## 4 Discussion and Study Implications
Current codes, such as Eurocode 8, generally do not differentiate between pulse-like and non-pulse-like motions in their design spectra, despite research showing that pulse-like motions can be more damaging. To address this, design provisions should include specific modifications or spectral shapes for pulse-type motions. Buildings near active faults or in regions prone to pulse amplification should be designed with greater ductility, energy dissipation systems, and sufficient separation to avoid bounding. Identifying pulse-like features early can also guide retrofitting and performance-based assessments, especially for older structures. Overall, PI analysis underscores the significance of pulse-sensitive design approaches, particularly in urban areas like Sofia, which are vulnerable to near-source seismic events. Although soil-structure interaction (SSI) is mentioned in the study, its effects have not been quantitatively evaluated. SSI can significantly influence the seismic response of buildings, particularly mid-rise reinforced concrete structures such as the one examined in this paper. The additional flexibility introduced by SSI reduces the system's natural frequency, potentially bringing it closer to resonance with the long-period components of seismic waves, leading to accelerations at upper floors being increased by as much as 20-40%. Given the geotechnical conditions typical of Sofia, future analyses should incorporate at least a simplified SSI model (e.g., an impedance-based approach) to enhance the reliability of response predictions and prevent overestimating structural safety. Amplification of the maximum accelerometric parameters reached values given in Table 1, indicating significant dynamic effects toward upper levels. The FSR method, which compares the amplitude spectra between the sixth floor (SGL2) and the basement (SGL1), revealed a dominant amplification peak at approximately 2.2 Hz, consistent across the horizontal components (EW and NS), and moderately visible in the vertical (UD). This dominant frequency corresponds to the natural mode of the structure identified through other analyses, such as H/V and response spectra. The amplitude of the FSR reached values that indicate significant dynamic amplification towards the upper floors. This observation is consistent with the findings based on PGAs and confirms the building's resonant behavior under seismic excitation. RDM analysis determined damping ratios ranging from 3.6% to 6.5%, showing frequency shifts across floors. This technique utilizes the decay of oscillatory segments of the response signal to estimate the damping ratio (\(\zeta\)) and dominant frequencies. The results indicated, for the basement level (SGL1) f\({}_{0}\approx\) 0.44-0.46 Hz and \(\zeta\approx\) 3.6-5.3%, and for the sixth-floor level (SGL2) f\({}_{0}\approx\) 2.3-2.7 Hz and \(\zeta\approx\) 4.3-6.5%. These values illustrate the frequency shift and increased damping as a function of height and match well with the building's flexible dynamic response as assessed in the Fourier and KY model interpretations. The sixth floor-to-basement PGA ratios for the EW, NS, and UD components--2.04, 3.27, and 2.11, respectively--are essential for assessing flexibility and torsional response. EC8 does not explicitly define acceptable amplification thresholds between floors but emphasizes modal analysis to capture floor-level responses. Ratios greater than 2 typically indicate a flexible structure with the potential for high inter-story acceleration differentials, which is relevant for equipment anchorage and non-structural performance checks. Eurocode 8 [58] recommends evaluating floor acceleration amplification and inter-story drift when the PGA amplification exceeds 1.5, as observed in the NS and EW directions in this case. For non-structural elements, ASCE 7-16 specifies that amplified floor accelerations must be considered when the sixth floor PGA > 0.4 g (\(\sim\)392 cm/s\({}^{2}\)).