COFFEE ZONE, COLOMBIA, JANUARY 25 EARTHQUAKE Observations on the Behavior of Low-Rise Reinforced Concrete Buildings

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1 INTRODUCTION COFFEE ZONE, COLOMBIA, JANUARY 25 EARTHQUAKE Observations on the Behavior of Low-Rise Reinforced Concrete Buildings By Santiago Pujol, Julio Ramírez and Alberto Sarria On January 25, 1999, at approximately 1:19 PM (local time), an earthquake of magnitude 5.9 m b 1, 6.0 M L 2, struck the coffee zone in Colombia. This paper presents observations on the damage suffered by low-rise reinforced concrete buildings in the cities of Armenia and Pereira. The observations presented are part of a study based on data collected during a five-day visit to the mentioned cities. Recommendations intended to avoid in the future the most frequently observed structural problems are presented. SCOPE The objective of the ongoing study, from which only the main field observations are presented here, is to correlate seismic vulnerability of reinforced concrete, low-rise, monolithic buildings with their dimensions and arrangement of columns and walls. SEISMICITY AND GEOLOGICAL CONTEXT Geographic Location and Regional Seismicity The location of the epicenter of the January 25, 1999, earthquake as estimated by Instituto de Investigación en Geociencias, Minería y Química, (INGEOMINAS), 2 is shown in Figure 1. Armenia, a 223,000 3 people city, is about 15 km north from the estimated epicenter (4.41N, 75.72W). Pereira, a 355,000 3 people city, is approximately 50 km north from the same point. Figure 1 Epicenter location relative to Armenia, capital of the Quindío department, and Pereira, capital of the Risaralda department. In Colombia, the Andes are divided into three mountain ridges. Both, Armenia and Pereira, lay on the western hills of the central ridge, which is characterized by the presence of several volcanoes. The Del Ruiz volcano, known for the catastrophic Armero mudslide of 1985, is among those. 1 Observatorio Sismológico del Suroccidente (OSSO), Instituto de Investigación en Geociencias, Minería y Química, (INGEOMINAS), From an official census made in 1993

2 Colombia and, in general, the northwestern corner of South-America, is a complex tectonic environment. Three tectonic plates interact there: Nazca, South-America and Caribbean (Sarria, 1995). The Nazca plate is believed to move from west to east at about 2.4 inches per year. The South-America plate moves from east to west at a relative velocity of 0.4 to 0.8 inches per year. The Caribbean plate moves in the WE direction at a relatively smaller velocity. The stress field generated by the interaction of these plates is evidenced by activity along several geological faults. Some of these faults have been well identified. That is the case of the Romeral fault (See Figure 2), which crosses Colombia from South to North along more than 1000 km and through six major cities: Pasto, Popayán, Armenia, Pereira, Manizales and Medellín (Sarria, 1995). A rupture along a branch of this fault generated the 03/31/1983, Popayán earthquake, which caused 300 casualties and losses of 300 million dollars (0.8% of Colombia s gross internal production in 1982). The January 25, 1999, earthquake seems to have been generated by a rupture along another branch of the Romeral fault: Cauca-Almaguer. Its strike and deep have been estimated to be N15ºE and 73ºE respectively 4. The rupture appears to be left lateral, which indicates a displacement in the NS direction of the Andes block with respect to the eastern planes of Colombia. Seismic History The coffee region has a rich seismic history. Table 1 contains data on the main seismic events occurred in the region in the last 20 years. Figure 2 shows the location of the epicenters of earthquakes with magnitudes (M s ) equal or larger than 4 occurred in Colombia between 1566 and Table 1. Main seismic events occurred in the Coffee Region in the last 20 years No Date Mm/dd/yy Magnitude Depth Km 1 11/25/ (---) /08/ (m b ) /25/ (m b ) 5-15 Figure 2 Epicenters of seismic events with M s 4 between 1566 and (From Colombian Association of Earthquake Engineering, 1998) 4 Espinoza A., Areas Ltda., Bogotá, Colombia. Personal communication, February 15,

3 Geology and General Soil Characteristics In general, the soils in the region affected by the earthquake may be described as deposits of volcanic ashes of about 60 ft laying on conglomerates or igneous rocks. These volcanic ashes are very cohesive soils with values of cohesion of about 7-14 psi 5. The zones that were affected the most in Armenia and Pereira coincide with those where buildings lay on old fills of bad quality. Ground Accelerations Measured The maximum horizontal accelerations measured in Pereira were 0.08G on rock and 0.30G on fills. 6 Preliminary information indicates that, in Armenia, values of 0.59G and 0.47G would have been measured at ground level for horizontal and vertical peak ground acceleration, respectively. 5 Aftershocks As of February 9, 1999, more than 90 aftershocks have been registered. The main aftershock occurred about 4h: 21min after the main shock and had a magnitude of 5.8 (M L ). 7 The location of the aftershocks has migrated towards the north, i.e., towards Armenia. No pre-shocks were recorded. EMERGENCY RESPONSE The January 25, 1999 earthquake affected 35 cities, caused more than 900 casualties and injured at least It has been estimated that about 200,000 people were left without shelter (El Colombiano, Feb. 1, 1999). Preliminary estimates indicate that reconstruction of the infrastructure of the cities affected will cost more than 500 million dollars. (El Tiempo, Jan. 29, 1999). This is about 0.5% of the 1996 gross national product o f Colombia; a country with an estimated fiscal deficit of 2% of this year s projected gross national product (El Tiempo, Jan. 29, 1999). In Armenia, the police headquarters suffered partial collapse. The fire station collapsed. The water, telephone and electricity lines suffered severe damage. Traffic through the main access roads and airport operations were interrupted. Government buildings were evacuated. Because of these events, the first days after the earthquake in the city were very chaotic. For similar reasons, the emergency in other cities of the region could not be properly managed either. In Circasia and Córdoba the hospital buildings collapsed. The Calarcá hospital suffered partial collapse. In Pereira, on the other hand, there was a basic infrastructure for emergency response in place. This permitted organized efforts to be conducted toward rescue and clean-up operations. This preparation can be attributed to the lessons learned from the earthquakes of 11/25/1979 and 02/08/1995. A daunting task now facing not only Pereira and Armenia but all 35 cities affected is that of reconstruction, particularly, of the low income housing infrastructure. PAST MITIGATION EFFORTS In the years from 1950 to 1980, the city of Armenia experienced the largest construction development in the last decades. But only since 1984 and in response to the damage caused by the 1983 Popayán earthquake, application of seismic design recommendations by the Colombian Association of Seismic Engineering is enforced by a law of the Republic of Colombia. A revision of these recommendations was made in 1998 (Colombian Association of Earthquake Engineering, 1998). Design provisions are made according to estimations of seismic risk that suggest a division of the Colombian territory into three zones with different hazard levels as shown in Figure 3. Observe that the cities of Armenia and Pereira lay on a zone with estimated high seismic hazard. Design ground acceleration in 5 Espinoza A., Areas Ltda., Bogotá, Colombia. Personal communication, February 15, OSSO, INGEOMINAS,

4 this zone varies from 0.25G to 0.4G. The recommended design ground acceleration for Pereira and Armenia corresponds to the lower bound of this range. The Calima, 1995 earthquake caused damage in Pereira. Consequently, a local mitigation program was deployed which benefits not only Pereira but also Dosquebradas and Santa Rosa de Cabal. Mitigation efforts before the 1999 earthquake also included retrofitting of some structures in Armenia. A detail showing how columns of a building at University of Quindío were upgraded is shown in Figure 4. In this figure, one of the original columns in the roof level is shown surrounded by steel in preparation for the casting of concrete around the original section. The behavior of this building during the January 25 earthquake was acceptable. It suffered relatively light damage of nonstructural masonry walls and moderate damage of the roof. In addition to triggering efforts to enforce the use of seismic design guidelines, the 1983 Popayán earthquake together with the 1985 Armero mudslide led to the development of the National Office for Emergency Response. Figure 3 Seismic Hazard in Colombia (Colombian Association of Earthquake Engineering, 1998). This office was successfully tested by the Paez, June 1994 earthquake. At the same time that the National Office for Emergency Response was established, a national network of seismographs together with over 100 accelerometers, most of them digital, began to be established throughout Colombia. Unfortunately, both emergency response and detection efforts have been underfunded in recent years. It is hoped that the lessons learned from the Jan. 25, 1999 earthquake will bring new impetus to both projects. OBSERVATIONS Most of the structural damage observed in low-rise, reinforced concrete buildings may be classified into four categories: Captive Columns Figure 4 Eng. Bldg., University of Quindío. It is common practice in Colombia to use unreinforced masonry walls, about 5 inches thick, as partitioning system. The interaction between these walls and the structure seems to be often ignored. This observation was frequently corroborated by the failures of many captive columns in Armenia as shown in Figures 5 to 8. In view of the potential consequences of ignoring the problem, an explanation is presented next. 4

5 Figure 5 Collapse of one building of Coproquin condominium, Armenia. Figure 6 Detail, Coproquin Buildings. Figure 7 Old Building at University of Quindío, Armenia. Figure 8 Detail of Column in the first story of ICBF Bldg., Armenia Exact determination of the forces induced in a column of a structure subjected to strong ground motion is not an easy task. A pragmatic approximation, very useful in design, is to assume that the element reaches its flexural capacity at both ends and under oposite curvatures. The maximum probable shear that can therefore act on a given 5

6 section of such an element is limited to twice the plastic moment capacity of the section divided by its clear height. When a nonstructural element restrains the column along part of its height only, the maximum probable shear increases almost in inverse proportion to the reduction in clear height. For instance, if a column is restrained by a retaining wall in a first story, a practice observed in several buildings in Armenia (Figure 6), so that its clear height is one fourth of its original clear length, the maximum probable shear that could eventually act on this element would be four times higher than that calculated ignoring the possible interaction between the wall and the column. Lack of transverse reinforcement Columns with very small amounts of transverse reinforcement, as the one shown in Figure 9, were observed to have experienced severe damage. Observe that, in this case, only one layer of transverse reinforcement was provided within the zone where the element developed large inelastic deformations. This was a 13.5 in x 13.5 in column and the spacing of the stirrups was about 12 in. Notice that besides the insufficient amount of transverse reinforcement, its anchorage was not adequate. Ninety degrees hooks not anchored in the core of the column could have limited the development of the yield capacity of the plain ties provided. Figure 9 Detail of Column of Los Balcones Bldg., Armenia. Damage related to the interaction between structural and nonstructural elements Figure 10 shows a column in the first story of a public school in Armenia. In this case, the location of the inclined crack seems to indicate that the direction of movement was such that one could not relate the failure of this column with the possible effect of the adjacent discontinuous wall. The slenderness of the member, in turn, indicates that the shear stress that could have been developed should not be expected to be high. However, it seems plausible that the discontinuity generated by the failure of the adjacent continuous wall may have triggered the failure of the column. Deficient detailing Figure 10 Santa Teresa de Jesus School, Armenia. Figures 11 and 12 serve to stress the importance of good detailing. Figure 11 shows a column in which the architectural flare at the top moved the critical section away from the joint region. This made any transverse reinforcement that could have been provided near the joint ineffective and, at the same time, the clear height of the column was reduced. 6

7 Figure 12 shows a construction joint at the base of a column in the first story of a reinforced concrete building in downtown Armenia. Observe the evident lack of continuity, the presence of rather unusual hooks, the use of plain bars, and the absence of transverse reinforcement. Figure 11 Detail of Column in the first story of La Suiza I Bldg. Figure 12 Detail of Column in the first story of Centenario Hotel, Additional observations Several additional observations are worth discussing: Figure 13 Masonry wall, 3 rd floor, Aguas & Aguas Bldg, Pereira. - Very fragile partitions consisting of unreinforced, brick walls were observed in Pereira and Armenia. Their use in very flexible reinforced concrete frames seemed quite common. Columns with aspect ratios ratio of distance between points of maximum and minimum curvature to effective depth- of more than 4 were frequently observed. In general, severe damage to the masonry was a common consequence of the Jan. 25, 1999 earthquake (See Figure 13). In the most fortunate cases, this only caused economical losses. Some of the 1998 revisions to the seismic design recommendations for Colombia (Colombian Association of Earthquake Engineering, 1998) were intended to remedy this. 7

8 - Figure 14 shows what was left of a residential building in Armenia after the January 25 earthquake. It was identical to the ones still standing next to the debris. It is obvious that the consequences could have been worse. The structure consisted of reinforced concrete flat slabs, slender reinforced concrete columns and unreinforced masonry walls. The inadequacy of the mechanism that could provide lateral resistance in these buildings was made evident by the January 25 quake. - The remarkable importance of structural redundancy cannot be overemphazised. It represents the difference between collapse and severe structural damage without collapse (Figures 15 and 16). - As important as design provisions, repair guidelines represent a critical component of any efforts toward the reconstruction of urban infrastructure. Figure 14 Maria Cristina Condominium, Armenia. -The absence of reinforced concrete shear walls in the twenty buildings surveyed was noted. Figure 15 Los Balcones Bldg., Armenia. Figure 16 Detail, first story column. 8

9 ACKNOWLEDGMENTS The writers want to express their gratitude with Mario F. De La Pava, Luz E. Ocampo and Luis C. Martínez, Society of Engineers of Quindío; Ana Campos, Margarita Ochóa and Jaime Guzmán, Project of Seismic Risk of Pereira, Dosquebrads and Santa Rosa De Cabal; Gabriel Fernández, University of Illinois; Adolfo Alarcón, INGEOMINAS; Jorge E Durán, Gómez-Cajiao y Asociados; Augusto Espinoza, Areas; Omar D. Cardona, Colombian Society of Earthquake Engineering; Josef Farbiarz and Jorge E. Polanco, National University of Colombia at Medellín; Martha C. Vélez, Integral; Pedro F. Pujol, Gerinsa; and Marcia Collins, Tracy Mavity, Vincent P. Drnevich and Mete A. Sozen, Purdue University. REFERENCES Armenia Despertó con Nuevas Réplicas, El Colombiano, February 1, Colombian Association of Earthquake Engineering, 1998, Normas Colombianas de Diseño y Construcción Sismo Resistente, 4 vols., Santa Fe de Bogotá, Colombia. Sarria Alberto, 1995, Ingeniería Sísmica, Second Edition, Ediciones Uniandes and ECOE Ediciones, Santa Fe de Bogotá, Colombia, 569 p. Se revisarán Proyectos de Desarrollo Acordados con el BID, El Tiempo, January 29,