Observations on the Southern Sumatra Earthquakes of September 12-13, 2007

Home , Banda Aceh, Bengkulu, Mukomuko Regency, Sumatra

EERI Special Earthquake Report — November 2007

Learning from Earthquakes

This report is derived from a longer All photos were provided by Build The second event, Mw7.9, struck at version by Elizabeth Hausler and Change, except where noted. The 6:49 a.m. local time on September Aaron Anderson of Build Change, publication of this Learning from 13; it was located 225 km northwest currently based in Banda Aceh, In- Earthquakes report is supported by of the first event, on the northern donesia. They conducted site visits the National Science Foundation end of the aftershock zone. As to Indonesia’s provinces of Beng- through grant #CMII-0131895. shown in Figure 1, the events oc- kulu and West Sumatra September curred on the boundary between 17-25, and to the Mentawai Islands Overview the Australia and Sunda plates. October 6-9. Their full report is BMG is in the process of installing available at http://www.eeri.org/ On September 12 and 13, 2007, two a network of strong ground motion lfe/indonesia_southern_sumatra. earthquakes struck off the island of instruments throughout Sumatra; html. Two additional reports are Sumatra, Indonesia, causing dam- however, no strong ground motion available at the same URL: one by age in the provinces of Bengkulu, recordings exist for the September consulting engineer Teddy Boen Jambi, and West Sumatra, and in the 12 and 13 events. Mentawai Islands. of Jakarta, Indonesia, and the Tsunamis materialized from both other by Dwikorita Karnawati, Iman The first event, wM 8.4, struck at 6:10 shocks and affected coastal areas Satyarno, and Subagyo Pramumi- p.m. local time on September 12. It between Padang and Benkulu and joyo of Gadjah Mada University in was centered approximately 130 km parts of the Mentawai Islands. Run- Indonesia. off the coast SW of the city of Beng- ups 2 m deep were observed in the kulu, at a depth of south part of the Mukomuko regen- 30 km (Figure 1). cy, damaging houses within 500 m Immediately after of beaches (Figure 2). the quake struck, a tsunami warning was The areas of concentrated damage issued by the Indone- include the urban centers of Beng- sian Meteorological kulu and Padang, a string of rural and Geophysics coastal communities along the main Agency (BMG). highway between Bengkulu and Padang, and the villages in the

Figure 2. House pushed off its foundation by the tsunami at the coast of Serangai Village in Batik Nau District, south Figure 1. Location of main events and after- part of Mukomuko Regency, S3.2538° E101.5335° (photo: shocks (USGS). Karnawati).

EERI Special Earthquake Report — November 2007

Figure 3. Confined masonry houses with failure in masonry walls and connections between tie columns and bond beams, Kec. Airnapal (North Bengkulu).

Mentawai Islands. The latest casu- coast north from Bengkulu and south masonry wall with varying de- alty figures stand at 25 killed and from Padang to the Pasisir Selatan grees and types of confinement, 100 injured (OCHA Situation Report District. In this area, foundations are including unconfined (unrein- No. 8). The Indonesian National generally shallow strip footings made forced) masonry walls, timber tie Development Planning Agency of rounded river stone masonry in ce- columns and bond beams; rein- (BAPPENAS) puts economic losses ment mortar. Roofs consist of timber forced concrete tie columns with at IDR 1.5 trillion (US $164 million). trusses supporting lightweight CGI timber bond beams; and rein- sheets, asbestos sheets, or clay tiles. forced concrete tie columns and As was the case in other recent Hipped and pitched roofs are both ring beams earthquakes in Indonesia, houses common; in the Bengkulu area, tim- were hit very hard by the two earth- • Timber frame with masonry skirt ber gable walls are more common quakes. On mainland Sumatra, around the lower third of the wall, than masonry gables. 17,695 houses were destroyed, and woven, plastered bamboo or 21,035 houses were severely dam- Wall systems for houses affected timber above aged, and 49,496 were mildly dam- on mainland Sumatra include the • Timber frame. aged (OCHA Situation Report No. 8). following structural types: Confined or partially confined

• Confined or partially confined ma- masonry: This structural type is Damage to Housing sonry. Full height fired clay brick supported by a shallow river stone On mainland Sumatra, housing damage was concentrated along the

Figure 5. Partially collapsed brick masonry house with RC tie columns and timber bond beams. Note partial collapse Figure 4. Partially collapsed unreinforced masonry of masonry gable wall and lack of inplane stiffness in front house with timber bond beam (North Bengkulu). wall, Kec. Lais (North Bengkulu), S3.53217° E102.03771°.

EERI Special Earthquake Report — November 2007

Figure 6. Confined masonry house with failure in mason- Figure 7. Partially collapsed unreinforced masonry house ry walls and connections between tie columns and bond with timber bond beam, Lempuing (Bengkulu), S3.82799° beams, Kec. Airnapal (North Bengkulu). E102.28473°. masonry strip footing, and may have Figure 3 illustrates the impor- span over 5 m without interior a reinforced concrete foundation tance of connections between crosswalls or bracing. (plinth) beam. Two types and sizes confining elements. Note that the (6) Use of timber for bond beams. of fired clay bricks were common. reinforced concrete columns and Common practice is to use rein- Traditional hand-molded and wood beams, despite their insufficient forced concrete for tie columns kiln-fired bricks are generally 9 cm and inconsistent section and use and timber for the bond beams. wide by 19 cm long by 4 cm in of small diameter bars and stir- This construction practice was height. Their quality and strength rups or ties at large spacing, re- not sufficient to prevent col- can vary considerably depending main largely intact, with failure lapse of walls of some houses on the type of clay used, duration occurring in the masonry wall and (Figure 7). of firing, and placement in the kiln. at the connection between tie col- Machine-mixed, molded and fired umn and bond beam. Several confined masonry houses perforated bricks, more common in (2) Poor quality workmanship in the located in a housing development and around Bengkulu, are 9 by 19 masonry wall. Figure 3 also illus- in Bengkulu were damaged for by 9 cm in size. Regardless of the trates the importance of good similar reasons (figures 8 and 9). type of bricks, houses typically use quality masonry; in this case, This sloping subdivision of at least running bond for the masonry wall, partial collapse was initiated in 60 houses was built by a developer, resulting in a half-brick wide wall (13 the walls. and the houses were subsequently cm with plaster, 10-11 cm without). purchased by owners. Although (3) Lack of sufficient stiffness inthe there were some variations in floor For confined masonry houses, col- in-plane direction of the front wall. plans, most houses were identical: umns are typically cast after the A common architectural prefer- two bedrooms with kitchen and in- masonry wall is built, flush with ence in Indonesia is to have many door bathroom, single story, and the wall, and thus the same width large openings at the front of the approximately 40 m2. as a brick or block (10 or 11 cm). house. This lack of stiffness in the Smooth reinforcing steel is com- in-plane direction contributed to Damage was common at the upper mon, typically 6-8 mm in diameter several partial collapses (Figures column-beam connection in the with stirrups ranging from 3-6 mm 4 and 5). open frame supporting the gable in diameter. Stirrups and ties are (4) Insufficient connections between wall above the covered terrace spaced at 15-35 cm intervals. tie columns and masonry walls. (Figure 8). The gable wall and the side walls collapsed completely in The most common reasons for (5) Tall, slender wall prone to out-of- several cases. The most signifi- damage or collapse of confined or plane failure. With plaster, the cantly damaged structure was the partially confined masonry houses common wall is only 13 cm thick, only one that was not yet plastered are the following: and walls tend to be 3 m in height. (Figure 9); in Indonesia, cement- (1) Insufficient connections In the case shown in Figure 6, the based plaster adds significant between confining elements. wall was over 3.5 m tall with a strength to the masonry wall. It is

EERI Special Earthquake Report — November 2007

Figure 8. Subdivision of confined masonry houses, dam- Figure 9. Subdivision of confined masonry houses, col- age to covered terrace, Bengkulu, S3.83218° E102.29287°. lapse of masonry wall exacerbated by insufficient connections, Bengkulu, S3.83218° E102.29287°. likely that some houses were more damaged than others because of base of the wall performed well in the The Mentawai Islands: These is- 1) variations in construction quality earthquake. Like confined masonry, lands are a predominantly rural, and workmanship, and 2) location this type of house is supported by a sparsely populated chain of four on the hillslope. These failures shallow stone masonry strip footing; major and many small islands ap- should be studied for the purpose however, there is rarely any connec- proximately 9 hours by boat from of improving guidelines for new tion between the timber posts and the mainland Sumatra. In many villages confined masonry construction. The footing, or the timber posts and the on the southernmost of the Menta- causes of these failures, particular- masonry panel. The damage to these wai Islands — North Pagai and ly workmanship and poor connec- structures, considered “semi-perman- South Pagai — 25%-90% of the tions, should be emphasized in the ent” in Indonesia, consisted of crack- houses were destroyed or unin- guidelines. ing of the masonry panel, failure of habitable. There are three primary Timber frame with masonry skirt: the masonry panel as a rigid block, structural housing types in the vil- Many timber frame houses with 40 and shifting and damage to the timber lages: 1) timber frame houses on cm to 1 m of masonry around the frame (Figure 10). stilts or shallow stub footings;

Figure 10. Timber frames with masonry skirt, Kec. Lais (North Bengkulu). Left: Superstructure has shifted off, although the relatively heavy roof structure remains largely intact, and the masonry panel is partially collapsed, Kota Agung, S3.55841° E102.08951°.

EERI Special Earthquake Report — November 2007

Figure 12. Confined concrete block masonry, partial collapse due to lack of sufficient connections between con- Figure 11. Timber frame with posts resting directly on fining elements, Barimanua, Pulau Sipora, Mentawai soft ground, Barimanua, Pulau Sipora, Mentawai Islands. Islands.

2) timber structures with a concrete and sites inundated by tsunami; this mainland, including lack of sufficient block masonry skirt up to 1 m; and foundation system is vulnerable to connections between confining ele- 3) confined or unreinforced hollow weathering of the timber due to the ments, and poor quality block ma- or cellular block masonry structures lack of isolation from water uptake sonry (Figure 12). Most concrete with timber truss roofs. In all cases, from the ground (Figure 11). Some blocks are cellular. There was a roof cover is lightweight, consisting houses with a stone or concrete stub wide variation in the strength of the of CGI sheets, asbestos sheets or foundation slid off the foundation. blocks used, some of them crushing locally made thatch (rumbia). Con- Superstructures lack bracing in the easily under hand pressure. This crete block structures were dam- in-plane direction, and members are lack of strength results from low ce- aged more significantly and in connected with nails only. Timber is ment content, use of beach sand, greater numbers than the timber not maintained or treated; newer and production by hand (without houses. houses with treated or painted timber mechanical vibration). performed better than those with old- Damage to the timber houses re- Liquefaction effects on houses: er, weathered timber. sulted from a few causes. First, Effects of liquefaction, in the form of timber posts rest directly on the Confined concrete block masonry differential settlement and founda- ground, which caused the posts to structures had similar problems to the tion cracking (Figure 13), vertical sink in the ground at soft soil sites confined masonry structures on the cracks in masonry walls, heave and cracking of interior floors, and reports of water and sand coming up

Figure 13. Cracks in foundation and walls associated with settlement and tilt on liquefiable soils, Lempuing Figure 14. Temporary houses, Barimanua Village (Bengkulu), S3.82799° E102.28473° (Mentawai Islands).

EERI Special Earthquake Report — November 2007

steel frame structures are under construction in Padang. In the cities, many multistory RC frame with masonry infill buildings had cracks; however, little structural damage was observed. In Padang, two buildings collapsed completely, both RC frame buildings with RC slab roof and floors. One of them, the Hyun- dai shop, is shown in Figure 15. Major structural damage to RC frame buildings in Bengkulu was similarly limited, observed at only two buildings, both “ruko” buildings — combination house (rumah) on upper floors, and shop (toko) on ground floor. The ground floor is an open frame at the front, with a brick infill wall at the back and possibly a brick infill wall halfway back with a door. Both damaged rukos were Figure 15. Hyundai Shop, Padang. located on sites that slope down- ward away from the road or parking through the cracks in the floors, Damage to Reinforced Con- area, and thus the ground floor is were found in at least six houses in crete Buildings supported by a frame on spread Lempuing, a village on the edge of footings bearing at different eleva- Bengkulu city. This low-elevation In the urban areas of Bengkulu and tions. Due to safety concerns, it was site consists of clean, fine to medi- Padang, RC frame with masonry infill not possible to do a detailed inspec- um sand near the surface, on top of is still the most common structural tion of settlement and deformation high groundwater. Floor slabs gen- type for multistory commercial build- of the foundation elements. erally consist of thin (5-10 cm) un- ings and hospitals, although a few reinforced concrete.

Transitional Housing and Reconstruction The Indonesian government has promised IDR 15 million (approximately US $1,700) for each family with a destroyed house, IDR 10 million for severely damaged houses, and IDR 5 million for mildly damaged houses. However, it is unlikely that this funding will be disbursed to the families until 2008. Homeowners are staying in temporary shelters consisting of recycled timber, thatch, tents and tarps (Figure 14) and starting to rebuild using recycled materials and their own resources. The Indonesian Red Cross and some international aid agencies are providing some basic materials, such as nails and tarps. In fear of a tsunami, some villages in the Men- tawai Islands have asked to relo- Figure 16. Two-story ruko, 3° tilt in first floor column, Bengkulu, S3.80362° cate to inland or higher ground. E102.27617°.

EERI Special Earthquake Report — November 2007

Figure 18. SMAK SNT Carolus School, pre-2000 earthquake building on right, post-2000 earthquake building on left, Bengkulu, S3.82846° E102.29587°.

ings, one of which was a new building also cracked. It appeared that they Figure 17. Three-story ruko, Beng- constructed after the previous build- had been previously repaired by a kulu, S3.80581° E102.27953°. ing collapsed in an earthquake in sealant or adhesive. Some masonry 2000. The other building was repaired interior walls were partially col- The ground floor of the five-bay, after the 2000 earthquake. Only hair- lapsed. line cracks were found at the edges two-story ruko shown in Figure 16 Confined masonry interior and end of masonry infill walls in the new dropped by approximately 1 m due walls collapsed at a rural single- building; however, the timber truss to failure at the column-slab inter- story school complex in Lais Sub- roof collapsed where it was joined face. The structure is tilting back by district (Figure 19), leaving the to or battered by the older building approximately 4º, and the first floor long walls standing and supported (Figure 18). The repaired building had columns are leaning to the left at against out-of-plane failure primarily new, hairline cracks in the same pat- 3º. Ribbed, 14 mm diameter steel through column steel tied around tern as those that were plastered and was used in the columns and the roof trusses. Without strong- painted after the last earthquake. The beams, with ties of 6 mm diam- motion recordings, it is difficult to eter spaced at 4 cm near the joint. tops of the first floor columns were Similar failure at the column-ground floor-beam interface occurred at a three-story, two-bay ruko (Figure 17). In this case, the failure was clearly exacerbated by slope failure towards the building. The columns were 32 cm x 32 cm and used up to eight smooth 15 mm diameter longitudinal bars. Nonstructural damage, primarily in the form of cracks in masonry infill walls, was common in multistory commercial buildings. Falling haz- ards existed at many frame structures in which the column was not located at the corner, and unreinforced masonry walls were used to extend the plan area. Schools: UN OCHA reports that 260 educational facilities were destroyed, and 450 severely damaged. The SMAK-SNT Carolus School in Bengkulu consists of two Figure 19. Sekolah Dasar Negeri No. 04, note steel splice location, use of separate two-story RC frame build- rounded cobbles in concrete, Kec. Lais (Bengkulu), S3.52958° E102.04525°.

EERI Special Earthquake Report — November 2007

plan with a covered terrace on one side, experienced cracking and tilt of the columns on the terrace. Cracking from deformation in the out-of-plane direction (perpendicu- lar to the long axis of the building) was observed. Hospitals: Nonstructural damage to hospital buildings in Bengkulu and Padang resulted in cracked masonry infill walls, chipped plaster, and dust (Figure 21); patients were evacuated to tents (Figure 22). The M. Yunus hospital in Bengkulu was not operational right after the earthquake due to repair work needed. The building code considers masonry infill walls to be nonstructural elements, which has important im- plications for critical facilities that need to remain operational after earthquakes. Figure 20. Collapsed confined masonry building, note lack of plinth beam and connection between column and plinth, Pal 30, Kec. Lais, (North Bengkulu), S3.53490° E102.05996°. Acknowledgements The field reconnaissance team of say how much directionality con- in the concrete, which was broken Dr. Elizabeth Hausler and Aaron tributed to the end and interior wall easily by hand pressure. Anderson from Build Change was collapses. The ring beams were assisted by Rina, a lecturer in civil not capable of spanning these wide A second school compound with sig- engineering, and Puput, a student classrooms. Ring beams were ap- nificant damage in Lais Subdistrict from Bengkulu University; they pro- proximately 15 cm x 11 cm in sec- consisted of two buildings. A single- vided translation and logistics ad- tion and used four 8 mm diameter story confined masonry building col- vice during the Bengkulu study. longitudinal bars, with 4-5 mm lapsed (Figure 20). The plinth was SurfAid International provided logis- diameter stirrups spaced at dis- built of rounded river stone masonry, tical and transportation support for tances of 20-30 cm. Similar to the without a plinth beam for the columns the reconnaissance on the Menta- problems observed for confined to tie into. Failure at these and the wai Islands. Dr. Sasimar Sangchantr masonry houses, connections be- upper column-beam joints likely led provided translation and guidance tween tie columns and bond beams to collapse. The columns themselves on the Mentawai Islands. Assistance were insufficient. Coarse gravel to appeared to be largely intact. A newer from all parties is greatly appreci- cobble-size aggregate was used single-story building, rectangular in ated.

Figure 21. Nonstructural damage at M. Yunus hospital in Bengkulu (photo: Boen). Figure 22. Patients were evacuated to tents (photo: Boen).