## 3 Description of the Case-Study Hospital Building
### As-Built State of the Hospital Building
The floor system comprises RC slabs of 15 and 12 cm thicknesses, with reinforcement assumed in accordance with the static design. Diaphragm action of the slabs may be expected. The building has two inner RC staircases and an elevator core consisting of coupled walls. Two stories below ground are partially embedded. An analysis of the embedding influence on the building displacements due to seismic loading was performed in the study. A nuclear shelter with structural walls with 50-60 cm thickness, and a 50 cm thick slab was built in the embedded part of the building. The foundation slab was 60 cm thick below the shelter. Stripped foundations of different dimensions, transferring loads from structural walls to the ground, are constructed on two levels (basement and ground floor). As it was previously mentioned, the building was designed and built in the mid-eighties of the 20 th century, before the implementation of modern European seismic regulations [29]. However, seismic regulations from 1981 [30] covered structural ductility demands, accounting for hinge yielding and energy loss due to the concrete cracking, in order to avoid structural failures. Moreover, this seismic code took into account the building location in the seismically active zone and the importance factor for hospital buildings. Therefore, in the original design, the base shear coefficient calculated according to the code amounted to 0.15G (G--the weight of the structure, approximately 15,000 t; comprises the walls, columns, beams and slabs with floor layers above, RC partition walls and additional dead load). According to available information, standard rules of practice for construction in the seismically active areas were regularly applied in the region at that time, bearing in mind the devastating consequences of the 1963 Skopje earthquake. The basic difference between the seismic code from 1981 and modern seismic regulations lays in the reference peak ground acceleration which was significantly  
Figure 4: Structural and non-structural elements: (**a**) at story 3 (ground floor); (**b**) at story 5 (the third story above ground level). increased. Consequently, the internal forces in structural elements due to the seismic loading in accordance with modern codes are much larger than calculated in the original design. A short description of the as-built minimum reinforcement is given in the following figure (Figure 5) and text. Walls and coupled beams have a longitudinal reinforcement of \(\emptyset\)8/15 cm and a shear reinforcement of \(\emptyset\)8/15 cm (A\({}_{\text{sw}}\) = 6.6 cm\({}^{2}\)/m). Columns have minimal reinforcement according to the original design (40/20 cm) and shear reinforcement (\(\emptyset\)8/15 cm). Depending on the position, geometry and internal forces, some elements are reinforced more heavily, even at the limit of maximum reinforcement. The built-in reinforcement was taken into account during the calculation of each element capacity curve. Soil Type Characterization, Seismological Ambient Noise Measurements, Seismic Hazard and Ground Motion Generation  
According to the original documentation, the building's foundations were constructed in a layer of karstified limestone--a rock that can generally be classified as the ground type A according to Eurocode 8 [29]. This was confirmed by seismological ambient noise measurements which were performed on seven free-field locations on the site of the hospital using a Tromino seismometer. The horizontal to vertical spectral ratio (HVSR), [31], of 16 min long measurements with a sampling rate of 128 Hz was computed for all free-field measurements in order to quantify the effects of the subsurface geology on the seismic ground motion, primarily to estimate the subsurface soil's resonant frequency and amplification, thus enabling ground type classification according to Eurocode 8.