Vulnerability Maps for Reinforced Concrete Structures for Mexico City s Metropolitan Area under a Design Earthquake Scenario

Transcription

1 Vulnerability Maps for Reinforced Concrete Structures for Mexico City s Metropolitan Area under a Design Earthquake Scenario Arturo Tena-Colunga, a) M. EERI, Eber Alberto Godínez-Domínguez, a) and Luis Eduardo Pérez-Rocha b) This paper presents vulnerability maps for reinforced concrete moment frames (RCMFs) in Mexico City s Metropolitan Area (MCMA) for a M s =8.1 earthquake scenario assessing peak nonlinear dynamic responses using the concept of displacement ductility demand spectra (DDDS). The considered earthquake scenario is the 1985 Michoacán Earthquake. Artificial and recorded ground motions were used. Takeda model was used to simulate nonlinear behavior of RCMFs. Seismic strength for existing RCMFs structures was estimated according to the nominal strength required by versions of Mexico s Federal District Code (MFDC) that were effective before the 1985 earthquake. Overstrength sources for RCMFs were also evaluated. With all of this information, DDDS were obtained for different sites in MCMA. Later, vulnerability maps based on contours of equal displacement ductility demands and nonlinear displacements for a given structural period and a version of MFDC were defined. These vulnerability maps were compared with the geographic area of severe structural damage and collapses mapped during the 1985 earthquake. Based on the results of this study, it is believed that this procedure can be useful to forecast demands for different structural systems for a given earthquake scenario, being therefore potentially useful for urban planning of cities located in zones of high seismic risk. DOI: / INTRODUCTION Damaging earthquakes that occurred during the last two decades in Chile, Mexico, Armenia, the United States, Japan, Peru, Bolivia, Egypt, Turkey, Iran, Philippines, Colombia, Greece, Taiwan, and Indonesia, among other affected nations, have warned the engineering community worldwide about the vulnerability of existing structures. Several research projects with the same final goal have been conducted during the last two decades: to mitigate the seismic hazard in the built environment. Among other issues, many research efforts have been directed from the structural engineering perspective to: (1) evaluate and improve existing guidelines available in seismic codes, (2) study a) Departamento de Materiales, Universidad Autónoma Metropolitana, Av. San Pablo # 180, México, DF b) Instituto de Investigaciones Eléctricas, Calle Reforma 113, Col. Palmira, Cuernavaca, México 809 Earthquake Spectra, Volume 23, No. 4, pages , November 2007; 2007, Earthquake Engineering Research Institute

2 810 A.TENA-COLUNGA ET AL. and develop modern technologies to improve the seismic performance of structures subjected to earthquakes (for example, base isolation, passive energy dissipation, and active control), (3) study and develop strategies for the seismic retrofit of structures, (4) improve methods for seismic analysis and design, (5) develop general guidelines for the seismic evaluation of existing structures, and (6) develop simple procedures to define the seismic hazard and vulnerability of the built environment of a region using seismic hazard maps. The seismic evaluation of existing structures is an issue of paramount importance in earthquake engineering. The evaluation of existing structures is not only important to assess the vulnerability of specific structures, but also to complement strategic plans directed to mitigate the seismic hazard in the built environment of a given region based on integral vulnerability studies. This paper summarizes an integral study where vulnerability maps for reinforced concrete moment frames (RCMFs) in Mexico City s Metropolitan Area (MCMA) were defined for a M s =8.1 earthquake scenario assessing peak nonlinear dynamic responses using the concept of displacement ductility demand spectra (DDDS) obtained from single-degree-of-freedom (SDOF) systems as presented in Tena-Colunga (1998, 2001). It is worth noting that the DDDS is equivalent to the constant strength response spectra (CSRS) formerly studied by others with other purposes (i.e., Mahin and Bertero 1981, Pal et al. 1987), which are based on the concept of inelastic design spectra (IDS) that can be traced back to the 1960s in the pioneering work of Veletsos and Newmark (1960). It is also worth noting that the concept of using recorded ground motions to compute inelastic response spectra to define spectral ordinates for various ductility ratios has been used before, for example, the pioneering work of Gómez-Bernal et al. (1991) for Mexico City, and the study presented by Bozorgnia and Bertero (2003) for Los Angeles using the recorded motions in the Northridge and Landers earthquakes. The considered earthquake scenario is an equivalent scenario to the M s = September 1985 Michoacán Earthquake that severely affected Mexico City. This study constitutes one step forward on previous vulnerability studies conducted for this region, as nonlinear dynamic behavior for the structural system considering stiffness degradation is directly considered. The methodology used in this study, as well as a summary of previous vulnerability studies conducted for this region, are addressed in the following sections. PREVIOUS VULNERABILITY STUDIES FOR MEXICO CITY S METROPOLITAN AREA Mexico City was severely affected during the 1985 earthquake because of, among other reasons, important amplifications in the ground motions due to site effects, particularly in the lakebed region, where very soft soil deposits (clay) of different depths exist. The lack of a reasonable seismic instrumentation in Mexico City at the time of the 1985 earthquake (only 11 instruments: four in firm soils [TACY, CU01, CUMV, CUIP],

3 VULNERABILITY MAPS FOR RC STRUCTURES FOR MCMA UNDER A DESIGN EARTHQUAKE SCENARIO 811 Figure 1. Recording stations in Mexico City s Metropolitan Area at the time of the 19 September 1985 earthquake. The zones where severe damage was observed are depicted with green and blue lines. six in the lakebed region [SCT1, CDAO, CDAF, TLHD, TLHB, TXSO], and one in what is known as transition soils (SXVI; Figure 1) allowed only a limited understanding of how site effects affected the existing built environment at the time, as it was difficult to correlate the observed damage (Figure 1, green, blue, and red line contours) with the recorded ground motions. From the lakebed instrumentation, only the SCT station (SCT1) was located in a zone where heavy damage and collapses were observed (Figure 1, green line contour). The other five stations were located in places where no severe damage was observed (other than pipelines system in Tláhuac, stations TLHD and TLHB). In fact, there were no instruments located in the zone where most collapses were observed during the earthquake: Roma and Condesa districts (Figure 1, red line contour), as well as downtown

4 812 A.TENA-COLUNGA ET AL. Figure 2. Strong motion array in Mexico City s Metropolitan Area available since area (Figure 1, green line contour). Given that the characteristics of the built environment and soil deposits near the SCT site were very similar to those found in Roma and Condesa districts, it was difficult to explain why there was much more collapse in these last two districts than near SCT. It was therefore clear that a densification of the instrumentation in Mexico City was needed to understand better why this happened during the 1985 earthquake (in addition to many other valuable applications, such as to improve the seismic zonation of the city for design purposes, etc). The most important densification of the instrumentation in Mexico City s Metropolitan Area occurred between 1986 and 1988, when more than 120 instruments were installed, about 90 of them in the free field (Figure 2). Most of the instruments were located in the lakebed zone, where the most severe damages and collapses were observed during the 1957, 1979, and 1985 earthquakes (Figure 2, blue, green, and red contours). The first important moderate earthquake recorded by this network was the 25 April 1989 earthquake M s =6.9, a benchmark event that allowed seismologists to understand better site effects in Mexico City s valley. As the network continued to perform well in recording earthquakes of interest M 6 to help understand the dynamics of this region, and the studies of the recorded ground motions provided consistent results, the interest on constructing vulnerability maps of Mexico City s Metropolitan Area to assess the performance of the built environment increased.

5 VULNERABILITY MAPS FOR RC STRUCTURES FOR MCMA UNDER A DESIGN EARTHQUAKE SCENARIO 813 Perhaps the first study with this orientation is the one presented by Gómez-Bernal et al. (1991), where the Arias Intensity tensors and ductility demands contours (incomplete) are shown for some areas of Mexico City s Metropolitan Area for elasticperfectly-plastic systems for a period range between 1.5 s to 1.8 s. They used for their study the recorded ground motions at 65 stations for the 25 April 1989 earthquake M s =6.9, scaling them to the 1985 earthquake based upon a simple factor f based on the Arias intensity: f = I Max85 I Max89 where I Max85 is the Arias intensity of the recorded site for the 19 September 1985 earthquake and I Max89 is the Arias intensity of the recorded site for the 25 April 1989 earthquake. Because in September 1985 there were only 11 stations in Mexico City, extrapolations on I Max85 were done for many stations, taking into account the nearest station and the soil profile type. Pérez-Rocha (1998) presented vulnerability maps based upon average pseudoacceleration spectral ordinates S a /g for systems with effective structural periods T = 1 ± 0.25 s and T = 2 ± 0.5 s for four different earthquake scenarios that may affect Mexico City s Metropolitan Area: (a) a M s =8.1 subduction earthquake from the coasts of Michoacán State, a similar scenario to the September 1985 earthquake; (b) a M s = 7.7 subduction earthquake from the coasts of Guerrero State, a scenario similar to the July 1957 earthquake; (c) a M s =8.2 subduction earthquake from the coasts of Guerrero State, the worst scenario expected by seismologists from the Guerrero Gap; and (d) a M s =6.5 normal faulting earthquake that occurred 80 km from Mexico City. The study of Pérez-Rocha (1998) is based upon scaling the seismic source to the target magnitude and distance R using the 2 model, and the information collected between 1988 and 1997 from the 120 stations from the strong motion array available in Mexico City s Metropolitan Area (Figure 2) using average transfer functions for each station, random vibration theory to define pseudo-acceleration response spectra, and a Bayesian interpolation method for the spatial definition of S a /g contours (Pérez-Rocha et al. 1995). At about the same time, Reinoso and Ordaz (1999) presented a study where spectral amplification ratios for acceleration for dominant periods T=1.5 s, T=2 s, T=3 s, and T=5 s are shown for the same region using a procedure similar to the one outlined in Pérez-Rocha (1998). Huerta and Reinoso (2002) presented maps for the elastic input energy E I for the 25 April 1989 earthquake M s =6.9 for systems with effective structural periods T =2 s, T=3 s, T=4 s, and T=5 s. They also presented maps for the Normalized Hysteretic Energy NE H for elastic-perfectly-plastic systems with µ=4 for the same earthquake scenario and structural periods. Finally, maps for the elastic input energy E I for systems with an effective structural period T =2 s were presented for the following additional earthquakes: (a) 14 September 1995 subduction earthquake M=6.4, (b) 30 September 1999 subduction earthquake M=7.5, and (c) 15 June 1999 normal faulting earthquake M=6.5. 1

6 814 A.TENA-COLUNGA ET AL. Table 1. Damage inventory of RC structures and bearing walls from the 19 September 1985 earthquake in Mexico City (adapted from Fundación ICA 1988) Number of Stories Structural System 1 to 2 3 to 5 6 to 8 9 to or more Total # of Structures Damaged Structures % Bearing walls 35,115 7, , RCMFs made , of waffle flat slabs RCMFs 1,961 4, , RC frames & walls Total # of 37,484 13,498 1, , structures Damaged structures % Therefore, previous studies have concentrated primarily on average elastic demand parameters to assess the vulnerability of Mexico City s Metropolitan Area (Gómez- Bernal et al. 1991, Pérez-Rocha 1998, Reinoso and Ordaz 1999, Huerta and Reinoso 2002). The only studies that considered inelastic response parameters (for elasticperfectly-plastic systems) are those presented by Gómez-Bernal et al. (1991), where the empirical scaling of the seismic source is somewhat questionable, and the work presented by Huerta and Reinoso (2002), where a moderate earthquake source was used and no details are provided about the interpolation method used for the spatial definition of N EH contours. This paper presents a study where vulnerability maps for reinforced concrete moment frames (RCMFs) in Mexico City s Metropolitan Area are defined for a M s =8.1 earthquake scenario assessing peak nonlinear dynamic responses using the concept of displacement ductility demand spectra (DDDS) based upon SDOF systems. RCMFs are studied as most of the medium-rise buildings that were severely damaged or collapsed during the 29 September 1985 earthquake had this structural system, as can be observed from Tables 1 and 2. This study constitutes a step forward from previous vulnerability studies as: (a) degrading characteristics of RC structures are considered using the Takeda model, and (b) minimum strength requirements for RC structures related to the versions of Mexico s Federal District Code (RCDF) that were effective before the 1985 earthquake are assessed. The methodology used in this study is briefly described in following sections.

7 VULNERABILITY MAPS FOR RC STRUCTURES FOR MCMA UNDER A DESIGN EARTHQUAKE SCENARIO 815 Table 2. Severe damage and collapse inventory of buildings for the 19 September 1985 earthquake in Mexico City (adapted from Instituto de Ingeniería 1985) Structural System Damage Year of Construction Number of Stories Total Reinforced concrete Collapse moment frames (RCMFs) Severe Steel moment frames Collapse (SMFs) Severe RCMFs made of waffle Collapse flat slabs Severe Masonry bearing walls Collapse Severe Other systems Collapse Severe Total Collapse Severe METHODOLOGY USED The methodology used in this study is described in detail in Godínez (2005) and schematically summarized in Figure 3. This methodology takes advantage of previous research conducted by Pérez-Rocha (i.e., Pérez-Rocha et al. 1991, Reinoso et al. 1992, Sánchez-Sesma et al. 1993, Pérez-Rocha et al. 1995, Singh et al. 1996, Pérez-Rocha 1998, and Pérez-Rocha et al. 2000) to estimate ground motions for the site for a given earthquake scenario based upon empirical data, and the vulnerability of the structural system is assessed by using the concept of displacement ductility demand spectrum (DDDS) as presented and discussed in detail elsewhere (Tena-Colunga 1998, 2001). A DDDS relates peak displacement ductility demands (and other important response quantities, e.g., displacements) with structural periods of nonlinear SDOF systems with a given yield strength. In order to compute a DDDS for a given building inventory located in a region of interest from a vulnerability-assessment viewpoint, the following information is needed: 1. Acceleration records for the sites of interest. 2. Estimates of natural periods (frequencies) of response for subject structures. 3. Estimates for the lateral load capacities for the building inventory of interest. 4. A suitable hysteretic model for the structural system of interest. The ground motion assessment for the sites of interest (item 1) is described in the next section. The characterization of the structural system (items 2 to 4) is described in later sections.

8 816 A.TENA-COLUNGA ET AL. Figure 3. Methodology used for the vulnerability assessment of structures using DDDS. GROUND MOTION ASSESSMENT As mentioned, Mexico City had only 11 stations available when the 19 September 1985 earthquake struck the city (Figure 1). These stations are as follows: (a) Firm soil sites: Tacubaya (TACY) and UNAM Campus (stations CU01, CUIP, and CUMV); (b) Transition soil sites: Viveros (SXVI); and (c) Soft soils: Ministry of Communications and Transportation (SCT), Tláhuac (TLHB and TLHD), Centralized Food Market (CDAO and CDAF), and Texcoco (TXSO). In order to estimate the intensity of the ground motions in Mexico City s Metropolitan Area for an earthquake scenario similar to the M s =8.1, September 1985 earthquake, a methodology to define additional artificial acceleration records related to this earthquake scenario was required. Therefore, the 92 free-field stations of the Strong Motion Array available in Mexico City since 1988 (Figure 2, Base, 2000) were also considered in this study. The steps required to obtain artificial acceleration records for each station under the considered earthquake scenario are described in following sections.

9 VULNERABILITY MAPS FOR RC STRUCTURES FOR MCMA UNDER A DESIGN EARTHQUAKE SCENARIO 817 Fourier Amplitude Spectra (FAS) for Firm Soil Sites This study used a method where the records of small M s 6 or moderate 6 M s 7 earthquakes are used to simulate the motions produced by greater earthquakes (e.g., Hartzell 1978). This method is based on the hypothesis that the complexity observed in the recorded ground motions will be present in earthquakes of greater magnitude that would occur in the same epicentral region. The effects attributed to the seismic source (discontinuities in the area of contact between plates, directivity effects, speed of rupture, particularities of the energy irradiation, etc.) and the distance from the source or trajectory (mainly attenuation) is preserved in small and strong earthquakes (Pérez-Rocha 1998). Singh et al. (1988) described quantitatively the relative amplifications of the ground motions by means of empirical transfer functions (ETF). According to that study, the relative amplifications of the ground motions (E-W and N-S) can be represented by means of the quotient: H i = A i A CU 2 where H i and A i are the ETF and the FAS for the site ground motion, respectively, while A CU is the FAS for the ground motion at the outcrop CU01 site (Figure 1). For any arbitrary site, FAS for the site is specified by means of the product between the ETF for the site and the FAS in firm soil for a postulated earthquake (Pérez-Rocha 1998). For the 92 free-field stations of the Strong Motion Array available in Mexico City since 1988 (Figure 2), Pérez-Rocha (1998) obtained statistical measurements from several earthquakes to describe the observed dynamic amplifications in the ETF. He calculated average ETF (two horizontal components) and coefficients of variation that are relatively small (between 0.1 to 0.3). The intention was to construct a database of ETF that would allow developing a spatial interpolation model (using Bayes method; Pérez- Rocha et al. 1995) with the purpose of having reasonable estimations for ETF for uninstrumented sites within Mexico City s Metropolitan Area. Generation of Artificial Acceleration Records The procedure used for obtaining synthetic acceleration records considers the use of the updated average ETF obtained by Pérez-Rocha (until 2004) and the average FAS corresponding to firm soils. Ordinates of ETF and FAS follow a lognormal distribution. For this study, the average FAS obtained for the outcrop CU01 station is taken as the average FAS for firm soils, in absence of recording stations at the base of the soft-soil deposits within Mexico City s Metropolitan Area. ETF are computed with respect to the average FAS for firm soils because the ob-

10 818 A.TENA-COLUNGA ET AL. tained amplification for most sites is reasonably constant from earthquake to earthquake, being relatively independent with respect to the magnitude, the epicentral distance, or the azimuth (Reinoso 1991, Reinoso and Ordaz 1999). During the scaling process of the seismic source, the recorded ground motions for the 92 stations for the 25 April 1989 earthquake M=6.9 were used as Green s functions (e.g., Hartzell 1978). Synthetic records (two horizontal components) for the 92 sites were obtained with a specialized program developed by Pérez-Rocha. The general procedure to obtain the synthetic records can be summarized in the following steps: 1. Define the average FAS for firms soils FAS FS for the earthquake being used as Green s function (e.g., Hartzell 1978). For our study, this corresponds to the CU01 station. 2. Selection of the average ETF corresponding to firm soils ETF FS. 3. Selection of the average ETF corresponding to the site of interest (ETF S ; e.g., Figure 4b), defined with respect to the average FAS for firm soils FAS FS. 4. Compute the average FAS for the site of interest FAS S, resulting from the product of the average FAS for firm soils FAS FS times the average ETF for the site of interest ETF S, divided by the average ETF corresponding to firm soils ETF FS. 5. Compute the average FAS for the site for the target earthquake magnitude of interest FAS STE from FAS S by scaling the seismic source. Compute the pseudo acceleration response spectrum Sa STE associated to FAS STE. 6. Compute the artificial record (AR) from FAS STE using random phases of Gaussian white noise (e.g., Boore 1983). The observed strong motion duration for the site is also considered and, in absence of information, the strong motion duration observed in an analogous site is considered. Normally, several artificial acceleration records are generated, but one of those selected should be one in which Fourier amplitude spectrum FAS AR and pseudo-acceleration response spectrum Sa AR are closer to the ones of reference (FAS STE and Sa STE, respectively). The final stages of the procedure used to obtain an artificial accelerogram after scaling the seismic source using the software developed by Pérez-Rocha are illustrated in Figure 4. The acceleration record for firm soils is depicted in Figure 4a. The average ETF for the site of interest is depicted in Figure 4b. The FAS S corresponding to the Green s function (Figure 4c) and its related pseudo-acceleration response spectrum, SA S (Figure 4d) are depicted with red ink. The target FAS STE (Figure 4c) and Sa STE (Figure 4d) obtained after scaling the seismic source are depicted with green ink. The generated artificial acceleration record (Figure 4e) and their corresponding FAS AR (Figure 4c) and Sa AR (Figure 4d) are depicted with blue ink. As can be observed from this figure, the generated artificial record matches well with the target FAS STE and Sa STE for this particular site. The spatial distribution of peak ground accelerations (PGA) and peak ground veloci-

11 VULNERABILITY MAPS FOR RC STRUCTURES FOR MCMA UNDER A DESIGN EARTHQUAKE SCENARIO 819 Figure 4. Final steps of the procedure used to compute artificial acceleration records. (a) Record for firm soil (CU01). (b) Average ETF S. (c) Fourier amplitude spectra for the site of interest (FAS). (d) Pseudo-acceleration response spectra associated to FAS. (e) Artificial acceleration record. ties (PGV) obtained from the methodology used in this study are depicted in Figure 5. The Kriging interpolation method available in the Surfer software (Surfer 1999) was used to define the contours. It can be observed that PGA (Figure 5a) are within reasonable bounds, taking into account that: (a) SCT station recorded a PGA of 0.18 g during the 1985 earthquake, similar to the one observed in this map around that site; and (b) higher accelerations have been recorded in other lakebed stations of the strong motion array depicted in Figure 2 for moderate earthquakes, where the highest recorded values for the PGA within the lakebed region have been as much as 50% higher than the ones recorded at SCT site (Base 2000). It is worth noting that the approach of using an average ETF and scaling the amplitude of the FAS has been shown to be extremely successful to estimate elastic response spectra at soft soil sites within Mexico City s Metropolitan Area before, and it has been

12 820 A.TENA-COLUNGA ET AL. Figure 5. Maps of PGA and PGV for the database generated for a M s =8.1 earthquake scenario in Mexico City s Metropolitan Area. used to define previous vulnerability maps for the studied region (e.g., Pérez-Rocha 1998, Reinoso and Ordaz 1999). However, in the opinion of some, it might not be appropriate enough to simulate realistic time histories, since the input ground motion is not the base of the soft soil deposit when one uses an outcrop station, particularly because only the amplitude of the Fourier spectrum is scaled, ignoring the complex part for phase shifts (as stated by an anonymous reviewer during the peer-review process, 2005). Recently, Bárcena and Esteva (2004a, b) have presented a semi-empirical model where they use a complex transfer function, defined as the Fourier transform of the ground acceleration time history at the soft soil site, divided by the Fourier transform of the acceleration record at the firm ground site. They state that working with both the real and the imaginary components of the transfer function, and not only with its modulus, serves to keep the statistical information about the wave phases (and therefore, about the time variation of amplitudes and frequencies) in the algorithm used to generate the artificial records. The model presented by Bárcena and Esteva is very promising and might be worth using for future hazard and vulnerability studies for Mexico City s Metropolitan Area. CHARACTERIZATION OF THE STRUCTURAL SYSTEM Since most of the RCMF buildings that experienced severe damage or collapse were medium-rise buildings between five and 15 stories built before 1976 (Table 2), from a global evaluation viewpoint it was necessary to estimate the approximate dynamic properties, to estimate the lateral strength required by building codes of the times, and to define a suitable hysteretic model for RCMFs, as described below.

13 VULNERABILITY MAPS FOR RC STRUCTURES FOR MCMA UNDER A DESIGN EARTHQUAKE SCENARIO 821 Table 3. Approximate formulas to estimate the fundamental period of vibration of RCMF buildings in Mexico City (adapted from Murià and González 1995) Structural System Firm Soils Soft Soils RCMFs T=0.100N T=0.126N N=Number of stories Estimates of Natural Periods The expressions proposed by Murià and González (1995) to estimate fundamental periods of vibration of RCMF buildings located in Mexico City s Metropolitan Area were used in this study (Table 3). These expressions were obtained from measurements of dynamic properties of buildings in Mexico City s Metropolitan Area from: (a) ambient vibration tests, (b) forced vibration tests, and (c) recorded motions of instrumented buildings. The expressions presented in Table 3 are valid for buildings up to 20 stories in height. It is worth noting that in these expressions, period elongation in soft soils due to soil-structure interaction is implicitly included. Fundamental periods of vibration for RCMFs buildings from five to 20 stories in height were estimated using the expressions of Table 3, taking into account the damage survey reported by the Institute of Engineering of UNAM in 1985 (Table 2). These estimates are reported in Table 4. Estimates of Lateral Load Capacities Given the absence of a detailed and reliable inventory of structures in the studied region at the time of the 1985 earthquake, lateral load capacities for existing RCMFs buildings in Mexico City s Metropolitan Area were crudely estimated according to the minimum nominal strength required by the building codes that were effective before the 19 September 1985 earthquake: RCDF-42, RCDF-57, RCDF-66, and RCDF-76 (Fundación ICA 1988). The seismic coefficient c for the elastic design spectra for the building codes of reference is reported in Table 5. For RCDF-57 and RCDF-66 codes, B1 Table 4. Estimates of fundamental periods of vibration for RCMF buildings in Mexico City RCMFs Firm Soils Soft Soils Number of Stories Period (s) Period (s)

14 822 A.TENA-COLUNGA ET AL. Table 5. Seismic coefficient c=v/w for the elastic design spectra for different versions of Mexico s Federal District Building Code (RCDF), for the RCMF buildings under study Firm Soils (Soil Profile Type I) Number of Stories Period (s) RCDF-42 RCDF-57 RCDF-57 RCDF-66 RCDF-66 RCDF-76 (B1) (B2) (B1) (B2) Transition Soils (Soil Profile Type II) Number of Stories Period (s) RCDF-42 RCDF-57 RCDF-57 RCDF-66 RCDF-66 RCDF-76 (B1) (B2) (B1) (B2) Soft Soils (Soil Profile Type III) Number of Stories Period (s) RCDF-42 RCDF-57 RCDF-57 RCDF-66 RCDF-66 RCDF-76 (B1) (B2) (B1) (B2) category stands for buildings with nonstructural walls properly linked to the structural system, and B2 category stands for buildings where nonstructural elements are properly separated from the structural system (Fundación ICA 1988). For illustration purposes, the elastic design spectra for soft soils are depicted in Figure 6, where the evolution of the seismic regulations of RCDF code from 1942 to 1976 can be observed. In fact, from 1942 to 1966, RCDF codes used a constant value for the seismic coefficient, independent from the building s period, but dependent on the soil profile type (Table 5); in addition, allowable stresses were used for the design of structures. The first regulation in Mexico to define elastic design spectra as known today and to use ultimate strength design criteria was RCDF-76 code (Figure 6 and Table 5). Response modification factors Q and strength reduction factors Q were first introduced in RCDF-76, and Q=4 and Q=6 were allowed for the design of RCMF buildings. Therefore, seismic coefficients reduced for inelastic response c/q for RCMF buildings designed according to RCDF-76 are presented in Table 6. The values marked with an as-

15 VULNERABILITY MAPS FOR RC STRUCTURES FOR MCMA UNDER A DESIGN EARTHQUAKE SCENARIO 823 Figure 6. Evolution of RCDF elastic design spectra from 1942 to 1976 for soil profile type III. terisk in Table 6 correspond to the value for the ground motion acceleration a 0 (for T =0), as the computed c/q values were smaller. It is established in RCDF-76 that c/q a 0. Overstrength sources associated to these structural systems were also crudely considered with an overstrength factor R =2. Therefore, lateral load capacities considering overstrength sources V/W= c/q R for the buildings codes of reference are reported in Table 7. It is worth noting that, for simplicity, in the new RCDF-2004 seismic provisions (NTCS ) an overstrength factor R =2 is proposed for the design of building structures with natural periods greater than 0.5 seconds. It is well known that overstrength in RC structures is not constant and depends on many factors (e.g., Park and Paulay 1978), such as variations in: a) stress-strain curves for the concrete and the steel reinforcement, b) dimensions of elements, c) axial stress level, and d) loading rate. Other very important factors are: e) the confinement of the concrete core, f) the participation of the slab reinforcement in beam strength, and g) the participation of nonstructural elements that are not properly separated from the structural system. In particular, the participation of nonstructural elements can be very important for old construction, especially if masonry walls were used as partition walls. It is also worth noting that it has been observed that overstrength developed by building structures in earthquakes may not be constant, even for buildings that have similar designs and that have been built in the same zone by the same constructor. For example, during the 1985 earthquake in Mexico City one of the most spectacular collapses in RC structures was the one of the three-building complex known as the Nuevo León Building (Figure 7a). However, there were nine other building complexes designed, constructed, and oriented identically to the Nuevo León Building in the Tlateloco urban complex that

16 824 A.TENA-COLUNGA ET AL. Table 6. Seismic coefficient reduced for inelastic response c/q for RCMF buildings designed according to RCDF-76 Firm Soils (Soil Profile Type I) Number of Stories Period (s) c (1976) c/q Q=4 Q= * * * 0.03 * * 0.03 * Transition Soils (Soil Profile Type II) Number of Stories Period (s) c (1976) c/q Q=4 Q= * * * * Soft Soils (Soil Profile Type III) Number of Stories Period (s) c (1976) c/q Q=4 Q= * 0.06* * * * experienced considerably less damage (Figure 7b). The nearest building similar to the Nuevo León building that did not collapse was about 100 meters away from and parallel to the Nuevo León Building. Therefore, assuming a constant overstrength factor R =2 for all the RC building inventory is a very crude approximation, so engineering judgment should be used when interpreting the results obtained from this study. Selected Hysteretic Model A variation of Takeda model as presented by Otani (Saiidi and Sozen 1979) was used to model the hysteretic behavior of RCMFs. Since special provisions for the ductile detailing of reinforced concrete structures were not available in RCDF-42, RCDF-57, and RCDF-66 regulations, B 0 =0.1 (stiffness degradation factor for large amplitude cycles) and B 1 =0.9 (stiffness degradation factor for small amplitude cycles) were considered in

17 VULNERABILITY MAPS FOR RC STRUCTURES FOR MCMA UNDER A DESIGN EARTHQUAKE SCENARIO 825 Table 7. Lateral load capacities considering overstrength sources V/W= c/q R for the RCMF buildings under study for different versions of Mexico s Federal District Building Code (RCDF) Firm Soils (Soil Profile Type I) Number of Stories Period (s) RCDF-42 RCDF-57 RCDF-57 RCDF-66 RCDF-66 RCDF-76 (B1) (B2) (B1) (B2) Q= Transition Soils (Soil Profile Type II) Number of Stories Period (s) RCDF-42 RCDF-57 RCDF-57 RCDF-66 RCDF-66 RCDF-76 (B1) (B2) (B1) (B2) Q= Soft Soils (Soil Profile Type III) Number of Stories Period (s) RCDF-42 RCDF-57 RCDF-57 RCDF-66 RCDF-66 RCDF-76 (B1) (B2) (B1) (B2) Q= the Takeda model for these building codes. As RCDF-76 had special provisions for ductile detailing of RCMFs (something similar to but below today s standards for intermediate moment-resisting frames as prescribed by UBC-97 or IBC-2000 codes), B 0 =0 and B 1 =1.0 were considered in the Takeda model for the RCDF-76 code. The selected values of B 0 and B 1 were based in available recommendations (Saiidi and Sozen 1979), as well as the observation on how these parameters impacted the characteristics of the hysteretic loops using sinusoidal pulses and recorded acceleration records. VULNERABILITY MAPS FOR RCMF BUILDINGS IN MEXICO CITY FOR A M S =8.1 EARTHQUAKE SCENARIO Provided that: (a) acceleration records within Mexico City s Metropolitan Area were obtained for a M s =8.1 earthquake scenario, (b) a detailed inventory of structures in the area of interest was not available from an engineering viewpoint,

18 826 A.TENA-COLUNGA ET AL. Figure 7. Structural performances of two of the 10 three-building complexes known as Buildings Type C at Tlatelolco urban complex during the 19 September 1985 earthquake. (a) Collapse of the Nuevo León Building ( (b) Twin of Nuevo León Building. (c) the natural periods for RCMF buildings from five to 20 stories in height could be estimated using approximate formulas obtained from experimental data for real buildings in Mexico City, (d) lateral load capacities of such buildings could be estimated according to minimum required strength capacities of RCDF codes of the times (including a crude estimate of overstrength sources), and (e) a modified version of the Takeda model could be used to model the hysteretic behavior of RCMFs, then DDDS were computed for each building category and code (RCDF-42, RCDF-57, RCDF-66, and RCDF-76) for all the acceleration records and soil profile types, considering nominal strength (Tables 5 and 6) and overstrength sources (Table 7) for a period range 0 T 5 s. From these DDDS, vulnerability maps based on contours of equal displacement ductility demands and nonlinear displacements were defined for the RCDF version and the natural structural period of interest. The fundamental structural periods of interest were T=0.5 s, T=1 s, T=1.5 s, and T=2 s, corresponding to RCMF buildings from five to 20 stories in height, where most of the severe damage or collapse was observed during the 1985 earthquake (Table 2). For each estimate of lateral load capacities of RCMF buildings according to a version of the RCDF code of interest, 16 different vulnerability maps were defined for ductility demands or nonlinear displacements (four combinations for each period of interest: nominal strength and overstrength, E-W and N-S component); thus, 32 maps were built for each version of interest. As the versions of interest to estimate lateral load capacities are RCDF-42, RCDF-57(B1), RCDF-57(B2), RCDF-66(B1), RCDF-66(B2), RCDF-76

19 VULNERABILITY MAPS FOR RC STRUCTURES FOR MCMA UNDER A DESIGN EARTHQUAKE SCENARIO 827 Figure 8. Boundaries defined for firm soil sites (green line) and transition soil sites (red line for T=0.8 s, blue line for T=1 s), where average DDDS values for such soil sites were imposed. Q=4, and RCDF-76 Q=6, a total of 224 vulnerability maps were constructed: 112 for ductility demands and 112 for nonlinear displacements, as presented in Godínez (2005). Given the lack of instrumentation for firm soil (hill zone) and transition soil sites east of Mexico City s Metropolitan Area (Figure 2), the average DDDS obtained for firm soils and transition soils (T=0.8 s and T=1 s) were used in these borders to help the contour curves to recognize the east boundaries of hill and transition soil sites, as depicted in Figure 8. A similar procedure has been used in previous studies (Pérez-Rocha

20 828 A.TENA-COLUNGA ET AL. Figure 9. Vulnerability maps based on contours of equal ductility demands µ for RCMF structures with T=0.5 s for a M s =8.1 earthquake scenario, considering a constant overstrength factor R=2 and the action of strongest ground components. 1998, Reinoso and Ordaz 1999, Huerta and Reinoso 2002). Some of the most interesting results are depicted in Figures 9 12, where the vulnerability maps based on contours of equal ductility demands µ for RCMF structures considering overstrength sources for each code version of utmost interest (RCDF-42, RCDF-57 [B2], RCDF-66 [B2], and RCDF-76 Q=4 ), and the strongest ground motion component are compared for each structural period of interest: T=0.5 s (Figure 9), T =1.0 s (Figure 10), T=1.5 s (Figure 11), and T=2.0 s (Figure 12). Vulnerability maps for B2-type structures are presented for RCDF-57 and RCDF-66, and for RCDF-76 with Q=4, as they are the most representative code category from the existing building in-

21 VULNERABILITY MAPS FOR RC STRUCTURES FOR MCMA UNDER A DESIGN EARTHQUAKE SCENARIO 829 Figure 10. Vulnerability maps based on contours of equal ductility demands µ for RCMF structures with T=1.0 s for a M s =8.1 earthquake scenario, considering a constant overstrength factor R=2 and the action of strongest ground components. ventory of the time. A color scale for µ values is depicted in each map. Elastic response is depicted with white (from 0 to 1), whereas important damage could be associated to µ 3. Because of the nonductile detailing of the time period, severe damage could be associated to µ 5 and collapses to µ 7. These target values were proposed based on previous studies where µ values obtained from DDDS have been compared with the results obtained from detailed MDOF nonlinear dynamic analysis of RC structures for responses such as peak story ductility demands and yielding mapping (e.g., Tena-Colunga 1998), besides of plastic rotations of beams and columns.

22 830 A.TENA-COLUNGA ET AL. Figure 11. Vulnerability maps based on contours of equal ductility demands µ for RCMF structures with T=1.5 s, for a M s =8.1 earthquake scenario, considering a constant overstrength factor R=2 and the action of strongest ground components. The resulting vulnerability maps are directly compared with the geographic area of heavy structural damage and collapses mapped during the 1985 earthquake, as the thick blue contour line depicts the zones where damage was observed during the 19 September 1985 earthquake and the thick green contour line depicts the zones where the most severe damage and structural collapses were observed. It is worth noting that, unfortunately, more precise and detailed maps for the location of damaged and collapsed buildings with information such as structural system, number of stories, year of construction, and type and amount of damage (that is, a reliable inventory of structures for the area of interest) are still unavailable to date. Therefore, only gross comparison can be made in

23 VULNERABILITY MAPS FOR RC STRUCTURES FOR MCMA UNDER A DESIGN EARTHQUAKE SCENARIO 831 Figure 12. Vulnerability maps based on contours of equal ductility demands µ for RCMF structures with T=2.0 s for a M s =8.1 earthquake scenario, considering a constant overstrength factor R=2 and the action of strongest ground components. this study for the recognized damaged area based upon: a) the information of Tables 1 and 2, b) the gross zones reported in the literature (depicted in the blue and green contour lines), c) the first author s engineering knowledge of the building inventory of the districts enclosed by the green and blue line contours at the time, and d) engineering judgment for interpreting the results obtained in this study, taking into account that, for simplicity, overstrength was crudely assessed with a constant factor R =2 for all the RC building inventory, and neglecting the fact that there are many factors that cause that overstrength to be inconstant even for buildings that are identical to the eyes of most

24 832 A.TENA-COLUNGA ET AL. Figure 13. Identification of the density of instrumentation within Mexico City s Metropolitan Area. engineers (e.g., Figure 7), as discussed in a previous section. Also, besides being a neighbor of the damaged zone through most of his life, the first author participated in reconnaissance teams immediately after the 1985 earthquake. Because of the characteristics of the strong motion array in Mexico City s Metropolitan Area (Figure 2), it is clear that there are zones where the resulting contours are less reliable, as depicted in Figure 13. The zone identified as Zone A (yellow zone),

25 VULNERABILITY MAPS FOR RC STRUCTURES FOR MCMA UNDER A DESIGN EARTHQUAKE SCENARIO 833 which is the most reliable zone for our study because of the high instrumentation density, is also depicted in the vulnerability maps of Figures 9 to 12 with a thick gray contour line. As it can be observed, the zones where the most severe damage or collapse was experienced during the 1985 earthquake are covered by Zone A; therefore, the contours depicted in this zone are reliable enough to compare grossly against the observed damage during the 1985 earthquake. It is worth noting that, at the time of the 1985 earthquake, the limits of Mexico City s Metropolitan Area to the south (crowded areas) were about ; below this latitude there were only old towns and scarce low-rise construction, usually housing made of masonry. That is, no RC buildings existed in that region, with a few exceptions. It can be observed from Figure 9 that for RCMF structures up to five stories in height T=0.5 s, ductility demands µ=10 or higher could be expected if they were designed to the minimum standards of RCDF-42 code and a constant overstrength R =2, particularly for the zone where most damage was observed. From a gross assessment viewpoint, this correlates well with the data presented in Table 2 for structures being built before 1957, as severe damage and many collapses were reported for such structures. However, it is worth noting that ductility demands for such building inventory could be considerably smaller, as actual overstrength for such structures could be much higher considering at least two key factors: a) gravitational loads have more impact on the overstrength in low-rise RC buildings, and b) masonry walls were frequently used as partition walls in RC structures in Mexico City before the 1980s, so their participation in taking lateral loading could be very important and difficult to assess. The participation of nonstructural masonry walls in the seismic response of buildings during the 19 September 1985 earthquake is a fact that was well documented in the literature by reconnaissance teams at that time (e.g., Instituto de Ingeniería 1985, Fundación ICA 1988). In addition, one could expect severe damage and collapse for five-story buildings designed according to RCDF-66 (B2) and RCDF-76 Q=4. However, other than some RCMFs made with waffle flat slabs (Table 2), not many buildings in that height and code category collapsed in 1985 in Mexico City because, since the early 1950s, medium-rise construction for RC structures was more popular in the city and, as a result, fewer buildings of that height category (five stories) were built at that times. Besides that, a higher overstrength could be developed in such structures for the reason mentioned above. It is worth noting that the smallest ductility demands for RCMFs up to five stories in height within Mexico City s Metropolitan Area are related to RCDF-57 (B2), because of the required higher strength for soil profile types II (transition soils) and III (soft soils), as can be observed in Table 7. One can also observe the evolution of RCDF codes to protect RCMFs with T =0.5 s against strong earthquakes within Mexico City s Metropolitan Area. For example, it can be observed in Figure 9 that RCDF-57 considerably reduced the hazard with respect to the previous RCDF-42 code. However, RCDF-66 increased the vulnerability of such structures, taking into account that allowable stresses were used for the design. RCDF-76 code is the first Mexican code that introduced strength concepts and a seismic design based upon inelastic deformation concepts in terms of the Q factor, and it was expected that this code would allow higher ductility demands than previous RCDF-66

26 834 A.TENA-COLUNGA ET AL. code, as can be observed in Figure 9. However, the inelastic demands are much higher than what was anticipated by the code committee in 1976, as the elastic design spectra defined in RCDF-76 were much smaller than those observed from the recording stations during the 1985 earthquake, as is well documented in the literature worldwide. Nevertheless, it is shown in these maps that the vulnerability of these 0.5-second period RCMF structures increased for RCDF-76 code, which was perhaps responsible for some of the collapses for the waffle flat slab systems (Table 2). It can be observed from Figure 10 that for RCMF structures about eight to 10 stories in height T=1 s, ductility demands µ 10 could be expected for the minimum standards of RCDF-42 code, but since there were only a few RCMF structures of such characteristics available in the city, this fortunately did not happen. Analyzing the data in Table 2, only four structures with severe damage could be associated to this category. Most medium-rise RCMF structures (standard frames and RC frames composed with waffle flat slabs) were built according to RCDF-57, RCDF-66, and RCDF-76 codes, so these maps are the most interesting to observe and to correlate with the heavy damage reported in Table 2 and depicted in the zones with thick green and blue line contours in the maps presented in Figure 10. It is observed that ductility demands 4 µ 10 are found within the blue and green contours for RCDF-57, RCDF-66, and RCDF-76 codes, so important and severe structural damage, as well as structural collapses, should be expected for such building categories if they developed an overstrength R 2. From a gross assessment viewpoint using engineering judgment, this fact correlates well with the data presented in Table 2 for RCMFs and RCMFs made of waffle flat slabs. Of course, ductility demands in this building inventory could be smaller if higher overstrength was developed, as it is known that overstrength is not constant even for structures that are very similar (Figure 7). From the evolution viewpoint of RCDF codes to protect RCMFs of T=1 s against strong earthquakes within Mexico City s Metropolitan Area, one can arrive at similar conclusions to those for T=0.5 s, as RCDF-57 considerably reduced the hazard with respect to the previous RCDF-42 code. However, RCDF-66 and RCDF-76 increased the ductility demands of such structures. There are more similarities in the ductility demand contours of RCDF-66 and RCDF-76 codes, but the main difference is that RCDF-76 code had improved provisions for the seismic detailing of RCMF structures with respect to previous codes. This fact may explain why fewer structures of this category built according to RCDF-76 collapsed or were severely damaged compared with those reported for previous code versions (57-76 category, Table 2), as higher overstrength and deformation capacities could be developed for the RC elements designed according to RCDF-76. It can be observed from Figure 11 that, again, for RCMF structures about 13 to15 stories in height T=1.5 s, ductility demands µ 10 could be expected for the minimum standards of RCDF-42 code. Since there were no RCMF structures of such characteristics in the city built according the RCDF-42 code, this damage was not observed. Most medium-rise RCMF structures 13 to 15 stories in height were built according to RCDF-57, RCDF-66, and RCDF-76 codes, so these codes must be compared and cor-

27 VULNERABILITY MAPS FOR RC STRUCTURES FOR MCMA UNDER A DESIGN EARTHQUAKE SCENARIO 835 related with the data reported in Table 2, and the thick green and blue line contours for their corresponding maps depicted in Figure 11. It is observed from Figure 11 that ductility demand contours for this structural period are considerably reduced with respect to previous structural periods (Figures 9 and 10), suggesting that a smaller amount of damage should be expected for the same building inventory of such height characteristics within the zone of interest if a constant overstrength R=2 is considered. In fact, one can observe from Table 2 that a smaller number of RCMF structures (standard and waffle flat slab) experienced severe damage or collapsed under this building category (13 to 15 stories in height), but certainly there were fewer 13- to 15- story than eight- to 10-story structures in Mexico City at the time, as can be inferred from Table 1. Therefore, the correlation with the damage report can be considered good from a gross assessment viewpoint, given the crude estimate of strength and overstrength considered in the present study and the absence of a detailed and reliable building inventory in the area of interest from an engineering perspective. From the evolution viewpoint of RCDF codes to protect RCMFs of T=1.5 s against strong earthquakes within Mexico City s Metropolitan Area, one can arrive to similar conclusions to those for structural periods of 0.5 s and 1.0 s: a) RCDF-57 considerably reduced the hazard with respect to the previous RCDF-42 code, b) RCDF-66 increased the vulnerability with respect to RCDF-57, and c) RCDF-76 increased the deformation demands of such structures with respect to RCDF-66, but since it had improved provisions for the seismic detailing of RCMF structures with respect to previous versions of RCDF codes, a smaller amount of damage and collapse was observed for this code version (Table 2). The equal ductility demand contours for RCMF structures with T=2.0 s (more than 15 stories in height) when R =2 is considered are depicted in Figure 12. Again, ductility demands µ 10 could be expected for the minimum standards of RCDF-42 code, but no damage was observed as RCMF structures of such characteristics designed according to this code did not exist in the city at the time of the 1985 earthquake. For the remaining codes, ductility demands are relatively small to moderate 1 µ 5 within the zones where most of the severe damage or collapses were observed during the 1985 earthquake; this correlates well with the available damage report survey (Table 2) from a gross assessment viewpoint. From the evolution viewpoint of RCDF codes to protect RCMFs of T=2.0 s against strong earthquakes within Mexico City s Metropolitan Area, one can arrive to similar conclusions to those for structural periods of 0.5 s, 1.0 s, and 1.5 s. It can also be observed from all the vulnerability maps depicted in Figures 9 12 that the highest ductility demand contours are usually found within: a) the center-northeast side of the maps, corresponding to Texcoco lakebed zone nearby Mexico City s International Airport and neighboring Nezahualcoyotl City, b) the center-south side of the maps, corresponding to Xochimilco lakebed zone, and c) the south-east side of the maps, corresponding to the Chalco-Tláhuac lakebed zone. However, as the density of the instrumentation in these zones is scarce (Figure 13), the magnitude and extension of such contours should be carefully examined before making strong remarks about them.

28 836 A.TENA-COLUNGA ET AL. One can conclude that the vulnerability maps based on contours of equal ductility demands presented in Figures 9 12 for RCMF structures under a postulated M s =8.1 earthquake scenario similar to the 19 September 1985 earthquake and considering a crude estimate of overstrength (constant for all the building inventory) because a detailed and reliable building inventory for the area of interest was not available had a reasonable correlation with the reported and observed damage from a general vulnerabilityassessment viewpoint. Finally, maps of equal nonlinear displacement demands for RCMF structures designed according to RCDF-76 for Q=4 for a M s earthquake scenario considering a constant overstrength R =2 for the building inventory are depicted in Figure 14 for structures with natural periods T=0.5 s, T=1 s, T=1.5 s, and T=2 s. It can be observed that, whereas relatively small peak displacements may have occurred in RCMF structures located in firm and transition soil sites of Mexico City s Metropolitan Area, RCMF structures located in soft soil sites may have experienced very large displacements (up to 100 cm, particularly nearby Xochimilco and Tláhuac zones, south, below ). It can also be observed that among the larger displacements in the zones identified as having experienced severe damage during the 1985 earthquake (thick green contours), the larger displacements are up to cm, being more important for structures with T =1.5 s and T=2.0 s. The maps obtained for other versions of RCDF code are somewhat similar, particularly for RCDF-66 code. CONCLUDING REMARKS Although a detailed and reliable inventory of building structures was not available for the area of interest, it can be concluded that the vulnerability maps (based on contours of equal ductility demands for RCMF structures under a postulated M s =8.1 earthquake scenario similar to the 19 September 1985 earthquake) presented in this study had a reasonable correlation with the reported and observed damage from a general vulnerability-assessment viewpoint (gross assessment). Therefore, it is believed that the procedure outlined in this study can be used with confidence to forecast the location of zones of high demands for different structural systems and earthquake scenarios, and is therefore potentially useful for urban planning of cities located in zones of high seismic risk. However, the applicability of the method to obtain a good forecast depends also on a reliable inventory of structures, from the engineering perspective, for the area of interest. As discussed, in addition to the well-known downtown area where most of the structural damage and collapses have been observed during the 1957, 1979, and 1985 earthquakes, the highest ductility demand contours in Mexico City s Metropolitan Area are usually found within: a) Texcoco lakebed zone near Mexico City s International Airport and neighboring Nezahualcoyotl City, b) Xochimilco lakebed zone, and c) Chalco- Tláhuac lakebed zone. These zones, particularly Xochimilco and Tláhuac (south, below ), were also recognized before as having a high amplification (Reinoso and Ordaz 1999) and a high energy input (Huerta and Reinoso 2002). However, as the density of the instrumentation in these zones is scarce, the magnitude and extension of such con-

29 VULNERABILITY MAPS FOR RC STRUCTURES FOR MCMA UNDER A DESIGN EARTHQUAKE SCENARIO 837 Figure 14. Maps of equal nonlinear displacement demands (cm) for RCMF structures designed according to RCDF-76 code, Q=4, under a M s =8.1 earthquake scenario, considering a constant overstrength factor R = 2 and the action of strongest ground components. tours should be carefully examined before making strong conclusions on this matter. Nevertheless, as the density of population and structures in those areas has considerably increased since the 1985 earthquake, a warning call should be made for Nezahualcoyotl City, Tláhuac, Chalco, and Xochimilco districts (in 1999, Reinoso and Ordaz made such a warning call for the new urbanized zones at Xochimilco lakebed). Therefore, it is believed that a densification of the seismic instrumentation is much needed in the east side of Mexico City s Metropolitan Area in order to improve the vulnerability assessment of the building inventory existing in these zones, using the methodology proposed in this study or other alternatives.