## 3 Results
### Evaluation of Amplification Effects Between Basement and Sixth-Floor Accelerations  
Understanding how buildings respond to seismic events is essential for minimizing damage and ensuring public safety. The real-time seismic monitoring data enable an accurate assessment of building behavior, support the refinement of building codes, and enhance design practices, ultimately improving urban seismic resilience in cities like Sofia. The analysis focuses on key aspects of seismic performance, including peak accelerations, velocities, and displacements at various levels within the building. A frequency response analysis is conducted to identify the building's natural frequencies and their correlation with the dominant frequencies of earthquakes. Amplification effects are evaluated by comparing acceleration responses at the basement and sixth floor. The graphs of the orientation-independent response spectra for the horizontal components of the motion, RotD100 max and RotD00 min ([[https://seismosoft.com/products/seismosignal/](https://seismosoft.com/products/seismosignal/)]([https://seismosoft.com/products/seismosignal/](https://seismosoft.com/products/seismosignal/)), accessed on 24 January 2025) [(48; 63)], which are independent of the site orientation relative to the epicenter and provide the maximum spectral amplitude for different rotation angles, are shown in Figure 7a,b. Figure 7a-d represents Response Spectra Acceleration (RSA). The RSA\({}_{\text{SGL1}}\) (Figure 7a) exhibits lower acceleration amplitudes, peaking at a maximum of 0.12 g for a duration of 0.25 s. In contrast, the RSA\({}_{\text{SGL2}}\) (Figure 7b) exhibits amplified accelerations of approximately 0.35 g for a period of around 0.4 s, corresponding to the building's natural frequencies. This amplification is caused by resonance in the building, which can be rated as a flexible structure. The sixth-floor RSA spectrum shifts slightly to longer periods, showing a 3-4 times increase in maximum acceleration compared to the basement. Understanding these differences is essential for structural retrofitting and seismic design, particularly in mitigating dynamic amplification effects on non-structural components and equipment on the sixth floor, especially if it is costly. Basement isolation or energy dissipation systems can help reduce amplifications on the sixth-floor level. The analysis examines the RSA for the UD component at two levels of the building: the base (Figure 7c) and the sixth floor (Figure 7d). The maximum RSA\({}_{\text{SGL2}}\) on the sixth floor is significantly higher (0.21 g) than at the base (0.062 g), indicating strong amplification of vertical accelerations as seismic waves propagate upward. The base RSA is concentrated in shorter periods (\(\sim\)0-1 s) and decays rapidly, demonstrating effective energy dissipation. In contrast, the sixth-floor RSA exhibits higher amplitudes over a broader period range, with  
Figure 6: Amplification factor FS\({}_{\text{SGL1}}\) vs. FS\({}_{\text{SGL2}}\). slower decay, reflecting dynamic effects such as resonance and vibration amplification. This result highlights the importance of considering vertical acceleration impacts in multi-story building designs to mitigate the damage and stress caused by amplified vibrations. The vertical PGA amplification on the sixth floor (2.11) and RSA-UD values (0.21 g) could contribute to column axial shortening, shear failures, or beam compression buckling. We now reference ASCE 7-16 SS13.5 and Eurocode 8 Part 1 SS4.3.3.5, emphasizing the need to explicitly account for vertical excitation in multi-story building design and retrofitting--especially for older RC frames lacking confinement and redundancy. The Spectral Acceleration-Spectral Displacement diagram (Figure 7e,f) highlights the structural responses to seismic waves, identifying the natural frequencies at which acceleration and displacement peak, thereby aiding in the detection of vulnerabilities and prioritization of improvements.