Seismic Vulnerability of School Buildings in Two Districts of Lima, Peru Using the Atc-21 Methodology

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 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. (http://www.eeri.org/earthquakes/Reconn/Arequipa_Peru/Bariola.html) 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. In the northern part of Chorrillos Distric t, 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 0.08s 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 materia ls 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 20.0 to 30.0 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, 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 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. . 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). 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 Barranco, Lima-Peru Percentages of School Populations