SEISMIC VULNERABILITY OF SCHOOL BUILDINGS IN TWO DISTRICTS OF LIMA, PERU USING THE ATC-21 METHODOLOGY

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1 SEISMIC VULNERABILITY OF SCHOOL BUILDINGS IN TWO DISTRICTS OF LIMA, PERU USING THE ATC-21 METHODOLOGY Z. Aguilar 1, J. Meneses 2, E. Bedriñana 3 and L. Pinto 4 (1) Associate Professor, (3) Student, Peru-Japan Center for Earthquake Engineering Research and Disaster Mitigation (CISMID), Universidad Nacional de Ingenieria, Lima, Peru, zaguilar@uni.edu.pe (2) Assistant Scientist, (4) Student, Dept. of Structural Engineering, University of California, San Diego, La Jolla, CA, USA, jfmeneses@ucsd.edu ABSTRACT The assessment of the seismic vulnerability of all school buildings located at Chorrillos and Barranco districts in Lima, the capital city of Peru, was conducted using the Rapid Visual Screening procedure of ATC-21. A total of 28 school buildings were evaluated in Barranco, and 80 in Chorrillos, comprising all kindergarten, primary, and secondary school buildings existing in these two districts. Even though some buildings are relatively new, their structural scores indicate that most of them show from medium to high seismic vulnerability. This information has been correlated with local soil conditions and seismic intensities observed in the past in the two districts. INTRODUCTION Lima, the capital city of Peru, is located in the Circum Pacific Rim where more than 80% of the world seismic activity occurs (Fig. 1). Lima experiences significant presence of traditional construction, vulnerable essential facilities (schools, hospitals, etc.), and little support from local governments for seismic risk management activities. Fig. 1 Location of Peru, Lima, and Chorrillos and Barranco Recent large earthquakes in Peru have shown the high vulnerability of buildings and facilities, and among them school buildings. During the 1996 Nazca earthquake, many school buildings were seriously damaged, including several newly constructed ones. After this, a new seismic code was issued in 1997, which is more demanding than the former 1977 code. The seismic

2 performance of school buildings constructed with this regulation was successfully tested during the June 23, 2001 Arequipa earthquake. In this event, these structures did not present damage at all even though they where located in cities as Moquegua, where the observed maximum seismic intensity was 8 on the Modified Mercalli scale. However others were severely damaged (Fig. 2). Most of the buildings do not meet the seismic requirements stated by the new seismic code, therefore, their seismic vulnerability need to be evaluated in order to establish retrofitting guidelines to reduce the seismic risk to acceptable levels. Since no methodology to evaluate the seismic vulnerability of a large number of structures is implemented yet in Peru, in this study the Rapid Visual Screening of Buildings for Potential Seismic Hazards of the Applied Technology Council (ATC-21), is used. An attempt to adapt this method to local construction systems and materials, local seismicity and soil conditions was done. As a pilot project, the assessment of the seismic vulnerability of all school buildings located at Chorrillos and Barranco districts in Lima, the capital city of Peru, was conducted using this methodology. These two districts out of the 43 existing in Lima were chosen for this study due to their: 1) large population, 2) large number of traditional buildings, and 3) high seismic intensities observed during past earthquakes. Damage distribution of past events shown that the seismic intensity recorded in these two districts is about one degree higher than the average one in downtown Lima. Fig. 2 Left: School designed with the 1977 Code. Right: School designed with the 1997 code. ( A total of 28 school buildings were evaluated in Barranco, and 80 in Chorrillos, comprising all kindergarten, primary, and secondary school buildings existing in these two districts. Even though some buildings are relatively new, their structural scores indicate that most of them exhibit from medium to high seismic vulnerability. This information has been correlated with local soil conditions and seismic intensities observed in the past in the two districts. GEOLOGICAL AND LOCAL SOIL CONDITIONS Chorrillos and Barranco Districts are located on quaternary deposits, mainly formed by the dejection cone of the Rimac River, with thickness ranging from 100 to 400 m. This cone is constituted by layered alluvial material, where gravel, sand, clay and silt deposits are heterogeneously superposed. The alluvial sediments were deposited during the last stage of the Pleistocene on the outcropping sedimentary rock from the Mesozoic. During the Holocene, the clay sedimentation on several areas of the valley was more intense, forming clay layers more than 10 m in thickness that cover the granular material.

3 In the northern part of Chorrillos District, there are some hills with outcropping rock conformed by quartzite, shale and sandstone from the El Fraile, La Herradura and Marcavilca Tertiary formations respectively. This lithology has influenced the morphology of the El Morro Solar massif, where the topography ranges from very steep to flat areas. In a Seismic Microzonation Study of Chorrillos and Barranco (Ayquipa, 1995), the local soil conditions of these districts were evaluated, and four geotechnical zones were identified, which are shown in Figure 3 and described below. Fig. 3 Seismic Microzonation of Chorrillos and Barranco Zone I. It is a limited area located around the outcropping rock of El Morro Solar massif formation in Chorrillos district. The soil profile is composed by layers of poorly graded sand, clayey sand and silty sand with lens of clay. At an average depth of 2.00 m, a layer of mid dense gravel, with silty sand matrix, is found. The natural vibration periods, determined by microtremor or ambient vibration measurements, ranges from 8s to 0.25s in this zone. Zone II. It mostly covers the Barranco district and the northern part of Chorrillos district. The soil profile shows randomly mixed layers of sand, clay and silt, with different thickness. Underlying these materials and at depths ranging from 2.0 to 8.0 m, a layer of gravel is found. The predominant natural vibration period range from 0.25s to 0.40s, with important amplification factors in this range of periods. The ground water level ranges from 2 to 3 m in depth. Zone III. It extends from south to southwest sector of Chorrillos District. The soil profile presents clayey silt and silty clay layers of variable thickness. Layers of organic silt and clay appear at depth from 0.50 m to 1.70 m, with high water content and thickness of 2.00 m. Underlying these materials are fine sands and silt with high organic content reaching depths of 5.0 to 7.5 m, where the alluvial gravel is found. The ground water table depth ranges from 1.0 to 3.5 m. This zone also includes the beach area, conformed by clean, loose and saturated sand. Predominant natural vibration periods in this zone ranges from 0.4s to 0.5s. Zone IV. It includes a relatively small area located in the southern part of Chorrillos District. It is formed by marshy ground named Pantanos de Villa. The soil profile consists of a thin layer of clayey silt followed by a black to yellowish green peat with fetid odor. From 6.0 to 7.0 m in depth appears a layer of compact sand inserted with lens of peat. The ground water level is shallow,

4 forming some ponds in the marshy area. The natural vibration periods in this zone are larger than 0.5s. STRUCTURAL TYPE OF SCHOOL BUILDINGS AT CHORRILLOS AND BARRANCO DISTRICTS The structural types of school buildings identified at Chorrillos and Barranco are mainly composed by concrete frame with unreinforced masonry infill walls (C3) and confined masonry (CM), which together comprises the 92% of the whole number of school buildings. A small number of unreinforced masonry structures (URM), reinforced masonry (RM), adobe (ADB) and wooden (W) structures were also identified. The typical C3 structure consist of a moment resistant concrete frame in the longitudinal direction and concrete frame with unreinforced masonry infill walls in the transversal direction. The CM structural type consists of clay brick bearing walls confined with caste in place concrete columns and beams, conveniently distributed to increment the structure ductility. These elements lightly contribute to increase the structure bearing resistance. The URM structures are those clay brick bearing walls that has no concrete columns confinement at all, or if they exist are widely apart that do not contribute to the structure ductility. Figure 4 shows photos of typical school buildings for each basic structural type. Fig. 4 Left: Virgen del Pilar (C3); Center: Mi Peru (CM); Right: San Luis (URM) The adobe and wood structures that are still being used as school buildings were constructed at the beginning of the XX century or earlier. As can be seen in the photos of Figure 5, these school buildings are becoming vulnerable due to aging and lack of maintenance; therefore some of them have changed its use to administrative office only. Fig. 5 Left: Jose Olaya (URM); Center: San Fernando (ADB); Right: San Julian (W) SEISMIC VULNERABILITY OF SCHOOL BUILDINGS To evaluate the seismic vulnerability of school buildings at Chorrillos and Barranco districts the Rapid Visual Screening of Buildings for Potential Seismic Hazards of the Applied Technology Council (ATC-21) was used. At this stage, the basic scores and modification factors proposed for

5 the several types of American structures were assumed to correspond to the equivalent Peruvian structures. A total of 108 school buildings were evaluated in the two districts, some of them having more than one structural type or blocks from different construction year. It is remarkable that near 50% of the schools have populations between 100 and 500 students, and 39% to 41% of the schools more than 500 in Barranco and Chorrillos respectively. Figure 6 shows the percentages of school populations of the evaluated school buildings.. Barranco, Lima-Peru Percentages of School Populations Chorrillos, Lima - Peru Percentages of School Populations 4 Schools 32 Schools 11 Schools 14% 39% 41% 14% 13 Schools 47% 11 Schools % 36 Schools Fig. 6: Percentages of School Populations in Barranco and Chorrillos Districts Figure 7 shows the percentage of basic structural types of school buildings in these two districts, where it is observed that 60% are concrete frame with unreinforced masonry infill walls (C3) and 32% are confined masonry (CM). Only the 8% of the buildings include Adobe, wood and unreinforced masonry structures. 71% of the school buildings are located in the geotechnical zone 2, where the soil conditions present an adequately good behaviour, 22% of them are located in the zone 3, where the soil conditions are unfavorable, and only 7% are located in the geotechnical zone 1 (Fig. 8). 2% 3% C3 URM S3 22% S1 7% 32% 60% CM Adob/Q. Madera S1 S2 S3 3% S2 71% Fig. 7: Percentage of Basic Structural Types in Barranco and Chorrillos Fig. 8: Distribution of School Buildings by Geotechnical Zones For the seismic vulnerability assessment of C3 structural type, the basic score and modification factors proposed by the ATC-21 for C3/S5 structures were used. Even though these values are being evaluated to analyze if they adequately represent the seismic behavior of Peruvian C3 type structures, on the basis of the damage to this kind of structures observed during recent past earthquakes, it seems that the basic score of 1.5 is representative for the expected probability of damage, which means that 3 of each 100 C3 structures will collapse. On the same basis it was assumed that a confined masonry structure (CM) would have similar basic score and modification factors of a C3 structural type. Only the modification factors that not apply to this structural type were eliminated. The others structural types were evaluated but their structural score do not adequately represent their seismic vulnerability, since the construction technique, materials and

6 maintenance do not correspond to the American ones; therefore a detailed analysis is required to estimate those structural score. This paper will assess the seismic vulnerability of the basic structural types C3 and CM only, since they comprise the 92% of the total school buildings in these two districts, and the structural score correspond more properly to those proposed by the ATC-21. Since several of the evaluated school buildings have more than one structural block, a total of 193 C3 type structure were identified in both districts. Figure 9 shows the percentage of school building and their ranking of final structural scores, where it is observed that 21% of the buildings have scores equal or less than zero, 37% rank from to 0.5 and 42% are greater than 0.5. Figure 10 shows the number of structures in each structural score rank. The large number of structures with structural score equal or less than 0.5 is explained by the short column modification factor, which is present in 111 of the evaluated structures. Plan irregularity and soil condition are other modification factors that affect the final structural score of the school buildings. C3 - Structural Final Score and Percentage of Buildings 21% 27% % 0 ó < 0 37% N. of School Buildings C3 - Number of School Buildings vs. Final Score Basic Score = ó < 0 Final Score (S) Fig. 9 Final Structural Score of C3 School Buildings in Barranco and Chorrillos Fig. 10 Number of C3 Structures by Final Structural Score in Barranco and Chorrillos Districts C3 -Number of School Buildings' Blocks and Modfication Factors N of Buildin High Rise Poor Conditi Vertical Irregula Soft Stor Torsion Plan Irregular Pounding Large Heavy Clad Short Colum Post Benchmark 14.0 S S2 S Fig. 11 Number of Structures vs. Modification Factors for C3 type school buildings Figures 12 and 13 show the percentage and the number of confined masonry school buildings (CM) and their ranking of final structural scores. For this structural type, it is observed that only 24% of the buildings have scores equal or less than 0.5, and 76% rank from 0.5 to 1.5. Plan irregularity and soil condition are other modification factors that affect the final structural score of these school buildings (Fig. 14). The final structural score of this CM type should also be decreased by modification factor such as lateral rigidity (walls density) and adequate confining

7 columns distributions, which were not included in this evaluation, because it implies to have access to the buildings interior. CM- Structural Final Score and Percentage of Buildings 1% 23% 43% ó < 0 33% N of School Buildings' Blocks CM- Number of Buildings' Blocks vs. Final Score Basic Score = ó < 0 Final Score Fig. 12 Final Structural Score of CM School Buildings in Barranco and Chorrillos Fig. 13 Number of CM Structures by Final Structural Score in Barranco and Chorrillos Districts CM - Number of Shool Buildings' Blocks vs Modification Factor N of Buildings' Blocks High Rise Poor Condition Vertical Irregularity Soft Story Torsion Plan Irregularity Pounding Large Heavy Cladding Short Columns Post Benchmark Year S1 S2 S3 Fig. 14 Number of Structures vs. Modification Factors for CM type school buildings Results show that a large number of C3 school buildings are moderately to highly vulnerable. The main constructive defect of this structural type is the short column problem, which not only appear in old construction but also in some new buildings constructed with no technical assistance, as could be observed in Figure 15, where a photo of an under construction school building is shown. Even though at the present time no structural score threshold has yet been defined to accurately identify the number of highly vulnerable structures, the ATC-21 procedure shows its effectiveness in classifying the more vulnerable ones. The selection of a cut-off structural score, i.e., a threshold differentiating adequate buildings from those potentially inadequate and thus requiring detailed review, is beyond the scope of this study. This requires more detailed studies on the structural systems, evaluation of observed seismic damage, adoption of seismic safety criteria among other considerations. Since the majority of large population school buildings in Barranco and Chorrillos districts are concrete frame with unreinforced masonry infill walls (C3) structures, and these are very popular

8 in urban areas in Peru, discussion on definition of degrees of seismic vulnerability will be focused on this type of school buildings. Thus, with the information gathered in this survey it is proposed four degrees of seismic vulnerability that are the result of combining school population with final structural scores. Table 1 shows the proposed classification and intends to provide criteria to prioritize action to reduce seismic vulnerability of school buildings. According to this table, 35% of the C3 school buildings present very high vulnerability, 18% high vulnerability, 32% medium vulnerability, and 15% low vulnerability. Due to the large number of student population of these C3 school buildings it is necessary to evaluate in detail the seismic vulnerability of the critical ones. Fig. 15 Defects of Short Columns in several C3 Structural Type School Buildings Table 1: Degree of Seismic Vulnerability for C3 School Buildings for both Districts School Population Final Score S > % (L) 8% (L) 15% (M) % (L) 4% (M) 10% (H) % (M) 8% (H) 35% (VH) Degree of Very High High Medium Low Vulnerability VH H M L CONCLUSIONS The Rapid Visual Screening of Buildings for Potential Seismic Hazards of the Applied Technology Council (ATC-21) was used to assess the seismic vulnerability of school buildings in Barranco and Chorrillos districts. This methodology has shown to be effective capturing basic features of the buildings for a rapid and economical assessment of the seismic vulnerability. From the basic structural types identified, the concrete frame with unreinforced masonry infill walls (C3) and the confined masonry (CM) are the predominants in these districts, and the more important modification factors detected were short columns, and plan irregularity respectively for each of these structural types. Unreinforced masonry (URM) and reinforced masonry (RM) are not commonly used for school buildings and only a small number of them were identified, mainly used as kindergarten and small schools. Most of the adobe and wood structures found are old structures, which are in poor condition and present plan irregularity. The most important school buildings in Barranco and Chorrillos districts, and the more popular in urban areas in Peru, are the concrete frame with unreinforced masonry infill walls (C3) structures,

9 which generally have large student populations. Combination of final structural scores and school populations defined different degrees of seismic vulnerability for this structural type school building. The findings set a basis for the implementation of suitable retrofitting projects of school buildings where economical analysis indicates retrofitting is less expensive than school replacement cost. Based on the results of this pilot project it is recommended the establishment of a standard rapid screening method that takes into account local structural characteristics of school buildings, and its implementation in other areas of Peru. ACKNOWLEDGEMENTS This study was funded by the University of California Pacific Rim Research Program, grant 02T-PRRP This assistance is greatly appreciated. REFERENCES Alva, J., Meneses, J., and Guzman, V. (1984). Distribution of Maximum Observed Seismic Intensities in Peru. Proc. Symposium on Seismic and Volcanic Hazard and Risk in South America; INPRES, CERESIS, Giesecke Alberto M., ed., pp Alva, J., Meneses, J., Martinez, J., and Huaman, C. (1991). Advances on the Seismic Microzonation of Lima city. Fourth International Conference on Seismic Zonation, Vol. III, pp , EERI, Stanford, California. ATC-21 (1988). Rapid Visual Screening of Buildings for Potential Seismic Hazards: A Handbook. Applied Technology Council, CA. FEMA 154/July Ayquipa, C. J. (1995). Microzonificación Sísmica de Chorrillos y Barranco. Thesis. Universidad Nacional de Ingeniería, Lima, Peru. (2001), Earthquake in Arequipa, Peru, June 23, EERI Special Report, June 23, 2001.