INFILL-FRAME INTERACTION: REFINED MODELLING FOR THE ANALYSIS AND THE ESTIMATION OF THE INTERNAL FORCES IN SEISMIC ASSESSMENT OF RC BUILDING STRUCTURES

The paper deals with the assessment of the internal force distribution in frame members due to masonry infills interaction under seismic loads. Special interest is addressed to shear demand, which cannot be evaluated by the common equivalent strut models, while refined finite element modelling is not computationally effective to be used in practice. With the aim to maintain the simplicity of the equivalent strut approach for the assessment of existing RC structures, the paper presents a detailed study of the infill-frame shear transfer mechanism. Refined 2D nonlinear models of real experimental tests on infilled frames have been defined using the OpenSees/STKO software platform. Shear demands at the columns ends of different case study tests have been extracted by integrating the nodal forces at specific section cuts of the RC members. An analytical relationship is finally proposed to correct the shear demand at the ends of the columns when using the equivalent strut approach. The new formula relates the additional shear demand to the current axial force on the equivalent struts and the geometrical and mechanical properties of the infilled frames. The formula is thought to be easily used to perform step-by-step shear safety checks in seismic safety assessments of existing RC structures.