Experimental investigation of effect of damage, repair and strengthening on dynamic behavior in r/c buildings

All structures are exposed to vibrations of greater or lesser intensity at any time due to environmental conditions. The source of vibration is time-dependent dynamic loads that can cause major damage to structures. Analyzing the vibration response in systems with time-independent static analysis methods is not very realistic and cannot fully reflect the behavior of structures. The reactions that occur in a system subjected to dynamic loading are greater than in systems subjected to static loading. Greater internal forces and displacements occur in structures. Considering all these, dynamic loads play a huge role in the design phase of structures. For these reasons, it is very important to examine the behavior of structures under dynamic effects and to carry out these examinations through dynamic solution methods. If we are to talk about a dynamic load for our country, which has even been described as an earthquake laboratory, it will undoubtedly be the earthquake load. Türkiye experiences approximately 30 thousand earthquakes every year. Some of these earthquakes are at a level that can damage structures. Of course, the condition and performance of the building are also decisive here. Structures damaged by earthquakes are often affected by aftershocks or new mainshocks before the strengthening/repair process has even started. Therefore, as the earthquake resistance of a structure that is already damaged at a certain level will decrease, new earthquakes may cause the structure to collapse. Unfortunately, as before, this situation was painfully experienced by our nation in the 6 February 2023 Kahramanmaraş earthquakes. In this study, the effects of damage caused by earthquakes in reinforced concrete buildings and the effects of repairs and strengthenings on the dynamic behavior were investigated. In the study, forced vibration tests were carried out on the shake table of three 1/3 scale, 3D, 2-storey-single-span reinforced concrete frame specimens produced in laboratory. The specimens were tested in undamaged, damaged, repaired, and strengthened conditions. In the damaged condition, the joint areas of the specimens were weakened. The damaged areas were subsequently repaired using repair mortar. Systemic techniques were used during the strengthening phase. At this stage, strengthening methods using in-plane reinforced concrete shear walls and X-shaped steel diagonal bracings were used. Also, at this stage, an improvement method with infill walls was applied in order to observe the contribution of infill walls to the dynamic behavior. As the excitation load, artificial ground motions of four different intensities were used, with the peak ground acceleration values of 0,19 g, 0,36 g, 0,46 g and 0,54 g. A total of more than 200 forced vibration experiments were carried out under these 4 different intensities of dynamic load in different conditions of the specimens. In the experiments, time-dependent acceleration data were recorded via vii accelerometers and time-dependent displacement data were recorded via potentiometric rulers. Based on these data, dynamic parameters (natural frequencies, natural periods, damping ratios, mode shapes) were determined with the experimental modal analysis method and acceleration and displacement response spectra were created. Also, in the time domain; story displacements, base shears, base moments and base shear-top displacement, base moment-roof drift ratio hysteresis curves, and lateral translational stiffnesses were obtained. On the other hand, using the ETABS structural analysis program based on the finite element method, numerical analyses were also conducted using time history analysis under the same conditions and loads. The effects of damage, repair and strengthenings on the dynamic behavior were examined in the light of all parameters and graphs obtained experimentally and numerically. As a result, it was observed that in the damaged condition, the lateral translational stiffness decreased significantly and the displacement demands increased considerably, and with the effect of the strengthenings, a large amount of stiffness was gained and the displacement values recorded under the same loads decreased substantially. Similarly, it has been calculated that at the 1st natural frequencies, the values increase by an average of 70% when transitioning from the damaged condition to the strengthened condition. In general, the best performances were achieved in all aspects as a result of the strengthening using in-plane reinforced concrete shear walls. In addition, the numerical analysis results were largely compatible with the experimental results.