Assist. Prof., Faculty of Engineering, Hokkaido University, Japan 3

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1 Field Investigation in affected area due to the 2015 Nepal Earthquake by AIJ reconnaissance team: Damage assessment and seismic capacity evaluation of buildings in Gongabu, Kathmandu Yo HIBINO 1, Naoki ONISHI 2, Akihiro NAKAMURA 3 and Yusuke MAIDA 4 1 Assoc. Prof., Graduate School of Engineering, Hiroshima University, Japan hibino@hiroshima-u.ac.jp 2 Assist. Prof., Faculty of Engineering, Hokkaido University, Japan 3 Assist. Prof., Graduate School of Environmental Studies, Nagoya University, Japan 4 Assist. Prof., Graduate School of Engineering, Chiba University, Japan ABSTRACT During the 2015 Mw 7.8 and Mw 7.3 Nepal earthquakes, reinforced concrete and masonry buildings were significantly damaged. After the event, Architectural Institute of Japan (AIJ) dispatched several reconnaissance teams to the affected area to conduct field investigation. The team of authors conducted field investigation in Gongabu, Kathmandu to collect information with respect to damage of buildings in Nepal to assess the damage of buildings and to predict seismic capacity of buildings in Nepal. Most of the severely damaged buildings, have similar structural details: reinforcement arrangements; anchorage; and joint, except difference of number of floors, dimensions and use application, were collapsed due to soft first story mechanism formed. According to damage classification of buildings, severely damaged buildings were concentrated in some small areas. The mechanism of soft first story in one of five story reinforced concrete buildings with non-structural brick walls was investigated by structural strength considering failure mechanism with Japanese structural calculation method and inelastic response analysis of members. The predicted failure mechanism was almost agreed with observation and seismic capacity of the building was predicted. Keywords: Seismic capacity, Reinforced concrete buildings, Masonry buildings, Damage evaluation, Failure mechanism 1. INTRODUCTION On 25 April 2015, Mw 7.8 earthquake occurred in the central region of Nepal and Mw 7.3 aftershock occurred on 12 May. During the earthquakes, more than eight thousand people were killed, 20 thousand people were injured, and damaged buildings spreaded throughout widespread areas around Kathmandu. After the event, a reconnaissance team organized by the Architectural Institute of Japan (AIJ) was dispatched to the affected areas to conduct field investigation. The team consisted of some professors and the team was divided into some groups according to their objectives. They specialize in reinforced concrete, masonry structures, earthquakes and ground, and these investigations were conducted under the support of many professors, local government officials and engineers in Nepal.

2 October 2015, Kathmandu, Nepal The team of authors conducted field investigation in Kathmandu to collect information, which helps understanding of the cause of damage to reinforced concrete buildings in Kathmandu due to the earthquakes. The objective of this study is to evaluate seismic performance of damaged buildings and find the cause of damage to buildings. 2. DAMAGE TO REINFORCED CONCRETE AND MASONRY BUILDINGS 2.1 Investigation and the area The authors investigated buildings in Gongabu, which is in the northwestern part of Kathmandu to collect information about damage to reinforced concrete and masonry buildings built with the local construction methods. In this area, most of low-rise buildings and houses did not suffer damage, nevertheless several mid-rise reinforced concrete and masonry buildings collapsed and suffered severe damage. Figure 1 shows map of Gongabu area. There are many buildings in the north area of the Ring Road and almost all of the buildings were non-structurally joined with each other. The red symbols indicate locations of damaged buildings which detailed structure was investigated. Figure 1: Investigated area and buildings in Gogabu 2.2 Structural details of damaged reinforced concrete buildings Figure 2 shows typical damage to reinforced concrete buildings in Gongabu. Damage pattern of suffered buildings were very similar and moreover, almost all of collapsed reinforced concrete buildings formed the soft first story mechanism. These buildings had similar stories: four and five stories, structural details in geometry of member, reinforcement arrangement, anchorage, and beam-column joint. The cross-sectional geometry of column was 9 in. (225 mm) x 12 in. (300 mm), the eight longitudinal deformed steel bars enclosed by hoops with 150 mm pitch spacing were placed. Longitudinal reinforcements of beam were not anchored in the core of the cross section of column at beam-column joint as shown in Figure 3, which may yield beam-column joint failure. New Technologies for Urban Safety of Mega Cities in Asia

3 Figure 2: Collapsed building Figure 3: Damage of beam-column joint 3. DAMAGE CLASSIFICATION In this investigation, damage classification based on European Macroseismic Scale (EMS-98) (Grünthal, 1998) were carried out for more than a thousand (1289) buildings in Gongabu. Damage to reinforced concrete and masonry buildings were classified into five levels according to this method. The investigated buildings and the damage grade were shown in Figure 4. The grades were defined according to damage level as shown in Figure 4. It appeared that the location of buildings which grade is more than Grade 3 (G3) concentrated in some small areas, which may indicate the effect of difference of ground characteristics. Figure 5 shows the ratio for reinforced concrete and masonry buildings. Total of the number of Grade 2 (G2) to Grade 5 (G5) for masonry buildings reaches 13 % of all the buildings (See Figure 5(a)), whereas for reinforced concrete buildings total of the number of those reaches 7 % (See Figure 5(b)). This means that the number of damaged reinforced concrete buildings is not small in relation to that of masonry buildings. Figure 4: Investigated buildings and damage level Field Investigation in affected area due to the 2015 Nepal Earthquake by AIJ reconnaissance team: Damage assessment and seismic capacity evaluation of buildings in Gongabu, Kathmandu

4 October 2015, Kathmandu, Nepal G5 (5%) G4 (5%) G3 (3%) G2 (12%) G5 G4 (3%)(2%) G3 (2%) G2 (8%) G1 (75%) (a) G1 (85%) (b) Figure 5: Damage classification of buildings: (a) Masonry; (b) Reinforced concrete 4. DETAILED INESTIGATION 4.1 Seismic capacity A reinforced concrete building, which had five and a half story forming the soft first story mechanism as shown in Figure 6, was selected to examine the seismic capacity and the cause of the failure mechanism. Figure 7 shows damage inside of the first floor, where the columns at the front of the building and two columns failed in shear due to be shortened by walls as shown in Figure 7. All the other columns on the first floor shows flexural yielding. Wide horizontal crack was observed on non-structural brick walls in the longitudinal direction on the first floor, however, diagonal cracks were observed on that in transverse direction, which indicates that response in the transverse direction dominated. The failure mode of columns was predicted by the ultimate strength with Japanese structural calculation method (JBDPA, 2005) using investigated structural properties and the mode agreed with observation. Additionally, base shear coefficient of was predicted as 0.25 assuming unit weight of 12 kn/m 2 on each floor, concrete strength of 40 MPa obtained by rebound hammer test and yield strength of steel of 415 MPa. Figure 6: Investigated building New Technologies for Urban Safety of Mega Cities in Asia

5 Figure 7: Damage of the first floor 4.2 Damage assessment Figure 8: Shear failure of shortened column The damage level of building was assessed according to the Japanese Damage Classification Method (JBDPA, 1991). The method classifies all columns into five damage levels, then calculates the ratio of residual seismic capacity to the original one. Figure 9 shows the damage level of each column on the first floor for both directions. ery heavy damaged columns which label is are observed on the west and north east corner on the floor. The ratio of residual seismic capacity was calculated as 37.5 % and 38.9 % in the longitudinal and transverse direction, respectively, and damage of this building was classified as destruction for both directions, which almost agreed with observation. Field Investigation in affected area due to the 2015 Nepal Earthquake by AIJ reconnaissance team: Damage assessment and seismic capacity evaluation of buildings in Gongabu, Kathmandu

6 October 2015, Kathmandu, Nepal Transverse direction Y1 Y2 Y X1 X2 X3 X4 X5 X6 Lomgitudinal direction Unit: mm 4.3 Analytical investigation Figure 9: Damage level of columns on the first floor To investigate collapse mechanism of aforementioned building, nonlinear static and dynamic response analyses were carried out. Three-dimensional frame constituted by beam element with rotational boundary springs, multi-spring model of column and three vertical line element wall model were used. Behavior of spandrel and breast walls on response of building were represented by shortening of column length. The hysteretic models of both beam and column springs and non-structural brick wall were assumed by Takeda model (Takeda, 1970) and bilinear with slip model (Sanada, 2009), respectively. Tangent stiffness-proportional damping of 3 % was assumed. The material properties was set to follow test results as discussed but the yield strength of steel of 490 MPa were used due to limitations of analytical software. Total weight of the building was assumed as kn assuming relative density of non-structural brick walls of Figure 10(a) shows relationships between story shear and interstory drift by pushover analysis, which terminated when the interstory drift reaches 2 %. The first and second floor s drifts dominate and the maximum base shear coefficient is 0.2. The base shear coefficient is smaller than the predicted value by assuming ultimate strength of columns but the soft story mechanism agreed with observation. Figure 10(b) shows story drifts obtained by dynamic response analysis for each direction with record at Kanti Path station (CESMD, 2015). It is observed that the story drift of the first story in the northsouth (NS) direction is dominant. Figure 11 shows failure mode of members obtained by dynamic response analysis under NS record. Most of columns suffered flexural failure at the top and bottom on the first floor and the failure mode and the locations almost agreed with investigated result. New Technologies for Urban Safety of Mega Cities in Asia

7 Story Shear (kn) th floor 5th floor 3rd floor 1st floor 2nd floor Floor NS EW 0 6th floor (a) Drift (%) (b) Drift (%) Figure 10: Pushover and dynamic analysis result: (a) Relationship story shear verses interstory drift; (b) Interstory drift distribution Shear failure Shear crack Flexural failure Flexural crack Figure 11: Failure mode of members 5. CONCLUSIONS The field investigation by AIJ reconnaissance team to assess damage of reinforced concrete and masonry buildings in Gongabu, Kathmandu was conducted. Damage classification and assessment for buildings were carried out. The main findings are summarized as follows: 1) number of the damaged buildings was investigated and the number for all buildings for reinforced concrete buildings was not smaller than that of masonry buildings; 2) base shear coefficient of five and a half story reinforced concrete building forming the soft first story mechanism was assumed as 0.25; 3) the ratio of residual seismic capacity became about 40 %, which classified as collapse and agreed with the observation; 3) analytical results almost agreed with observation in terms of base shear coefficient, failure mode of members, and failure mechanism of investigated building. Field Investigation in affected area due to the 2015 Nepal Earthquake by AIJ reconnaissance team: Damage assessment and seismic capacity evaluation of buildings in Gongabu, Kathmandu

8 October 2015, Kathmandu, Nepal SYMPATHY NOTE AND ACKNOWLEDGEMENT The investigation members would like to express deepest sympathy to all victims. The investigation was organized by Architectural Institute of Japan. We appreciate Prof. Toshimi Kabeyasawa, the chair of research committee on earthquake disaster for his arrangements. The Japan Building Disaster Prevention Association also financially supported the investigation. Dr. Prakirna Tuladhar, Senior Divisional Engineer, Department of Urban Development and Building and Dr. Krishna Bhetwal, Assistant Professor, Tribhuvan University with his colleagues kindly contributed our onsite investigation. REFERENCES Center for Engineering Strong Motion Data (CESMD), Cooperative effort of USGS, CSG and ANSS, Retrieved from Grünthal, G. (ed.), European Macroseismic Scale 1998, Cahiers du Centre Européen de Géodynamique et de Séismologie, Luxembourg, ol.15. Sanada Y., Tanjung J. and Choi H, Experimental evaluation of the seismic performance of nonstructural brick walls used in developing countries: findings from damaged buildings due to the 2007 Sumatra, Indonesia earthquakes, Journal of Housing Research Foundation, No.36, pp (in Japanese) Takeda, T., Sozen, M. A., and Nelsen, N. N., Reinforced concrete response to simulated earthquakes, ASCE Journal of the Structural Division, ol.96, No.ST12, pp The Japan Building Disaster Prevention Association (JBDPA), Guideline for Post-earthquake Damage Evaluation and Rehabilitation. (in Japanese) The Japan Building Disaster Prevention Association (JBDPA), Seismic Evaluation and Retrofit. New Technologies for Urban Safety of Mega Cities in Asia