Structural identification of the dynamic behavior of floor diaphragms in existing buildings

The deformability of floor diaphragms plays a primary role in the structural behavior of existing buildings. Nonetheless, few structural identification procedures are available to investigate this matter from in-situ experimental measurements. Ambient vibration tests can be very useful to the purpose, allowing to assess the importance of the floor deformability in operational modal analyses through model-driven approaches. This information is particularly valuable for unreinforced masonry buildings, often characterized by deformable diaphragms whose effective stiffness is commonly unknown and hard to be evaluated. Based on these motivations, in this paper, a discrete linear model of deformable diaphragm is formulated in a novel fashion. The modal properties governing the free undamped dynamics are analytically determined through a fully general perturbation technique (direct problem). Therefore, a model-based structural identification procedure is proposed to analytically assess the inertial and elastic properties of the deformable diaphragm (inverse problem), assuming the outcomes of experimental modal analyses as known input. Consistently with the perturbation approach, explicit formulas are determined for low-order minimal models and higher-order model updating, accounting for mass and inertial eccentricities. Among the other identifiable mechanical parameters, the focus is put on the first and second-order identification of the in-plane shear stiffness of the diaphragm. The theoretical developments are successfully verified on pseudo-experimental and experimental bases, by applying the identification procedure to (i) the computational model of a prototypical steel frame structure, (ii) the large scale laboratory model of a two-story composite structure with mass eccentricities, (iii) a permanently monitored masonry building recently struck by the 2016-2017 Central Italy earthquake sequence.