Chapter 5 Structural Assessment of a School Building in Sankhu, Nepal Damaged Due to Torsional Response During the 2015 Gorkha Earthquake Supratik Bose, Amin Nozari, Mohammad Ebrahim Mohammadi, Andreas Stavridis, Moaveni Babak, Richard Wood, Dan Gillins, and Andre Barbosa Abstract This paper discusses the structural assessment of a red-tagged four-story school building in Sankhu, Nepal. The building had a masonry-infilled reinforced concrete frame which was severely damaged during the 2015 Gorkha Earthquake. The concentration of damage in the west end of the first story indicates that the frame exhibited torsional response to the ground excitation. The authors visited the structure 2 months after the earthquake, collected LiDAR scans, and recorded the ambient vibrations of the damaged structure. The LiDAR data has been used to create a three-dimensional point cloud of the building which has allowed the identification of the locations and geometry of the major cracks but also the measurement of the permanent deformations of the building. The structure was also instrumented with four unidirectional accelerometers on every floor; two at opposite corners, to capture the translational and torsional motion. The translational and torsional modes have been identified with an operational modal analysis method and have been used to validate a finite element model of the structure. The comparison indicates that the model can capture the modal properties of the structure utilizing the strut modeling approach for the infill panels. Keywords Infilled RC frame • 2015 Nepal Earthquake • LiDAR scan • Stochastic subspace identification method • Finite element model 5.1 Introduction On April 25, 2015, a devastating, 7.8 Mw, shallow earthquake with a focal depth of 15 km [1] struck Nepal. In Kathmandu, which is at a distance of 82 km from the epicenter, the ground motion shown in Fig. 5.1 was recorded at the KATNP station (27.7 N, 85.3 E). As it can be seen in the figure, the horizontal peak ground acceleration (PGA) in the North–South direction, during the main-shock was equal to 0.164 g. The response spectra of the three components of the recorded ground motion are illustrated in Fig. 5.2a, where it can be observed that in the short-period range which is typical for low-rise infilled reinforced concrete (RC) frames, the spectral acceleration exceeded 0.6 g, while unusually large spectral amplification was observed in the long-period range (3–6 s). The isoseismal map presented in Fig. 5.2b indicates widespread damage with a maximum magnitude of IX in Mercalli scale. The earthquake was followed by 400 after-shocks of magnitudes larger than 4.0 Mw, including two of magnitudes of 6.6 Mw and 7.3Mw and epicenters in the districts of Gorkha and Dolakha, respectively. The seismic sequence caused more than 9000 fatalities, almost 25,000 injuries and damaged beyond repair over 500,000 buildings [1, 4]. Between June 5 and July 8, 2015 the authors, along with other researchers from the US, Italy, Portugal and Nepal visited the affected regions to conduct rapid and detailed damage assessment of buildings. Part of the goal of this visit was to obtain S. Bose • A. Stavridis ( ) Department of Civil, Structural and Environmental Engineering, University at Buffalo, Buffalo, NY, USA e-mail: astavrid@buffalo.edu A. Nozari • M. Babak Department of Civil and Environmental Engineering, Tufts University, Medford, MA, USA M.E. Mohammadi • R. Wood Department of Civil Engineering, University of Nebraska Lincoln, Lincoln, NE, USA D. Gillins • A. Barbosa Department of Civil and Construction Engineering, Oregon State University, Corvallis, OR, USA © The Society for Experimental Mechanics, Inc. 2016 S. Pakzad, C. Juan (eds.), Dynamics of Civil Structures, Volume 2, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-29751-4_5 31
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