## 5 Comparison and Discussion  
The main objective of the comparative case study was to assess the extent of SSI effects on two different steel typologies (MRF and BF) in order to learn how different structural systems and lateral load distributions are affecting the overall seismic response of the analyzed topologies. Changes in the story drift for different levels of peak ground acceleration (PGA) were observed. The target displacements for each seismic acceleration were determined based on the N2 method [53]. Limit values of the story drifts for the two analyzed typologies are given in terms of limit states by Eurocode 8--Part 3 [54] or FEMA as follows: (i) Damage  
Figure 11: Numerically obtained results for the braced frame. \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Story**} & \multicolumn{2}{c}{**Modal Distribution (N)**} & \multicolumn{2}{c}{**Uniform Distribution (N)**} \\ \cline{2-5}  & **Fixed Base** & **Model on the Soil** & **Fixed Base** & **Model on the Soil** \\ \hline Foundation & - & 1.6 & - & 50 \\ \hline
1 st story & 16.3 & 18.2 & 50 & 50 \\ \hline
2 nd story & 36.1 & 36.7 & 50 & 50 \\ \hline
3 rd story & 58.8 & 57.1 & 50 & 50 \\ \hline
4 th story & 76.7 & 74.4 & 50 & 50 \\ \hline
5 th story & 94.3 & 91.5 & 50 & 50 \\ \hline \end{tabular}
\end{table} 
Table 5: Lateral load distributions for braced frame. limitation (DL or IO), (ii) Significant damage (SD or LS), and (iii) Near collapse (NC or CP). According to Croatian annex, two limit states should be observed, which means that a return period of 475 years should be checked for the SD limit state and a return period of 95 years for the DL limit state. The analyzed moment resisting frame was designed without consideration of SSI effects. The building is located on a moderately active seismic area with a peak ground acceleration (PGA) equal to 0.22 g and a soft soil site that corresponds to sub-soil of class D in accordance with Eurocode 8--Part 1 [52]. A comparison between the pushover curves of the SSI system (coupled systems) is already presented in Section 4. The capacity curves for both lateral load distributions are plotted in Figures 9 and 11. Changes of the capacity for the moment resisting frame (MRF) coupled systems present an increase of the capacity up to 25% when uniform load distribution is compared to the modal lateral load distribution. Uniform and modal lateral load distributions for the braced frame (BF) on the soil resulted with differences in the capacity up to 35%. Furthermore, Figure 12 presents the story drift for a seismic event with a 475-year return period. When the MRF is observed, increase in the story drifts for the systems on the soil are highlighted. Substantial differences are found in higher stories, while the lower stories present a smaller increase for the soil systems. It is important to emphasize that fixed-base model loaded with uniform load showed larger drifts than allowed within the Eurocode regulations. When the soil is included, the damage limit is not reached. In other hand, there are no safety reserves when the soil is taken into consideration. It is noticed that a large increase of the story drifts is found for the BF of the soil compared to fixed-base BF, especially for the modal lateral load distribution which exceeds the significant damage (SD) state according to Eurocode. This implies that BF is underdesigned if the soil part of the system is considered. Damage within the building floors is kept similar for each lateral load distribution case when the frame is modelled as fixed at the base and founded on compliant soil. Overall, comparing the two lateral load distributions for MFR and BF, larger story drifts are distinguished for the modal lateral load distribution than the uniform load distribution no matter the foundation case. Results for the incremental N2 method are presented in Figure 13. The PGA was increased incrementally from zero to 0.50 g, with a step size of 0.05 g. Fixed-base models and models on the soil with varying lateral load distribution were considered.