## 4 Discussion and Study Implications  
The building, typical of mid-20 th-century residential or mixed-use structures in Sofia, features a reinforced concrete frame and slabs. While designed to resist lateral forces, such as wind and earthquakes, its age is approximately 70-80 years, and the design may not align with modern seismic codes. As such, this building can represent many similar regional structures that lack retrofitting for contemporary earthquake standards. Assessing the response of such buildings during seismic events highlights vulnerabilities and informs future retrofitting strategies to enhance resilience. The comparison of response spectra shows that the vertical acceleration on the sixth floor of the building is significantly amplified compared to that at the base. These findings underscore the importance of considering vertical acceleration effects in structural design to prevent potential damage, particularly in multi-story buildings. Figure 10: Evaluated damping coefficient and predominant frequency using the RDM: **(a)** SGL1 in the basement; **(b)** SGL2 on the sixth floor. Black line—free response extracted from the acceleration record, red line—exponential decay curve fitted to the free response. Figure 9: Evaluated Floor Spectral Ratio (FSR) according to Equation (6). In this study, we apply the KY approach [22] to compare accelerometric data from the basement and the sixth floor of a six-story building located in the central part of Sofia, Bulgaria. Analyses were performed based on the comparison, evaluating the differences in seismic response between the basement and the structure's sixth floor. The sixth floor experienced more significant oscillations and higher amplitudes than the basement due to dynamic amplification. The basement primarily reflects the direct seismic input, while the sixth floor exhibits amplified and prolonged oscillations due to structural dynamics. This highlights the importance of evaluating resonance and damping mechanisms in tall structures to effectively mitigate seismic effects. The KY formula provides a simplified method for estimating a building's seismic response using sixth-floor-level acceleration data; however, it has notable limitations. It assumes a linearly elastic, one-dimensional shear beam model, which overlooks non-linear behaviors like cracking or plastic deformations that occur during strong earthquakes. The model does not account for irregularities in mass or stiffness, nor does it consider soil-structure interaction (SSI), which can significantly affect a building's response, especially in areas with soft soil, such as the Sofia Graben. It also assumes uniform material properties and vertical shear wave propagation, ignoring torsion, asymmetry, and other complex dynamics present in real structures. These simplifications can lead to inaccurate predictions of inter-story drifts and acceleration amplifications. Future enhancements to the KY model should aim to incorporate non-stationary ground motion characteristics, site-specific geotechnical profiles, and multidirectional dynamic inputs to better reflect the actual seismic demand on buildings in complex urban environments. The PI analysis of accelerogram data from the basement (SGL1) and sixth floor (SGL2) provides valuable insights into seismic ground motion characteristics and their impact on structural behavior. It classifies seismic events as pulse-like (PL) or non-pulse-like (NPL) based on the presence of velocity pulses, which are typical in near-fault earthquakes. For the 22 May 2012 Pernik earthquake, pulse-like features were notably present in the horizontal (NS) components, exhibiting strong velocity peaks around 6-8 s and a gradual decay, particularly at the sixth-floor level. These concentrated energy bursts can significantly affect the performance of flexible, mid- to high-rise buildings. Classifying ground motions as pulse-like has significant implications for seismic design and building codes.