Non-ductile Reinforced Concrete Frames (RCF) with and without Unreinforced Masonry (URM) infills can be found in many places around the world including the Western United States, Eastern Europe, Asia and Latin America. These structures can have an unsatisfactory seismic performance which may even lead to collapse due to brittle failure modes. Furthermore, the effect of the infills on the seismic response of the structural system is not always accounted for in analysis and design. At present, there is no consensus on whether masonry infills are beneficial (by increasing the resistance of the system) or detrimental (by leading to brittle failure modes) for RCF construction.

This study focuses on the development of a simplified modeling approach for non-ductile RCF with URMI that combines the simplicity of strut-and-tie models with the accuracy of Nonlinear Finite Element Analysis (NLFEA). Despite the fact that NLFEA procedures are the most advanced way to address the structural analysis of RCF with URM infills, their conceptual complexity and computational cost may hinder their widespread adoption as an analysis and design tool. At the same time, simplified methods, such as those based on the equivalent strut concept, may be overly crude and neglect essential aspects of the nonlinear response. To address the need for an adequately accurate, but computationally and conceptually efficient analysis method, this study establishes a novel method for planar RCF with URM infills subjected to lateral loads. The method, which is based on the Nonlinear Truss Analogy (NLTA) is shown to have an accuracy comparable to that of NLFEA. Specifically, the method is shown to adequately capture the strength and stiffness degradation and the damage patterns while entailing a reduced computational cost (compared to that of NLFEA). The proposed method is expected to bridge the gap between overly crude equivalent strut models and computationally expensive NLFEA.