1985+Mexico+City+Summary+&+Lessons+Learned

1985 Mexico City Earthquake Summary & Lessons Learned toc // Nicole B. Trujillo, BAE, Penn State, 2012 //

Keywords: Earthquake, Mexico City, High Rise on Soft Soils, Building Collapse

=Introduction=

On September 19th, 1985, at 7:17 A.M., a magnitude 8.1 earthquake occurred on the pacific coast of Mexico [See Figure 1]. A major aftershock occurred about 36 hours after on September 20 (Suzuki 1986). It was of 7.5 magnitude (Esteva 1988). The epicenter of the earthquake was located by the national seismological service at 17.6°N, 102.5°W in the Pacific Ocean, in a subduction zone near Michoacán (U.S. Dept. of Commerce 1990). It is one of the most devastating earthquakes in the Americas (Gunn 2008). An estimated 300,000 square miles were affected throughout Mexico with severe damage caused in parts of Mexico City. There were about 10, 000 causalities and about 400 structures that were severely damaged or destroyed (Esteva 1988). It caused over $3 billion US dollars in damage and was felt by 20 million people (Gunn 2008).

 The earthquake affected the downtown of Mexico City that lies on an ancient lakebed. Much damage was caused due to the underlying layers of soft clay soil in the downtown of Mexico City (Bertero 1989). This weak soil allowed much more ground movement than engineers had expected for such a distant earthquake. In the central district of Mexico, buildings of 5 to 7 stories matched the 2-second natural period of the earthquake causing 400 to collapse, and more than 700 buildings were damaged (National Bureau of Standards 1985). These structures oscillated violently, lengthening their periods of vibration, and entered the range of resonance with the ground (Nieto 1985).

In addition, there were failures of unreinforced masonry buildings and of non-ductile lightly reinforced concrete buildings, including the Central Communications Center. This building failure caused a collapse of communication between Mexico City and the rest of the world. As a result, there were many lessons learned from the Mexico City Earthquake; including changes to building codes and a further development of earthquake resistant design.

= = =Seismological and Geotechnical Aspects=

Mexico City lies on the Cocos Plate, which is a zone of intense seismicity (Romo 1988). The 1985 earthquake was caused by the subduction of the Cocos plate beneath the continental plate of North America. It ruptured a segment of the Michoacán gap (Suzuki 1986). It is the most active subduction thrust fault in the Western Hemisphere (Esteva 1988). In comparison to California, which in the 20th century experienced five earthquakes of magnitudes greater than 7, Mexico had forty-two (Gunn 2008).

There are three zones in Mexico City on the basis of geotechnical characteristics. These include the Lake zone, transition zone, and hilly zone. The Texcoco and Xochimilco-Chalco Lakes were connected and the lake zone includes both basins (Romo 1988). The most damaged areas were those in the bed on historic Lake Texcoco, where there are silt and volcanic clay sediments (Gunn 2008). The deep lake deposits amplified the ground motion (National Bureau of Standards 1985). There was little or no damage to structures that were in the transition zone or on firm soil (Nieto 1985). It was due to the distance between the earthquake’s epicenter and Mexico City that greatly reduced the amplitude of the seismic waves so that very few structures that were built on firm soil and rock suffered damage (Gunn 2008). The severe damage was limited to less than 3 percent of the total inhabited area of Mexico City (Nieto 1985). Cinna Lomnitz Aronsfran, Professor of Seismology at the National Autonomous University of Mexico (UNAM) said that only 16% of the total buildings in Mexico City collapsed (A Film Media Group Company 2008).

Due to the amount of damage that Mexico City experienced due to the 1985 earthquake. Researchers investigated ways to detect when an earthquake might occur, in order to give a warning. Researchers found a radon anomaly in a nuclear track detector placed on the fault prior to the earthquake (Segovia 1989). Since no other phenomenon can be found as a possible cause for the radon anomaly, it was suggested that radon measurements could provide a good precursor in high seismic risk areas (Segovia 1989).

=Building Performance=

 Before 1985, the worst earthquake occurred in 1957 (Castaños 1995). In the 1985 earthquake, the downtown area located on soft lake mud was the part of Mexico City most affected (Castaños 1995). In the ancient lakebed, shaking from the earthquake lasted from 3 to 4 minutes (Gunn 2008). The strong shaking in the Mexico City earthquake with a magnitude of 8.1 caused a great amount of damage (Salvadori 1992). Since, t he earthquake hit early in the morning before schools and offices in Mexico City were occupied the death toll was lower than possible. The greatest death toll occurred when buildings collapsed (Gunn 2008).

A majority of the population, who lived in the downtown area, resided in former 19th century unreinforced brick or stone masonry, yet little damage occurred in this type of housing (Castaños 1995). Also, old Colonial palaces were also undamaged. Damage in traditional masonry construction was lower in 1985 than it had been for some earlier seismic events on record (Castaños 1995). The older the building, the better it performed in the earthquake because the structures reached an equilibrium position with the soft soils (Castaños 1995).

 It was high rise buildings, which experienced much of the building failures. Most of the casualties occurred in high rise buildings. High-rise buildings had been designed by professional engineers in accordance with federal building codes and regulations, which featured special provisions for seismic forces and for soft-ground conditions (Castaños 1995). In 1940, sky-rocketing real estate values caused the construction of many concrete-frame high-rise buildings (Castaños 1995). Yet, t he earthquake disaster was a result of high rise buildings on soft soil.

**// Mexico City Building Code //** The code in effect in 1985 was the 1976 Mexico City building code. Most buildings were all in compliance with the code. Yet, the 1985 earthquake tested buildings in Mexico City and engineers were forced to evaluate the building practices at the time to ensure building performance during a following quake.

Many damaged buildings were built before the 1976 seismic code for Mexico City was issued. These buildings were designed in accordance with the 1942, 1957, and 1966 design codes which did not provide sufficient strength against an 8.1 magnitude earthquake (Suzuki 1988). Some buildings were not even designed for earthquake resistance (Suzuki 1988).

The Mexico City building code was modeled after American codes rather than local experience (Castaños 1995). When high-rise buildings were again destroyed in the 1985 earthquake, the code became more stringent instead of modifying the high-rise buildings (Castaños 1995). As a result, t he base design acceleration was increased by 80%. Yet, this was not sufficient since the actual acceleration produced by the 1985 earthquake had exceeded the building code specifications by 315% (Castaños 1995).

Building performance varied. It depended on the type of structure and location. In high rise buildings, the inverted pendulum effect and the unusual flexibility of Mexico City structures caused upper floors to sway as much as 3.28 feet (U.S. Dept. of Commerce 1990). Differential movements of adjacent buildings also resulted in damage (U.S. Dept. of Commerce 1990). Also, two adjacent building that swayed and came into contact often were damaged or collapsed. In addition, corner buildings were vulnerable to damage (U.S. Dept. of Commerce 1990).
 * //Cases// **

There were five parameters that affected the seismic performance (Bertero 1989). These included the degree of regularity, redundancy of structure, relation between the effective natural period of the structure and the expected predominant period of the seismic motion, the real strength of the structure, and the ability to sustain cycles of inelastic deformation without a loss in strength (Bertero 1989).

<span style="font-family: Arial,Helvetica,sans-serif;">Some of the most notable cases are described below:

<span style="font-family: Arial,Helvetica,sans-serif;">Nuevo León building
<span style="font-family: Arial,Helvetica,sans-serif;"> <span style="font-family: Arial,Helvetica,sans-serif;">The Nuevo León building was located in the Tlatelolco apartment complex [See Figure 2]. Tlatelolco is an area in the Cuauhtémoc borough of Mexico City. The complex consisted of 102 buildings, housing approximately a hundred thousand residents (Walker 2009). Many of its residents were government bureaucrats, doctors, university professors, and other professionals (Walker 2009).

<span style="font-family: Arial,Helvetica,sans-serif;">On the morning of September 19, 1985, the fifteen-story reinforced concrete structure collapsed (National Bureau of Standards 1985). The building had independent wings that were separated by construction joints (National Bureau of Standards 1985). The two northern wings had 196 apartments in each wing. The building overturned due to an asymmetrical foundation failure. The supporting columns were pulled from the foundation on the west side which led to crushing of the western columns and toppling of the structure (National Bureau of Standards 1985).

<span style="font-family: Arial,Helvetica,sans-serif;">Tlatelolco became a national and international symbol of devastation. The residents of the Nuevo León building demanded an investigation of the collapse because the building was in need for repair for many years prior to the quake. In 1979, the building was leaning 27.6 inches (Walker 2009). This exceeded the maximum allowed by city safety regulations for a building of its height. The maximum horizontal inclination should have been 12.6 inches. By early 1982, the Nuevo Leon leaned over 39.4 inches. In March of 1982 repair work reduced the inclination to 31.5 inches (Walker 2009). In1983, the state agencies had refused repair on the apartment buildings. When the agencies finally agreed to repair, they only made superficial repairs and continued poor maintenance. State agencies investigated the collapse and concluded that the residents who modified their apartments by removing walls altered the structure leading the loss of structural capacity (Walker 2009). Yet, foreign technicians found that it was poor construction that led to the collapse of the apartment building (Walker 2009).

<span style="font-family: Arial,Helvetica,sans-serif;">Hotel Continental
<span style="font-family: Arial,Helvetica,sans-serif;"> Hotel Continental was constructed in 1950 [See Figure 3]. The top floors failed during the 1985 earthquake (U.S. Dept. of Commerce 1990). There were four aspects of the building that made it vulnerable to failure. First, the upper floors displaced more than the ground floors during the tremors. Next, the resonance frequencies of the building coincided with the ground vibrations leading to large amplification of oscillations. Third, the upper floors had weaker and smaller load-bearing components. Finally, the length of the earthquake event gave time for the development of torsional vibrations due to the asymmetric distribution of masses and elasticity in the high rise buildings (U.S. Dept. of Commerce 1990).

<span style="font-family: Arial,Helvetica,sans-serif;">The seven-story Ministry of Labor building that collapsed during the earthquake offers one example of the disregard for safety standards: it had been built on a concrete base designed to hold only three stories (Walker 2009). Public officials were responsible for approving building licenses and performing inspections, yet they approved the design. These types of failures led to the current practice of external supervision for every project.

<span style="font-family: Arial,Helvetica,sans-serif;">The Central Communications Center was a twelve story reinforced concrete building used by the Ministry of Com munications and Transport [See Figure 4]. During the 1985 earthquake, the top floors of the building failed. The failure of the building caused the lack of long-distance communication between Mexico City and the coordination of international rescue efforts (U.S. Dept. of Commerce 1990).

<span style="font-family: Arial,Helvetica,sans-serif;">The period of vibration of the Communications Center was 2 seconds, with negligible vertical acceleration. The 0.17g horizontal ground acceleration persisted for 30 seconds. This caused the building to be driven to resonance (National Bureau of Standards 1985). The first mode of torsional oscillations caused the failure of the columns. Then, the upper floors collapsed due to the crushed north-west corner of the building (National Bureau of Standards 1985).

Conjunto Pino Suárez consisted of a 5-steel building complex [See Figure 5]. It was a government office building. There were two southern towers, a 14 story building and a 21 story building (National Bureau of Standards 1985). The buildings utilized an exterior welded box column frame and integrated composite slab system. The building used heavy floor truss beams. Yet, there was very little ductility that developed due to a buckling of the truss braces under reverse loading (National Bureau of Standards 1985).

<span style="font-family: Arial,Helvetica,sans-serif;"> The major problem was axial overloads in columns. The collapse seems to have been initiated by column buckling in the 21-story tower (National Bureau of Standards 1985). The 21 story tower collapsed, then leveled the 14 story tower in the process (National Bureau of Standards 1985). There were few steel structure failures as a result of the earthquake. Two of the buildings in the complex did not collapse, but experienced severe damage. This can be attributed to redundancy (Bertero 1989). So, it was recommended that redundancy should be included in the code design after 1985.

=Overall Patterns and Lessons Learned=

**Failure Types**
<span style="font-family: Arial,Helvetica,sans-serif;">There were five common patterns of failure for the severely damaged buildings. These patterns included: 42% were corner building failures, 40% were a collapse of intermediate floors, 38% were a collapse of upper floors, 15% were due to pounding, and 13% were foundation failures (U.S. Dept. of Commerce 1990).

<span style="font-family: Arial,Helvetica,sans-serif;">//Corner building failures// The main cause of failure was due to torsion and orthogonal effects (Suzuki 1986). <span style="font-family: Arial,Helvetica,sans-serif;">//Collapse of intermediate floors// <span style="font-family: Arial,Helvetica,sans-serif;">In many damaged buildings, just one floor had collapsed. But in some cases, the damage was caused by the top of a lower, adjacent building crashing against the walls and the supporting columns of its taller neighbor building (Bertero 1989). Eventually, the columns gave way causing the intermediate floors of the taller building to collapse. In some cases, this caused many stories collapsed. For example, a multi-story parking garage experienced a Progressive collapse while some neighboring buildings remained undamaged. Many of these buildings utilized flat cement slab or waffle slab floor systems.

<span style="font-family: Arial,Helvetica,sans-serif;">//Collapse of upper floors// <span style="font-family: Arial,Helvetica,sans-serif;">Failure of upper stories of buildings [See Figure 6] was due to pounding between adjacent buildings or by large torsional motions (Bertero 1989). The formation of plastic hinges in the upper stories of the structure when the second or third natural period of the structure is shorter than the dominant period of the ground motion in its second or third mode after it suffers some stiffness deterioration.

<span style="font-family: Arial,Helvetica,sans-serif;">//Pounding// <span style="font-family: Arial,Helvetica,sans-serif;">Pounding is when one building repeatedly striking another during the earthquake (Bertero 1989). This resulted to buildings required to be adequately separated to prevent pounding (Bertero 1989). <span style="font-family: Arial,Helvetica,sans-serif;">//Foundation failures// <span style="font-family: Arial,Helvetica,sans-serif;"> These failures occurred mainly on very soft ground [See Figure 7]. Foundations on friction piles experienced a reduction in the shear strength due to many cycles of alternating loading (Esteva 1988). Also, settlements of 2 feet under buildings that were supported on point- bearing piles were evidence of failures in the clays (Nieto 1985). The nature of the soil had changed in downtown Mexico City, which influenced the amplification factors. This led to the amplification factors for structures in the zone of transition or in the zone of the lake to require a more stringent zone classification for structures in the downtown area (Nieto 1985).

<span style="font-family: Arial,Helvetica,sans-serif;">Many of the observed structural failures were caused primarily by human errors of the following types: errors in design, change of use, building alterations, poor quality of materials, and errors of execution (Nieto 1985).

<span style="font-family: Arial,Helvetica,sans-serif;">//1. Errors in design// <span style="font-family: Arial,Helvetica,sans-serif;">There were a number of failures in irregular buildings on oblique corners where the lack of structural symmetry made the seismic torsion more severe (Nieto 1985). Also, the lack of attention to structural details was blamed for the collapses (Castaños 1995).

<span style="font-family: Arial,Helvetica,sans-serif;">//2. Change of use// <span style="font-family: Arial,Helvetica,sans-serif;">Some buildings designed for lighter loads or for residential purposes and had been converted to clothing factories with machinery operating (Nieto 1985).

<span style="font-family: Arial,Helvetica,sans-serif;">//3. Building alterations// <span style="font-family: Arial,Helvetica,sans-serif;">Many failures were due to significant structural modifications which were carried out by building owners after the original construction without appropriate permits (Nieto 1985).

<span style="font-family: Arial,Helvetica,sans-serif;">//4. Poor quality or deterioration of materials// <span style="font-family: Arial,Helvetica,sans-serif;">The quality of some reinforcing steels and some concretes obtained from collapsed structures was of poor quality (Nieto 1985).

<span style="font-family: Arial,Helvetica,sans-serif;">//5. Errors of execution and lack of supervision// <span style="font-family: Arial,Helvetica,sans-serif;">There was evidence of poor construction practices. The positioning of reinforcement was poor. Also, there were insufficient anchorages (Nieto 1985). Cold joints were made with foreign material such as paper or wood (Nieto 1985). Concretes was badly vibrated or segregated during placement and there were bad connections of infill walls (Nieto 1985).

**Lessons Learned**
<span style="font-family: Arial,Helvetica,sans-serif;">The magnitude 8.1 earthquake was one of the most unique and severe earthquakes in history. After the 1985 earthquake, many researchers conducted tests to understand why some structures experienced damage due to the earthquake. For example, a test model of a one-tenth-scale reinforced concrete building that was seriously damaged during the 1985 Mexico earthquake was tested on a shaking table (Xianguo 2004). The input ground excitation used during the test was from the records obtained near the site of the prototype building during the 1985 Mexico earthquake (Xianguo 2004). The tests showed that the damage pattern of the test model agreed well with that of the prototype building. The comparison of the analytical results and the shaking table test results indicated that the response of the reinforced concrete building to moderate earthquakes can be predicated well (Xianguo 2004).

<span style="font-family: Arial,Helvetica,sans-serif;">Lessons learned from the patterns of earthquake damage were applied to prevent another disaster when an earthquake hits Mexico City (U.S. Dept. of Commerce). The lessons of the Mexico earthquakes consisted mostly in raising the design parameters (Castaños 1995).

<span style="font-family: Arial,Helvetica,sans-serif;">//**Building Code Revisions**// <span style="font-family: Arial,Helvetica,sans-serif;">The Mexico City Building Code is the model code for the country. In 1985, the 1976 Mexico City Building Code was used. As a result of the earthquake, Mexico City put into effect the Emergency Regulations of 1985 (Alcocer 2008). These new regulations were given in order to design rehabilitation projects of damaged structures and for designing new structures in the soft soil zones. Some important changes included the increase of elastic design seismic shear coefficients in soft soil zones (Alcocer 2008). Due to several column failures, there was a reduction of the strength reduction factor, stricter requirements for transverse steel, and the minimum cross sections were increased. Live loads for offices were doubled. For the simplified static method, the maximum height for structures was reduced and seismic coefficients were increased. Finally, the maximum torsion eccentricity at each story had to be smaller than 20 per cent of the largest in-plan dimension.

<span style="font-family: Arial,Helvetica,sans-serif;">Also, other countries such as the United States learned lessons from the 1985 Mexico City earthquake. The main lesson was that the partial or total collapse of buildings is the main cause of earthquake deaths and injuries. Seismic resistant design is very important to incorporate into building codes. Therefore, there is a need to incorporate seismic resistant design provisions in State and local building codes through high seismic risk areas in the country (U.S. G.P.O 1985).

<span style="font-family: Arial,Helvetica,sans-serif;"> There were one million structures in Mexico City that performed well (U.S. Dept. of Commerce 1990). Well designed masonry infill frame structures safely dissipated the energy in shear walls (National Bureau of Standards 1985). Also, m any tall concrete structures whose designs met the requirements of the building code performed well (National Bureau of Standards 1985).
 * <span style="color: #000000; font-family: Arial,Helvetica,sans-serif;">//Successes// **

<span style="font-family: Arial,Helvetica,sans-serif;"> Torre Latinoamerica survived (Hartman 2004). The 44 story Torre Lationoamerica office building remained almost totally undamaged, as it did in the 1957 earthquake. The building is a symmetrical steel frame structure built to resist earthquakes with a frequency near 4s (National Bureau of Standards 1985). It has 200 piles extending down about 115 feet to the topmost stable layer of soil (U.S. Dept. of Commerce 1990).

<span style="font-family: Arial,Helvetica,sans-serif;">Corruption in the construction industry had been a known problem. Before a project began, the contractor had to apply for a license and submit architectural plans. City authorities then filed these but rarely confirmed that the building had been built according to specifications; often contractors used lower-grade materials than those originally proposed (Walker 2009). Today, it is common practice for the client to hire the contractor and external supervision for each project. The external supervision is hired to ensure that the code is followed and that the materials specified on the drawings are being used on site.

<span style="font-family: Arial,Helvetica,sans-serif;">
===<span style="font-family: Arial,Helvetica,sans-serif;">** Current Seismic Design ** === The current code used in Mexico City is the 2004 Mexico City Building Code (Alcocer 2008). Some major revisions from the 1987 Mexico City Building Code include recent research developments on the design of steel, reinforced concrete, masonry and timber structures (Alcocer 2008). Due to the peculiar characteristics of the Mexico City soft soil area, comprehensive requirements are given for foundation design in the 2004 code (Alcocer 2008). Also, the 2004 Mexico City Building Code states that for buildings constructed prior to 1900 and that have suffered no damage, an assessment on their structural safety is not needed (Alcocer 2008). The lessons learned from the 1985 earthquake were applied, and all damaged buildings are now required to be reported. If their rehabilitation is needed, the building must meet the requirements established for new construction in the code (Alcocer 2008). In addition, buildings many high-rise buildings have been retrofitted to adhere to the new code [See Figure 8].

<span style="font-family: Arial,Helvetica,sans-serif;">Torre Mayor raises 55 stories, as the tallest tower in Mexico City. It was completed in July 2003 (Hardman 2004). The architect was Zeidler Roberts & Partners. Torre Mayor is located in the Reforma Centro District of Mexico City, which is where much damage occurred during the Mexico City earthquake of 1985. Therefore, it was designed in order to counteract seismic forces. Torre Mayor uses a double triangle diamond pattern, an ultra-solid combination of bracing and dampers, and buildings concrete-encased steel columns to provide an earthquake resistant design (A Film Media Group Company 2008). It uses viscous dampers that are placed in the diagonal braces to act as shock absorbers (Anders 2003). It is the first tall building to utilize large fluid viscous dampers as a primary means of seismic dissipation (Taylor). There are a total of 98 dampers throughout the building (Taylor). The building can withstand an 8.5 magnitude earthquake (A Film Media Group Company 2008).
 * <span style="font-family: Arial,Helvetica,sans-serif;">//Torre Mayor// **

<span style="font-family: Arial,Helvetica,sans-serif;">Torre Mayor’s lower floors had just been occupied, on January 21st, 2003, when a magnitude 7.6 earthquake hit. Yet, most all occupants were oblivious of the tremor (Post 2003). Torre Mayor raises the bar in earthquake resistant design.

<span style="font-family: Arial,Helvetica,sans-serif;">The 2003 Colima, Mexico earthquake had a magnitude of 7.6 and killed 18 people (Hardman 2004). Mexico’s City mud vibrates at the same frequency as certain earthquake waves which is one cycle every 2.5 seconds (Hardman 2004). This is dangerous for buildings between 10 to 15 stories which have a 2.5 second period as well. Therefore, there is a value of using seismic dampers for buildings built on soft soil (Hardman 2004).
 * <span style="font-family: Arial,Helvetica,sans-serif;">//2003 Colima, Mexico Earthquake// **

=Conclusion=

Over 400 multistory buildings collapsed in Mexico City. There were many different types of damage observed. In many damaged buildings, just one floor had collapsed. But in some cases, the damage was caused by the top of a lower, adjacent building crashing against the walls and the supporting columns of its neighbor. Eventually, the columns gave way. Also, there were foundation failures due to substantial reductions in soil capacity in shear under cyclic loading. In addition, there were thousands of failures of non-ductile lightly reinforced concrete buildings, including the Central Communications Center. This building failure caused a collapse of communication between Mexico City and the rest of the world.

As a result, Mexico City made emergency building code revisions due to damage surveys. Revisions to design and construction specifications included: an increase in seismic coefficients, reduced strength reduction factors, and drift limitations and clear-spacing between buildings were stressed. Despite all the failures, there were also success stories. Such as Torre Latino, this building withstood the earthquake forces with ease.

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