Seismic Strengthening for Low-rise Bamboo- and Masonry-wall Residential Buildings in Colombia

Transcription

1 Seismic Strengthening for Low-rise Bamboo- and Masonry-wall Residential Buildings in Colombia Koji YOSHIMURA*, Lizhen LIU**, Tania CROSTON** and Lin MA*** Key words: Seismic strengthening, low-rise masonry building, Colombia Earthquake, unreinforced masonry wall. 1. INTRODUCTION At 1:19 p.m. on Sunday of the 25th of January 1999, an earthquake of Richter Magnitude of M=6.2 occurred with the epicenter located about 16 km to the South of the City of Armenia, which is located approximately 180 km to the West South from Santa Fe de Bogota, the capital of the Republic of Colombia. The depth of the main shock of this earthquake was about 10 km, and due to this shallow-focus earthquake, more than 1,150 people were killed and approximately 5,000 persons were injured. Principal cause to those extensive human damage was caused mainly by the complete collapse of about 21,000 building structures and 32,000 partial collapse of the buildings [Reference.1]. One of the authors of the present paper conducted the field investigation on severely damaged buildings as a member of The Reconnaissance Team from the Ministry of Education of Japanese Government, which arrived at Santa Fe de Bogota on March 3, 1999 and spent eleven days in Colombia in order to investigate the effects of the earthquake on the ground, lifelines and building structures. Herein, based on the field investigation results obtained from the severe structural damage especially in the lowand/or medium-rise masonry buildings [Reference.2 and 3], principal causes to those building damage are presented first, and then the importance of the development and conducting the effective seismic strengthening for unreinforced masonry buildings as well as the confined masonry wall building structures is described. Received June 1999 * Professor of Department of Architectural Engineering, Oit University, Oita, Japan ** Graduate Student of Department of Architectural Engineering, Oita University, Oita, Japan *** Senior Structural Engineer, Building Science Research Institute of Liao Province, Shenyang, P. R. of China TYPICAL STRUCTURAL SYSTEMS IN SEVERELY DAMAGED BUILDINGS Although a considerable amount of damaged building structures were observed within the City of Armenia where most severe earthquake damage was concentrated, typical structural systems adopted in the severely damaged building structures are can be classified into followings: 2.1. BAMBOO-WALL STRUCTURES: A traditional wall structure in some Latin American countries, where structural and non-structural (or partition) walls are composed of bamboo (or timber) frames, mud or clay fillers and chopped straws as wall surfaces in some cases, and are usually finished by plastered material (Photo 1). In most of this kind of walls, there are no wall girders along the top of each wall. Most of the structural members or frames in roofs and ceilings adopted are also composed of bamboo and/or timber members. Photo 1 Damage to Bamboo-Wall Structure 2.2. UNREINFORCED CLAY BRICK MASONRY WALL STRUCTURES: One of the popular boxed-wall structural systems widely observed and constructed all over the world, in which all

2 the structural walls are composed of clay brick masonry units and mortar, and there is no wall reinforcement such as the steel reinforcing bars (Re-bars) within the walls in both horizontal and vertical directions (Photo 2). Photo 2 Damaged Unreinforced Masonry Wall 2.3. CONFINED MASONRY WALL STRUCTURES: One of the typical masonry wall systems widely accepted and constructed in Latin American countries and South- East Asian countries such as the People's Republic of China for low- and medium-rise houses or residential buildings, where cast-in-place R/C small columns are provided at the extreme wall edges and intersections of masonry walls as well as the cast-in-place R/C collar beams, wall girders or floor slabs along top of each masonry wall (Photo 3). In most of the cases, longitudinal Re-bars and hoops in R/C columns are installed before masonry wall units are placed within the wall plane. Therefore, after brick or block masonry units are placed with mortar in one or half story height, then concrete in columns is cast, and R/C collar beams, wall girders or floor slabs are finally constructed by cast-in-place concrete after all the masonry walls and R/C cast-in-place columns are completed. A total of four longitudinal (or main) Re-bars are usually provided in each of the R/C column sections. This type of masonry walls are called as "Confined Masonry Walls" (or "Muros Confinados de Mamposteria" in Spanish) in Latin American countries MASONRY FILLED SLENDER R/C FRAME STRUCTURES: Relatively smaller size and shape of moment-resisting frames which are composed of R/C slender columns and beams (or cast-in-place flat slabs or waffle slabs in some cases). After moment-resisting frames with slender members are completed, then all the exterior and partition walls are filled with unreinforced masonry brick or block units and mortar (Photo 4). Most of the perimeters in each masonry wall panel are usually not connected firmly to the attached R/C column and beam (or floor slab) members by using the connecting pieces such as small size of Re-bars as being observed in Chinese Construction methods. This type of structural systems seem to be widely adopted in Colombia especially in medium-rise residential buildings less than five stories. Photo 4 Masonry Filled Slender R/C Frame Structure Photo 3 Confined Masonry Wall 2.5. OTHER STRUCTURAL SYSTEMS: In addition to the above structural systems adopted widely to the severely damaged building structures in Armenia City, considerable structural damage was observed in some of the ordinary R/C moment-resisting frames in which unreinforced masonry brick or block walls are also provided within the plane enclosed by the R/C column and beam or slab members (Photo 5). In most of those cases having extensive earthquake damage to the structural or non-structural members, inadequate structural designs or structural systems such as the extremely irregular structures or framing systems were adopted, for example, buildings with ir- -10-

3 regular configuration, buildings with abrupt changes in lateral resistance and/or lateral stiffness, buildings with unusual size and shape, buildings on steep hillsides or on soft soils, and so on. ing structures were coming from the followings: All or most of the clay brick or block masonry units in masonry wall parts were not connected each other or not connected so firmly to the attached or surrounding R/C members by wall reinforcing materials such as high quality mortar and/or connecting pieces like steel Re-bars (Photo 7). Photo 5 Ordinary R/C Moment-Resistant Frame Filled with Unreinforced Masonry Wall 3. PRINCIPAL CAUSES TO SEVERE STRUC- TURAL DAMAGE 3.1. BAMBOO-WALL HOUSES: Most of the severe structural damage observed in this type of low-rise house buildings was caused by the following reasons: All of the vertical wall-edges at the L -, T - and + - shaped intersections in plan were not connected each other or inadequately connected. In addition, there were no continuous or monolithically constructed collar beams provided along the top of each wall. In case of this type of boxed-wall structures, it is necessary to connect the vertical connections between adjacent walls, and horizontal connections between wall and adjacent strong collar beam firmly UNREINFORCED MASONRY WALL STRUC- TURES: Destructive damage to the unreinforced clay brick masonry wall structures was mainly caused by the facts that each of the masonry units as well as the masonry wall as a whole panel were not reinforced. In addition, vertical wall-edges in each of the unreinforced masonry walls were not connected each other especially at the wall intersections in plan (Photo 6). Furthermore, there were no collar beams or no horizontal floor diaphragms provided or connected along the top of all the masonry walls. Photo 6 Damage to Unreinforced Masonry Wall Structure Photo 7 Separation of Masonry Wall Units from R/C Column As the results, remainder R/C columns reduced to the slender independent columns, some of which were buckled, broken or crushed, and finally the columns lost their gravity load- carrying capacity that resulted in the total or partial collapse of the whole structures (Photos 8 and 9). In other cases of the structural damage observed in confined masonry wall structures or masonry filled slender R/C frame structures, shear cracks or partial damage which occurred within the unreinforced masonry wall panels propagated into the adjacent confining R/C column-members, that resulted in the local collapse of the R/C columns such as inplane or out-of-plane buckling or crushing, and finally the 3.3. CONFINED MASONRY WALL OR MASONRY columns lost their vertical and lateral load-carrying capacity. Main cause to this kind of damage is due to the relatively FILLED SLENDER R/C FRAME STRUCTURES: Principal causes to the severe damage to this type of buildsmall size and shape of the R/C columns and inadequate reinforcing details provided into the slender column sec- -11-

4 tions. Other causes to the earthquake damage to the R/C columns, beams and beam-to-column connections of the con- Photo 8 Total Collapse due to 2nd Story R/C Column Failure Photo 9 Damage to R/C Column in Confined Masonry Wall Building fined masonry wall structures or masonry filled slender R/C frame structures were due to the lack of concrete sections and/or discontinuity of concrete section at around the beamto-column connections, which are mainly caused by the leakage of water and cement-mortar from the concrete forms during concrete casting. In addition, poor reinforcement details in beam-to-column connections were one of the other causes to the severe damage occurred in the confined masonry wall structures and masonry filled slender R/C frame structures (Photo 10). Photo 10 Poor Reinforcing Details in R/C Beam-to-Column Connection 4. SEISMIC STRENGTHENING FOR BAMBOO AND MASONRY WALL STRUCTURES 4.1. BAMBOO (OR TIMBER) WALLS: In bamboo (or timber) wall structures, most of the walls behaved independently during the earthquake, some walls separated from other wall(s) at their vertical connections and finally overturned in out-of-plane directions which resulted in the severe structural damage such as the total or partial collapse of the buildings. Therefore, in strengthening this type of traditional boxed-wall structures, it is necessary to connect the vertical joints between adjacent structurally important walls first, as well as to connect the horizontal connections between wall and newly added or newly constructed adjacent strong collar beams firmly. Once this type of building structure is strengthened so as to keep its original boxshaped configuration of the building, then each of the walls can develop its excellent in-plane lateral anti-seismic resistant capacity, which results in much more safer buildings against future earthquakes. Some of the experimental studies are expected how to connect the adjacent walls firmly and effectively against the coming next earthquakes. Carbon fiber reinforcement method is considered as one of the effective methods for such types of wall strengthening [Reference 4] UNREINFORCED MASONRY WALLS: According to the present Colombian Seismic Design Standards: NSR-98 [References 5 and 6], this type of unreinforced masonry buildings are not permitted to construct in such high seismic risk regions as the Cities of Armenia and Pereira [Reference 3]. However, there are still a large number of same or similar types of existing unreinforced masonry buildings -12-

5 in Colombia and other earthquake countries all over the world, where tremendously numerous people are living and, if such types of unreinforced masonry wall structures are not strengthened by the appropriate and effective rehabilitation methods, then considerable amount of severe structural and human damage will be again and again expected to occur during next big earthquakes, therefore seismic strengthening for the existing unreinforced masonry residential buildings is one of the most important and urgent problems to be solved to minimize the human and building damage against coming big earthquakes. As being mentioned in earlier Sections, destructive structural damage to this type of unreinforced brick or block masonry wall structures were caused due to the facts that; most of the masonry units were only connected partially by the low quality mortar and not connected each other by the reinforcing materials such as the steel reinforcement both in in-plane and out-of-plane directions. In addition, each of the unreinforced masonry wall panels are not connected firmly along the vertical wall-edges especially at the wall-towall intersections in plan. Furthermore, it seems that there are no continuous or monolithic collar beams, or no horizontal floor diaphragms provided along the top of the existing walls, especially at the uppermost stories. As the results, each of the masonry units and masonry walls behaved independently during the earthquake, some of the masonry units were cracked, crushed or broken or separated each other, or some of the non-damaged masonry wall panels separated from the adjacent wall(s) at their vertical joints, and finally overturned in out-of-plane directions which results in the severe structural damage such as the total collapse or partial collapse especially in the low-rise houses or residential buildings. By considering those extensive human damage and severe structural damage to the unreinforced masonry walls, effective seismic strengthening methods must be developed and performed as early as possible. One of the effective methods now being considered is a structural and seismic strengthening method adopted to many existing unreinforced masonry building structures in the People's Republic of China [Reference 7], where vertical and horizontal wall reinforcing bars (or welded wire fabrics in some cases) are firstly provided and fixed on the surfaces of the existing unreinforced masonry walls by using the connecting steel bars, and then mortar or shotcrete is put on the surfaces of the masonry walls (refer to Figure 1 and Photo 11). This seismic strengthening method for unreinforced masonry brick walls seems to be effective both in in-plane and out-ofplane directions of the masonry walls. Also seismic strengthening method by using carbon fiber sheets is proposed for such an existing unreinforced masonry wall (refer to Photo 12 [Reference 4]). In any cases, effectiveness for severe earthquake in both in in-plane and out-of-plane directions of the masonry walls must be investigated furthermore by using the full-scale (and three-dimensional if possible) specimens. Figure 1 Example of Masonry Wall Strengthening Method in P. R. of China Photo 11 Seismic Strengthening for Unreinforced Masonry Wall under Construction (P. R. of China) 4.3. CONFINED MASONRY WALLS OR MASONRY WALLS FILLED IN SLENDER R/C FRAME STRUC- TURES: In severely damaged building structures where this type of -13-

6 structural system was adopted, some or most of the unreinforced clay brick masonry units were separated each other and /or removed from the attached R/C column- and beam- or slab-members, and some of the masonry parts were dropped down from their original position into out-of plane directions. This is because each of the clay brick masonry units was not connected each other as well as not being connected to the attached or surrounding R/C members so firmly. As the results, R/C columns became to the slender independent columns, some of which were buckled, broken or crushed, and finally the columns lost their gravity loadcarrying capacity that resulted in the total or partial collapse of the whole structures. Figure 2 and Photos 13 and 14 show some illustrative examples for construction details between clay brick masonry wall-edges and adjacent cast-in-place R/ C columns in confined masonry walls recommended in Colombian Standards [Reference 6] and Mexico. It can be observed in these figures that there are not connecting pieces such as the steel Re-bars provided between masonry brick wall-edges and attached R/C column section. On the contrary, Figure 3 shows one of the connection details between masonry wall-edges and adjacent cast-in-place R/C columns in confined masonry walls recommended in Chinese Standards [Reference 8], where U-shaped connecting steel Re-bars with bar size of 6 mm must be provided Photo 12 Unreinforced Masonry Wall Specimen Strengthened by Carbon Fiber Sheets [4] Photo 13 Connection Details between R/C Column and Masonry Wall (CENAPRED in Mexico City) Figure 2 Construction Details in R/C Columns and Beams in Colombia (NSR-98: Chapter E) [6] -14- Photo 14 Connection between R/C Columns and Masonry Units under Repairing (Colombia)

7 every 50 cm between masonry wall-edges and attached R/C totally collapsed. Herein, based on the field investigation column section (see Photo 15). It seems that these connecting pieces are effective for preventing the masonry wall- of Education of Japanese Government, principal causes to conducted by the Reconnaissance Team from the Ministry edges separating from the attached R/C columns, although the severe damage to the building structures, especially lowand medium-rise masonry buildings in the City of Armenia additional experimental studies must be needed furthermore. Since there have been quite few experimental studies on the were discussed. Although a considerable amount of building structures were damaged during the earthquake, most of seismic safety of this kind of confined brick or block masonry walls, further researches especially using three-dimensional full-scale specimens such as a series of experilapse of the relatively low-rise buildings, especially one- the human damage was caused by the total or partial colmental studies conducted in CENAPRED (National Center and two-story dwelling buildings which were constructed for Disaster Prevention) in Mexico and in CISMID (Japan- by unreinforced masonry or bamboo (or timber) wall structural systems. In addition, since most of the existing build- Peru Center for Earthquake Engineering Research and Disaster Mitigation) in Peru, must be required. ing structures are one- and two-story residential buildings which are constructed by unreinforced masonry buildings or bamboo wall houses in Colombia, most of the description on the principal causes to the severe earthquake damage as well as strengthening for seismic safety and future research needs were mainly focused on those most popular low-rise buildings in Colombia. In order to mitigate the severe earthquake damage to those human and building structures, it is necessary to perform the followings as soon as possible: (1). In bamboo (or timber) wall structures, it is necessary to connect the vertical connections between adjacent structurally important walls first, as well as to connect the horizontal connections between wall and newly added or newly Figure 3 Recommended Details in Masonry Wall-to- R/C Column Connection in P. R. of China [8] constructed adjacent strong collar beams firmly. Some of the experimental studies are expected how to connect the adjacent walls firmly and effectively against the coming next earthquakes. (2). According to the present Colombian Seismic Design Standards, unreinforced masonry buildings are not permitted to construct in the high seismic risk zone. However, there are numerous unreinforced masonry buildings existing in Colombia and other earthquake countries where considerably large number of people are living, and once a big earthquake occurs, then severe human and structural damage is expected to occur, therefore seismic strengthening for the existing unreinforced masonry residential buildings is one of the most important and urgent problems to be solved to minimize the Photo 15 U-Shaped Steel Re-bars for Connecting human and building damage against coming big earthquakes. Masonry Wall and R/C Column (P. R. of China) (3). By considering those extensive human damage and severe structural damage to the unreinforced masonry walls, 5. CONCLUDING REMARKS effective seismic strengthening methods must be developed On the 25th of January 1999, an earthquake of Richter Magnitude of 6.2 occurred in Colombia, and more than 1,100 (4). One of the effective methods now being considered is a and performed as early as possible. people were dead, and about 21,000 building structures were structural and seismic strengthening method adopted to -15-

8 many existing unreinforced masonry building structures in China. This seismic strengthening method seems to be effective both in in-plane and out-of-plane directions for the existing unreinforced masonry walls. (5). Also structural and seismic strengthening method by using the carbon fiber sheets seems to be effective for the existing unreinforced masonry walls. In any cases, effectiveness against severe earthquakes both in-plane and outof-plane directions of the masonry walls must be investigated furthermore by the experimental studies. (6). In confined masonry wall structures or masonry walls filled in slender R/C frame structures, unreinforced clay brick masonry units were separated each other and /or removed from the attached R/C column- and beam- or slab-members during the earthquake, and some of the masonry parts were dropped down from their original position into out-of plane directions. According to the present Colombian Standards, connecting pieces such as the steel Re-bars are not recommended to provide between masonry brick wall-edges and attached R/C column sections. According to the Chinese Masonry Standards, however, U-shaped connecting steel Re-bars must be provided every 50 cm between masonry wall-edges and attached R/C column sections. It seems that these connecting pieces are effective for preventing the masonry wall-edges separating from the attached R/C columns, although additional experimental studies must be needed furthermore. (7). Since there have been quite few experimental studies on the seismic safety of the confined brick or block masonry walls, more extensive researches are needed in order to develop much more seismic masonry building structures. 6. ACKNOWLEDGEMENTS All the investigations presented herein were made possible by the financial support from the Grant in Aid for Scientific Research of the Ministry of Education of Japanese Government. Authors thank Mr. Teruhisa YUTAKA of Japanese Embassy in Colombia, Mr. Toshiaki FURUYA and Mr. Ukihiko KASAMA of the Colombian Office of the Japan International Cooperation Agency (JICA). In addition, all of the other members of The Reconnaissance Team shown below are acknowledged by their help and supports for this research: Dr. Hiroshi KAGAMI and Dr. Yuji ISHIYAMA of Hokkaido University, Dr. Yasuhiro UMEDA:,Dr. Kinya NISHIGAMI, Dr. Haruo HAYASHI of DPRI(Disaster Prevention Research Institute) of Kyoto University, Dr. Hiroshi SATO of Earthquake Research Center of Tokyo University, Dr. Hitoshi TANIGUCHI of Earthquake Disaster Mitigation Research Center, Dr. Masakatsu MIYAJIMA of Kanazawa University and Mr. Takao HASHIMOTO of Chiyoda Engineering Consultants Co., Ltd. Special thanks are to Dr. Hironori KAWAKATA, Dr. Zenon AGUILAR B and Mr. Nelson E.P. HERNANSDEZ of DPRI of Kyoto University. 7. REFERENCES 1) Ministry of Interior of the Republic of Colombia, Technical Evaluation of Infrastructures, Damage Occurred by the Earthquake in the Coffee Grower Zone, Report No.16, March 3, 1999, in Spanish 2) Koji Yoshimura, Tania I. Croston D., Hiroshi Kagami and Yuji Ishiyama, Report on damage to building structures caused by the 1999 Quindio Earthquake in Colombia, Inhouse Report, May 31, ) Koji Yoshimura, Tania I. Croston D., Hiroshi Kagami and Yuji Ishiyama, Damage to Building Structures Caused by the 1999 Quindio Earthquake in Colombia, Faculty of Engineering Research Report, Oita University, No.40, September, ) Koji Yoshimura, Kenji Kikuchi, Masayuki Kuroki, Lizhen Liu, Shunji Koga and Lin Ma, "Seismic Strengthening of Clay Brick Masonry Walls for Preserving an Old Masonry Building in Japan", will be published in the Proceedings of the First International Conference on Advances in Structural engineering and Mechanics (ASSEM'99), Seoul, Korea, August, ) Colombian Association of Seismic Engineering, NSR-98: Colombian Standards for Earthquake-Resistant Design and Construction of Building Structures,1998, in Spanish. 6) Colombian Association of Seismic Engineering, Manual of Minimum Specifications for One- and Two-story Housings, Chapter E of NSR-98, Technical Bulletin No.52, February, 1999, in Spanish 7) National Standards in the People's Republic of China, Seismic Strengthening Recommendations for Building Structures, 1986, in Chinese 8) National Standards in the People's Republic of China, Seismic Design Standards for Building Structures (GBJ 11-89), 1989, in Chinese. -16-