## 4 Comparative Analysis of Proposed Design Spectra and Code Spectra  
The amplification factor of the site profiles was computed, as shown in Figure 11. The black solid lines in the plots indicate the mean amplification at all periods. It is evident from the results that Site Class D profiles (Figure 11a-d) were amplified at shorter periods, whereas Site Class E profiles (Figure 11e,f) were amplified at longer periods. The increase in amplification can be attributed to the elongation characteristics of the soft soils observed in the site response analysis, particularly beyond the site periods. The mean amplification factor of the site profiles versus the spectral period was plotted, as shown in Figure 12. The amplification peaks were observed at \(\sim\)0.47 s for the Site Class D profiles and \(\sim\)1.63 s for the Site Class E profiles. The seismic site coefficients for the short period (Fa) and long period (Fv) were defined as the ratio of response spectral accelerations in the short and long periods, respectively. Fa ranged between 0.1 and 0.5 s, while Fv ranged from 0.4 to 2.0 s; these parameters were evaluated using Equations (5) and (6), as per the recommendations in [74]. \[\text{Fa}=\frac{1}{0.4}\int_{0.1}^{0.5}\biggl{(}\frac{\text{RS}_{\text{soil}} \text{ (T)}}{\text{RS}_{\text{bedrock}}\text{ (T)}}\biggr{)}\text{dT} \tag{5}\]  
\[\text{Fv}=\frac{1}{1.6}\int_{0.4}^{2}\biggl{(}\frac{\text{RS}_{\text{soil}} \text{ (T)}}{\text{RS}_{\text{bedrock}}\text{ (T)}}\biggr{)}\text{dT} \tag{6}\]  
Figure 11: Amplification factors for site profiles S-1 to S-6, showing mean responses (black solid lines). Site Class D profiles (**a–d**) were amplified at shorter periods, while Site Class E profiles (**e**,**f**) exhibited amplification at longer periods due to soft soil elongation effects. A similar approach was employed in [75] for the development of Korean site classification, and it was adopted in the present study. The computed mean values for Fa and Fv were found to be 1.691 and 1.476 for the Site Class D profiles and 0.598 and 2.035 for the Site Class E profiles, respectively. After computing the seismic site coefficients, the design spectra based on the NL analyses (design spectra NL--Soil Type D and design spectra NL--Soil Type E) were calculated and compared with the BCP spectra, as shown in Figure 13. It was evident that the spectral acceleration values specified by BCP:2021 for Site Class D were lower compared to the proposed design spectra for spectral periods of less than 0.32 s, while higher values were observed beyond these spectral periods. In contrast, the spectral acceleration values adopted by BCP:2021 for Site Class E were higher compared to the proposed design spectra. This trend persisted throughout the entire period range. Figure 12: Amplification versus spectral period for (**a**) Site Class D profiles and (**b**) Site Class E profiles. Figure 13: Proposed design spectra versus code spectra for (**a**) Soil Type D and (**b**) Soil Type E.  
Improving Seismic Design: The Need for Site-Specific Spectra and Code Updates in Earthquake-Prone Areas  
The observed trends in spectral acceleration can be attributed to differences in the seismic site coefficients Fa (short-period amplification) and Fv (long-period amplification). For Soil Type D, the lower spectral accelerations in BCP:2021 for periods of less than 0.32 s suggest that Fa values may underestimate short-period amplification, while higher accelerations at longer periods indicate conservatively elevated Fv values. In contrast, for Soil Type E, consistently higher spectral accelerations in BCP:2021 across all periods reflect more conservative Fa and Fv values to account for the significant amplification effects of softer soils. These differences highlight the need for site-specific coefficients for accurate design spectra. Generally, site-specific spectra are superior to code-based spectra, as they are tailored to local conditions, offering a more accurate representation of seismic behavior.