Seismic Vulnerability Index for Low-Rise Composite Reinforced Concrete and Masonry Buildings in Nepal

Abstract eng:
Three- to five-storey high reinforced concrete (RC) frames with brick masonry infill walls have been predominantly used for housing construction in urban areas of Nepal since the late 1980s. These buildings are typically constructed without detailed engineering input during design and construction. Several buildings of this type were damaged and/or collapsed in the April 25, 2015 Gorkha earthquake (M 7.8), even in areas characterized with moderate and low shaking intensity such as Kathmandu Valley (MMI intensity of VI and VII). Due to inadequate column size, amount of reinforcement, and an absence of seismic detailing, RC components were not effective in resisting seismic loads. As a result, these buildings behaved essentially like shear wall structures and their lateral load resistance depended mostly on the shear capacity of unreinforced brick masonry walls. This building typology will be referred to as "composite RC and masonry buildings" in the paper. The paper describes the results of a study of 98 buildings of this typology at three different sites that were affected by the Gorkha (Nepal) earthquake. The main objective of the study was to correlate the observed damage grades for individual buildings with the corresponding wall density index, that is, a ratio of the sum of cross-sectional areas of all walls in each major direction of the building plan and the total floor plan area. The damage classification was based on the European Macroseismic Scale (EMS-98) with some modifications. The data collection was performed using electronic survey tools developed for the Global Earthquake Model (GEM). Each building was characterized by its location (latitude and longitude), and 13 attributes from the GEM Building Taxonomy describing details of the lateral load-resisting system, prevalent construction materials, building height, shape of the building plan, type of floor/roof system etc. In addition, earthquake damage photographs and floor plans with relevant dimensions were mapped for each building. This study builds on the approach taken in previous studies from Chile which used wall density as a seismic vulnerability index for masonry buildings. The results presented in this paper may be relevant for assessing seismic vulnerability of existing RC buildings.

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