Mechanisms of Seismic Failure in Multistory Masonry and Infilled Frame Buildings: Field Investigation and Numerical Validation from the 2022 Luding Earthquake

Multi-story buildings in seismic regions are susceptible to earthquake-induced damage; however, the direct correlation between observed damage patterns and underlying failure mechanisms remains insufficiently understood. The Ms6.8 Luding earthquake, which struck Luding County, Sichuan Province, China, in September 2022, offers a unique opportunity to investigate this relationship, as it affected a concentrated area with diverse building types and preserved a wide range of damage states. This study leverages the distinctive conditions of the Luding earthquake to elucidate the influence of wall element distribution on structural failure modes under seismic loading. To elucidate the underlying mechanisms, three representative buildings were analyzed using a one-dimensional numerical model. The simulations yielded shear force distributions, shear ratios, and displacement ratios across structural components, enabling a detailed assessment of failure modes. The results indicate that torsion-dominated structures are susceptible to premature failure of low-stiffness components due to excessive displacement, whereas high-stiffness components generally remain intact owing to their ductility. In contrast, translation-dominated structures fail when high-stiffness components fracture at small displacements, resulting in global collapse without substantial ductility or load-bearing contribution from other elements. Structures that remained undamaged exhibited a relatively uniform stiffness distribution, enabling them to resist seismic forces primarily through overall capacity rather than ductility. The numerical results closely reproduced the observed damage patterns, thus validating the proposed mechanisms for the three structural categories. These findings contribute to a deeper understanding of seismic damage processes and provide a basis for enhancing seismic design and retrofitting strategies for both new and existing structures.