Seismic Vulnerability Assessment of Heritage Masonry Building Bal Mandir

Abstract

Heritage masonry buildings in Kathmandu Valley are highly vulnerable to earthquake damage because of their age, unreinforced masonry construction, weak mortar, irregular geometry, and limited seismic detailing. This study evaluates the seismic vulnerability of Bal Mandir, a historic Rana-era masonry building in Kathmandu, Nepal, using a 3 Dimensional macro finite element model (FEM) developed in ETABS v20. Material properties were obtained from relevant literature, and seismic loading was defined in accordance with NBC 105:2020. The building was analyzed using response spectrum analysis and linear time-history analysis with seven earthquake ground-motion records matched to the target response spectrum. The first-mode period obtained from the numerical model was 0.348 s, which was close to the empirical value recommended by NBC 105:2020. Fragility curves were developed using roof-displacement demand obtained from the time-history analyses and damage-state thresholds adopted from the FEMA HAZUS-MH methodology for the URMM (Unreinforced Masonry, Mid-Rise) building class. At the Kathmandu design PGA of 0.35 g, corresponding to a 475-year return period, the probabilities of exceedance for Slight, Moderate, Extensive, and Complete damage were estimated as 100%, 99.32%, 84.03%, and 55.17%, respectively. The results indicate a high likelihood of severe damage under the design-level earthquake and highlight the need for appropriate seismic strengthening and conservation measures for Bal Mandir. The assessment is based on linear structural analysis and generalized HAZUS damage thresholds and should therefore be interpreted as a global vulnerability estimate rather than a detailed prediction of local nonlinear damage mechanisms.