Relevant Topics on the Strength of Existing Reinforced Concrete Buildings: - In-plane deformability of orthotropic floors - Shear strength of interior beam-column joints under cyclic loads - Seismic retrofitting of a building with strongly different stiffness in the main directions - Destructive and non-destructive tests on concrete - Assessment of the residual strength after fire

The present research work deals with five important topics relevant to the structural strength of existing Reinforced Concrete (RC) buildings. The first topic concerns the use of equivalent 2D elements with assigned thickness to reproduce the in-plane deformability of orthotropic floors in the Finite Element (FE) model of the building, for analyses under seismic actions. An advanced FE-based procedure aimed at determining the elastic properties of the homogeneous material constituting the equivalent 2D elements is proposed for the first time. A discussion on the applicability of the simplified procedure commonly adopted by engineers, based on the equivalence between the transverse areas of the RC elements of the real floor and those of the 2D elements, is also provided. A case study in which the simplified procedure is used to effectively reproduce in-plane deformability of floors is presented. The second topic addressed in this thesis concerns beam-column joint shear behavior under cyclic loads. On the basis of a mechanical analysis and the use of 69 test results available in the literature, a direct formula for the shear strength prediction of interior beam-column joints with deformed bars is proposed. The formula accounts for the resisting contributions of three inclined concrete struts and of joint reinforcements. The influence of the upper column axial load on the inclination of the concrete struts is taken into account. Also a design formula is proposed. On the basis of the model proposed for joints with deformed bars, a shear strength formula and a design formula for beam-column joints with plain bars are also derived. Another relevant topic addressed herein is the design of the retrofit intervention of an existing RC building whose displacement capacity does not satisfy the seismic Italian Code requirements. The modal analysis highlights that the structure has strongly different translational stiffness along its main directions and low torsional stiffness. To improve the seismic behavior of the building, it is proposed to install an external steel frame with stiff braces along the side of the structure which is the most affected by torsional effect. Conversely, in the orthogonal direction, where the ductility demand is much higher, external steel frames with dissipative braces are used. Then, the topic concerning the assessment of in-place concrete strength of existing buildings is addressed. The results of an experimental campaign performed on column and cube specimens made with the same concrete mix are presented, to assess the capacity of coring destructive test and non-destructive tests to predict the in-place concrete strength. The non-destructive predictive methods are performed also during the curing of the concrete at different ages. A discussion on drilled cores’ strength, determined using provisions of different building codes, relevant to cube specimens’ strength is provided. The effects of the core diameter and the position of test execution along the columns’ height, on in-place concrete strength, are investigated. The minimum number of cores to adequately predict the in-place concrete strength by means of non-destructive test methods is also determined. The last topic addressed in this thesis concerns experimental tests to perform for assessing residual strength of RC structures subjected to fire. The test program, with laboratory and in situ tests, used to determine the residual strength of steel reinforcements and concrete of a case study is described. The program includes test methods that are able to catch the variability of concrete degradation with distance from the heated surface. Test results are presented and their acceptability in relation to Italian Code is discussed. A discussion on the advantages and limitations of the adopted test methods, useful for the designers, is performed. To restore the load-bearing capacity of the structure, repair interventions are also proposed.