Clay masonry-infilled reinforced concrete (RC) frame structures are a widespread construction practice globally. Nonetheless, the brittle behavior of these infill panels, along with the complex interplay between their in-plane (IP) and out-of-plane (OOP) responses, can markedly increase a building’s seismic vulnerability influencing both local and global structural performance. To address this issue, there is a pressing need for reliable modelling tools capable of capturing the coupled IP/OOP behavior of infills under seismic loading. In pursuit of this aim, this paper presents a macro-model implemented within the OpenSees-STKO environment to simulate the IP/OOP response of infill walls. The model comprises four diagonal struts—two per diagonal—and two concentrated OOP masses positioned at the panel’s center. Each strut is discretized using two beam-column elements, enabling the formation of a plastic hinge at mid-span. This hinge concentrates the nonlinear behavior due to IP/OOP interaction, represented using a fiber-section model.

The model has been calibrated and validated against a series of experimental studies involving both unreinforced and strengthened thin clay masonry infill panels. The model is then employed in a series of non-linear time history analyses within a broader parametric study investigating different thin infilled frame configurations. Results confirm the model’s effectiveness in capturing complex interaction mechanisms and highlight the positive impact of the strengthening techniques in mitigating adverse IP/OOP coupling effects.