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Vulnerability Assessment and Retrofitting of School Buildings in Indonesia
Kusumastuti, D. Institute of Technology Bandung
(email: [email protected]) Pribadi, K.S.
Institute of Technology Bandung (email: [email protected])
Rildova Institute of Technology Bandung (email: [email protected])
Boen, T. PT Teddy Boen Konsultan (email: [email protected])
Ando, S. United Nations Centre for Regional Development Hyogo Office
(email: [email protected])
Abstract
Past earthquake disasters in Indonesia reveals that large number of fatalities and excessive economic losses were due to damage and/or collapse of structures. Hence, earthquake mitigation efforts in Indonesia should include activities on ensuring that structures perform satisfactory under earthquake loading to minimize the impact of the disaster on the community. Aside from its function that school buildings in Indonesia are public facilities, these buildings are considered non engineered structures, i.e., buildings that were built traditionally with very little or no assistance from qualified engineers. The large number of occupants for school buildings highlights the importance that these buildings should ensure the safety of the students and the teachers. The viable solutions on improving structural performance of school buildings are that new buildings comply with building codes, and existing school buildings should be evaluated to verify their vulnerability against earthquakes and if necessary, retrofitted/strengthened to improve the structural performance and to reduce earthquake vulnerability. A pilot project of vulnerability assessment and retrofitting activities was conducted for two school buildings in West Java, Indonesia. The project was a collaborative work of United Nations Centre for Regional Development (UNCRD) and Center for Disaster Mitigation (CDM) ITB. The vulnerability assessment and retrofitting works are presented in this paper. The vulnerability assessment was carried out using field and laboratory investigations, and the building evaluation was completed with structural analyses. Next, the retrofitting strategies of the structures were designed and selected. The issues considered for retrofitting strategies were the structural deficiencies/weak parts and their accessibilities, weighing in factors of retrofit on buildings’ life time, earthquake resistance capacity, buildings’ function, and appropriate retrofit strategy/techniques. The design of
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retrofit strategy also considered factors of continuation of normal function, availability of materials and skilled construction workers, needs of upgrades for non structural components, and total costs. The construction works were then conducted based on the designed retrofit strategies. Recent earthquake in the region reveals that the retrofitted structures performed satisfactorily under seismic load.
Keywords: non-engineered structures, school buildings, earthquake vulnerability, retrofitting
1. Background
1.1 Geological and geotechnical aspects of Indonesia
In the past five years, more than 10 large earthquakes with magnitude of more than 7.0 in Richter scale have occurred in Indonesia, causing more than two hundred thousands fatalities and colossal damages on structures and infrastructures. Geologically, five tectonic plates intersect in Indonesia. The tectonic plates’ movement is the main source of seismic activities in the region, as in the case of 26 December 2004 Aceh earthquake, 2 September 2009 West Java earthquake, and 30 September 2006 Padang earthquake. Moreover, the intraplate faults also act as earthquake sources. Figure 1 shows the seismicity map of Indonesia.
Figure 1: Seismicity Map of Indonesia (1990-2006)
(http://earthquake.usgs.gov/earthquakes/world/indonesia/seismicity.php)
The seismic risk is introduced into structural design through the application of earthquake forces. A seismic zonation map of Indonesia is established to determine the PBA (Peak Base rock Acceleration)
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for various locations in Indonesia. The PBA coefficients are calculated with a 10% of exceedance in 50 years, or a return period of approximately 500 years.
Studies reveal that damages on structures and infrastructures are the most probable cause for casualties due to earthquakes. Therefore, it is crucial that structures perform satisfactory under seismic loads. Based on the available building codes, the seismic design criteria for buildings state that buildings may have some damage, but should not collapse due to earthquakes. For important facilities that are expected to be used immediately after the disaster, buildings should not have major damage.
Figure 2: Seismic Coefficients for Various Locations in Indonesia for 500-year Return Period for Hard Soil (SNI 02-1726-2002)
1.2 Ty pical school buildings in Indonesia
School buildings are considered as important facilities due to their vital roles in the communities. Since children are the main occupants of the buildings and they are more vulnerable to disasters than adults, building safety is a must. Moreover, school buildings play important roles in post disaster activities. Unfortunately, past earthquake reveal that many school buildings were damaged due to earthquake hazards. Since the occupancy rates for these structures can be very high, if the buildings behave poorly under earthquake loads, the occupants are susceptible to earthquake.
Typical school buildings in Indonesia are one-story reinforced concrete (RC) frames with masonry walls, consisted of three to four classrooms for each block. Most school buildings were built in 1970s and 1980s. With many buildings were designed for 30-50 years of life time, those buildings may have been in their old age and require major repair and maintenance. To complicate matters, school buildings in Indonesia are typically non engineered structures, i.e., were built traditionally with very little or no assistance from qualified engineers. These buildings are frequently found to have poor detailing, wide variety of quality of materials, and wide variety of construction methods.
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Due to minimum reference to building standards/codes, non-engineered buildings are usually constructed without consideration of the level of damage. The structures are usually built by local masons/workers, using local materials and traditional construction methods. Therefore, the performance of these buildings varies greatly, depends on the quality of the materials, and the construction methods, which makes the vulnerability assessment more difficult.
The earthquake mitigation efforts for existing school buildings in Indonesia should become priority due to the building functions and the great number of such structures. The procedure should include vulnerability assessment of structures in resisting design earthquake forces based on current building codes. Based on the assessment, if the performance of the structure is not satisfactory, retrofitting strategies should be designed to strengthen the buildings.
2. Retrofitting of school buildings: SDN Cirateun Kulon 1-2, Bandung
From the background, it is clear that school buildings need to perform well under earthquake loads, children are more vulnerable during earthquakes, and that school buildings may be used for emergency facilities or shelters in post-earthquake recovery efforts, thus need to have minimum damage (behave elastically) under earthquake loading.
Following recent disasters, a pilot project in evaluating the structural performance of school buildings in resisting earthquake loads have been carried out in 2007-2008. The project was a collaborative effort between United Nations Centre for Regional Development (UNCRD) and Center for Disaster Mitigation, Institute of Technology Bandung (CDM-ITB), under the corridor of the School Earthquake Safety Initiative project. The objectives of the project are to reduce vulnerability of school children to earthquakes, to reduce number of victims due to earthquakes, and to prepare school communities/elements in facing earthquake disaster.
The city of Bandung was selected as the location of the project due to its moderate seismicity level, with a PBA of approximately 0.2 g, and the perception that there is a possibility of large earthquake in the near future. SD Cirateun Kulon 1-2 in Bandung City, West Java, was selected as one of the two school buildings to be retrofitted due to the dire needs of improvement and severe deficiencies of earthquake resistant systems. The school is located at Jalan Dr Setyabudhi KM 10.7, Sukasari District, Bandung. The population consists of 423 students and 14 teachers that reside in the building from 7AM to 5PM, thus represents high number of possible injured/fatalities if earthquake occurred.
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Figure 3: SD Negeri Cirateun Kulon 1-2
The retrofitting project for SDN Cirateun Kulon 1-2 includes 3 steps, that is: (1) Vulnerability Assessment and Analysis, (2) Design of Retrofitting Techniques/Approach, and (3) Construction/Implementation of Retrofitting.
3. Vulnerability assessment
Vulnerability assessment was carried out to determine the level of risk associated with loss of serviceability and severe damage or collapse. With the risk quantified, rational decisions can be made as to whether the buildings should be retrofitted or replaced. Several aspects were look at, which include technical aspect, cost and benefit, importance of building, availability of adequate technology, skilled labor to implement the retrofitting strategies, and duration of works.
The first step in vulnerability assessment was to check the location. The site of the buildings was evaluated to determine the soil properties and to check for other hazards such as landslide or flood for ensuring the stability of the foundation. The study reveals that the building is located on a relatively moderate slope, yet stable. The bearing capacity of the soil was adequate for the existing foundation.
The existing structures were then investigated to determine the layout, structural type and quality of materials used, as well as the existing lateral resisting system. The school buildings were found to be of two blocks, with different materials and structural systems.
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Building I
Building II
Figure 4: Building Layout of SDN Cirateun Kulon 1-2
The first block (Building I) was L-shaped, and built in the 1970s. The building has four classrooms, each is 7 x 8 m. The structural system of Building I was made of pilaster masonry columns and masonry walls. No tie beams were found on the structure, and R/C ring beams were found on some locations, not on all perimeter of the structure. Poor materials were used for the structure, especially for R/C members. Poor detailing (longitudinal reinforcement of 4φ8, fy=240 MPa, and stirrups of φ8-400/500mm) was also found for ring beams. The roof truss system showed damage with some deterioration occurred on the timber truss elements, and the top chords showed some sagging. The shallow foundation was 25 x 65 cm, which is rather small compared to the minimum dimension required by the code.
The second block (Building II) was rectangular, built in 1994. Three classrooms, one teacher’s room and toilet are located in this block. The typical classroom for this building is 7 x 7.5 m. Building II had RC tie beams, ring beams, and columns, with poor materials and poor detailing (longitudinal reinforcement of 4φ8, fy=240 MPa, and stirrups of φ8-400/500mm). The tie beams were exposed and the shallow foundation was barely supported on some places due to scouring. Roof truss system was found to have some damage.
Visual inspections revealed that both blocks suffered damages, with cracks and concrete spallings found on structural elements. Both buildings showed poor beam-roof connection and poor roof truss element and connection. The structural investigations concluded that the school buildings were in dire need of structural repair, if not retrofitting/strengthening, considering the risk of seismic activity and
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the high number of building occupants. Therefore, the second stage of vulnerability assessment was conducted, that is structural analysis.
Figure 5: Existing Conditions of SD Negeri Cirateun Kulon 1-2
The structural analysis was conducted using the material properties obtained from actual condition. The elements were modelled, and the structural deficiencies were included in the model. The design earthquake used was based on the building code, which considers seismic risks from potential earthquake sources and local soil conditions. The structure was expected to behave elastic under the applicable loads, with the target performance level of immediate occupancy. The results from the structural analysis revealed that the performance of both buildings were inadequate in resisting lateral loads. Therefore, retrofitting should be conducted on both buildings.
4. Retrofitting strategies
The retrofitting work was conducted with funding from Hanshin Department Store Labor Union, Japan, for structural works, and from local government funding for finishing works. The target was to improve the structural quality and to reduce earthquake vulnerability of the structure. The weak aspects of the existing structures were considered as the focus of the retrofitting strategy. Other aspects that were considered are the effect of retrofitting on the buildings’ life time, available resources (materials and skilled labors), repair of non structural components, duration of physical work, and the total cost. The cost benefit analysis was also conducted to ensure that the selected retrofitting strategy would be able to ensure that the buildings will fulfil its function and will benefit the community.
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Finally, the retrofitting was designed based on the structural deficiencies/weak parts and their accessibilities, weighing in factors of retrofit on buildings’ life time, earthquake resistance capacity, buildings’ function, and appropriate retrofit strategy/techniques. The design of retrofit strategy also considered factors of continuation of normal function, availability of materials and skilled construction workers, needs of upgrades for non structural components, and total costs. Since the two buildings had different structural systems and different weaknesses, two retrofitting strategies were designed for the buildings.
The retrofitting strategy of Building I employed installing confining frame elements of RC ring beams, tie beams, and columns. The strategy was selected due to lack of lateral resisting system in the structure, and damages found on the structural members. The new columns were of two types, 30 x 30 cm installed on the corners, and 20 x 20 cm as practical columns. The columns were installed with footings to support the new frame. The proper detailing of the columns to tie beams and ring beams was introduced, considering the development length, lap splices, and seismic hook. To provide connection from columns to the wall panels, short anchorage was provided using φ8 mm bars of 40 cm length for every 50 cm height. The roof trusses were repaired by replacing deteriorated timber elements and providing proper connections. To improve the overall structural performance, seismic gap (dilatation) was provided in the structure by separating the wall of the one class room from the row of three classrooms, thus obtaining rectangular layout. Therefore, the L-shaped building became two regular structures.
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Figure 6: Retrofitting Strategy for Building I
Figure 7: Retrofitting Works for Building I
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The retrofitting strategy for Building II took into account that the overall existing condition of the building was adequate for resisting gravity forces. Therefore, the improvement was designed to improve the lateral capacity of the building. Iron wire mesh was provided to add strength and stiffness of the existing confined frames columns. The wire mesh was also installed on both sides of the walls, on the locations of columns and ring beams, as well as on the gable wall. To improve the foundation system, double tie beams were added adjacent to the existing ones. The beams were tied together at every one meter using a cross beam underneath the walls. The shallow stone foundation was also repaired to ensure the bearing capacity. The repair for roof trusses was also conducted by replacing deteriorated timber elements and providing proper connections.
Figure 8: Retrofitting Works for Building II
with iron wire on the both side of wiremesh
(Pour with concrete)
On the both edge, there is approximately 20 cm of longitudinal barappeared, the purpose is for connecting to the other tie beams
The iron wiremesh should be cut,if it meets the hoops
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Figure 9: Retrofitting Strategy for Building II
Upgrade and repair of non structural components and health facilities were also carried out. Structural openings (doors and windows) were repaired. New roof tiles, ceilings, and floor tiles were installed to replace the old ones. Toilets and sanitation facilities were also fixed. Cosmetic repair including paint job was conducted to improve the structural appearance, thus making the buildings more comfortable for the occupants. The physical retrofitting works was completed in five months with good results, and the structures were expected to have adequate quality in resisting applicable loads, including seismic loads.
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Figure 10: Retrofitted School Buildings
5. Performance of buildings due to earthquake
On Wednesday, September 2, 2009, approximately at 3PM, the West Java earthquake shook the region. The earthquake was measured at magnitude 7.3 on the Richter scale, with the epicentre at 8.24°S-107.32°E, and the depth of 30 Km. The peak base acceleration (PBA) of the earthquake in Bandung was estimated at 0.12g, which is lower than the design PBA based on codes of 0.20g. Although the West Java earthquake was less than the design earthquake, a number of buildings were found damaged or even collapsed due to the earthquake.
The earthquake posed as a real test for the newly retrofitted buildings. Since the structures were improved in lateral load capacity and building performance, it is expected that the structures could survive the West Java earthquake with minor damage.
A survey was conducted on September 3, 2009, to evaluate the conditions of SDN Cirateun Kulon 1-2. The survey revealed that the performance of SDN Cirateun Kulon 1-2 was satisfactory during the earthquake. The structure did not collapse, and damage on structural elements was prevented. Some minor cracks were found on the structure after the earthquake, especially near openings (window and door frames area). The crack depth and width were insignificant thus do not affect the structural strength and stiffness. No other damage was found on the structure. The retrofitting work seems to be able to improve the structural quality and to reduce the earthquake vulnerability of the structure. If retrofitting works had not been conducted, structural damage was possible, thus increasing liability of the students who were studying at the time of the earthquake.
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Figure 11: Post earthquake Conditions of SDN Cirateun Kulon 1-2
6. Conclusion
Recent earthquakes in Indonesia highlighted the importance of good structural response due to seismic loads. The seismic performance of school buildings becomes a major issue due to the difficulties in assessing the seismic vulnerability, the large number of such buildings in the country, and the high occupancy ratio of the buildings. Although school buildings are public facilities, it is a common practice in Indonesia that one- and two-story school buildings are built without assistance from a qualified engineer. Therefore, the quality of the structures varies greatly depending on the quality of materials and the construction methods. The strategy for obtaining earthquake resistant school buildings includes ensuring new buildings to comply with building codes, and evaluating the existing school buildings to verify their vulnerability against earthquakes. If the performance of the existing structures is not adequate, retrofitting/strengthening should be conducted.
A collaborative work in reducing the seismic vulnerability was conducted by United Nations Centre for Regional Development (UNCRD) and Center for Disaster Mitigation (CDM) ITB. The project focuses on improving the school safety against earthquake. The selection process was conducted based on needs of the school, and SDN Cirateun Kulon 1-2 in Bandung was selected for the project. The school has two buildings with different structural systems and materials.
The vulnerability assessment of the building was conducted, which includes field and laboratory investigations, and structural analyses. Based on the deficiencies and weaknesses of the structures, the retrofitting strategies of the structures were designed and selected. The issues considered for retrofitting strategies were the structural deficiencies/weak parts and their accessibilities, weighing in
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factors of retrofit on buildings’ life time, earthquake resistance capacity, buildings’ function, and appropriate retrofit strategy/techniques. The design of retrofit strategy also considered factors of continuation of normal function, availability of materials and skilled construction workers, needs of upgrades for non structural components, and total costs.
The construction works were conducted based on the designed retrofit strategies. Good quality of materials and skilled labors were provided to ensure the quality of the structure. The newly retrofitted structures underwent a real test when the West Java earthquake occurred. Post earthquake investigation revealed that the retrofitted structures performed satisfactorily under seismic load. The structure only suffered minor cracks due to the earthquake.
References
Building Research Institute and National Graduate Institute for Policy Studies. (2006). A Study on Development of Earthquake Disaster Mitigation Policy in Developing Countries, Technical Report, Japan, 81-114.
Departemen Permukiman dan Prasarana Wilayah. (2002). Standar Nasional Indonesia (SNI) 02-1726-2002 Standar Perencanaan Ketahanan Gempa untuk Struktur Bangunan Gedung, Indonesia.
United Nations Department of Economic and Social Affairs, United Nations Centre for Regional Development (UNCRD) Disaster Management Planning Hyogo Office. (2008). Reducing Vulnerability of School Children to Earthquakes, Japan, 21-23.
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