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Performance of Structures in the Mw

6.1 Mae Lao Earthquake in Thailand onMay 5, 2014 and Implications for FutureConstructionPanitan Lukkunaprasita, Anat Ruangrassameea, Tirawat Boonyateea,Chatpan Chintanapakdeea, Kruawun Jankaewb, NuttawutThanasisathitc & Tayakorn Chandrangsud

a Department of Civil Engineering, Chulalongkorn University,Bangkok, Thailandb Department of Geology, Chulalongkorn University, Bangkok,Thailandc Department of Civil Engineering, King Mongkut’s University ofTechnology North Bangkok, Bangkok, Thailandd Department of Public Works and Town & Country Planning, Ministryof Interior, Bangkok, ThailandPublished online: 12 Aug 2015.

To cite this article: Panitan Lukkunaprasit, Anat Ruangrassamee, Tirawat Boonyatee,Chatpan Chintanapakdee, Kruawun Jankaew, Nuttawut Thanasisathit & Tayakorn Chandrangsu(2015): Performance of Structures in the Mw 6.1 Mae Lao Earthquake in Thailand on May5, 2014 and Implications for Future Construction, Journal of Earthquake Engineering, DOI:10.1080/13632469.2015.1051636

To link to this article: http://dx.doi.org/10.1080/13632469.2015.1051636

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Journal of Earthquake Engineering, 00:1–24, 2015Copyright © Taylor & Francis Group, LLCISSN: 1363-2469 print / 1559-808X onlineDOI: 10.1080/13632469.2015.1051636

Performance of Structures in the Mw 6.1 Mae LaoEarthquake in Thailand on May 5, 2014 and

Implications for Future Construction

PANITAN LUKKUNAPRASIT1 , ANAT RUANGRASSAMEE1,TIRAWAT BOONYATEE1, CHATPAN CHINTANAPAKDEE1,KRUAWUN JANKAEW2, NUTTAWUT THANASISATHIT3,and TAYAKORN CHANDRANGSU4

1Department of Civil Engineering, Chulalongkorn University, Bangkok, Thailand2Department of Geology, Chulalongkorn University, Bangkok, Thailand3Department of Civil Engineering, King Mongkut’s University of TechnologyNorth Bangkok, Bangkok, Thailand4Department of Public Works and Town & Country Planning, Ministry of Interior,Bangkok, Thailand

An Mw 6.1 earthquake struck northern Thailand on the 5th of May 2014. The epicenter was locatednear Mae Lao district in Chiang Rai province. The earthquake caused unprecedented damage tostructures, the most damaging earthquake ever in recorded Thai history. Five hundred and ninety-fourbuildings out of 10,863 were damaged to the extent that they were unsafe for occupancy. This articlepresents a reconnaissance investigation of damage to buildings and bridges in the two districts—Phanand Mae Lao—which suffered the most damage. Attention is paid to the performance of buildings withsimilar configurations and structural design, but with different layout of unreinforced masonry infillsas non-structural components.

Keywords Reconnaissance; Moderate Earthquake; Reinforced Concrete; Buildings; Performance;Masonry; Non-Seismic Design

1. Introduction

On May 5, 2014 at 11:08:43 UTC a strong earthquake struck Chiang Rai province in north-ern Thailand. It was reported to have a local magnitude ML of 6.3 with the epicenter atLatitude 19.748◦N, Longitude 99.692◦E by the Seismological Bureau, Thai MeteorologicalDepartment (TMD) with a depth of 7 km [Thai Meteorological Department, 2014], whilethe USGS reported Mw 6.1 at Latitude 19.656◦N, Longitude 99.670◦E at a depth of 6 km[US Geological Survey, 2014]. USGS ShakeMap indicates an Instrumental Intensity of VII(moderate damage) at the epicenter. It was the biggest instrumentally recorded earthquakeever in Thailand. Shaking from the main shock was felt by people in many provinces,including Chiang Rai, Lampang, Lamphun, Chiang Mai, Nan, Phayao, Nong Khai, andLoei, and in high-rise buildings in Bangkok. The most severe damage to structures was wit-nessed in Mae Lao and Phan districts. Less damage occurred in nearby districts including

Received 27 October 2014; accepted 11 May 2015.Address correspondence to Anat Ruangrassamee, Department of Civil Engineering, Faculty of Engineering,

Chulalongkorn University, Phayathai Road, Pathumwan, Bangkok 10330, Thailand. E-mail: anat.r@chula.ac.thColor versions of one or more of the figures in the article can be found online at www.tandfonline.com/ueqe.

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FIGURE 1 Epicenters and surrounding areas.

Mae Suai and Muang of the Chiang Rai Province. The location of the epicenter was south ofMae Lao district. Figure 1 shows the topographic map of the areas surrounding the reportedepicenters with fault segments depicted.

The earthquake caused unprecedented devastation. A total of 10,369 private build-ings were reported to have suffered various degrees of damage, with 475 of them beingunsafe for occupancy, 2180 potentially repairable, and 7,714 safe for occupancy with minordamage [DPT, 2014]. Out of 494 public buildings (including temples), the corresponding

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Performance of Structures in the Mw 6.1 Mae Lao Earthquake 3

FIGURE 2 Locations of surveyed structures. (Details are in Table 1.)

figures were 119, 196, and 179 for the three categories, respectively. In this event, oneperson was killed by a collapsed masonry panel.

While numerous reports exist for high seismicity regions, there is relatively little infor-mation for events around Mw 6, especially in a region where most buildings have not beendesigned for earthquake resistance like Thailand. This article presents damage to buildings,bridges, and other structures mainly in the Mae Lao and Phan districts, most of which areof non-seismic design. Performance of non-engineered buildings is also covered. Locationsof structures presented in this article are shown in Fig. 2. Detailed locations and damagelevels of surveyed structures are summarized in Table 1. The damage levels were classifiedbased on performance levels defined in ASCE41-13 [2014]. Preliminary analyses were con-ducted on two structures to gain insight into the cause of damage to the structures. Lessonslearned and implications for future construction are addressed, which should be valuablefor countries of similar seismicity and socio-economic settings.

2. Seismicity in the Area and Observed Ground Motions

Phan district and its vicinity had been seismically quiescent until an earthquake of Mw5.2 hit the district on September 11, 1994. The epicenter of the 1994 earthquake wasat 19.586◦N, 99.526◦E located on Mae Suai Fault Segment of the Phayao Fault Zone[Earthquaketrack, 2014]. The 1994 Phan earthquake caused minor to moderate damageto more than 50 buildings including schools and hospitals. Structural damage includedshear cracks in short columns as well as boundary columns of reinforced concrete (RC)frames infilled with unreinforced masonry panels, and flexural cracks in soft story columns.However, no building collapsed or was even on the verge of collapse.

There were no earthquakes with a magnitude larger than 4 in the area of Phayao FaultZone between September 11, 1994 and May 5, 2014 earthquakes. Phayao Fault Zone is90 km long and is composed of 20 fault segments, namely: (1) Mae Tak; (2) Doi KunMae Suk; (3) Sai Ngern; (4) Ban Rong; (5) Pa Faek; (6) Pang Daeng; (7) Pa Boon Nak;(8) Phayao; (9) Phan; (10) Wang Nuea; (11) Haui Mae Toom; (12) Haui Ton Pueng;(13) Wang Tong; (14) Huai Sai; (15) Wiang Kalong; (16) Wiang Pa Pao; (17) Mae Korn;(18) Mae Lao; (19) Mae Suai; and (20) Mae Jaydee. The location of the May 5, 2014 earth-quake epicenter was just north of the Phan Fault Segment, an oblique slip + normal fault

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Performance of Structures in the Mw 6.1 Mae Lao Earthquake 5

with a Maximum Credible Earthquake (MCE) of Mw 6.7, deduced from surface rupturelength interpreted from the satellite image [DMR, 2009].

Department of Mineral Resources [DMR, 2009] published results from active faulttrenching along Phayao Fault Zone in the area close to the epicenter. There were fourtrenches in this area. Ban Pang Moong trench, in Mae Korn Fault Segment, was reportedto be an oblique slip with reverse fault which had the last movement about 8,000 yearsago with horizontal movement (interpreted from offset stream) of about 0.156 mm/yr. BanHaui San Yao trench of Mae Lao Fault Segment was concluded to be an oblique slip withnormal fault with 2 movements, one dating back more than 5,300 years and the other about5,300 years. The horizontal movement is reported to be 0.110 mm/yr. Ban Pa Jorh trench ofPhan Fault Segment is reported to be an oblique slip with normal fault containing one faultmovement about 5,200 years ago with a horizontal movement of about 0.175 mm/yr. BanPa Neng trench of the Wang Tong Segment is reported to be an oblique slip with normalfault which experienced 2 movements about 5,000 and 4,000 years back. The reportedhorizontal movement is 0.342 mm/yr.

Seven TMD stations could detect strong motions of the main shock. Figures 3 and4 show the ground acceleration and spectral acceleration, respectively, at Phayao Station(epicentral distance about 40 km). The ground condition in the area is classified as ClassD according to ASCE 7-10 [2010] based on shear wave velocity tests [DMR, 2011]. Thepeak ground acceleration was about 0.07 g.

Comparison of observed accelerations to attenuation models by Sadigh et al. [1997],Boore et al. [1997], Abrahamson and Silva [1997], and Idriss [1993] is shown in Fig. 5.The attenuation models by Sadigh et al. and Idriss fit the observed data reasonably well.The peak ground acceleration at 10 km from the epicenter is predicted to be about 0.2–0.3 g from the attenuation relations.

3. Building Codes and Local Design and Construction Practice

Buildings designed as per different standards perform differently under the same action.Therefore, an attempt is made to identify if the building surveyed was designed for seismicaction. The Ministerial Regulation No. 49 [Ministry of Interior, 1997] under the BuildingControl Act [Ministry of Interior, 1979] concerning seismic resistance design of buildingshas been promulgated just only since 1997. It was based on the 1985 Uniform BuildingCode, with Zone II designated for 10 seismic prone provinces in Thailand, includingChiang Rai. Public and essential buildings of any height are required by the regulation

FIGURE 3 Strong motion acceleration at Phayao Station.

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FIGURE 4 Acceleration response spectrum for Phayao Station (5% damping ratio).

FIGURE 5 Attenuation of peak ground accelerations.

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Performance of Structures in the Mw 6.1 Mae Lao Earthquake 7

to be seismic resistant against a peak ground acceleration of up to 0.15 g (g is the accel-eration due to gravity) on rock site. However, private buildings not taller than 15 m arenot required to have any seismic resistance design. Consequently, a large building stockcontains inadequate seismic resistant structures. Therefore, unless specifically noted, thebuildings reported herein fall into this category.

Most of the residential houses are two stories high, featuring a soft story with smallreinforced concrete (RC) columns. Typical widths of columns are 150 mm and 200mmfor two and three story buildings, respectively, with light transverse reinforcement, typi-cally 6 mm diameter round bars spaced approximately at 150–200 mm. Unreinforced infillmasonry (URM) panels, generally 100-mm thick (including 15–20 mm cement plasteringon each face), are extensively used as non-structural partitions, with a small number ofdowel bars (if any) connecting the panels and the boundary RC frames. Often the dowelbars are not provided. The ultimate compressive strength of concrete in buildings is nor-mally in the order of 18 MPa (or much less for non-engineered buildings). Reinforcing barsusually have specified yield strengths of 240 MPa for plain bars and 300 MPa for deformedones. Generally, bricks, cement blocks (for infill panels) and mortar are of unknown qual-ity since they are used as non-structural elements. Low-rise residential/commercial RCbuildings (2–3 stories high) are generally constructed without involvement of engineers.

4. Observed Performance of Buildings

4.1. Residential Buildings

A common feature of traditional Thai houses in rural areas is the elevated first floor aboveground to avoid blocking flow in the event of flooding. These non-engineered buildings arecustomarily built without engineering drawings. As such, only important features affectingstructural performance are discussed. These buildings are often supported by small non-ductile concrete columns at the ground level with open space, creating a soft story systemwhich is vulnerable to damage. Those non-engineered residential buildings have been builton small non-ductile columns with cross section size of 150 × 150 mm. Figure 6 shows onesuch building with the first floor sitting on the pan-caked ground floor columns. This housewas located at about 10 m from a surface crack caused by the earthquake which can be seenin the foreground. Figure 7 depicts a typical wooden house elevated on RC columns andspread footings, located about 40 m from the building in Fig. 6. Some flexural cracks devel-oped in the columns which were of poor quality. The building was leaning to one side, andthe owner put up shoring, out of common sense, to prevent it from collapse. However,he used timber planks instead of timber posts (obviously because of lack of resourcesdue to poverty). Furthermore, some braces were wrongly placed against the flimsy tim-ber walls (see Fig. 7a). Thus, dissemination of basic technical knowledge is important tobetter prepare laymen for damage reduction in future earthquakes.

In many cases, part of the open space is utilized for occupancy or for other purposes,usually with the utilized space enclosed by URM partitions. Such an enclosed space isrelatively rigid in comparison with the RC frames, and can create torsional irregularity ifplaced away from the center of mass. An example is shown in Fig. 8a. The newly builtone-story RC elevated house exhibited several types of damage. The 200 mm × 200 mmcolumns at the ground level were severely damaged. The masonry walls at the ground levelattracted a large seismic force and were mostly destroyed (Fig. 8b). The beam-columnjoints at corner columns witnessed joint distress (Fig. 8c). Severe shear failure with verticalbar buckling occurred due to the well-known short column effects where the masonry paneldid not fill all the way through the column height (Fig. 8d).

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FIGURE 6 A collapsed house in Mae Lao district, Chiang Rai province. The elevated firstfloor collapsed down to the ground.

FIGURE 7 Typical timber house near surface crack in Mae Lao district swayed due topermanent displacement: (a) shoring was attempted but not properly applied and (b) crackin low quality precast column.

Besides the traditional elevated house, many new single story houses are built withthe first floor resting on ground (Fig. 9). Columns are customarily made of precast concretewith a small cross section of 120 mm × 120 mm, usually not meeting proper standards evenfor gravity load requirement. These non-engineered columns have very little reinforcementand are not suitable for seismic-prone areas. Although the columns are not very strong, thesteel roof and cement tiles are rather lightweight. The masonry walls using hollow cement

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Performance of Structures in the Mw 6.1 Mae Lao Earthquake 9

FIGURE 8 (a) An elevated one-story reinforced concrete house in Dong-mada sub-district, Mae Lao district; (b) eccentrically placed URM infills and failure; (c) flexure failureof un-braced columns and damage in beam-column corner joint; and (d) shear failure inshort columns.

FIGURE 9 (a) A one-story house in Dong-mada sub-district, Mae Lao district built onground using precast concrete columns and steel roof truss; (b) partial out-of-plane collapseof masonry wall due to lack of anchorage to the columns.

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FIGURE 10 (a) A three-story RC building in Mae Lao district with soft first story and tor-sional irregularity; (b) shear failure of ground floor columns (courtesy of Police LieutenantColonel Anandech Yavichai).

blocks are popular. The prevailing damage on this type of house is partial out-of-plane col-lapse of masonry walls due to lacking of anchorage between the wall and column (Fig. 9).This failure mode caused the only fatality in Thailand for this earthquake event as theearthquake occurred during the day time. If it had occurred during night time, many morefatalities might have resulted as the walls would have collapsed onto sleeping residents.

Two- to three-story RC residential buildings mostly have non-seismically designedcolumns/beams and open space on the ground floor. Two of the collapsed buildings areshown in Figs. 10 and 11. The two-story building with an extended portion on top inFig. 10a was about 3 km from the epicenter reported by TMD. Besides the soft-storystructural irregularity, the building also had torsional irregularity due to the presence ofURM partitions at one end of the structure on the ground floor (left-hand side in Fig. 10a).The building was severely damaged and was on the verge of collapse on the day of themain shock with shear failure in the unrestrained interior columns. The perimeter columns,which were partially restrained by URM panels, also exhibited severe shear failure in theshort column portions adjacent to the cement block louvers (see Fig. 10b). It is noteworthythat all occupants managed to evacuate safely. On the next day after strong aftershocks (inthe order of Mw 5.0), the building totally collapsed.

To ascertain the cause of collapse of the three-story RC building in Fig. 11a, a sim-plified analysis was conducted with one bay of the structure modeled as a 2-D momentframe, as shown in Fig. 11d. The dimensions were estimated as close as possible to theactual condition. The frame was subjected to base excitation with a peak ground acceler-ation of 0.20 g. The ground floor columns were 200 mm × 200 mm reinforced with six12-mm diameter longitudinal bars and 6-mm diameter ties spaced at 200 mm. The beamswere rectangular in cross section, 200 mm wide and 400 mm deep. Field evidence revealedthat the columns failed at column ends without development of ductile failure modes (seeFig. 11c). The building was not designed nor detailed for seismicresistance. Hence, its

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Performance of Structures in the Mw 6.1 Mae Lao Earthquake 11

FIGURE 11 (a) Collapse of a three-story RC building with a soft first story under con-struction in Mae Lao district; (b) collapse of the first story; (c) failure of column; and (d)2-D structural model.

members could be put in the force-controlled category according to ASCE41-13 [ASCE,2014]. Following the seismic evaluation standard ASCE41-13, a demand-to-capacity ratioof 5.5 was obtained for flexural response and the corresponding value for shear responsewas 1.4 in the most critical column. Thus, the column was obviously not adequate forthe seismic demand. Note that the simplified analysis did not consider any torsional effectwhich actually existed in the building and, obviously, would increase the seismic demandon the members. In fact, for a newly designed building, a column size of 300 mm × 300 mmwith a longitudinal reinforcement ratio no less than 1% and 9 mm diameter ties at 150 mmspacing in the critical zones near beam-column joints is needed for the building to safelymeet the seismic demand with limited ductility.

4.2. School Buildings

School buildings in Thailand typically have a long rectangular floor plan with single-bayframes of 6–10 m span in the transverse direction and multi-bay frames of 4–4.5 m spansin the longitudinal direction. Most schools have 2–4 stories where the ground floor has alarge open space, creating a soft-story system. Unfortunately, many recently built schoolbuildings (including the ones reported in this article) have not been constructed with properseismic design even though the regulations call for design for a peak ground accelerationof about 0.15 g. Consequently, several buildings with a soft story system coupled withtorsional irregularities suffered significant damage, as evident in Fig. 12 which shows abuilding in Wat Muang Nga kindergarten school in Phan district that suffered moderate

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FIGURE 12 (a) A three-story RC school building (Building 2) with a soft first story cou-pled with torsional irregularity at Wat Muang Nga kindergarten school in Phan district,Chiang Rai province; (b) damage in soft story columns; (c) corner column most severelydamaged; (d) Building 1 with URM infilled RC frames at center of one side; and (e)Building 3 with well distributed URM panels.

structural damage.The three-story RC building (labeled Building 2) has an open space onthe ground floor, except the two end bays to the north (left side of Fig. 12a) of the buildingwhich accommodate a staircase and a restroom with URM partitions. Due to torsionaldeformation, the column farthest from the stairwell was under the most critical seismicdemand, resulting in crushing and splitting cracks at the bottom (see Fig. 12c). Altogether,14 columns were damaged to varying degrees, mainly in flexural mode.

It is interesting to note that two other RC buildings nearby, labeled as Buildings 1 and3 in Figs. 12d and 12e, respectively, exhibited much better performance with minor non-structural damage in URM infills. All buildings have approximately the same height and thesame span lengths between columns in both orthogonal directions. The structural designsof the main structural components are basically similar. Building 1, adjacent to Building2 with RC precast slabs connecting the two, has a stiff stairwell at the center on one side

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Performance of Structures in the Mw 6.1 Mae Lao Earthquake 13

FIGURE 13 (a) A four-story RC school building with a soft first story at Ban Don Tanschool in Phan district, Chiang Rai province; (b) URM infill in the end bay to the right; (c)damage condition of URM infill at stairwell; and (d) shear failure in column.

of the longitudinal direction. In addition, several rooms with URM partitions exist on theground floor. Building 3, opposite to the others, has well-distributed URM panels all aroundon the ground floor. Obviously, the contribution of well configured URM infills in Buildings1 and 3 greatly reduce the adverse effect of soft story and torsional irregularities.

The two RC school buildings in Figs. 13 and 14, located about 3 km apart in Phan dis-trict, were constructed using the same structural design in general, i.e., same dimensions,member sizes, and reinforcement. The difference is that the first one- Ban Don Tan schoolessentially has an open space with one end frame filled with a URM panel on the groundfloor (Fig. 13b), whereas the other one —Tesaban 1 school–does not have such an infill(Fig. 14b). Instead, the open space is utilized for different functions using light aluminumframed partitions. The RC frames with infills at the stairwell, placed near the opposite end,are practically the same. It should be noted that the stairwell, which is relatively stiffer thanthe individual frames, is located way off the center of mass of the structure, resulting in asevere torsional irregularity in the second building whereas the URM infill at the end bayof the first building contributes significantly to reducing the torsional irregularity. Contrastin performance is evident. The contribution of the URM infill panel mentioned led to lessseismic demand on the components in the first building. Consequently, the RC frames andthe URM infill panels of the stairwell remained much less damaged than the second build-ing in general (compare Fig. 13c with Fig. 14c). However, the two boundary columns ofthe URM infill panel on ground floor were severely damaged by the strut force exerted bythe large infill panel which is 10m long (Fig. 13d). These columns, being 300 mm wide ×500 mm deep with 9 mm diameter tie bars at 200 mm spacing, failed in shear as a result of

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FIGURE 14 (a) A four-story RC school building with a soft first story together with tor-sional irregularity at Tesaban 1 school in Phan district, Chiang Rai province; (b) open endbay to the right; (c) damage condition of URM infill at stairwell; and (d) severe damage ofcorner column.

short column effect following corner crushing of the infill. On the other hand, 8 columns inthe second building suffered moderate to severe damage, and one corner column (Fig. 14d)was stressed beyond Life Safety Performance Level.

Only one of the four major RC buildings at Phan Pittayakom school, the main schoolof the Phan district, was damaged due to this earthquake event. The reason is that theother buildings are shorter, three-stories or less, and the floors are made of timber planksinstead of concrete slab, so the seismic force would be much less than that in the damagedbuilding which is four stories tall with RC slabs. The damaged building (Fig. 15) essentiallyexhibited a soft first story with short column effect due to the fact that the 3 m ground floorcolumns were practically restrained by URM infill at the lower 1 m, and by the aluminumframing for fixed window panes at the top. Whereas the URM infilled RC framing of thetwo stairwells provided significant lateral force resistance in the transverse directions byvirtue of the orientation of the stiff components in that direction, the lateral force resistancein the longitudinal direction had to rely solely on the RC columns. Consequently, almostall ground floor columns, 300mm × 450 mm in cross section with 6mm round bar tiesspaced at 270 mm, severely failed in shear. Clearly, they were stressed beyond Life SafetyPerformance Level (Fig. 15b). The building was subsequently demolished.

Two buildings at Mae Lao Witayakom School, the main school of Mae Lao district,were heavily damaged (Fig. 16). These buildings did not have soft first story because the

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Performance of Structures in the Mw 6.1 Mae Lao Earthquake 15

FIGURE 15 (a) A four-story RC school building at Phan Pittayakom school in Phandistrict, Chiang Rai province; and (b) typical shear failure of ground floor columns.

FIGURE 16 (a), (b) Two of the damaged three-story RC school buildings at Mae LaoWittayakom school in Mae Lao district, Chiang Rai province.

space at the ground level was used for offices and classrooms with URM infills as partitions.Similar to the Phan Pittayakom school, columns partially restrained by URM infills overpart of the column height suffered shear failure caused by the short column effect. Onebeam-column joint of an end RC frame with URM infill panel was so severely damagedin shear caused by the huge strut force from the URM panel that the column was almostpulled out of the joint (Fig. 17a). Joint failure due to poorly constructed cold joint was alsoobserved at some beam-column joints (Fig. 17b).

It is noteworthy in passing that our field observations on contribution of masonryinfill to the performance of RC frames are in agreement with previous findings of sev-eral researchers. As Fardis and Panagiotakos [1997] said, “. . . with very few exceptions,the presence of infills is beneficial for the global seismic response and performance of thestructure”. Pujol and Fick [2010], based on their full-scale test of a 3-story RC building withinfills and some field evidence, suggested that masonry infill walls might be potentiallybeneficial, especially for regions where masonry is widely used. However, the possibil-ity of shear failure in columns and beam-column joints should be carefully considered indesign. An appropriate measure should be taken to avoid such brittle shear failure, e.g.,by employing the scheme proposed by Srechai and Lukkunaprasit [2013] whereby URM

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FIGURE 17 (a) Shear failure of beam-column joint induced by strut force from URMpanel; and (b) failure at poorly constructed joint.

panels are separated from the boundary columns and steel brackets are provided to transferthe interactive forces between the URM panel and the boundary beams.

Table 2 summarizes the structural details of the schools whose drawings are avail-able, including observed damage classified according to ASCE41-13. It is seen that thevolumetric ratio of tie reinforcement is less than 1%.

4.3. Hospitals

Two main hospitals in Phan and Mae Lao districts have remained operational after theearthquake incident, although there was panic and the patients had to be evacuated rightafter the main shock. All buildings are 1–2 stories high and are built without any seis-mic resistance provision except the newest building in Phan hospital which has beenconstructed to replace the one severely damaged by the 1994 Phan earthquake. The newbuilding, with proper seismic resistance provisions, performed satisfactorily without anydamage. For other buildings, the structural components were not damaged except for theseparation joints between the corridors and the main buildings. Since only small gaps ofabout 15 mm were generally provided, pounding between the corridors and the connectingbuildings occurred, causing damage such as that shown in Fig. 18. However, the damagewas only moderate at worst and very much localized without affecting the integrity of thestructures.

5. Observed Damage of Bridges

5.1. Pounding

Pounding damage was found in several bridges in Chiang Rai province. Figure 19 showscontact damage caused by pounding between a bridge cross beam and a pedestrian stair.The relative displacement between the cross beam in the bridge transverse direction andthe stair was larger than a gap of 20 mm provided. Hence, there was damage to concretesurface. Figure 20 shows the damage caused by pounding between adjacent spans of abridge crossing the Mae Lao river. The bridge has three 10-m long spans on the approach

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Performance of Structures in the Mw 6.1 Mae Lao Earthquake 17

TABLE 2 Structural details of schools surveyed

Details Fig. 12 Fig. 13 Fig. 14 Fig. 15

Number of Floors 3 4 4 4Roof Type Steel Steel Steel WoodBeam Span

(Longitudinal) [m]6 9.4 9.4 10.5

Beam Span (Transverse)[m]

4.5 4.5 4.5 4

First Floor Area [m2] 476 402 402 955.8Total Floor Area [m2] 1397 1883 1883 3648First Floor Column Area

[m2]2.3 3.2 3.2 5.2

Masonry Wall Area(Longitudinal) [m2]

− 7.2 7.2 −

Masonry Wall Area(Transverse) [m2]

17.1 26.8 26.8 38.0

Thickness of masonry[m]

0.10−0.12 0.10−0.12 0.10−0.12 0.15

First Floor Column -Average MainReinforcement Ratio[%]

3.38 2.16 2.16 2.34

First Floor Column -Average TieVolumetricReinforcement Ratio[%]

0.3 0.6 0.6 0.18

RC Damage Moderate Severe Severe SevereMasonry Wall Damage Moderate Severe Severe Severe

FIGURE 18 Pounding damage at Mae Lao hospital.

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FIGURE 19 Pounding of a pier and a staircase.

FIGURE 20 Pounding of bridge girders of a bridge crossing the Mae Lao river.

side and the inner spans have a span length of 20 m, as shown in Fig. 21. A finite elementmodel of the 10-m-span and 20-m-span segments was developed to determine the relativemovement between the girders. The ground motion shown in Fig. 3 was scaled to havean estimated maximum ground acceleration of 0.20 g. The relative movement of 35 mmis predicted from the analysis. Since the gap between girders is usually about 20–30 mm,pounding occurs between two adjacent girders. Since seat widths of about 200–300 mm areprovided, unseating is not likely to occur considering the relative displacement alone. But

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Performance of Structures in the Mw 6.1 Mae Lao Earthquake 19

FIGURE 21 Plan and elevation of the bridge crossing the Mae Lao river.

FIGURE 22 Damage to shear key block.

it is important to note that damage to columns or foundations can also lead to unseating ofsuperstructures.

5.2. Shear Dowels

The shear dowels are usually provided between a girder and a crossbeam to limit the move-ment of the girder. The girder end treated as a longitudinally fixed support is provided withshear dowels anchored to diaphragm beams. Since the force transferred from the girderis larger than the resistance of the shear dowel block, the damage occurs, as shown inFig. 22. Hence, sufficient reinforcement and confinement shall be provided to the sheardowel block.

6. Geotechnical Aspect

6.1. Liquefaction

Liquefaction was widely witnessed in the hardest hit districts. However, no severe dam-age to buildings was caused by liquefaction. Liquefaction was observed along the RongThan canal, a branch of the Mae Lao river in Phan district. Typical soils in this area are

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of a light grey, silty fine to coarse grain, loose to medium dense sand at depths of 2–8 mbelow the ground surface. The water table in this area is at around 1 m below the groundsurface. Based on four boring logs in nearby areas, the uncorrected SPT values of the sandlayers ranges from 4–31 blows/ft. The content of fine particles (F200) ranges between8–30%. Under a maximum ground acceleration of 0.20–0.25 g, the excess pore water ratioof 0.1–0.4 can be estimated according to the study by Teachavorasinskun et al. [2009].In other words, partial liquefaction can occur in this area.

Liquefaction was found in Sai Kao subdistrict, of Phan district, where a small villagewas populated along the Rong Than canal. The water table in the canal was relatively closeto the ground surface, therefore, a high ground water table could be expected. From the fieldsurvey, traces of liquefaction were observed in various areas. Sand boil from water wellswas also observed (Fig. 23). In the same area, there was a two-story house which lightlysettled due to liquefied ground (Fig. 24). Traces of sand were found beside the house wheresettlement occurred, leading to cracks in walls and slabs.

FIGURE 23 Muddy sand after bursting from a water well.

FIGURE 24 (a) Trace of liquefaction outside a house and (b) crack on building slab andwall due to liquefaction-induce subsidence.

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Performance of Structures in the Mw 6.1 Mae Lao Earthquake 21

6.2. Slopes and River Banks

Slopes along roads and rivers were also affected by the earthquake. Slope failures due tolateral acceleration occurred in some areas in Phan and Mae Lao districts. As most of failedslopes were man-made, they may be less consolidated than natural ground. It is also notedthat failed slopes are relatively high (height > 5 m) and steep (slope > 45◦). Small crackswere also observed in steep slopes along rivers and canals (Fig. 25). A bearing failureof a river bank was observed in Phan district (Fig. 26). Traces of sand were found alongthe shoreline of the river bank. Hence the failure may be due to the lateral spreading ofthe underlying sand layer. Cracks across roads were also observed in areas close to theepicenter.

Besides the slope failure and liquefaction, damage was observed in the approach ofbridges (Fig. 27). Dynamic lateral earth pressure and the difference in dynamic response ofthe bridge and embankment were considered as the causes of pounding between the bridgeand approach slab.

7. Power Supply Facilities

Power outage occurred for a couple of hours in the Mae Lao district. It is normal prac-tice in Thailand that the transformers supplying electricity to households are placed on

FIGURE 25 Slope failure.

FIGURE 26 A river bank failed by lateral spreading of underlying sand layer: (a) Settledarea; (b) Trace of sand on the free face.

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FIGURE 27 Failures at approach embankments.

FIGURE 28 (a) Un-anchored transformer, Mae Lao district, Chiang Rai; and (b) toppledtransformer (Courtesy of Mr. Assadakorn Ragsapainai).

cross-beams sitting on electricity poles without any anchorage to the supporting beams.Some transformers almost toppled (Fig. 28a) and some actually fell down to the ground(Fig. 28b).

The substation building, a two-story RC building in the Mae Lao district, was intact.However, some instrument cabinets were displaced horizontally by about 20 mm. A hugehigh voltage transformer outdoor, placed on grade without anchorage to the ground slabwas also displaced by about 60 mm (Fig. 29). This is a good indication of the severity ofthe ground shaking.

8. Concluding Remarks

As always, the Mae Lao earthquake has brought about valuable lessons as well as cluesfor rehabilitation and future design. Although several lessons have been well recognizedin a high seismicity region, they do re-iterate the fact that a poor structural system is alsovulnerable to damage under moderate hazard. Thus, buildings featuring poor structuralsystems, such as soft stories with small columns, torsional irregularity, and short columns

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Performance of Structures in the Mw 6.1 Mae Lao Earthquake 23

FIGURE 29 (a) The high voltage transformer at Mae Lao substation; and (b) evidence ofhorizontal displacement due to ground shaking.

are prone to be severely damaged. As reported in other events, poor detailing and sub-standard construction further aggravate the problem (e.g., Scawthorn, 2000; Kawashimaet al., 2010). Because such weak systems are abundant in poor villages, there is a need toretrofit them for safe occupancy which poses a big challenge with regard to effective andaffordable retrofit of these buildings.

Field evidence from this event has again confirmed the beneficial effect of URM infillin RC frames in enhancing performance of RC buildings, with which the collapse of somebuildings might have been deferred, thereby allowing safe evacuation. However, the possi-bility of shear failure in columns and beam-column joints should be carefully consideredin design.

Bridges in surveyed areas were not designed for seismic resistance. Damage mainlyoccurred at movement joints of superstructures. It is a general practice to provide sheardowels to allow limited movement between a pier and a girder supported by rubber bear-ings. The shear dowel could limit the relative displacement of the girders. However, theshear dowel blocks needs to be improved to have a sufficient capacity for transferring shearforces. Further study is needed to understand the interaction of the girder, rubber bearings,and shear dowels under seismic actions.

Acknowledgments

The authors would like to express their sincere thanks to Department of Public Works andTown & Country Planning, Department of Rural Roads, and Department of Highways forcoordination and support in the reconnaissance survey.

Funding

The prompt financial support from Chulalongkorn University is highly acknowledged.

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