## 1 Introduction
However, no experimental validation was carried out to check that such strengthening methods were indeed effective. In our opinion, it is necessary to develop experimental methods that allow engineers to determine a building's dynamic parameters before and after strengthening. Other examples of seismic resistance evaluation, strengthening, and upgrading are the old Yokufu-en building in Japan, which was constructed in Tokyo about 100 years ago [12], Bnai Zion Hospital in Haifa, Israel [13], a retrofitted church in Slovenia [14], etc. Presently, seismic strength planning is based on the numerical analysis of a building, aimed at finding the optimal strengthening and upgrading options from technical and economic feasibility viewpoints. In some cases, the main question is to prove the benefit of strengthening and upgrading an existing structure compared with the construction of a new building [15]. However, the real state of the existing as well as the strengthened structure can be obtained experimentally. Therefore, we propose measuring structural dynamic parameters before deciding whether strengthening or upgrading is required, as well as at all stages of strengthening and upgrading. As described below, for the building investigated in the frame of the present study, 11 stages are included. The response of a real 11-story RC building to seismic-type loadings was investigated experimentally and numerically under dynamic horizontal loads applied by a vibration machine placed on the roof of the building [16]. Preliminarily, a three-story fragment of this building was tested by free vibration, created by an initial force equal to the expected horizontal seismic load [17]. Damages and cracks in structural elements were inspected, and the natural vibration period of the structure was obtained. Following the preliminary design, braces were added in the short direction of the building. Experiments were carried out on a fragment without braces to prove that the bracing was really required. It was concluded that X-shaped braces in this direction are effective and should be used. An experimental program was carried out to investigate the seismic behavior of structural and nonstructural elements [18]. The research included shake table testing of a full-scale five-story reinforced concrete building with nonstructural components (NSCs). The building was subjected to earthquake motions with increasing intensity. The major components of the structure, its NSCs, and the monitoring systems were described. The response and damage to the structure were presented. Damage observed in various NSCs and its correlation with the structure's response was discussed. Numerical values of structural dynamic parameters do not always correspond to experimental values [19]. It was demonstrated that for a 10-story building, slab flexibility should be considered, and for a 12-story building, masonry infill walls should be taken into consideration in dynamic analyses. This supports our idea that the dynamic parameters of a strengthened or upgraded building should be measured and not only calculated. It will allow us to verify if a building's strengthening and upgrading is indeed effective and corresponds to the design of the building.