconds, and those at

the two Central Market stations were close to 3 seconds

at frigorifico and close to 4 seconds at the central

office. It can be seen from figures 3.18 and 3.19 that

the low frequency components of the ground motion in

the lake zone were damped out and during part of the

record the dynamic response was almost perfectly

sinusoidal

.

The dynamic response of the soil column can be correlated with

two parameters: its damping ratio and its natural frequency. As

previously noted the damping ratio of the clay deposits at a shear strain of 0.1% is approximately 6% of critical

damping. Strains actually experienced during the earthquake may

have exceeded 0.1% and thus the damping ratio may have been

somewhat higher. Assuming that it was 6% or more, the maximum

possible amplification of the ground motion by the soil column

(as measured by the ratio of peak accelerations) would be

830%. The actual amplification of any particular ground motion at

the base of the column would depend on the frequency content of

that ground motion. The fact that a 500% amplification occurred

indicates that the ground motion of the bedrock at the base of

45
EW

0 1 2 3 14

0 12 3 4 HZ

VERT.

0 12 3 4 HZ

Figure 3.20 Frequency-Dependent Spectral Ratios Determined

from the Strong -Mot ion Acceleration Records of

the September 19, 1985 Mexico City Earthquake

from the SCT and UN AM Stations. (Ratios are

plotted as SCT acceleration/UNAM acceleration).

46
the lake region had a significant energy content close to the 0.5

Hz frequency of the soil column.

The frequency contents of the ground motions at SCT, UNAM, and

Tacubaya, which were obtained by a spectral analysis of the

acceleration records, are shown in figures 3.21 to 3.23. As

expected, the north-south and east-west components of the SCT

motion shown in figure 3.21 had a dominant frequency of 0.5

Hz. The frequency content of the vertical component of the SCT

motion, which is in part attributable to compression waves^ is

much more scattered and generally indicates that the natural

frequency of the soil column in compression is much higher than

that in shear. This is attributable to the low compressibility of

the saturated clay which was previously discussed.

All three components of the Tacubaya spectra shown in figure

3.22, but particularly the vertical component, have a dominant

0.5 Hz frequency. Since the Tacubaya station is in the foothill

zone and on shallow compact soil (the natural frequency of the

soil column is much higher than 0.5 Hz) this is taken as an

indication that the motion of the underlying bedrock had a dominant 0.5 Hz frequency. Response spectra developed for the

ground motion at the UNAM site [15] show significant

amplification at the 2s period.

The UNAM record in figure 3.23 also shows that in the east-west

and the vertical direction significant energy was concentrated in

the 0.5 Hz frequency range. In the north-south direction the

dominant frequency is not as pronounced.

Even though the ground motions in this earthquake had a larger

amplitudes than those observed in previous earthquakes, the

amplification and frequency responses associated with the deep

lake deposits can be compared with previous observations.

47
SCT1__N90U

(East -West)

I I I I I 1

I

I I I I \ I 0 2.1 4.2 6.2 8.3 10.4 12.5 14.6 16.7 18.7 20.8 22.9 25.0

SCT1_S00E

(North-South)

LUQ

Ilyiliii iii I i^.-i-i .i.. -^1 ..L^A.,A,L.l^...^^ A—ki

I I I \ I I I I 1 I I 1 I 0 2.1 4.2 6.2 8.3 10.4 12.5 14.6 16.7 18.7 20.8 22.9 25.0

SCT1__VER'

(Vertical

UJQ3

_1 I I I I I I I I I I I 2.1 4.2 6.2 8.3 10.4 12.5 14.6 16.7 18.7 20.8 22.9 25.0 FREQUENCY (Hz)

Figure 3.21. Spectral analysis of the ground motion at the SCT

Station„

48
TACY N90U

(East-West)

0 2.1 4.2 6.2 8.3 10.4 12.5 14.6 16.7 18.7 20.8 22.9 25.0

TACY_S00E

(North -South)

0 2.1 4.2 6.2 8.3 10.4 12.5 14.6 16.7 18.7 20.8 22.9 25.0

UJQ3

CL<

TACY_ VERT

(Vertical)

0 2.1 4.2 6.2 8.3 10.4 12.5 14.6 16.7 18.7 20.8 22.9 25.0 FREQUENCY (Hz)

Figure 3.22. Spectral analysis of the ground motion at Tacubaya.

49
HIQ

ID CUIP_N90U

(East -West)

I I I I I ! I I I 0 2.1 4.2 6.2 8.3 10.4 12.5 14.6 16.7 18.7 20.8 22.9 25.0

CUIP„N00E

(North -South)

CL<

0 2.1 4.2 6.2 8.3 10.4 12.5 14.6 16.7 18.7 20.8 22.9 25.0

CUIP_VERT

(Vertical/

0 2.1 4.2 6.2 8.3 10.4 12.5 14.6 16.7 18.7 20.8 22.9 25.0 FREQUENCY (Hz)

Figure 3.23. Spectral analysis of the groun(d motion at UNAM„

50
Rosenblueth (1979) [16] stated that "soil amplification, particularly for long periods, is extreme in the soft clays of the

Valley of Mexico. In acceleration spectra prevailing periods of

2-2. 5s are common, and at one site (the sports palace) they

reached 5s." The 100 year return period maximum ground

accelerations and velocities for the lake zone in Mexico City

have been estimated by Faccioli, et al . (1975) [17] to be 0.14G

and 0.81 m/s, respectively. These can be compared with the 0.17G

maximum acceleration and 0.61 m/s maximum velocity shown in

figure 3.18. Reference [18] lists accelerations measured in the

March 14, 1979 earthquake at the UNAM site and in the basement of

the National Lottery Building in downtown Mexico City (Reforma at

Juarez) as 21 gal and 55 gal, respectively, in the east-west

direction. This represents an amplification of peak

accelerations in excess of 250 percent. A free field measurement

would have probably recorded a larger acceleration response than

that in the basement of a building.

Thus the amplitude of the ground motion measured at the SCT site,

and the amplification effect and prevailing period associated

with that ground motion are not at all inconsistent with previous

observations and estimates.

A boring in the vicinity of the SCT station (#231 in Ref.[ll])

shows the following soil profile: a 6m surface layer; a 2 6in

thick upper clay layer with a water content of 27 5 percent; a 6m

thick capa dura with an SPT blowcount of 30 blows per 0.3m; a 3m

thick lower clay layer with a water content of 2 00 percent; and

deep deposits at 41m. The boring was not continued beyond the

42m depth. However, other information indicates that the thickness of the deep deposits is approximately 38m. If it is assumed

that: at the strain level experienced in the earthquake, the

shear wave velocities of the two clay layers are 70 m/s for the

upper and 12 0 m/s for the lower (UNAM data indicate that shear
wave velocities range from 60 m/s to 120 m/s) ; the shear wave

velocities for the upper layer and the capa dura are 250 m/s; and

the deep deposits are about 38m deep and have a shear wave

velocity of 3 00 m/s (these are estimates, since data on dynamic

moduli are only available for the clay) ; then the natural period

of the site would be about 2 to 2.1 seconds which is the dominant

period of the SCT record.

Figure 3.24 shows a plot of the locations of severely damaged or

collapsed buildings, on which depth contours for the top of the

capa dura and the top of the deep deposits at the boundary of the

damage zone are superimposed. From this plot it is apparent that

most of the damage area is between the 3 0m and the 38m depth

contours for the top of the capa dura and the 38m and the 50m

contours for the top of the deep deposits. There is another

limited area of damaged buildings to the south, outside the area

covered in figure 3.24 (between the UNAM stations and Cerra del

Estrella in figure 3.8). That latter area is also in the lake

zone. If it is assumed that: the thickness of the deep deposits

is 3 0m at the shallow end of the damage zone and 40m at the deep

end of the zone; the shear wave velocities of the first clay

layer is 70 m/s at the shallow boundary and 60 m/s at the deep

boundary; and the shear wave velocity of the second clay layer

was 12 0 m/s; then the earthquake damage was concentrated in a

zone where the estimated natural period of the site was between

1.9 and 2.8 seconds. Not much damage occurred outside that

zone. The damage can be attributed to both the amplification of

the ground motion by the soil column and the natural period of

the damaged buildings.

3.3.3 Comparison of Response Spectra With Design Spectra

Used in Mexico City and the U.S.

The significance of the ground motion record in the Lake Zone is

' illustrated by the response spectra shown in figures 3.25 and

3.2 6 for the SCT and the Central Market sites, respectively. The

52
0 Kilometers 5

ure 3.24. Soil depth contours at the boundary of the damag

zone.

53
0 1 2 3 4 5 6

NAT. PERIOD (s)

Figure 3.25. Comparison of acceleration response spectra

calculated from 5% damping for the SCT site (taken

from Mena, et a! [13]) with the design spectrum for

nonductile structures in the lake zone included in the

1976 Mexico Federal District Building Code.

54
1.0

0.9

0.8 -

0.7 -

0.6 -

0.5 -

Response spectra calculated from ground

motion for 5% damping

Frigorifico

'^"S Central office

Design spectrum

for non-ductile

buildings

2 3 4

NAT. PERIOD (s)

Figure 3.26. Comparison of acceleration response spectra

calculated for 5% damping for the Central Market site

(taken from Quaas, et al [14]) with the design spectrum

for nonductile structures in the lake zone included in

the 1976 Mexico Federal District Building Code.

55
response spectra, which were taken from References [13] and [14],

are for a single degree of freedom resonator with 5 percent

viscous damping, and provide a measure of the magnitude of the

base shear that the ground motion would produce in buildings, as

a function of their fundamental period. For comparison, the

design spectrum for nonductile buildings incorporated in the

Mexico Federal District Building Code [19], which was developed

on the basis of previous seismic events and was used as a basis

for design of many of the modern buildings, is also plotted in

the figures. It can be seen that buildings with natural periods

close to 2 seconds located in the zone which experienced ground

motions similar to that at SCT were subjected to shear forces far

in excess of the forces specified in the design spectrum. Safety

margins inherent in the applicable design procedures would not

compensate for this large discrepancy. This probably accounts

for the observed selective failure pattern, where buildings

between 7 and 18 stories high suffered the greatest damage. It

should be noted that the natural period of buildings in the

affected area was lowered by the interaction between the

structure and the supporting soil and by initial damage. An

examination of figure 3 . 18 indicates that there were 9 load

cycles in which the acceleration was close to the maximum. Thus

buildings subjected to this ground motion had to survive 9

reverse cycles of maximum lateral forces. Ground motions similar

to that at the Central Market site would have been critical for

taller buildings. Fortunately, there are no tall buildings in

that area. Had there been such buildings, they would have been

subjected to 3 0 reverse cycles of maximum lateral acceleration-'-.

Figures 3.27 shows a comparison of the response spectra for the

SCT site, as well as those calculated for the observed ground

motions from the Central Produce Market site, with design spectra

Reference 6 notes that the Central Office record may have

been affected by the building. The other record from that site

(Frigorif ico) does not cover the entire duration of the earthquake.

56
0 I \ \ \ \ \ L

0 1 2 3 4 5 6

NATURAL PERIOD (s)

Figure 3.27. Comparison of design spectra for non-ductile

structure stipulated in present and proposed U.S.

design provisions with response spectra for 5%

damping calculated for the ground motions observed

during the September 19, 1985 earthquake at the SCT

and Central Produce Market sites in Mexico City.

57
developed in accordance with presently used or proposed

U.S. design provisions. These include the 1985 edition of the

Uniform Building Code (UBC) [20], the NEHRP "Recommended

Provisions for the Development of Seismic Regulations for New

Buildings" [21], and the Tentative Lateral Force Requirements of

the Structural Engineers Association of California (SEAOC) [23].

The notations used in Figure 3.27 are taken from the referenced

documents: = coefficient representing effective peak acceleration and A^ = coefficient representing effective peak velocity

from Ref . [21] ; S = coefficient for soil profile characteristics

from Refs. [20,21, and 22]; R and R^ are seismic response

modification coefficients taken from Refs. [21] and [22],

respectively.

Te upper NEHRP spectrum and the UBC and SEAOC spectra are for the

most severe condition envisioned for the U.S [a site with 30ft

(10m) or more depth of soft to medium stiff clays or sands in the

San Francisco Bay area or an area with similar anticipated ground

motion severity] . The NEHRP and SEAOC spectra were obtained by

plotting the equations for the coefficients for seismic base

shear, calculated for R or R^ values of 1. It is reasoned that

these spectra represent ultimate loads for non ductile structures

(the NEHRP provisions specify strength design; the SEAOC provisions are written for working stress design, but they use larger

response modification factors than those in the NEHRP provisions

and thereby compensate for the difference between working load

and ultimate load) . The UBC spectrum was calculated for a hypothetical K value of 0.67x12. The rationale for this K value

is that 0.67 is the UBC K value for ductile moment resistant

space frames, for which the SEAOC document specifies an R^ value

of 12. Since both the SEAOC and the UBC documents are written for

working stress design, the SEAOC response spectrum for Rw=l would

correspond to the UBC response spectrum for K=0. 67x12. The lower

NEHRP spectrum is for a site for which the seismic coefficient

representing the effective peak velocity is 0.3, and that

58
representing the effective peak acceleration is 0.2, which could

conceivably have been chosen for Mexico City before the last

earthquake occurred [refer to Ref. 21 for definition of the

seismic coefficients)

A comparison of the response spectra calculated for the Mexico

City earthquake for a 5% damping ratio with the design spectra

drawn in figure 3.27 gives a reasonable indication how structures

designed by present or proposed U.S. provisions may have

performed in the Mexico City earthquake. It is evident that

structures with natural periods ranging from 1.8 to 3 seconds

could have failed when subjected to ground motions similar to

those measured at the SCT site, even if they were designed for

the most severe U.S. conditions envisioned. The situation would

have been even more critical, had these structures been designed

in accordance with the design spectrum shown by the lower NEHRP

curve (or curves for similar conditions derived from the UBC or

SEAOC documents) , which most probably would have been the case in

absence of the benefit of the hindsight we now posses. In the

latter case, structures with natural periods ranging from 3.4 to

4.4 seconds may also have performed poorly in the vicinity of the

Central Produce Market.

It is important to examine the reasons for the conclusions that

can be drawn from figure 3.27. The design spectra in References

20 to 22 were developed on the basis of generalized assumptions

on ground motion characteristics, derived from information from

many sites, which apply to most situations likely to be

encountered in the U.S., but do not apply to the conditions

encountered in the Lake Zone of Mexico City, where

large-amplitude ground motions occurred at low frequencies. This

situation can occur when two conditions exist: 1. The site has a relatively long natural period and the soil deposits have a low

damping ratio; and 2. the underlying bedrock transmits ground

motions whose frequency content it such that they can be

59
amplified by the soil column. The question arises whether these

conditions are predictable.

The first condition is amenable to analysis, since its parameters

can be estimated. We can in most instances estimate the natural

period of sites with deep clay deposits similar to those in the

lake region of Mexico City on the basis of past strong motion

records or appropriate in situ and laboratory measurements. We

can also get an indication of damping ratios from laboratory

tests on undisturbed samples, which usually can be obtained in

cohesive soils. We can also determine the natural period of

buildings with a reasonable allowance for the

structure/ foundation interaction, and thus predict which

buildings would be vulnerable to the ground motion.

The second condition is much more elusive. It seems much more

difficult to estimate the probable frequency content of the

anticipated ground motions of the bedrock underlying the soil

column. Incorrect assumptions could lead to unduly conservative

requirements or may cause us to underestimate seismic risks.

Some consideration should be given to the introduction of an

alternate method of calculating design amplification factors as a function of the ratio between the natural period of the site and

that of the supported buildings. This would permit us to

selectively increase lateral load requirements for buildings

which have natural periods close to that of the site and thereby

prevent the kind of selective damage pattern that occurred in

Mexico City (the 1975 edition of UBC used a "site-structure

resonance coefficient", S, but the resulting response spectrum

did not exceed the UBC spectrum drawn in figure 3.2 6). This

method could then be applied in areas of soft to medium stiff

deep clay deposits which are thought to be at risk [clays, rather

than granular deposits, are of concern because of the low damping

ratios observed in some clays (for instance 6% for shear strains
in the order of 10"" for the Mexico City clays) ] , and design

criteria could conceivably be pre-determined by microzonation

61

4. FOUNDATION FAILURES

4 . 1 General

Table 4.1 summarizes building characteristics which were observed

to contribute to failures during the earthquake. The percentages

given in the table add up to more than 100%, since several causes

can be observed to contribute to any one building failure. It can

be seen from the table, that in about 15% of all failures

unsatisfactory foundation performance or prior differential

settlements were observed to be major contributing causes. There

may have been other instances where foundation failures, such as

shear failures in piles, occurred and were not observed because

they were below ground. In addition, stresses introduced by

differential settlements which were not visually observable

probably reduced the load resistance of many structural

members. Nevertheless, the percentage of foundation related

failures was surprisingly low, given the difficult foundation

conditions in the lake region of Mexico City, even if an

allowance is made for possible unobserved failures or causes of

failure. To fully appreciate this fact, one should examine the

response spectra shown in figure 3.25, which give an indication

that many building foundations were subjected to overturning

forces far in excess of those assumed in their design.

63
I

Table 4.1 Causes of Building Failures in Mexico City (from | Ref.[23])

j

Observed Characteristics Contributing to

Failure

1 Percent of Cases

Corner buildings 1 42

Intermediate story failure 1 40

Upper story failure 1 38

Pronounced asymmetry in stiffness 1 15

Pounding 1 15

Unsatisfactory foundation performance 1 13

Excessive mass 1 3

Weak first story 1 8

Previous earthguake damage 1 5

Punching in waffle slab 1 4

Short columns 1 3

Previous differential settlement 1 2

There are several reasons for this relatively good performance of

structural foundations:

(1) The permeability of the lacustrine deposits is very

low. Thus, volume change which requires the

displacement of pore water cannot occur within the time

frame of an earthquake. Major settlements are therefore

inhibited (according to information obtained from UNAM

faculty minor settlement can however occur as a result

of fissures in the clay)

.

(2) The soils in the lake region of Mexico City are

predominantly cohesive and will therefore not liquefy

during an earthquake. In many instances, liquefaction

64
would cause major devastation in an earthquake of that

magnitude.

(3) Shear deformations and shear failures which do not

require volume change (Poisson's Ratio = 0.5) can and

did occur, however the time element involved in the

development of major rotational slides in clays mayhave inhibited or limited many shear failures which

otheirwise would have occurred under the prevailing

equilibrium conditions.

4.2 Loss of Shear Resistance of Piles

Figure 4.1 shows a view of the sidewalk next to the Bank of

America building on 122 Reforma. Point A is on a sloped portion

of the sidewalk pavement which is attributed to differential

settlement distortions between the building and the surrounding

area which predated the earthquake (some cosmetic repairs are

evident) . Point B shows a drop of approximately 80mm between the

sidewalk and the building which is attributed to the earthquake.

Figure 4 . 2 shows some disturbance in the sidewalk pavement near

the building, that probably resulted from the 80 mm drop in the

pavement. Similar patterns of distress were observed in many

other locations, for instance, the differential slips shown in

figures 4.3 and 4.4 which occurred in two buildings at the

intersection of Reforma and Calle Versalles.

These disturbances did not cause major damage, but the

interpretation of the mechanism that caused the disturbances has

important implications. As previously noted, piles in the Lake

Region (particularly end bearing piles) are subjected to progressively increasing negative friction forces (downdrag exerted by

the surrounding soil) as the surrounding area subsides. These

friction forces cause an upward warping of the surrounding soil

layers, as their downward displacement is inhibited. If these

negative friction forces between the soil and the pile are

65
Figure 4.1. Differential displacements at the sidewalk near the

Bank of America Building.

66
Figure 4.2. Disturbances caused by differential displacement

between the sidewalk and the Bank of America Building.

67
Figure 4.3 Differential displacement between sidewalk and building

at the east corner of Versalles and Reforma.

68
Figure 4.4. Differential displacement between sidewalk and

building at the west corner of Versalles and Reforma.

69
released, the soil surrounding the pile will displace downward,

and the pile itself will slightly rebound elastically. In figure

4.1, the slope at point A is an indication of the past emergence

of the building from the surrounding terrain which in turn is a

proof that negative friction forces were acting on the piles

prior to the earthquake. The 8 0mm displacement at point B, as

well as the ground disturbances shown in figure 4.2, are the

result of the combined elastic rebound of the pile (which is

estimated to be less than 10mm and the downward displacement of

the soil surrounding the piles. Even though the saturated clay

cannot develop consolidation settlements during the duration of

an earthquake, soil deformations not associated with a volume

change (Poisson's Ratio = 0.5), such as those associated with the

relaxation of shear stresses, can occur instantaneously.

Thus, it is reasoned that the phenomena observed in figures 4.1

through 4.4 are the result of a relaxation in the negative

friction forces acting on the piles. This relaxation is the

result of loss of shear strength. The loss of shear strength

occurred because the clays are sensitive (refer to figure 3.9)

and was caused by shear deformations associated with the relative

motion between the piles and the surrounding soil. From available data [8] it is estimated that between 85 and 88 percent of

the shear strength can be lost. This loss of shear strength does

not necessarily lead to an overall foundation failure (it did not

cause such a failure in the buildings shown in figures 2 0 to 23),

particularly, since it also results in the relaxation of negative

friction forces which the pile had to resist, but it is an

important limit state, which should be considered in the design

of earthquake-resistant structures founded on sensitive clays.

70
4 . 3 Tilting or Overturning

Figure 4.5 shows a tilted ten-story building in the southwest

corner of the intersection of Lazaro Cardenas and Fernando.

Relative displacements between the building and the sidewalk to

the north are shown in figure 4.6, and figure 4.7 shows damage

caused to adjacent two-story buildings by the tilting structure.

This building is in the vicinity of the severely damaged Communications Center (SCT) where several buildings collapsed. Figure

4.8 shows another tilted building in the same general area in the

northwest corner of the intersection of Xola and University.

This ten-story building also suffered severe structural and

cladding damage. Figure 4.9 shows two- and three-story buildings

a short distance to the south on University and opposite the

severely damaged Communications Center which apparently did not

suffer damage. Likewise, many two-story residential buildings on

University to the north of Xola, where two multi-story reinforced

concrete buildings (one of them post-tensioned) collapsed, also

appeared undamaged.

On the other hand, to the east of the building in figure 4.5 on

Toledo and Galicia there are evidence of severe ground disturbance and also of damage to many of the two-story residential

buildings. Some of the ground disturbance is shown in figure

4.10. There is some indication that, at least in part, the ground

disturbance is attributable to differential movement between

utilities and the surrounding soil.

Tilting of structures did not always result in adverse effects.

The 20-story microwave tower next to the Communications Center

which before the earthquake was reported to have a tilt

displacement of 600mm experienced a substantial reduction in this

tilt displacement as a result of the earthquake (information

obtained from UNAM) . Soil conditions in the vicinity of SCT were

71
Figure 4.5. Tilted ten story building at Lazaro Cardenas and

Fernando.

72
Figure 4.6. Relative displacement between sidewalk and tilted

building at Lazaro Cardenas and Fernando.

73
Figure 4.7. Damage to adjacent structures caused by the tilting

of a ten story building at Lazaro Cardenas and

Fernando.

74
Figure 4.8. Tilted ten story building at University and Xola.

75
Figure 4.9. Undamaged two and three story buildings on

University Boulevard opposite the severely damaged

Communications Center.

76
Figure 4.10. Ground disturbance on Galacia viewed from

intersection of Toledo in a westerly direction.

77
previously discussed and include a 6m surface layer (some of it

fill) and a 26m upper clay layer with a water content of 275

percent.

An example of an overturned building is shown in figures 4.11 to

4.13. The building is located on the southeast corner of the

intersection of Zacatecas and Orizaba, an area where there is

some irregularity in the surface elevation of the capa dura,

which dips from a depth of 32m to almost 40m (see A in figure

3.6). Figures 4.12 and 4.13 show the exposed pile foundation and

a pulled out pile. Inspection of the dowels connecting the piles

to the grade beam indicated that the connection was very weak.

Apparently no tensile forces were anticipated at that point.

Since only one pile was actually pulled from the ground, it is

possible that the failure was triggered by a failure of the

connections between the piles and the stiff foundation.

Tilting was observed in many severely damaged or collapsed

buildings, but it was difficult to determine to what degree

settlements contributed to the structural failures which caused

the damage. One example is the office complex at Pino Suarez,

where steel frame buildings collapsed. The buildings which

collapsed are discussed elsewhere and collapse is attributed to

column failures. However, other buildings in the same complex

close to the collapsed buildings, which did not collapse, were

observed to be considerably out of plumb (figure 4.14). In this

case, it is unlikely that tilting contributed to the collapse

since the collapsed buildings were located on top of an

underground subway station which suffered relatively minor

damage. However, the stiff base provided by the subway station

may have caused an increase in the shear forces, which otherwise

would have been attenuated by structure foundation interaction.

In other cases, the P-delta effect associated with tilting

78
ure 4.11. Oveturned building at southeast corner of Zacatecas

and Orizaba.

79
Figure 4.12. Exposed foundation of overturned building at

Zacatecas and Orizaba.

80
Figure 4.13. Exposed pile and dowels of the foundation of

overturned building at Zacatecas and Orizaba.

81
during the earthquake and lack of plumbness prior to the

earthquake substantially contributed to structural failures.

4.4 Warping Effects

Many buildings were damaged as a result of distortions caused by

differential foundation settlements. An example is shown in

figures 4.14 and 4.15. The far corner (SW) of the building shown

in figure 4.14 which is located on the south side of Teresa De

Mier just to the east of Medico Militar settled more than the

other corners. This differential settlement resulted in a

warping distortion which can be seen in the wall shown in

figure 4.15.

The building shown in figure 4.14, as well as the Pino Suarez

complex discussed in Chapter 5, the collapsed Juarez Hospital,

and the San Pablo Cathedral, one tower of which overturned, are

all in an area of a few blocks, where other tilted buildings were

observed. In this area at boring #391 [11] which is at the

Juarez Hospital the capa dura is at a depth of 3 6m and the upper

clay layer is 29m thick and has a 3 00 percent water content

(which is relatively high) . The subsurface conditions may have

played a critical roll in all the structural failures in that

area.

4.5 Other Observed Soil Disturbances

In many locations the ground surface over utility lines was

uplifted. An example is shown in figure 4.17 which shows a

photograph taken at the intersection of Monterrey and Obregon.

Figure 4.18 shows old trolley rails which had been covered over

by pavement. The rails buckled as a result of the earthquake

motion. This phenomenon repeated itself on Alvarado Obregon from

Monterrey to the east, always at the same location, just to the

east of every road intersection. The buckling is attributed to

82
83
Figure 4.15. Warped wall on the north side of building on Teresa

de Mier.

84
Figure 4.16. Soil disturbance over utility line at the intersection

of Monterrey and Obregon.

85
Figure 4.17. Buckled old trolley line on Obregon.

86
accumulated compressive stress in the rail lines, caused by

sliding interaction with the surrounding soil as a result of the

horizontal earthquake motion. The rails buckled upwards because

of lack of confinement in that direction and probably failed at a weak spot created by the excavation and backfilling of utility

lines. Heave at the utility line may also have contributed to

the buckling.

4.6 Discussion of Foundation Failure Mechanisms

Since volume change in the soil is inhibited by low permeability,

and liquefaction or cyclic mobility are ruled out as failure

mechanisms, it is reasoned that the observed soil displacements

associated with tilting, differential settlements (warping) and

the release of negative friction forces acting on piles were

associated either with shear failures or with shear deformations,

and only minor settlement could occur as a result of compressive

forces

.

However, since the compressive forces generated by the earthquake

were resisted by excess porewater pressures, it is not ruled out

that gradual settlements could occur over a ' period of time, as

the excess porewater pressures generated by the earthquake

gradually dissipate. Similar phenomena were observed after

previous earthquakes [11].

Special importance is attributed to the previously discussed

observation that many foundation piles lost most of their

frictional resistance during the earthquake. This limit state,

as well as the fact that the clays lose much of their shear

strength when large shear deformations occur, should be

considered in the design of earthquake-resistant foundations.

In view of the potential magnitude of the shear forces and

overturning moments that this earthquake could generate (refer to

87
figure 3.7) it is noteworthy that many buildings apparently

escaped serious structural or foundation damage. This phenomenon

is attributed to the unique foundation conditions in Mexico City:

(1) The structure-foundation interaction will tend to limit

the magnitude of the shear forces that can be resisted at

the foundation level.

(2) Most taller buildings have stiff mat or box foundations

(refer to figure 4.12). The settlement and tilting of these

foundations would require long time increments since any

volume change in the supporting clay layer is inhibited by

its low permeability.

(3) Shear failures which may have occurred in foundation

piles may have escaped detection.

88
5. STRUCTURAL FAILURES

5.1 History of Building Regulations

Historically, Mexico City has been influenced by long distance

effects of earthquakes on the coast. Since 1845, the most recent

ground motion to have caused the collapse of a colonial church, a great number of quakes have registered magnitudes in excess of

7.5 on the Richter Scale along the Middle American Trench (See

Fig. 2.2). It has been recorded that "While local officials were

awaiting the entrance into the City of the leader of the

revolution on June 7, 1911, there was a major earthquake at 4:26

A.M. The local barracks of San Cosme were demolished as were

other buildings and walls in different parts of the city. There

were 39 badly wounded and 49 deaths. Other earthquakes were

subsequently felt and church bells rang by themselves." [33] The

earthquake of June 7, 1911 was the most intense since that of

1845.

The first building code containing earthquake-resistant design

provisions in Mexico dates from 1942 [25]. This code, while

provided minimal guidelines for seismic design. Prior to the

1950 's there were few buildings in the city over 12 stories. The

1942 code required that joints between two adjacent structural

elements be designed to have similar strength as the elements

themselves. Each structure was to be designed and built to

89
oscillate as a unit. Foundations were required to be designed so

that the structure would not slide or overturn and that joints

between piles and foundation girders or slabs would not fail.

Maximum permissible pile overstressing during an earthquake was

33%. This same percentage applied to any point of the bearing

area of the ground or the foundation elements, as well as

structural joints. Seismic design computations were required for

structures over 16m high, those with a height-width ratio greater

than two, and any building intended for use by large groups of

people.

Seismic design coefficients varied from zero for unimportant

structures to 0.1 for the most important. Houses, hotels, and

office buildings had a coefficient of 0.025, which was constant

for every story.

The 1942 code was found to be deficient following large damages

sustained as a result of the July 28, 1957 earthquake. This

shock had a Richter magnitude of 7.5 with peak accelerations of

between 0.05 - O.lg and dominant periods of 0.9 to 1.8 seconds

[28]. This was the worst shock the city had experienced since

records were kept beginning in 1900. Emergency regulations were

promptly issued. These regulations established base shear

coefficients in terms of microzonation and recognized ductility.

Equivalent base shears of 0.17G were stipulated for ordinary

frame structures in the lake zone. An importance coefficient of

2 for critical structures was recommended. However, a 50%

overstress (working) was allowed for combined gravity and

seismic loads. The 1957 regulations were later replaced by the

1966 code which reduced the importance factor to 1.3, allowed

ultimate strength design, explicitly recognized slenderness

effects, and recommended dynamic analysis. Buildings with floor

areas larger than 10,000 m^ or higher than 45m were required to

be instrumented (a provision that was not implemented in

practice). These provisions were subsequently revised in 1969. In
the 1976 code, the three zones dividing Mexico City as in 19 57

were readopted. The 1976 code [19] represented a substantial

improvement, reflecting the state of the art in earthquake

resistant design. Ultimate strength design provisions were

recommended with a load factor of 1.4 for earthquake load.

Lateral load coefficients reflected the known problem of spectral

amplification. Design story shears were obtained by specifying

horizontal accelerations proportional to elevation above ground,

similar to those used in U.S. codes. Detailing requirements

were practically comparable to that of Appendix A of ACT 318-8 3

[26] provisions. However, hoop reinforcement was still

substantially less than that required in ACI provisions. The

maximum design base shear coefficient for non-ductile frames with

5% damping in the lake zone was 0.24. The maximum reductions in

design lateral shear forces permitted for ductile structures was

by a factor of 6. Permissible story drift was limited to 0.008

times the story height. This drift limitation can be compared

with the UBC value of 0.005. Thus the Mexican code permitted more

flexible structures than the Uniform Building Code.

It was with this background, that Mexico City was prepared for

the earthquake of September 19, 1985. Following the earthquake,

on October 18, 1985, emergency regulations were issued. In these

regulations the design response spectrum for the lake zone was

increased to a maximum base shear force coefficient of 0.4 for

non ductile structures in the lake zone, and the maximum

reduction factor for ductile structures was changed from 6 to 4

.

Figure 5.1 shows a comparison of the 1976 and 1985 design spectra

for non ductile structures.

91

5.2 Failure Patterns

Table 5.1 shows the summary of a building damage tally for the

Federal District carried out by UNAM [1]. Damage figures as a percentage of buildings in the surveyed area are given below:

Buildings up to 2 stories 0.9%

Buildings from 3 to 5 stories 1.3%

Buildings from 6 to 8 stories 8.4%

Buildings from 9 to 12 stories 13.6%

Buildings over 12 stories 10.5%

Total number of buildings in surveyed area 53,356

Percent of total buildings heavily damaged 1.4%

There is no information and no reasonable way to find out what

percentage of these buildings actually complied with the regulations which were in force at the time of their construction.

While other statistics on building characteristics that

contributed to the failures were provided in Table 4.1, the most

striking feature of the earthquake damage observed by teams

surveying the earthquake damage was the selective damage

pattern. Most buildings in the 5 to 2 0 story range either

collapsed or were severely damaged, while the smaller 2 to 4

story buildings suffered little or no damage. Modern tall

buildings (more than 3 0 stories) did not sustain any severe

damage.

The cause for this damage pattern is the unique character of the

ground motion in the lake region. As exemplified in the timehistory plots of ground motions in the lakebed zone in figures

3.3 and 3.4 and the response spectrum in figure 3.27, large

spectral accelerations within the dominant ground periods of 1.5

and 2.5 seconds were experienced. Therefore, buildings with

fundamental periods in this range were subjected to very large

93
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94
shear forces. Furthermore, since the fundamental period of

structures are increased due to structural damage sustained by

the building components and soil-structure interaction, there is

every reason to believe that many structures experienced further

damage as a result of this shifting of the fundamental period.

Another feature of the damage pattern is that in several cases in

urban development areas identical structures were built at a particular site and only one or two of the group of structures

collapsed. Some of the remaining buildings suffered only

negligible damage. Irregular subsurface conditions, uneven

construction quality control, and distribution of gravity loads

can be cited as possible reasons for the variation of damage in a vicinity.

5.3 Performance of Different Structural Systems

The great majority of buildings that collapsed were concrete

structures (Table 5.1), which is economically attractive in

Mexico City. Few steel frame structures failed as indicated by

Table 5.1. However, steel construction represents only a small

percentage of Mexico City's building inventory, due to the high

cost of fabricated sections.

5.4 Reinforced Concrete Frame Structures

5.4.1 General

Two types of reinforced concrete frame systems have been used in

Mexico City. These can be classified as those in which: (1) The

frame provides support for vertical loads only; seismic

resistance is provided by shear walls or braced panels; and (2)

the frame resists the vertical, as well as the seismic forces

(Figure 4.2).

95
i

Figure 5.2. Schematic drawing of typical reinforced concrete

framing system.

96
In case (1) the frame is designed to carry the vertical load and

only a small percentage of the earthquake load. Most of the older

concrete frame structures in Mexico City are of this

type. Shearwalls are nomally constructed of brick, concrete

masonry, or reinforced concrete. In some taller buildings the

structures have been stiffened by diagonal reinforced concrete

bracing members as shown in figure 5.3. Many of these buildings

performed well. However, some of these structures collapsed when

the shear-walls and/or infill walls failed (sometimes by out of

plane bending) . A special problem associated with this type of

building has been the tendency to omit the shear-walls in the

lower story, to leave room for storefronts, lobbies, and parking

facilities. This practice generated a soft story, and resulted in

unsymmetrically braced buildings which are subjected to torsion.

In case (2) the earthquake load is resisted entirely by frame

action. In an earthquake the girders are required to carry the

vertical slab loads to the columns, as well as absorb seismic

energy through the formation of plastic hinges. Properly

designed reinforced concrete frames are expected to behave in a

ductile manner, and are more flexible than shear wall type

structures. In any event, hinging in the columns at a

column-beam connection is not desirable. Frame systems in which

columns are permitted develop plastic hinges can result in

stability failures during stress reversals, through mechanisms

incapable of resisting lateral loads. Therefore, behavior of the

frame in the inelastic range becomes an important feature in

resistance strategy which relies on energy absorption capability

while deforming beyond the elastic limit.

Consequently, in a reinforced concrete frame, it is desirable to

have plastic hinges form in the beams rather than in the columns.

Special transverse confining reinforcement — in the form of a spiral or closed rectangular ties — is necessary to insure the

required ductility should plastic hinges form at column ends

97
Figure 5.3. Typical reinforced concrete diagonal bracing with

masonry infill used in Mexico City.

98
during a seismic event. The ties compensate for strength loss

should spall ing occur in the concrete cover and serve as all or

part of the web reinforcement. Furthermore, ties prevent buckling

of compression reinforcement. Both ACI 318-8 3 Appendix A [26]

and the 1976 Mexico City Building Code [19] contain provisions

which recognize these factors. With the above background we can

now examine the performance of a number of reinforced concrete

frame structures.

5.4.2 Hospitals

Major buildings at three of the City's largest hospital complexes

collapsed, thus impairing use of these critical facilities. All

were reinforced concrete frames and typified a common failure

mode for this type of construction. Some 12 00 people were buried

as six buildings collapsed at the Centre Medico and Mexico

General Hospital, one of the largest medical facilities in Latin

America. Figure 5.4 shows an aerial view of this complex looking

towards the southwest and the street intersection of Cuatemoc and

Cohuila boulevards. The central (easternmost) structure

collapsed towards the east exhibiting a pancaking type collapse

similar to those normally associated with flat plate

construction. However, the failure mechanism is quite different

in this case. Examination of debris from this structure (figure

5.5) showed negligible damage in the beams and girders. The edge

spandrel beams were, in contrast to the above discussed

weak-beam/strong-column philosophy, quite stiff in comparison to

the columns. Consequently, failure was likely initiated by the

formation of plastic hinges at the column ends, both above and

below the floor level joints with the girders.

Figure 5.6 shows a close-up view of one of the edge columns.

These were typically 14-in (30cm) deep by 12-in (25cm) wide.

Closed stirrup confinement consisted of approximately #2 bars

(visual evaluation) at 4-6-in (10-15cm) spacing. Following

99
Figure 5.4. Aerial view of Mexico General Hospital.

100
Figure 5.5. Debris from the collapsed Mexico General Hospital.

101
Figure 5.6. Edge column of collapsed General Hospital.

102
current Mexico City building code specifications of 1976 [19],

the required diameter of these stirrups, given the same spacing,

should have been on the order of a #5 bar, which is five times

the cross sectional area of steel that was provided. . However,

this same code restricts the maximum hoop spacing to 2-3/4-in

(7cm) . The General Hospital structures were built prior to the

adoption of the 1976 code. Of particular interest was the

performance of the new parking garage (upper center, figure 5.4)

also a frame type structure. In contrast to the personnel

building, however, the corners of this building contained

reinforced concrete shear walls (see figure 5.7) which safely

withstood the lateral forces during the quake. Figure 5.8 shows

hairline cracks near beam column connections of the parking

garage building.

Yet in another hospital building, the Maternity wing of the

Juarez Hospital, eight blocks south of the Presidential Palace,

400 medical personnel and patients, notably infants, were

trapped. This nine story reinforced concrete frame structure

collapsed towards the east also (figures 5.9 and 5.10) exhibiting

localized failures at the beam to column joints of each floor.

A close-up of various joint details (figure 5.11), indicates that

the columns were generally well-proportioned to resist gravity

loads. However, figure 5.11 and 5.13 clearly shows minimal to

nonexistent confinement through the joints. Furthermore, many

joints had longitudinal bar splices at these same critical moment

locations. These factors probably contributed to joint failures

after a relatively small number of cyclic load reversals.

The lack of such confinement, and the presence of splices in such

critically underconf ined joints — which could lead to a loss of

vertical load bearing continuity as soon as the bond strength is

lost following core breakup — directly contributed to the

failure of the Juarez Hospital. This particular structure

received considerable media attention as rescue personnel

103
Figure 5.7. Reinforced concrete shearwalls in the corners of the

parking garage of the iVIexico General Hospital. The

group standing in the foreground was part of the

dog-rescue team from France. These teams successfully tracked down several survivors trapped in rubble

of the buildings that collapsed.

104
Figure 5.8. Pattern of hairline cracks near the beam column

connection of the parking garage (cracks were

highlighted for better visibility).

105
Figure 5.9. View of collapsed maternity wing of the Juarez

Hospital looking south.

106
Figure 5.10. View of collapsed maternity wing of the Juarez

Hospital looking east.

107
Figure 5.11. Close up joint details of collapsed Juarez Hospital

indicating lack of sufficient confining reinforcement

in the vicinity of the joint.

108
Figure 5.12. Joint details of collapsed Juarez Hospital indicating

adverse effect of a splice.

109
Figure 5.13. View looking south at Juarez Hospital showing how

column hinging permitted rotation of successive

floors toward the east.

110
continued to retrieve survivors as late as ten days after the

earthquake by tunneling through the debris between the floor

slabs (see figure 5.10).

5.4.3 Apartments

Heavy damage was also sustained in the Urbana Juarez apartment

complex, just west of the General Hospital (figure 5.14). One

eight-story, one ten-story, and one thirteen-story reinforced

concrete frame structures with brick masonry shear walls

collapsed. From figure 5.14 it can be seen that three identical

ten story apartments bound the southern edge of the complex.

Only the middle one collapsed and damage in the remaining two

appeared limited. A four-lane divided highway runs from south to

north beneath the next row of apartments. These consisted of

four identical eight-story structures, the northernmost of which

collapsed. Finally, on the northern boundary of the complex,

there were three low-rise (four-story) apartment buildings

bounded to the west by two apartments identical to the southern

group of three, and to the east by the thirteen story high-rise.

The majority of the apartment structures at the Urbana Juarez

site possibly had natural periods of vibration ranging between

1.1 and 1.6 seconds (calculated as 0.12xstory height). When

allowance is made for soil-structure interaction, this is well

within the resonance range of spectra of motions experienced in

the lakebed zone. In general, these buildings were relatively

narrow: average width was approximately 8-10 meters (one bent

frame) , while the length was typically 80 to 100 meters. Access

to higher stories was usually by means of a central elevator, or

an external stair tower on the north side of the structure.

The southern two-thirds of the thirteen-story complex, separated

by a construction joint from the rest of the structure, remained

standing but tilted out of plumb and was considered unstable (see

figure 5.15). This was one of the first structures to be

111
Figure 5.14„ Damaged Urbana Juarez Apartment Complex west

of Genera! Hospital.

112
Figure 5.15. The southern two B towers of the Juarez Apartment

complex which remained standing.

113
dynamited approximately three weeks following the quake. Figure

5.16 shows a close-up of the first two stories of this structure

following removal of debris. It can be seen that the column width

was increased in the east-west direction in an effort to stiffen

the resistance of the structure in the narrow direction.

Nonetheless, based on the debris pattern following the collapse,

it appeared that failure of this building was initiated by

hinging of the columns in the east-west direction. It is likely

that the presence of the thin connecting walkway (see aerial

photo, figure 5.15) with the neighboring two towers to the south

permitted sufficient independence for each tower to oscillate

individually in the east-west direction. This explains why the

northern tower was able to collapse without affecting the two

southern towers.

Figure 5.17 shows an aerial view of the collapsed eight-story

apartment immediately to the south of the thirteen story complex.

This structure was similar in many respects to the one described

above, with the notable exception that the aforementioned four

lane highway ran directly underneath the center of the building.

The reinforced concrete tunnel structure and road foundations may

have stiffened the overall foundation, and hence increased the

efficiency with which ground accelerations were transmitted to

the frame. Each member in this group of four buildings was cast

as a continuous unit with no obvious construction joints that

might otherwise cause separate portions of the building to act

independently under seismic loading. Figures 5.18 shows the

"pancake" nature of the collapse, indicating hinging in columns

during the earthquake. The skewed stacking of the slabs towards

the east indicates that ultimate failure likely occurred during

an eastward excursion. Figures 5.19 and 5.2 0 show views of this

same structure looking north-east and east, respectively. The

exterior stair tower, which experienced a shear failure in the

support pylon at the first story, can be seen on the left side of

114
Figure 5.16. First two stories of Juarez apartment structure.

115
Figure 5.17. Aerial view of collapsed eight story apartment

building.

116
Figure 5.18. Pancake type collapse of eight story apartment

117
Figure 5.19. View of collapsed apartment looking northeast.

118
Figure 5.20. View of collapsed apartment structure looking east.

) 119
beam and girder floor framing plan for a typical story after the

deck slab has been jack-hammered free.

It is of particular interest to know why this particular building

collapsed, while its three sister structures (figure 5.21)

immediately to the south, did not. Part of the reason may be

local variations in the soil consolidation, or perhaps

differential-settlement induced stresses, as mentioned in Section

5.2.

Reference [28], in August of 1957, states "a separate commission

also was appointed to determine whether the Urbana Benito Juarez

housing project for 4,7, and 10 story buildings should be

condemned. These structures, built of reinforced concrete, are

long and narrow and have brick walls above the second floor.

Each of the taller buildings has an inside, centrally located

elevator and an outside stairway, with steps cantilevering from

and circling a vertical concrete shaft. During the quake, one of

these shafts collapsed, toppling the stairs to the ground. The

stairs of several others became partly detached from the

buildings but did not fall over. The pile-supported tall

buildings in this group settled at the ends. Because of the

skew, elevators jammed in their shafts. The lower buildings,

probably on spread footings, sank 12-inches or more, tilting

adjacent sidewalks about 4 5 degrees. Long narrow buildings

subdivided by joints were damaged when the sections pounded each

other during the quake and were left with a permanent lean.

However, surface examination of exposed structural members and

masonry end walls reveals little damage to these components."

From the above it is apparent that significant settlement-induced

stresses, and perhaps permanent, though unseen, structural damage

was locked into these structures as a result of the 1957 quake.

120
Figure 5.21. Undamaged apartment building near identical

structure whicli collapsed.

121
5.5 Flat Plate and Waffle Slab Buildings

5.5.1 General

By suitably proportioning and reinforcing the slabs, it is

possible to eliminate supporting beams altogether. This type of

construction is often referred to as "flat slab" construction.

The slab is supported directly on the vertical columns, which

transfer the load to the foundation. Figure 5.22 shows a typical

flat slab floor. Of particular importance in this type of

construction are the thickened portions of the slab (drop panels)

at the column, and the enlarged portions of the top of the

columns (capitals) . Both the drop panels and the column capitals

serve the primary purpose of reducing the shear stress in the

slab near the column supports. They also provide greater

effective depth to resist negative bending moments.

Very few of the slab structures which collapsed were observed to

have drop panels and column capitals. However, many of the

buildings that sustained significant damage utilized 2-way ribbed

slabs, a different type of design. As shown in figure 5.23, it is

possible to greatly reduce the dead weight of the slab by

creating void spaces at the underside of the slab. For the most

part, the concrete removed by the voids is in tension and

therefore does not contribute to the strength of the slab. The

lighter floor, therefore, has approximately the same strength as

a solid floor which is as thick as the ribs. Voids are usually

shaped by using dome shaped steel forms (fig. 5.24), which are

removed after the concrete slab had hardened. The lower flange of

each dome contacts that of the adjacent dome, so that the

concrete is poured against a metal surface, resulting in good

finish appearance of the slab. A waffle-like appearance, which

is often used to architectural advantage, is imparted to the

underside of the slab. Because of this appearance two way ribbed

slabs are commonly termed rib slabs or waffle slabs.

122
!

Figure 5.22. Schematic drawing of typical reinforced concrete

flat slab floor.

123
Schematic drawing of typical reinforced concrete

ribbed slab.

124
Figure 5.24. Steel forms for ribbed slab.

125
Alternatively, another slab system that was commonly employed in

Mexico City was flat plate construction. A "flat plate" floor is

essentially a flat slab floor with the drop panels and column

capitals omitted, so that a floor slab of uniform thickness is

carried directly by prismatic columns (See figure 5.25). The

construction depth for each floor is held to the absolute

minimum, with resultant savings in the overall height and weight

of the building. Flat plate apartment structures have thus

found popularity in Mexico City, both from the viewpoint of their

relative economy over other forms of concrete construction, and

because of their ease of construction, and maintenance (e.g.

painting or plastering) . However, there is a price for such

economy. Shear stress in the concrete is typically very high in

the vicinity of the column-slab junction. Unless special

supplementary reinforcement is provided in such areas

(e.g. figure 5.26, from ref. 34) one runs the risk of a slab

punching failure. Such problems have contributed to several

collapses of flat plate structures acted upon only by the force

of gravity [29, 30]. The need for this extra reinforcement, and

the confinement of the joints, as mentioned under the discussion

of RC frames above, is imperative in locations where the

structure may be subjected to numerous cycles of lateral load

reversals.

5.5.2 Column Failures

Taller concrete structures with flat plates, such as apartment

buildings over five stories, were particularly susceptible to

earthquake damage. The 14 -story Nuevo Leon building was part of

the Tlaltelalco housing project (figure 5.27) near the

intersection of Guerrero and Flores-Magon, approximately 2km

north of the area most heavily damaged by the quake. This

structure consisted of three independent wings separated by

construction joints. The two northernmost wings, each containing

126
Figure 5.25. Typical flat plate reinforced concrete floor system.

127
m

/^Welded

I or W

sections

Standard deformed

reinforcing bars

(a) (b)

^Welded

cfiannel sections

, Steel column

1

(c)

/ \

Stiffening ribs

Steel flanged

collar

^ steel column

(d)

Figure 5.26. Typical shear reinforcement in a flat plate slab.

128
196 apartments (see figs. 5.28 and 5.29), apparently collapsed

due to overturning when a failure was initiated on the east side

of the structure near the foundation level. Supporting columns

were then sheared or pulled from the ground on the west side of

the structure (figure 5.30), leading to the toppling of the

building towards the east.

Although the floor levels behaved as flat slabs, they were in

fact a special variety of waffle slab. This can be verified by

inspection of figure 5.3 0 which shows the presence of porous

blocks used to form the slab, revealed after the ceiling plaster

spalled off during the final impact. Figure 5.31 shows the

presence of regularly spaced reinforced concrete x-bracing as

well as masonry infill walls which would have served to resist

lateral forces during the earthquake. Figure 5.32, in fact,

shows four structures identical to the Nuevo Leon building in the

same vicinity which performed better during the quake.

Significant controversy has arisen over the Nuevo Leon building

and in general all of the Tlaltelalco government housing project.

Following the 1979 Petatlan event, residents complained of

noticeable settlements. Subsequently, some foundations, notably

that of the Nuevo Leon building, were underpinned. The resulting

difference in foundations may have contributed to the collapse:

the stiffening of the foundation on the east side of the building

led to an asymmetrical response condition whereby the east side

columns would have been subjected to relatively greater lateral

forces. Figure 5.3 3 shows failures that have occurred in the

short column line on the east side of the remaining southern

apartment tower, particularly on the level used as a passageway

between blocks.

129
Figure 5.27. The TIaltelaIco Apartment Complex. Nuevo Leon

building is located at center of photo. Aerial view

looking northeast.

130
Figure 5.28. Aerial view looking northwest of the Nuevo Leon

Apartment Complex.

131
i

Figure 5.29. Aerial view looking south of tlie Nuevo Leon

Apartment Complex.

132
Figure 5.30. West side of the collapsed Nuevo Leon Apa

Complex.

133
Figure 5.31. Concrete X-braces in remaining tower of Nuevo

Leon Apartment Complex.

134
Figure 5.32. Four undamaged structures in the TIatelaIco

Apartment Complex.

135
Figure 5.33. Failures in columns of the southern

Nuevo Leon apartment tower (the only unit of the

group of three to remain standing).

136
This asymmetry in column loading may have induced a failure of

the first story east column line in the two northern structures,

due to the relatively lower stiffness associated with the first

floor lobby. Shearing of the columns would then occur either at

underconf ined joints or at the story mid-heights between the

X-bracing. That the x-bracing proved effective is demonstrated

by the relatively intact nature of large sections of the

structure after failure. This is to be contrasted with figures

5.34 and 5.35 which show an office structure near the corner of

Neva and Barragan streets, similar in manny respects to the Noeva

Leon building, with the exception of the lack of reinforced

concrete x-bracing. The pancaking phenomenon is clearly evident.

5.5.3 Punching Shear Failures

A typical example of flat plate structural failure is shown in

figure 5.36, of the ten story Bancomer Building at thecorner of

Ave. Mier and 2 0 de Noviembre. This building was constructed

recently, after the 1976 design code changes. In many cases the

use of thin slabs precluded the placement of sufficient shear

reinforcement in the vicinity of the slab-to- column junction.

Under the reversed cyclic loading imposed by the earthquake,

punching shear failures initiated a vertical progressive collapse, leaving the columns standing. Such failures were also

seen at several other collapse sites involving flat plate

construction. With the exception of projecting reinforcement at

approximately 4 meter intervals, the columns appear bare in

figure 5.36. Closer inspection showed that the floor plates slid

down the columns, one on top of the other. The classical "shear

cone" failure, shown in figure 5.37, is not visible here because

the upper floors successively sheared off these cones from the

top side as they fell.

The fact that there were a large number of flat-plate failures

during the earthquake can be attributed to two reasons: a) the

137
138
Figure 5.35. Collapsed office building near the corner of Neva

and Barragan Streets.

139
Figure 5.36. Punching shear failure of the 10 story Bancomer

Building.

140
Figure 5.37. Shear cone resulting from punching shear failure.

Ref. [24].

141
relatively low ductility at the slab-to-column junction which

could have been improved through the use of special shear head

reinforcement, as was mentioned in section 5.4.1 above, b) the

popularity of this form of construction — due to its economy —

following the 1976 design changes, giving rise to a larger

percentage of structures in the 6-15 story range.

5.6 Waffle Slabs

5.6.1 General

It has previously been mentioned that the vast majority of

structural failures during this quake occurred in concrete

structures. Although the total number of concrete frame structures which were damaged was high, proportionally there were a greater number of failures in buildings which employed waffle

slab construction (see Table 5.1). It has been reported [31]

that 2.9% of all concrete frame structures in the zone of major

damage were either destroyed or required demolition. For waffle

slab construction the percentage reported was 5.9%.

5.6.2 Column Failures

Waffle slab structures generally have heavier floor dead weights

than structures using beam/girder type floors. The waffle floor

slab itself is quite stiff in its plane and therefore transfers

by diaphragm action all of the seismic energy into the columns as

concentrated story shears. As a consequence, the column ductility

demand in this type of structure, is high.

In many cases, the extent of damage to this type of structure was

determined by the presence or absence of masonry exterior infill

shear walls, as demostrated in figures 5.38 a and b. The infill

walls shown in figure 5.38a failed, and in the process absorbed a

142
substantial amount of energy prior to buckling outward. On the

other hand, the Banco Vencen building near the corner of Ave.

Trujano and Hidalgo (figure 5.38 b) was under construction and

masonry walls were in the process of being added to some of the

upper floors, but were not complete enough to prevent column

shear failures from occurring along the entire eastern side of

the frame at the third story. Those portions of the building

where the infill walls had been completed remained standing. A

typicalexample of a failure of a waffle-slab structure with

flexible columns was the 10-story apartment building at the

corner of Ave. Dinamarca and Chapultepec shown in figures 5.39 a and b.

5.6.3 Porous Block Formed Waffle Slabs

Two unusual variations in waffle slab construction bear mentioning, as they proved to be particularly poor performers during

this earthquake. Figure 5.4 0 shows a new parking garage north of

the U.S. Embassy on Ave. Reforma under construction, using

rectangular cinder blocks in place of the traditional metal pan

forms. The rationale behind this technique was to use readily

available, inexpensive cinder block to form the blockout, thereby

avoiding the expense of an inventory of metal pan forms while

also leaving a smooth ceiling surface for finishing. These

cinder blocks are spaced similarly to their metal counterparts,

but are left in the structure permanently. Because they are cast

into the slab the cinder blocks add to the mass of each floor

level

.

Yet another variation of this form of construction employed

standard hollow cinder building block, as shown in figures 5.41.

This type of slab was particularly prone to bond failure during

reversed cyclic loading, leading to the "floor drop out"

pheonomenon shown in figures 5.4 2 and 5.43. In most cases, these

same structures contained minimal slab reinforcement placed with

143
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TD CD 0 "D

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CO

CO JzE is > ^

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00

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in

D)

144
Figure 5.39= Collapsed 10 story apartment building in the corner

of Dinamarca and Chapultepec. This waffle slab

structure failure is attributed to base column failure

and possible pounding with the neighboring structure.

145
Figure 5.40. Parking garage under construction on Reforma,

north of the U.S. Embassy.

146
Figure 5.41. Waffle slab construction incorporating concrete block,

147
Figure 5.42. The "floor dropout" phenomenon observed in waffle

slab structures with concrete block for pan forming.

148
I
negligible cover. As the building underwent successive cycles of

lateral loading, the bottom (tensile) cover concrete spalled off,

leaving the tension reinforcement exposed in a grid-like pattern.

With further shaking the cinder blocks simply began falling out,

eventually leading either to the "floor drop out" phenomenon if

sufficient shear reinforcement was present to resist a punching

shear failure or, more likely, a progressive punching shear

failure (see figure 5.43) similar to that observed in flat plate

construction. It was apparent from the extensive and specific

damage that special design procedures should be developed for

this method of construction in future design codes.

5.7 Steel Structures

5.7.1 General

The historical background of steel construction in Mexico City

was discussed by Hanson et al. [25]. In the early part of this

century some steel structures with masonry infill walls were

constructed. Multistory building construction started in the

1950's. The Robles Building (24 stories), the Latino Americana

tower (43 stories) , the Abed building (35 stories) the Banco

International (32 stories) , and the Seguros Anhuac and La

Comercial buildings (27, 28, and 25 stories) are from that era.

These moment - resisting frame buildings used imported wide

flange beams and columns, and often columns consisting of two

channels with welded cover plates. These buildings generally

performed well in several earthquakes. However, we should keep in

mind that, because of their longer natural periods, these

buildings were not subjected to resonating seismic forces typical

of the sites they were constructed on.

Failures in steel structures accounted for only ten out of the

hundreds of buildings which collapsed during the earthquake.

150
This can likely be attributed to two factors. The first is that

as a percentage of the total, there are relatively few steel

frame buildings in the Valley of Mexico. The second factor

contributing to the low number of steel building failures is that

steel, whether in the form of heavy rolled sections (imported) or

built-up plate sections (domestic) , was preferred in tall

structures. These same tall structures had natural periods of

vibration significantly greater than the predominant 2 second

ground motion period and therefore were not likely to resonate.

The study of those steel structures which did fail during this

earthquake is thus of considerable importance.

During the last three decades the importation of large steel

profiles were limited and Mexican mills produced rolled sections

up to 18" (4 60mm) in depth. However, rolled plates of up to 5/8"

(16mm) in thickness were readily available. In several cases this

led to steel construction using welded box columns andopen web

joists in composite action with concrete decks.

5.7.2 Steel Frame Structures That Failed

Perhaps the most impressive failure resulting from the September

19 earthquake was that of the two southern towers of the five

tower Consume Pino Suarez, a government office complex built in

the early 1970 's, located just west of the Juarez Hospital (See

Fig. 5.44). The northern-most and southern-most buildings were

15 stories in height, while the central three buildings were 2 3 stories high (figure 5.45) all oriented linearly from north to

south. Underlying the buildings were barrel arch type reinforced

concrete tunnels housing the Pino Suarez Metro station, a connecting station at the intersection of two subway lines. The

weight of the buildings served to prevent uplifting of the metro

station, a practice that is common in the lake bed of Mexico

151
Figure 5.44. Southern towers of the Consumo Pino Suarez

Complex shortly after collapse.

152
I I I I I

•—Roof

2 1 St Floor

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Figure 5.45 Plan and elevation of Pino Suarez Office Complex,

153
city. The main line of the Metro runs approximately east west

along Ave. Obregon, the street bordering the Pino Suarez block on

its northern side. The exact configuration of the foundation

system beneath the southern half of the structure is unknown at

the time of this writing, however all five buildings made use of

a common two-story mezzanine and lobby (figure 5.46) and it is

known that some of the building columns extend through the subway

station itself* Therefore, the entire complex was subjected to

the same base accelerations during the earthquake.

Each of the 2 3 story towers had two bays in the north-south

direction (total width 12 meters) , and four bays in the east-west

direction (total length 28 meters) , which in part helps to

explain the collapse of the structures along the north-south axis

despite the predominance of the east-westacceleration component

of the quake. The third bay on the south side of each of the 2 3

story structures contained a braced elevator shaft and stairwell,

as can be seen in figures 5.47 a and b, taken nine months after

the earthquake following the removal of cladding and debris.

All five of the Pino Suarez towers utilized a welded box column

frame (see figure 5.48) and an integral composite slab system

supported by truss beams (figure 5.49) fabricated from tubular

sections with steel angles forming the inclined truss members in

the shape of a simple Warren truss. The truss-to-column

connections were designed to be moment resisting (See Fig.

5.50a). Lateral bracing for the building consisted of one bay of

"v" braces along the exterior south wall adjacent the elevator

shaft and stairwell, and two north-south oriented lines of

X-bracing on either interior side of the elevator shaft and

stairwell bay.

* Pers. Comm. Ing. Enrigue Del Valle-Calderon, August 1986

154
Figure 5.46. Middle 23-story tower and connecting ground level

mezzanine for Pino Suarez Office Complex.

155
(a) Remaining three Pino Suarez towers.

(b) Central 23-story tower (reduced to 14 stories) at Pino Suarez.

Note "C" clamps at bottom right.

Figure 5.47. Remaining Pino Suarez structures shown during

July 1986.

156
Figure 5.48. Typical box column cross section used in Pino

Suarez Complex, (courtesy E. Martinez-Romero)

157
Figure 5.49. Truss beam detail used in Pino Suarez Connplex.

(courtesy E. Martinez Romero)

158
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The columns were hollow box sections made of four plates with

exterior fillet welds along the edges. Internal stiffner plates

spaced at regular intervals and welded to three sides along the

interior length and along all four sides at the column ends.

Prior to the September 1985 earthquake, through ambient vibration

tests performed at the Pino Suarez complex for 2 3 story towers,

the fundamental period of vibration was determined as 2.0

seconds, clearly in resonance with the period of the ground

accelerations at the SCT site.

From the plan view of the complex presented in figure 5.45, it is

seen that the lateral bracing surrounding the elevator shaft of

each tower created an eccentric center of stiffness, thus

resulting in torsional vulnerability. This was confirmed through

a simplified three dimensional computer model (figure 5.51). The

first mode of vibration in the narrow direction was scaled to

match the known period of 2 seconds, since precise element

stiffnesses and masses were not available at the time of the

analysis. The same scale factor was used in making a bestestimate of the periods for the higher order modes in the real

structure. The first mode in the wide direction had a calculated

period of 1.17 seconds and a distinct twisting component; and the

pure torsional mode had a calculated period of 0.64 seconds,

sufficiently long to have been excited by the September 19

earthquake. Column axial and bending forces would have been

generated in the structure due to components from each of these

modes of vibration and, during certain phases, these forces would

be additive on certain structural elements, producing loads in

excess of those that would be indicated from two dimensional

dynamic analyses commonly employed in structural design.

Although the structure was designed as a moment resisting frame,

it is evident that not much inelastic deformation developed in

the beams. Rather, the individual truss diagonal elements

160
buckled (see figure 5.50) indicating that they were not able to

dissipate energy through inelastic action in the manner of

typical rolled beam sections. This in turn had two adverse

effects: primary ductility demand was transferred to the columns

(for which they were not designed) and large lateral deflections

gave rise to large P-delta effects, all of which served to

overstress the southern, stiffer, column line. Eventually, local

plate buckling — amplified by weld tears at the plate joints —

occurred in the most highly stressed columns along the south face

near the top of the common mezzanine.

Buckling of columns, and deformations caused by other actions,

induced substantial tilt towards the south, as can be seen in

figure 5.52. It is highly probable that this situation existed in

all three of the 2 3 story towers prior to the ultimate collapse

of the southern-most tower. To complete the collapse mechanism,

however, an additional failure was required. Figure 5.53

indicates that the final straw was likely a series of tensile

fractures in the box-columns on the north side of the frame,

presumably at a joint line near the second story. Lacking this

crucial moment resisting element (viewing the structure as a whole) the tower was free to rotate southward, further crushing

the southern column line in the process, and eventually impacting

on its neighboring 14 story building. Both buildings were

subsequently destroyed, leaving the two collapsed buildings in

the configuration previously shown in figure 5.44.

Information which became available in January of 1986* indicated

that the upper floors of the southern-most 2

3 -story structure

contained a substantial amount of heavy goernment equipment which

would have adversely affected the performance of the structure

under the influence of lateral loads. Likewise, the presence of

* Personal communication: Ing. Enrique Martinez Romero

161
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Figure 5.52. Tilted central 23-story tower at Pino Suarez

Complex. Southern side of structure is to left side.

163
Closeup of fractured column line near second story,

north side, of the southernmost 23-story structure

in Pino Suarez Complex, shortly after collapse.

164
Figure 5.54. Aerial view of the Atlas Building, a typical steel

frame structure constructed during the 1940's.

165
Figure 5.55. The 27 story triangular plan National Lottery

Building performed well.

166
the Pino Suarez Metro station beneath the complex may also have

had a complicating effect on the dynamic response of the towers,

since it created a more rigid foundation system, thus more

efficiently transmitting earthquake energy into the towers.

Other steel frame structures performed quite differently from the

Pino Suarez Buildings, depending on their location, elevation,

and proximity to adjacent buildings. One such example is the

Atlas Building (figure 5.54), a 14-story concrete-encased steel

frame structure constructed during the 1940 's and located just

south of Alemeda Park on Avenida Lazaro Cardenas. During the

1957 quake this building suffered significant damage to

moment-resisting knee braces (steel angles) and was later

retrofitted with the diagonal cross bracing visible in figure

5.54.

During the 1985 earthquake the upper seven stories of the

building collapsed. The front of the adjacent seven story

structure was also destroyed by falling debris from the Atlas

building. In the case of the Atlas Building, field evidence

indicated that pounding was the principal cause of collapse of

the upper floors.

5.7.3 Tall Moment Resisting Steel Structures That Performed Well

High rise towers performed well during the September 19, 1985

event. Several examples of tall buildings include the Latin

American Tower (43 stories) , the Pemex Tower (54 stories) , and

the National Lottery Building (27 stories, figure 5.55), a triangular shaped structure 110 meters high with construction

similar to that of the U.S. Steel Building in Pittsburgh.

The Torre Latino Americana (Latin American Tower) is undoubtedly

the most famous steel structure in Mexico City (figure 5.56).

The 1985 earthquake produced non-structural damage estimated at

167
Figure 5.56. 43-story Torre Latino Americana, a steel frame

structure constructed In the early 1950's. It was

designed by Adolpho Zeevaert with consultations

from N. Newmark and Leonardo Zeevaert. The

structure survived the 1957, 1978, 1979 and the

September 1985 earthquakes with minor nonstructural damage.

168
$5000. A visual and X-ray examination of the structural frame by

the building engineers revealed no structural damage. The Torre

Latino Americana is constructed from rolled structural shapes

with lateral resistance being provided by riveted moment

connections and composite floor beams.

The approximately five second natural period of the Torre Latino

Americana placed it well outside the resonance period of the

quake, and therefore outside the range of the most heavily

damaged structures. The apparent lesson so clearly demonstrated

by the performance of this structure (and other tall steel

buildings in Central Mexico City) is the need to "detune" the

structural system based on subsoil dynamic response

characteristics for each specific construction site.

5.8 Building Configuration and Design Problems

5.8.1 Building Separation (Hammering)

(1) General Discussion

"Pounding" or Hammering" has been observed routinely by

earthquake investigators over the past several decades. For

example, in the 1972 Managua earthquake, the five story Gran

Hotel suffered a complete collapse of its third story when the

battering ram phenomenon occurred at the roof level of the

adjacent two story building. [24]. In the Alaska 1964

earthquake, the fourteen story Anchorage Westward Hotel pounded

against its low-rise ballroom and an adjoining six story wing,

even though separated by a 4 -inch gap [32].

The problem of two adjacent buildings or blocks of the same

building pounding together during an earthquake is related to two

basic issues: the width separating the two structures (or the

width of a seismic "separation joint" if the buildings are

169
architecturally connected to form one building) , and the relative

stiffnesses and vertical configuration of the two structural

systems. Building codes address the separation problem in the

form of required drift limits. For example, the 1985 Uniform

Building Code [20] prohibits drift from exceeding 0.5% of the

story height. In the case of a 16 story height, this drift would

equal about 10 inches (254 mm) . The 1976 Mexico City building

code [19] permitted a value of 1%. The proposed NEHRP provisions

also permit 1% [21] . Reference [35] mentions a Russian text as

suggesting 1-1/4 inches (32 mm) of separation for building

heights of up to 16 feet (4.9 m) and 3/4 inch (19 mm) additional

separation for every 16 feet (4.9 m) above this.

Buildings sway during earthquakes. Adjacent buildings can sway

out of phase — first away from each other and then toward each

other, each in its own natural period of vibration. This

phenomenon is particularly pronounced where the difference in

height between two adjacent structures is great and the duration

of the earthquake is long. . . thus permitting each to reach

maximum response (see computer model in figure 5.57). It is

evident, therefore, that to prevent pounding, the clear space

between buildings should be the sum of the individual maximum

dynamic lateral deflections at a given story height. Whether or

not this criterion is met by existing single-number code drift

separation requirements (calculated as a percentage of building

height) depends to a large extent on the ability of designers to

predict actual deflections caused by an earthquake, taking into

account the effect of inelastic excursions.

(2) Separation Problems in Mexico City (1985)

In more than 40 cases of heavily damaged buildings and in 15% of

the damaged buildings examined (see table 4.1) there was evidence

of pounding adjacent structures against one another. On some

occasions the pounding caused only local damage in the structure

170
Figure 5.57. Computer simulation showing the pounding

phenomenon when two buildings of unequal height

vibrate out of phase.

171
or in the cladding. However, there were several cases in which,

due to non-alignment of story levels of adjacent buildings, the

impact disabled the mid-story columns and was the primary cause

of collapse. Pounding was responsible for many of the failures

observed in upper stories as well. Building separations, in

almost all instances, were less than that specified by the

buildig code. The separation between adjacent buildings was

commonly on the order of 10cm which, according to building

regulations, would be permitted for buildings of 10m (about 4

stories) or less height.

Figure 5.58 illustrates a typical example of the pounding

phenomenon. A seven story office building (figure 5.58a) on Ave.

20 de Noviembre near the corner of Izazaga suffered a partial

structural failure due to impact damage from the neighboring nine

story building to the north (figure 5.58 b) . A complicating

problem in this particular case (and in many other buildings

where pounding was observed) was the non-alignment floor level

elevations between the two structures. This is effectively

illustrated in figure 5.58a where the second floor slab level of

the southern building is seen to intersect the second story

column of the northern structure at approximately mid-height, the

worst possible situation for resisting lateral impact loads.

Equally damaging shear loads were being imparted to the columns

on the southern building eventually leading to the progressive

collapse shown in figure 5.58a.

In most cases, the building which collapsed during pounding was

the taller structure. An example of such a collapse is

illustrated in figure 5.59 where a nine floor clothing store at

the corner of Ave. Jose Maria Izazaga and Isabel la Catolica

impacted against its neighboring three story structure on the

west. Figure 5.59b shows that the separation between the two

buildings was in the order of 10cm and clearly insufficient to

prevent contact when both buildings were vibrating out of phase.

172
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Figure 5.59. Pounding damage between buildings of dissimilar

heiglits and stiffnesses. Tine upper floor of tlnis 9-

floor clothing store collapsed following pounding

with its much stiffer neighboring 3-story structure.

The separation between the structures was

approximately 10 cm.

174
One variant on the pounding problem occurred where expansion

joints existed within a structure and where it was possible for

the separated substructures to vibrate independently, but where

an insufficient "seismic gap" was provided to prevent pounding.

An example of this type of failure was the Hotel Continental on

Avenida Reforma. The building was V-shaped in plan, largely

because of its street corner location, with two independent wings

forming the "V" (see figure 5.60). The wing sections, each as

tall as the main structure, apparently vibrated out of phase

with the stiffer primary building and eventually suffered column

failures due to pounding. This, in turn, led to the collapse

shown in figure 5.61. Adequate separation between the wings of

the building may have prevented such a failure.

5.8.2 Torsional Effects

If the center of mass of a structure does not coincide with its

center of resistance the inertial loads imparted to the structure

during an earthquake have a torsional component. Unfortunately

this is often the case in actual structures because of lack of

symmetry. It also can be induced by unsymmetrical arrangements

of shear walls, lateral bracing or other horizontal-load

resisting elements which may occur in corner buildings, first

floor lobbies, or open areas facing a street. Torsion may also

result from other causes, such as uneven load distribution,

failure of structural members during an earthquake and

differential foundation settlements.

Buildings that are either T-shaped, or L-shaped are unsymmetrical

and create several problems: (1) Even if these buildings would

act as one rigid unit, they would be subjected to large torsional

moments. (2) Different portions of the buildings tend to have

different rigidities. Stated differently, if either a T or L

175
Figure 5.60. Aerial view showing "V" shape of Continental Hotel

on Paseo de la Reforma.

176
Figure 5.61. View looking north of collapsed southern wing of

the Continental Hotel on Paseo de la Reforma. The

entire complex was subsequently dynamited.

177
shaped building were divided into symmetrical components each of

the component structures would likely have vibrational modes

which differ significantly from those of the others in any given

direction. Combining these incompatible pieces into one

structure produces large stress concentrations at the junction

between the two parts of the building, and particularly at

re-entrant corners.

An example in which building configuration appeared to have led

to torsional response during the Mexico Earthquake is illustrated

by the 12-story SCT (Secretaria de Communicaciones y Telegrafos)

complex (figure 5.62) on Ave. Lazaro Cardenas, approximately two

kilometers south of the central historic portion of the city. The

earthquake precipitated a progressive collapse which started near

the top of the building at the inside northwest (re-entrant)

corner as shown in figure 5.63. The collapse was probably

triggered by failure of one of the upper columns (which are often

of smaller cross-section and lighter-weight concrete in Mexico

City) and then propagated by means of corner column failures

along the north face of the west wing and the west face of the

north wing of the T as can be seen in figure 5.64. The resulting

progressive slab collapse was arrested at the 8th floor, where

the 7th floor columns were sufficient to resist the impact load

from the collapsing upper stories.

Because of the unusual geometry of the building (essentially a Tshape) and because of the asymetrical failure mode, a preliminary effort was made to understand the dynamic performance of

the structure shortly after the NBS/USGS team returned to the

United States in October 1985. A modal analysis was conduted on

a simplified version of the building (as shown in the computergenerated image in figure 5.65), under the assumption that the

complex could be modeled as a continuous T-shaped structure.

This analysis indicated that the fundamental mode of vibration

for the SCT building was torsional. From this it was concluded

178
(a) Aerial view looking north.

(b) Aerial view looking southwest.

Figure 5.62. Aerial view of SCT Communications Complex.

179
Figure 5.63. Northwest re-entrant corner of SCT Communications

Building showing likely initiation point of collapse.

180
(b) West side of SCT Building.

Figure 5=64. SCT Communications Building showing extent of

progressive collapse.

181

that large shearing forces would have been generated at the reentrant corners leading to the initiation of the collapse.

Later discussions with Mexican engineers [36] indicated the

presence of a vertical expansion joint between each of the wings,

which partly explains why the damage did not progress into the

eastern wing of the structure. This would have permitted

independent vibration to take place in the various sections of

the building leading inevitably to collisions between the various

components (i.e., pounding). In either case, T-shaped section or

construction joint with insufficient separation to prevent drift

interaction, the brunt of the damage would have been sustained at

the same junction location. The complex is undergoing renovation

and the height was reduced to a uniform 5 stories as of September

1986. The nearby SCT tower sustained negligible damage and

offices were in use shortly after the earthquake.

There are also examples of failures associated with asymmetry in

structural stiffness, rather than shape. The first of these was

alluded to in Section 5.7.2 under the discussion of the dynamic

mode shapes for the Pino Suarez towers. Figure 5.51a indicates

that the principal mode of vibration in the east-west direction

(along the long axis of the structure) has a definite torsional

component. The cause for this torsional response is the presence

of the "V" and "X" bracing surrounding the stairwell and elevator

shaft on the southern side of the structure. These braces, which

have no symmetrical counterparts on the northern side of the

buildings, serve to create an eccentricity between the center of

rigidity and the center of mass causing torsional vibrations.

While it is clear that other complicating factors (local

buckling) initiated the collapse of the Pino Suarez towers it

seems likely that torsion was in large part responsible for the

structural overload on the southern column line.

183
If there is asymmetry in the arrangement of the perimeter walls

of a building, the center of mass will not coincide with the

center of resistance, and torsional forces will tend to cause the

building to rotate around the center of resistance. This is

particularly acute in corner buildings — such as shops, offices

etc. which have shear walls on the interior edges and curtain

walls or glass on the street sides. When shaken by an

earthquake, the rear and side walls are quite rigid but the front

wall is very flexible, and as a result the upper part of the

building tends to rotate [24]. Columns in the front walls of the

ground floor are highly stressed because they resist torsional

shear in addition to the normal deflection loads and shears. The

rotational motion of the building also induces stresses in the

columns and shearwalls at higher floor levels where they may not

have been anticipated in the design. Sometimes these stresses are

in part the result of pounding of adjacent structures.

During the Mexico City earthquake a great number of corner

buildings failed (42% of the total — see table 4.1). For the

most part these collapses can be attributed to the torsional

effects just mentioned. Even though the lower floor columns are

highly stressed in such a situation, failure of these columns was

a relatively rare event. More generally, failure was initiated

in the upper stories of the building and propagated downward

until the collapse was arrested , usually several floors above

street level . An example of such a failure is the building shown

in figure 5.66, which is located just south of Alemeda park near

the Torre Latino Americana. In such situations two additional

characteristics were usually present: thinner columns in the

upper floors (a practice permitted by the code) , and generally

very small separation gaps between adjacent buildings (a practice

not permitted by the code) . When such a corner structure is

excited by earthquake motion, lateral displacements caused by its

torsional response are greatest in the top story and, in most

cases, this was apparently sufficient to permit pounding to

184
Figure 5.66. Torsionally induced pounding in corner building south

of Alameda Park, near Torre Latino Americana.

185
occur with adjacent structures. At this point one of two

possibilities existed, depending on the relative stiffnesses of

the impacting structures: either the building undergoing torsion

sustained a failure in one or more of its upper level columns and

a progressive collapse ensued to a level where the column size

was sufficiently large to resist the load, or the adjacent

structure underwent a similar fate. In either event, such

undesirable response characteristics could be mitigated by

enforcing adequate separation gaps and by stiffening the portal

frames for the open first floors of the corner buildings to

provide coincidence between the center of mass and the center of

rigidity.

5.8.3 Upper Story Collapses

Lateral forces that a building must resist during an earthquake

are greatest near the base of a structure. It is thus somewhat

surprising that the third largest category of structural

collapses listed in Table 4.1 was that of failures in the upper

stories of buildings. In many cases observed during the field

investigation (as for example the structures shown in figure

5.67), pounding was not a factor since there were no adjacent

strutures. This left only two likely reasons for the ubiquitous

nature of this failure mode. The first is the apparently common

practice of reducing column dimensions with story height in an

effort to save on material costs (which in Mexico are of greater

concern than labor costs) . This practice is clearly demonstrated

in figures 5.68 and 5.69 which show the upper story collapse

initiating at the point of a general change (reduction) in

column stiffness. In the case of the building shown in figure

5.68 (the eight story LACSA Building on the corner of Reforma and

Marne Streets) the upper two story steel-framed penthouse was a later addition to the lower six story concrete frame. Figure

5.68 a indicates that most of the damage was confined to the

penthouse

.

186
(a) Upper story failure in office building on Ave. Lazaro Cardenas.

(b) Upper story failure in clothing store.

Figure 5.67. Typical upper story failures in building where pounding

did not occur.

187
Figure 5.68= LASCA Building on Paseo de la Reforma and Marne,

showing large change in colunnn stiffness in upper

two floor penthouse.

188
Figure 5.69. Commercial building showing large changes in

column stiffness in upper stories.

189
The tendency to make room for additional office space by

constructing "add-on" upper floors is common practice throughout

Mexico, particularly where land is at a premium in urban

centers. Because of the desirable nature of the lower stories

for office space and living quarters, the add-on upper stories

are often used to store merchandise or archival records,

especially in the case of federal government buildings.

5 . 9 Masonry Structures

Some of the most effective "survivors" in terms of structural

performance during the 1985 earthquake were the historic stone

masonry churches and colonial office buildings (see figure

5.70) . These buildings have withstood numerous previous

earthquakes. In terms of structural behavior the endurance of

these buildings may be attributable primarily to their

exceptional stiffness. Since the height of these buildings is

limited, these structures in general have short fundamental

periods of vibration. Therefore, in the lakebed of Mexico City,

these structures were out of the range of the spectral peaks of

the earthquakes. Furthermore, over the years, powerful

earthquakes removed the less earthquake resistant structures.

There are few modern buildings in Mexico City which employed

reinforced masonry, as is commonly used in other parts of the

world. Low rise, unreinforced masonry construction is very

common for houses, apartments, and commercial buildings. The

most general use of masonry in Mexico City, however, is for

unreinforced infill walls for high rise frame structures (figure

5.71) . While a number of medium and high rise buildings did

suffer infill wall damage (figure 5.72), infill walls generally

had a beneficial effect on structural performance [27]. This was

accomplished by increasing the structure's lateral stiffness and

thus reducing its fundamental period of vibration, and by

providing or augmenting lateral-load resistance. Both of these

190
Figure 5.70= Colonial stone masonry structures (National Palace

and Cathedral) have short natural periods of

vibration and performed well in the earthquake.

191
192
Figure 5.72. Significant diagonal shear cracking in a masonry

infill structure on Ave. T. de Mier. Enough energy

was dissipated in the walls to prevent collapse of

the building.

193
effects served to improve performance under earthquake loading.

Stiffening of structures was an advantage in Mexico City

primarily due to the unusual soil conditions which produce ground

motions with periods considerably longer than those experienced

in most urban centers around the world. In those areas where

relatively firm subsoil conditions exist, such stiffening could

actually increase the lateral loads.

Certain types of reinforced concrete braced frames also made use

of masonry infill, as shown in figure 5.3, These were typically

employed in structures ranging from 7 to 12 stories and consisted

of reinforced concrete "X" braces with brick infill. They were

apparently very effective at resisting lateral loads, and no

out-of-plane infill wall failures were observed with this type of

bracing. One notable structure which used this system was the

Nuevo Leon building described previously in Section 5.5.2.

Figure 5.32 shows a typical placement pattern for the X-bracing

in the remaining unit of the Nuevo Leon building. Failure in

that particular building was surmised to have been initiated at

the lobby-level crushing of the eastern column line, rather than

by any localized failure on the part of the infill walls.

194
6. FINDINGS

The September 19, 1985 Mexico City Earthquake provided a great

deal of information in the areas of seismology, the effect of

subsurface conditions on ground motions, the protection provided

by existing and proposed design provisions and the performance of

structures and foundations. It confirmed the effectiveness of

some of the current practices in earthquake engineering and the

validity of the underlying hypotheses, while giving us cause to

question others. In terms of loss of human life, this was a devastating event, and one which the authors, who visited many

sites where victims still lay trapped in the rubble, will not

forget. It will always remind earthquake engineers of the

importance of the efforts to further and perfect the state of the

art in earthquake resistant design.

Earthquake Origin and Characteristics

The earthquake originated in the state of Michoacan, about 4 00km

to the east of Mexico City, and occurred as a result of the

subduction of the Cocos plate beneath the North American

plate. It ruptured much of the Michoacan gap, providing further

confirmation of the validity of the hypothesis that major

earthquakes can be predicted by identifying seismic gaps. The

earthquake was well documented by the Guerrero array of

instruments which was installed in anticipation of a major

seismic event in one of two seismic gaps identified in the

area. The primary earthquake had a Richter magnitude Mg = 8.1 and

a focal depth of approximately 18 km. An aftershock of Mg = 7.5

occurred a day and a half later.

Ground Motion Characteristics in Mexico City

A record from the area of deep lake deposits where most of the

structural damage occurred indicates that the strong motion had a

195
duration of approximately 60 seconds, and peak accelerations of

168 gal ( lgal=lcin/s2=:i . 025xlO~-^G) in the east-west direction, 98

gal in the north-south direction and 3 6 gal vertically. The

resultant peak acceleration was calculated as 198 gal in the

south 60° east direction. There were 16 cycles of more than 1/2

the peak acceleration in the east-west direction. Maximum

velocities and displacements in the east-west direction were 61

cm/s and 21 cm, respectively. The ground motion had a dominant

period of 2s and was nearly sinusoidal. Longer dominant periods

(up to 4s) , and longer durations of strong motion, (in excess of

160s) but lower peak accelerations were recorded in areas of

deeper and less consolidated lake bed deposits. Much lower peak

accelerations (of the order of 40 gal or less) were recorded in

the foothill zone of Mexico City where soil deposits are shallow.

From a record of the subsurface deposits underlying the lake bed

region of the City it is estimated that the ground motion in the

most damaged area had dominant periods ranging from 1.9 to 2.8s.

Effect of Subsurface Conditions on Ground Motion

Part of the area of Mexico City is located in the bed of Lake

Texcoco which was drained after the Spanish occupation and is

underlain by deep deposits of soft clays and silts. Other parts

are located in the foothill zone where there are shallow soil

deposits over volcanic rock, and there is a transition zone

between the lake bed region and the foothill zone. These

variations in subsurface conditions account for the great

differences between the ground motions recorded in different

parts of the city and the resulting differences in earthquake

damage. It has been shown that in the lake bed region the ground

motion transmitted through the underlying bedrock was amplified

by as much as a factor of 5 or more. The great amplification of

the ground motion in the part of the lake bed region which has a

fundamental site period of vibration close to 2 seconds is

196
attributed to the frequency content of the strong motion of the

underlying bedrock. It is inferred from spectral analyses of the

strong motion recorded in the foothill zone that the motion of

the underlying bedrock had a dominant period close to 2 seconds

(unfortunately no strong motion records were taken directly on

competent bedrock)

.

The amplification of the ground motion by the lake bed deposits

is attributed to two factors: The low damping ratio of the clay

deposits (about 6% at shear strains of 10~^%) and the dominant

period of the ground motion transmitted through the underlying

bedrock. Similar amplifications have been observed in previous

earthquakes, and ground motions of magnitudes similar to those

observed in the most critical area have been previously predicted

to have a 100 year mean recurrence interval.

While the subsurface conditions of Mexico City are in many

respects unique, it cannot be ruled out that similar ground

motion amplification could occur in other areas underlain by

deposits of saturated clays.

Implications for Existing Design Provisions

Elastic response spectra for single degree of freedom resonators

with 5% damping were compared with design spectra for non ductile

structures specified in the 1976 Mexico Federal District Building

Regulations, and with design provisions presently used in the

U.S. or proposed for U.S. practice. The comparison indicates that

even well designed buildings following the provisions of the

Mexico code or those specified for the most severe conditions envisioned in present or proposed US practice could have failed in

the Mexico City earthquake if they had natural periods of vibration close to the 2s site period. The implication of this finding

is that perhaps special provisions for site - structure resonance

197
I

should be considered for areas underlain by deep clay deposits

similar to those found in Mexico City.

Damage Patterns '

j One of the unique aspects of this earthquake was the selective

damage pattern. While some areas of Mexico City suffered

devastating structural failures, other areas were only slightly

affected. This phenomenon is attributed to the great

amplification of the ground motion in areas of lake deposits

which had natural periods close to 2 seconds. Within the danger

zone, the damage pattern was also selective. A high percentage of

the buildings in the height range of 6 - 17 stories failed, while

lower buildings and taller buildings performed relatively

well. This phenomenon is attributed to the large number of cycles

of nearly sinusoidal ground motion, which greatly affected

buildings with fundamental periods close to that of the ground

motion. In some locations the damage pattern was erratic. Some

structures collapsed, while almost identical structures in the

immediate vicinity suffered little damage. In one instance this

phenomenon could be traced to the effect of past differential

foundation settlements. Other possible causes suggested are

uneven soil consolidation patterns caused by structural loads and

groundwater pumping, poor construction quality control, and heavy

live load concentrations in upper stories.

Foundation Performance

In spite of the difficult foundation condition prevailing in the

lake region of Mexico City and the severity of the ground motion,

structural foundations on the whole performed surprisingly well,

even though some spectacular foundation failures occurred. This

record is attributed to the low permeability of the clay

deposits, which precludes large settlements associated with

volume change of the subsoil within the time span of a seismic

198
event, and to the cohesive strength of the clays which precludes

liquefaction. However, settlements and rotational slides

associated with shear deformations and failures did occur.

Foundation failures and distress observed included tilting and

overturning of structures and loss of shear resistance of

foundation piles which manifested itself as settlement of the

soil surrounding pile supported buildings. The latter phenomenon

is attributed to the sensitivity of the clay (it loses much of

its shear strength when subjected to large shear deformations)

.

In some cases of overturning, the connection between some of the

foundation piles and the stiff box foundations they supported

failed, leaving some piles in the ground and pulling others

out. Several structural failures resulted from prior stressing of

structural elements due to differential foundation settlements

and tilting prior to the earthquake (Table 4.1). These

conditions, together with irregular consolidation patterns

caused by structural loading, excavation and groundwater pumping

prior to the earthquake may have caused failure of some

structures, while identical structures in their immediate

vicinity survived. Some foundation failures, such as shear

failures in piles, may have escaped detection.

Structural Performance

The severity of structural response (and similarly the degree of

damage) of the buildings was directly related to the proximity of

their fundamental period of vibration to the period of the ground

motion. In the historic lake bed region of Mexico City, which

experienced the most severe ground motions, this period was

approximately two seconds. Failures were thus concentrated in

buildings with story heights in the range of 6-17 stories, which

had vibration periods close to that of the ground motion. Free

vibration measurements taken after the earthquake indicated

typical periods of 0.12s per story. These periods tended to

199
lengthen as a result of structure-foundation interaction and

inelastic deformations in the structure (with an ensuing

reduction in stiffness) . These latter factors help to explain

why so many structures with natural periods of 1-1.5 seconds in

the elastic range were ultimately destroyed: the dilation in

period during the earthquake allowed these structures to vibrate

themselves into the "zone of danger" where lateral accelerations

were greatest.

The observed structural failures generally fall (in order of

descending frequency) into the following categories:

1. Lack of ductile design characterized by inadequate

detailing of beam-column and slab-column connections.

Specifically the lack of sufficient confining

reinforcement in concrete column joints, the use of

vertical lap splices in column joints, and a lack of

sufficient shear reinforcement connecting flat slabs to

their respective columns in flat slab and flat plate

construction

.

2. Lack of plan symmetry in either structural

configuration or structural rigidity. This led to

torsional response mechanisms and subsequently high

localized stresses, and possibly dynamic impacting of

adjacent structures. This was particularly evident in

corner buildings, where open store fronts on the first

floor, combined with infill walls on the interior

sides, produced torsional response leading to pounding

against adjacent buildings.

3. Pounding of adjacent structures due to insufficient

separation. The problem was complicated by the

tendency of structures of dissimilar natural

frequencies to vibrate out of phase with one another,

200
such that at any given point in time the maximum

dynamic lateral deflection of each structure could be

towards the adjacent structure. Separation based only

on the maximum deflection of one structure or the other

would not have been sufficient to prevent collisions.

4. Abrupt changes in structural stiffness (or strength)

with story height. Common practice in Mexico City has

been to decrease column dimensions in the upper floors

of multistory buildings. Also prevalent is the

addition of flexible stories to the top of a structure

at some time after completion of the original

structure. Both of these can lead to upper-story

collapses if pounding occurs or if high live loads

(for example archives, as mentioned in 5. below) are

stored in the upper floors.

5. Large dead loads in upper floors. Office space in

urban Mexico City is at a premium and there is a

tendency to use lower floors for business (either

offices or stores) and relegate storage to the upper

floors. This practice serves to lengthen the period of

vibration and increase the shear forces in the upper

stories, since more mass is concentrated in the upper

floors

.

6. Unnecessarily heavy floor systems. Several types of

concrete slab systems, produced unnecessarily heavy

story loads which in turn led to high overturning

moments and story shears.

Tall structures, in general, responded well to the earthquake,

owing perhaps more to their natural periods of vibration which

placed them well outside the two second "zone of danger" than to

201
excellence in structural design. A great many of the taller

buildings in Mexico City, such as the Torre Latino Americana (43

stories) and the Pemex Tower (53 stories) , utilize steel frame

construction. When assessing the good performance of the tall

structures, many of which were in locations where widespread

damage was caused in adjacent, lower-height structures, the

importance of the relationship between the natural period of the

structure and that of the site should be kept in mind. These

same tall buildings may not have faired well at all had they been

constructed in the virgin lake bed area near the sports palace,

where periods of vibration in the vicinity of five seconds were

recorded, or near the Central Produce Market, where the period of

the ground motion was four seconds and the duration approached

three minutes. It is also reasoned that a 50-story concrete

structure in the historic district would probably have performed

as well as the. tall steel structures. The underlying argument is

that dynamic "detuning" of a structure to avoid resonance with

the site period could be as important as the choice of the

structural system.

One particular type of reinforced concrete slab system, known as

Losa Reticular (waffle slabs) was responsible for a large portion

of the cases of total collapse. This system employed cinder

block (either solid or hollow) rather than metal pan forms during

the casting of the waffle slab. These blocks were cast into the

structure leading to high floor weights. Waffle slab structures

suffering the greatest damage during the earthquake generally

exhibited a lack of sufficient concrete cover on slab

reinforcement and a lack of sufficient slab-to-column shear

reinforcement. These factors led to numerous punching shear type

failures, pancake collapses, and "drop out", a phenomena peculiar

to this type of construction where the cinder blocks simply fell

out of the floors after a number of inelastic load cycles. This

type of construction practice should be subject to stringent

specifications in future Mexico City building codes.

2 02
Older, low-rise masonry buildings generally performed well, as

did the massive stone masonry colonial churches and government

offices. These structures have short periods of vibration and

thus were not very vulnerable to the ground motion in the lake

region of Mexico City. Modern high-rise framed structures which

contained masonry infill walls (or shear walls) generally

performed better than similar structures which did not. Infill

walls increased the lateral stiffness of the structures and

therefore served to decrease their natural period of vibration,

thus making them less vulnerable to the long period ground

motion. In most cases such walls were unreinforced and often

suffered severe damage during the earthquake. However, in doing

so they dissipated a substantial amount of energy and probably

prevented collapse of numerous buildings.

203
REFERENCES

[1] Serna, J.F., The Mexico City Earthquake: A technical

challenge; an example of human solidarity, copy transmitted

to the National Bureau of Standards on Dec. 27, 1985 by

Francisco J. Serna, International Director, ASCE.

[2] Suzuki, S, Kiremidjian, A.S.,The Mexico Earthquake of

September 19, 1985, the John A. Blume Earthquake Engineering

Center, Report No. 77, Stanford University, January 1986.

[3] Institute de Ingenieria, UNAM, Effecto de los Sismos de

Septiembre de 1985 en las Construcciones de la Ciudad de

Mexico, Aspectos Estructurales , Segundo Informe de Ingeniria

de la Universidad Nacional Autonoma de Mexico, UNAM, Mexico

City, November, 1985.

[4] Anderson, J.G. et al.. Aspects of Strong Motion From the

Michoacan, Mexico Earthquake of September 19, 1985.

October 1985 (unpublished)

.

[5] Singh, S.K.,L. Astiz, and J. Harskov, Seismic Gaps and

Recurrence Periods of Large Earthquakes Along the Mexican

Subduction Zone: a re-examination. Bull. Seism. Soc. Am..

71, pp 827-843, 1981.

[6] McNally, K.C. and Minster, J.B., Nonuniform Seismic Slip

Rates Along the Middle American Trench, J. Geophysical

Research . 86, pp 4949-4959, 1981.

[7] Mena, E. , Carmona, C, Alcantara, L. , Delgado, R. , Analysis

Del Accelerograma "Zacatula" del Sismo del 19 de Septiembre

de 1985, Informe IPS lOE, Institute de Ingenieria UNAM, 1985.

[8] Zeevaert, L. , Strong Motions Recorded During Earthquakes of

May the 11th and 19th, 1962 in Mexico City, Bull. Seismic

Society of America . 54, pp 209-231, 1964.

[9] Marsal, R. J. The Lacustrine Clays of the Valley of Mexico,

International Clay Conference, Mexico City, Universidad

National Autonoma de Mexico, Institute de Ingenieria, E16,

Mexico City, July 1975.

[10] Marsal, R.J. and Mazari, M. , The Subsoil of Mexico City . Universidad National Autonoma de Mexico, Institute de

Ingenieria, Mexico City, September 1959 (Second Edition,

1969) .

204
[11] Resendiz, D. , Springall, G. , Rodriguez, J.M., Esquivel,R.,

Informacion Reciente Sobre Las Caracteristicas del Subsuelo

y la Practica de la Ingenieria de Cimentationes en la Cindad

de Mexico, 5th National Meeting of the Mexican Society of

Soil Mechanics, 1970.

[12] Prince, J., et al., Accelerogramas en Ciudad Universitaria

del Sismo del 19 de Septiembre de 1985, Informe IPS-lOA,

210985 Institute de Ingenieria UNAM, September 20, 1985.

[13] Mena, E., et al., Accelerograma en el Centre Scop de la

Secretaria de Comunicaciones y Transportes, Sismo del 19 de

Septiembre de 1985, Informe IPS-lOB, 200985 Institute de

Ingenieria UNAM, September 21, 1985.

[14] Quaas, R. , et al . , Los Dos Accelerogramas de Sismo De

Septiembre 19 de 1985, Obtemidos en La Central de Abastos en

Mexico, D.F., Informe IPS-IOC, 230985 Institute de Ingeneria

UNAM, September 23, 1985.

[15] Anderson, J.G., Bedin, P., Brune, J. N. , Prince, J., Singh,

S. K. , Quaas, R. , Duate, M. , Strong ground motions from the

Michoacan, Mexico, Earthquake, Science Magazine, Vol 233,

pp. 1043-1049, Sept. 5, 1986.

[16] Rosenbluth, E., Seismic Design Requirement in a Mexican 1976

Code, Earthquake Engineering and Structural Dynamics,

Vol. 7, 1979.

[17] Faccioli, Ramirez Flores, J., Repuestas Sismicas Maximas

Probables en las Ascillas de la Ciudad de Mexico, Institute

de Ingenieria UNAM, Report No. 359, 1975.

[18] Forell, F. , Niceletti, J. P., Mexico Earthquakes, Oxcalla,

Nov. 29, 1978 , Guerrero, March 14, 1979, Earthquake Engineering Research Institute, October 1980.

[19] Manual De Diseno Per Sismo, Segun el Reglamento de Construcciones Para el Distrito Federal, Series del Institute de

Ingenieria, No. 406, July 1977.

[20] International Conference of Building Officials, Uniform

Building Code, Whittier, CA, May 1985.

[21] Building Seismic Safety Council, NEHRP Recommended

Provisions for the Development of Seismic Regulations

for New Buildings, Washington, D.C.,1985.

[22] Seismology Committee, Structural Engineers Association of

California, Tentative Lateral Force Requirements, SEAOC,

Sacramento, CA, October 1985.

205
[23] Rosenblueth, E. and Meli, R. , The Earthquake of

19, September 1985: Effects in Mexico City, Report by

Subcommittee on Norms and Construction Procedures, Committee

for Mexico City's Metropolitan Area.

[24] Arnold, C. , Reiterman, R.. Buildincf Configuration and Seismic

Design . J. Wiley & Sons, 1982.

[25] Hanson, R. , Martin, H. , Martinez-Romero, e.. Performance of

Steel Structures in the September 19, and 20, 1985 Mexico

City Earthquakes, Proc. Natl. Earthquake Engineering Conf.,

. AISC, Nashville, TN, June 1986.

[26] American Concrete Institute, Building Code Requirements for

Reinforced Concrete, ACI 318-83, American Concrete

Institute, Detroit, MI, Nov. 1983 ( as well as 1979 edition)

[27] International Masonry Institute, Mexico Earthquake,

September, 1985, Washington, D.C., 1986.

[28] Merritt, F.S., Earthquake Revealed Defects in Design and

Construction, Engineering News Record, August 15, 1957.

[29] Leyendecker , E. V. and Fattal, S . G ., Investigation of the

Skyline Plaza Collapse in Fairfax County, Virginia, NBS

Building Science Series 94, Natl. Bur. of Stand.,

- February 1977.

[30] Lew, H.S. et al ., Investigation of Construction Failure of

Harbor Cay Condominium at Cocoa Beach, Florida, NBS Building

Science Series 145, Natl. Bur. of Stand., August 1982.

[31] Meli, R. , Efectos de los Sismos de Septiembre de 1985 en las

Construcciones de la Ciudad de Mexico—Aspectos

Estructurales, Segundo Informe de Institute de Ingenieria,

UNAM, November 1985.

[32] Blume, J.A. , Newmark, N.M. , Corning, L.H. , Design of

Multistory Concrete Buildings for Earthquake Motion,

Portland Cement Association, Skokie, IL, 1961.

[33] U.S. Senate Committee on Commerce, Science and

Transportation, , Subcommittee on Science, Technology and

Space, Testimony Presented by Dr. Dallas L. Peck, Director,

United States Geological Survey, in October 3, 1985 hearing

on the September 19, 1985 Mexico City Earthquake.

[34] Winter, G., and Nielson, A.H. , Design of Concrete

Structures, Mc Graw Hill, New York, NY, 1972.

206
[35] Newmark, N.M. , and Rosenbluth, E. , Fundamentals of

Earthquake Encrineering , Prentice-Hall, Englewood Cliffs, NJ,

1971.

[36] Vazquez-Vera, A. , and del Valle-Caldron, E. , Damage Pattern

and Characteristics, Proc. Int. Conf. on September 19, 1985

Mexico City Earthquake, Mexico City, September 19-21, 1986,

American Society of Civil Engineers, New York, NY.

207

NBS-lUA (REV. 2-BC)

'J.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBS/BSS-165

2. Performing Organ. Report No. 3. Publ ication Date

May 1987

4. TITLE AND SUBTITLE

Engineering Aspects of the September 19, 1985 Mexico Earthquake

5. AUTHOR(S) William C. Stone, Felix Y. Yokel, Mehmet Celebi, Thomas Hanks,

Edgar V. Leyendecker

6. PERFORMING ORGANIZATION (If joint or other than NBS. see in struct/on s) U.S. Department of Commerce U.S. Department of the Interior

National Engineering Laboratory Geological Survey

National Bureau of Standards Office of Earthquakes, Volcanoes,

Center for Building Technology and Engineermg

Gaithersburg, MD 20899 Menlo Park, CA 94025

^ Golden, CO 80401

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP) U.S. Department of Commerce

National Engineering Laboratory

National Bureau of Standards

Center for Building Technology

Gaithersburg, MD 20899

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 87-619819

[]J Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Following the September 19, 1985 Mexico earthquake, a team consisting of four

engineers and one seismologist from the National Bureau of Standards (NBS) and

the United States Geological Survey (USGS) was dispatched to Mexico City to provide

technical advice to the US rescue effort and to assess structural damage. _ This

report is primarily based on data gathered by the team, but it also contains a compilation of other available information. The report addresses the origin and

characteristics of the observed ground motion, the ability of buildings designed

in accordance with present and proposed seismic design provisions to resist

this type of ground motion, and observed data on structural and foundation

fail ures

.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) building codes; earthquake; foundations; geology; geotechnical engineering;

ground motion; response spectra; seismology; seismic design; standards; structural

engineering

13. AVAILABILITY

Unl imited

I I For Official Distribution. Do Not Release to NTIS

|V] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 2216 I

14. NO. OF

PRINTED PAGES

215

15. Price

* U.S. Government Printing Office: 1987 0-181-076 (60083) USCOMM-DC 6043-P80



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