The need for repairing and strengthening RC buildings and their structural elements occurs when their elements do not possess sufficient strength, stiffness, and/or ductility due to damage mostly caused by earthquakes. Within the scope of this study, special emphasis will be given to RC buildings where the built-in concrete has not achieved the designed concrete class during construction and/or buildings that cannot satisfy the required strength, stiffness, and deformation characteristics particularly in earthquake conditions owing to construction of additional storeys or enlargements. In such cases, it is necessary to repair and strengthen both individual structural elements and whole structures using traditional (concrete jacketing) and innovative (FRP) materials.

In a large number of seismic-prone regions, the reinforced concrete frames of the buildings are filled with masonry and in this way, a composite constructive composition is created whose behaviour can be hardly predicted and modelled, which leads to flaws in the design regulations. Masonry infill is often presented as a non-constructive element and often is not included in the analyses, which creates a problem, a constructed building, in reality, can have different dynamic characteristics from the project ones, and thus different seismic behaviour than predicted. Therefore, the aim of this paper is to investigate how buildings behave in seismic activities when the masonry infill is included in the model and calculations. A comprehensive comparative analysis has been performed involving linear and nonlinear-Pushover static analyses and the obtained results from hypothetical models with 5 and 8 floors, with and without modelling the infill, according to the Macedonian regulations and according to Eurocodes were compared. For modelling the infill, the recommendations according to FEMA 356 for macro-modelling were used, where the masonry is modelled with an equivalent diagonal strut. From the conducted analyses, it can be generally concluded that the masonry infill has a very favourable effect on the behaviour of the buildings when it is evenly distributed. Although the ductility of the building decreased, however, the initial rigidity is substantially increased (almost 2 times in the presented paper). It can be freely recommended that when we have such a case of evenly and regularly distributed masonry infill, it should be included in the analysis as the first line of defence in overcoming seismic forces. In the model Ekopen 1-st story, it can be seen that the infill has a negative impact on the overall behaviour with a tendency to form a soft story on the ground floor.

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Existing reinforced concrete structures are subject to damage and performance degradation under external loads. It is then necessary to improve their performance using appropriate strengthening techniques. Over the last decades, new construction materials for strengthening and design have emerged. These are referred to as fiber-reinforced polymer (FRP) composites and have unique mechanical and in-service properties. Investigation of FRP-strengthened RC members subjected to impact loads is warranted. In addition, there is also a lack of finite element (FE) studies concerning this topic. FE analysis is a powerful and economical tool to investigate the response of structures under various loads and examine virtually endless number of variables that would be otherwise very difficult to be performed experimentally, due time, cost, and laboratory limitations. The existing RC column was strengthened with CFRP sheets, installed at the site where plastic hinge may occur. First of all, three-dimensional FE CFRP confined column was designed. Due to the complexity of the strengthened element, every element with different material characteristics was designed separately. Every element has different meshing. The nonlinear behavior of the concrete and steel material is accounted in the modeling by introducing the actual inelastic properties in the model. The boundary conditions between the concrete and CFRP material also were taken into account. At the end, the strengthened column was subjected to cyclic loading. As a result of the FE analysis, hysteresis loop was obtained. In order to see the benefits of strengthening with CFRP materials using FE analysis, comparison between hysteresis loops for strengthened and unstrengthen column was performed.

It is a usual practice that traditional methods with traditional materials (most frequently jacketing of elements) are used for repair and strengthening of structures. However, lately, particularly in the last two decades, there have occurred new construction materials intended for strengthening and design referred to as composites strengthened by polymer fibers (CFRP). These materials have special mechanical properties and special properties. Within the frames of this paper, special emphasis will be put on RC buildings where, during construction, the built-in concrete has not achieved the designed concrete class and/or buildings that cannot satisfy the required strength, stiffness and deformation characteristics, particularly in earthquake conditions. In these cases, it is necessary to take measures for repair and strengthening of both individual structural elements and whole structures. In the paper part of the analytical, laboratory and quasi-static experimental investigations of designed models of RC columns will be presented. Particular attention will be paid to behaviour of these columns under cyclic loads, whereat a number of comparative analyses of a number of parameters obtained from the experimental investigations of the tested models will be carried out. It will be pointed out that the use of these innovative CFRP materials greatly contributes to the strengthening of RC columns and significantly increases the ductility capacity, which is of great importance in seismically active regions. Some recommendations and outcomes will be given as to the approach and technology of practical application of these materials.