Methods for modeling in‐plane deformability of orthotropic RC floors and analyses of the effects of floors deformability on multi-story buildings. Study of the ageing effect on the mechanical behavior of BFRP-reinforced RC beams

The present research work deals with two important topics regarding the safety assessment of existing Reinforced Concrete (RC) structures. The first topic focuses on the Finite Element (FE) modeling of in-plane flexibility of orthotropic floors and on the effects produced on multi-story building. The study is divided into three stages. The first stage is dedicated to develop simplified two-dimensional finite elements to reproduce the in-plane flexibility of a single floor cell. To obtain the mechanical parameters of the two-dimensional FEs, a numerical procedure based on the comparison between the simplified 2D model and a reference model of the floor cell made with solid FE is proposed. For the comparison. the displacements under three different deformed configurations of the floor cell are considered. A simple case study is then investigated whose floor’s mechanical properties are determined by means of a simplified analytical method, usually adopted by practitioners to reproduce in-plane flexibility. The purpuse of this analysis is determine the potential and limitation of this approach. The second stage is dedicated to develope more sofisticated methods and compare them in terms of accuracy and efficiency. Two different approaches are developed based on the homogenization theory, one analytical and one numerical. The analytical one is developed using the Voigt and Reuss bounds to obtain a close form to calculate the floor in-plane elastic parameters. The numerical approach, based on the standard static homogenization, is used to determine, through an iterative procedure, the floor elastic parameters. The mechanical parameters obtained from the two methods and from the method developed in the first stage are used to model the in-plane floor flexibility in one- and six-story building models. Lateral forces, constant and proportional with the building height, are applied to the models and the response in terms of base shear and in-plane floor displacement is observed. The accuracy of the homogenization methods is assessed based on the comparison with the models with solid FE floors. The models where floors are modeled with the homogenization method are compared with the model with rigid diaphragms and the model where floors are modeled considering the concrete slab only. In the third stage the effects of floors’ in-plane flexibility on an existing RC building with an elongated shape in plan are investigated. The case study is a school building located in Teramo, Italy, built at the end of the 1960s. The investigations are carried out for the as-built configuration and the building retrofitted with external steel braced frames and CAM system. To model in-plane floor flexibility, the analytical method (ANH) is used. For the investigations, A non-linear dynamic analyses are carried out. The results in terms of total base shear, base shear in the columns and floors’ in-plane displacements, obtained from the model with rigid diaphragms and the model with in-plane flexibile floors are compared. The second topic regards the study of the environmental actions and ageing effects on RC beams strengthened with Basalt Fiber Reinforced Polymers (BFRP) fabric. An experimental campaign was carried out on seven couple of identical RC beams strengthened with BFRP fabric for flexure. The campaign, consisting of four-point bending tests, was performed in two time stages. The first set of tests was performed at 28 days after the casting and the second 6 years later, after the exposure to environmental action and ageing process. The beam behavior was investigated in terms of bearing-load capacity and deflection. Strength, stiffness, ductility and failure mode of the beams before and after the exposure to ageing effects are compared.