Seismic Evaluation of the Experimentally Calibrated Multi-tiered Historical Masonry Structure

Abstract

Historical structures in seismically active regions endure recurrent seismic events, posing significant challenges for their long-term preservation. Despite high seismic vulnerability, the dynamic properties of brick-mud mortar pagoda structures on soft soil remain largely unexplored through ambient vibration testing (AVT). This study presents a seismic evaluation of the Bhairabnath temple through an integrated AVT and nonlinear dynamic analysis framework. AVT identified fundamental frequencies of 1.58 Hz and 2.5 Hz in predominant directions, revealing inherent directional asymmetry. The identified frequencies were used to calibrate the finite element model through iterative updating, converging on the masonry material properties. Nine near-fault records with velocity pulses, spectrally matched to site-specific spectra, were applied in nonlinear time history analyses along both principal directions. Results revealed pronounced directional vulnerability, with the East-to-West (EW) direction exhibiting larger displacement demand and lower secant-to-elastic stiffness ratios than the North-to-South (NS) direction. The wide hysteresis loops and nearly twice the cumulative hysteretic energy dissipation in the EW direction indicate that it absorbed seismic demand primarily through inelastic damage rather than elastic resistance. Damage patterns confirmed tensile cracking as the dominant mechanism, concentrated mostly at the tapering upper section due to a disproportionate stiffness-to-mass ratio, at the fifth-floor openings caused by geometric stress concentration, and at the base corners due to cumulative overturning effects. These findings highlight the directionally asymmetric response of pagoda-style structures, providing a foundation for retrofitting strategies that prioritize direction-specific vulnerability alongside heritage conservation guidelines.