Seismic Assessment of Thin Masonry-Infilled RC Frames via a Coupled In-Plane/Out-of-Plane Macro-Model

Thin clay masonry infills are commonly employed in reinforced concrete (RC) frame buildings across the globe. Despite their widespread use, these elements exhibit a brittle response and engage in complex in-plane (IP) and out-of-plane (OOP) interaction under seismic loading—often leading to a significant increase in the structural vulnerability of infilled frames. Accurately representing this coupled behaviour remains a key challenge for seismic assessment and design. To address this, a new macro-modelling strategy is developed and implemented within the OpenSees-STKO framework. The proposed model aims to simulate the seismic response of infill panels by explicitly accounting for the interplay between IP and OOP actions. The infill is represented by four diagonal struts—two per diagonal—combined with a pair of lumped masses placed at the centre of the panel to reproduce inertial OOP demands. Each strut is divided into two beam-column elements, with a mid-span plastic hinge that governs the nonlinear coupling effects through a fibre-based section formulation. Model calibration and validation are carried out using a comprehensive set of experimental results on both unreinforced and retrofitted thin clay masonry panels. Once validated, the model is applied in a series of nonlinear time-history analyses as part of a parametric investigation involving different infill typologies and strengthening strategies. The outcomes highlight the model’s capability to replicate critical IP/OOP interaction mechanisms, while also demonstrating the effectiveness of retrofitting solutions in mitigating adverse seismic effects.