Modeled Seismic Interaction Alters Fragility of Adjacent Masonry Building Clusters

A computational study of low-rise unreinforced masonry buildings found that seismic interaction among adjacent units can protect a weaker building while making a stronger neighbor more fragile. The model also showed that failure could begin in the most vulnerable unit and develop into progressive collapse affecting adjacent structures that had remained stable. These results indicate that estimating the seismic fragility of such buildings as isolated structures can produce unrealistic demands and fragility estimates, particularly when neighboring units differ in strength and stiffness.

Unreinforced masonry buildings are prevalent in historic urban centers and often form clusters of independent structural units built directly next to one another. Conventional seismic performance assessments commonly analyze each building as though it were isolated, overlooking the interaction that can occur between neighboring structures. The research examined this coupling by modeling three distinct unreinforced masonry archetypes and arranging them into seven clusters composed of either two or three units. The configurations made it possible to evaluate the influence of each building’s position and the differences in strength and stiffness between adjacent structures.

To represent the masonry structures, the researchers used a three-dimensional equivalent-frame approach capable of capturing both in-plane and out-of-plane wall behavior. They first developed fragility curves for each archetype when treated as an isolated structural unit. Those curves were derived from nonlinear response-history analyses using a suite of actual ground motions and the multiple stripe analysis methodology. The researchers then constructed fragility curves for buildings located within the two- and three-unit clusters and compared them with the corresponding isolated estimates. This comparison quantified how the modeled seismic interaction changed the estimated likelihood of different damage states across the range of ground-motion inputs represented in the analyses.

The effects depended on the configuration rather than following a single uniform pattern. Units with lower in-plane stiffness and lower strength capacity could benefit from the support of stronger neighbors. That same interaction increased the fragility of the stronger buildings, and the increase became more pronounced as the stiffness disparity between neighboring units grew. Position also influenced the response of individual buildings, but the modeled collapse sequence consistently began with the most vulnerable unit, regardless of whether it occupied an end or interior position. Failure of that unit could trigger progressive collapse that compromised adjacent, otherwise stable structures, showing that a building’s properties and position alone did not determine its behavior during shaking.

The findings are numerical estimates for the selected low-rise masonry archetypes and cluster arrangements, not direct observations of damage following an earthquake. Within the scope of the models, however, the results provide quantitative evidence that assessing adjacent unreinforced masonry buildings in isolation is unconservative. The authors conclude that their seismic fragility cannot be adequately quantified in isolation and that realistic performance assessments for historic urban centers require cluster-level models that explicitly represent configuration, differences in structural properties and mutual interaction. The study thus shifts the assessment from a set of unrelated individual buildings to a system in which the seismic response and possible collapse sequence of one unit can affect its neighbors.

Subject of Research: Modeled seismic fragility of interacting low-rise unreinforced masonry building clusters

Article Title: Impact of pounding interaction on the seismic fragility of low-rise adjacent unreinforced masonry buildings

Article References: Gallardo, J. A., Tomić, I., Palazzi, N. C., de la Llera, J. C., & Beyer, K. (2026). Impact of pounding interaction on the seismic fragility of low-rise adjacent unreinforced masonry buildings. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02722-3

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02722-3

Keywords: unreinforced masonry, seismic fragility, building clusters, pounding interaction, progressive collapse, earthquake engineering, structural dynamics, nonlinear analysis, masonry walls, ground motions, Impact, pounding