Calibration of a Multi-strut Macro Model for Seismic Analysis of Infilled Reinforced Concrete Frames

Reinforced concrete (RC) frames often incorporate unreinforced masonry infills which are composed of either solid or hollow clay bricks or concrete units and have been widely used for many decades around the world. Researchers have discovered that masonry infill enclosed in RC frames has a considerable stiffening and strengthening effect, which can be beneficial or adverse in the case of seismic action and influences global displacement capacity. Although numerous experimental and numerical studies have shown in different literature that masonry infills modify the behaviour of framed structures under lateral loads, the contribution of panels is generally neglected in the common structural analysis due to modeling complexity. However, internal force modifications caused by infill-frame interaction may be incompatible with the strength of surrounding frame elements, especially when additional shear forces arise at the end of beams and columns leading to unexpected brittle failures. Micro-modeling and macro-modeling techniques are used for the finite element analysis of infilled frame structure. Different codes and regulations enforce the single equivalent diagonal compression strut approach to consider the elastic in-plane stiffness of the masonry panel. However, the additional shear demand due to the frame-infill interaction cannot be evaluated using standard equivalent strut models, and the micro-modeling approach is too computationally demanding to be employed in practice. This leaves open the issue of determining the additional shear on columns and, as a result, selecting the appropriate eccentricity for the diagonal struts. This study aims to calibrate an equivalent three-struts macro-model and validate outcomes with refined micro-model and experimental data from the various literature to reach acceptable results both in global and local analyses. The outcome of this calibration study will determine the equivalent width of each strut and the position of the non-diagonal strut for maximum shear demands at the column ends of various case study tests. A genetic algorithm is used to find the optimum struts position to optimize both local and global response. When conducting seismic assessments of existing RC structures for practical applications, the findings of this research study can be conveniently applied to perform shear safety checks at the column's ends.