Assessment of dynamic characteristics and seismic vulnerability of a six-storey reinforced concrete building in Armenia using the Nakamura method
Abstract
This study addresses the seismic vulnerability of a mid-twentieth-century reinforced concrete building in the Republic of Armenia, a region of high seismic hazard. The structure was erected prior to modern seismic design codes and has undergone structural modifications during service, leading to changes in its dynamic characteristics and a reduction in load-bearing capacity. Determining the actual dynamic parameters of existing buildings is a key task in seismic risk mitigation and the rationale for retrofitting. The aim is to quantitatively assess the dynamic characteristics and seismic vulnerability of a six-storey reinforced concrete building constructed in the 1960s through integral stiffness indicators, natural periods of vibration, and vulnerability indices. Methods: multi-point recording of microseismic vibrations was performed at representative points within the building. Time series were processed using the horizontal-to-vertical spectral ratio (HVSR) method following the Nakamura technique, identifying the principal natural periods and mode shapes of the load-bearing system. Vulnerability coefficients were then computed for individual structural zones. The approach provides non-destructive diagnostics of stiffness changes induced by operational loads and reconstruction interventions. Results revealed pronounced heterogeneity in stiffness distribution along the height and in plan, with reducedstiffness zones localised in specific sections. The vulnerability indices indicate increased susceptibility of these zones to seismic actions, including resonant amplification when the natural frequencies of the structure coincide with dominant excitation frequencies. The data confirm the effectiveness of HVSR for rapid diagnosis of dynamic characteristics of reinforced concrete buildings and assessment of seismic resistance. Recommendations are formulated for local strengthening of critical zones and continuous monitoring to minimise seismic risk and ensure safe operation.
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