Height-Dependent Seismic Vulnerability of Masonry Buildings: Validation of Fragility Screening Equations Using Post-Earthquake Data

Abstract

The 2019 Mw 6.4 Durrës earthquake in Albania caused 51 fatalities due to the collapse of unreinforced masonry (URM) structures and revealed a pronounced height-dependent vulnerability pattern not captured by conventional single-curve fragility models. This study presents a statistically validated, height-stratified fragility framework for URM buildings, demonstrating the need to explicitly account for building height as a primary fragility parameter and quantifying the prediction errors incurred when it is neglected. The framework is developed using 307 laboratory material characterization tests and a post-earthquake dataset of 187 structures. Its validity is assessed through post-earthquake field observations, equivalent frame modeling in TREMURI, and incremental dynamic analysis (IDA) of five height-based archetypes subjected to a suite of 22 ground motion records. Results show that increasing building height significantly reduces normalized lateral strength (Vmax/W = 0.584 − 0.072N; R² = 0.89, RMSE = 0.023, p < 0.001), while displacement demand increases nonlinearly with height (δ ∝H¹.⁸²). A five-story building exhibits approximately 56% lower strength and up to 16 times higher displacement demand compared to a single-story structure. Drift concentration at the ground floor (CFdrift = 1.32 + 0.46N) promotes soft-story collapse mechanisms, with drift amplification increasing from 1.68 to 2.50 between one- and five-story buildings, highlighting the limitations of pushover analysis for N ≥ 4. The probability of collapse under the design earthquake ranges from less than 1% for single-story buildings to 45% for five-story structures. The proposed height-dependent screening criteria reduce damage-grade prediction error (RMSE) by 42–65% compared to three state-of-the-art methods, demonstrating improved predictive performance.