# Seismic Performance of Modal Transfer Stations on Soft Clays
## 5 Seismic Tunnel-Bridge-Building Interaction
Furthermore, multiple numerical analyses of several authors [14; 48; 49] have demonstrated that the influence of the underground structure on the aboveground seismic response decreases gradually as the horizontal distance between them grows, being detrimental to the aboveground structure in most cases. Figure 14: _Cont._  
Figure 15a,b present the calculated peak acceleration at the urban bridge deck and its foundation, and Figure 16a,b at the building roof and its foundation. From the accelerations computed at the bridge deck in the transverse direction, it can be clearly noted that the most critical scenario is when the building is one diameter away. Thus, the amplification is almost 50% in comparison to the independent structure scenario. This effect decreases as the building moves away from the bridge. Furthermore, it can be highlighted that the tunnel effect is almost negligible when the building is one diameter away from the bridge, because the computed accelerations for TBU and BU cases are similar. Nevertheless, for the TU cases (i.e., when the building is not present), there is a sudden reduction in the acceleration, which continuously decreases as the tunnel moves away from the bridge. Thus, it can be concluded that the structure that affects the seismic response of the bridge the most is the building. Regarding the performance of the building, the maximum detrimental interaction observed can be associated with the tunnel, which leads to an amplification of the maximum acceleration computed at the building roof in the TB_B2 case (Figure 16a). In addition, the same effect is noticed for the scenario considered at the building foundation level (Figure 16b), which suggests that additional energy comes from the soil below. This can be related to the wave diffraction and reflection effect that occurs at the tunnel lining, causing a larger impact on the building. On the other hand, it seems that the presence of the bridge leads to a reduction in the computed acceleration TBU_B2 (i.e., beneficial interaction), which is the same scenario as previously discussed, but with the presence of the bridge. In previous research [50,51], it has been established through analytical and numerical models that when the natural frequencies of two aboveground structures are different, there is an energy transfer between them, causing a beneficial interaction for the structure with a smaller frequency, and detrimental for the other one. In addition, this trend is exacerbated when the masses of the structures are different, being significantly detrimental for such structures with small masses and higher natural frequency. Therefore, this can explain why the building amplifies the seismic response of the bridge, while the building's response is decreased. According to the above definition, the bridge structure is the most affected because its natural frequency (2.22 Hz) is higher than that of the building (0.99 Hz) and it is almost five times more lightweight than the building. The maximum accelerations were also computed at the ground surface on the space between the two aboveground structures, called \"near field\" here. Figure 17 shows the comparison of the values obtained in each scenario with the free-field case (the scenario without structures). Interestingly, the maximum acceleration for the transverse direction appears between the tunnel and the building, but without the bridge, in the TB_C1 and TB_C2 cases. In these scenarios, the building and the tunnel seem to contribute to the  
Figure 14: Maximum relative displacements in the transversal (X) and longitudinal (Y) directions. Computed at the (**a**) urban bridge and (**b**) building. generation of surface waves, associated with the building rocking back and forth and the wave scattering by the presence of the tunnel, leading to ground motion amplification in this zone. On the other hand, in the longitudinal direction, there is a decrease in accelerations when the tunnel is just below the point of analysis.