# Seismic Performance of Modal Transfer Stations on Soft Clays
## 5 Seismic Tunnel-Bridge-Building Interaction
Figures 10 and 11 show the interaction factor for the bridge; it is clear that for frequencies around 1 and 2 Hz, the amplification potential is affected by the presence of the building, while for higher frequencies, the presence of the tunnel increases the amplification potential. Also, the interaction among the three structures decreases in the frequency range of interest when the tunnel moves away from the bridge (case TBU_C). This can be clearly appreciated by comparing the black line (\"BU\", which represents the case of the urban bridge and building without the tunnel) with the red lines (\"TBU\", which represent the cases with the three structures in their different positions). This can be explained by considering that in almost all the cases studied, these lines are very similar within the relevant frequency range. This suggests that the presence of the building has a greater impact on the seismic response of the bridge within these frequencies; nevertheless, as the frequency increases, the red lines tend to be more similar to the blue lines (\"TU\", which represents the case of the urban bridge with the tunnel in different positions), which suggests that  
Figure 13: Interaction factor for the building in the longitudinal (\(\Upsilon\)) direction under different scenarios for (**a**) the CU17 and (**b**) the CU85 synthetic ground motion. the tunnel has a greater impact at higher frequencies. Similar effects can be observed on the building response (Figures 12 and 13), and in addition, there is a clear amplification potential (i.e., detrimental interaction) in both directions as the tunnel gets closer to the building underneath its foundation, as shown by all the discontinuous lines (i.e., cases B and C). The above results agree with the relative maximum displacements of the bridge and the building (Figure 14). In order to evaluate the seismic demand on the structures in more detail, the relative displacement between the base and the top of the structures was monitored during the seismic analyses for all the scenarios. Figure 14 depicts the numerical results for each case and the comparison with the independent structure case for each seismic motion. A detrimental interaction can be clearly observed on the maximum relative displacement of the upper deck and the building roof (Figure 14), computed as the maximum absolute value of the instantaneous relative displacement between the center of the upper deck bridge and the building roof and the base of their box foundations. In general, the maximum relative displacement is greater in the transverse (x) direction than in the longitudinal (y) direction. For subduction earthquakes, the maximum relative displacement is observed in the transverse direction when the tunnel is under the bridge. This amplification increases when the distance between the bridge and the building reduces (i.e., one diameter, 1D, in comparison with three diameters, 3D). The amplification in relative displacements is higher for the building in cases TB_C1 and TB_C2, which corresponds to the tunnel beneath the building. In addition, the differences between the relative displacements with normal and subduction earthquakes are more evident in the building, with greater values for CU85 observed, which can be associated with its frequency content which is closer to that of the structure. Thus, for the bridge performance, the tunnel induces up to 20% of amplification potential when it is located beneath the bridge; nevertheless, such an effect is exceeded by the presence of the building that generates an increase in the column drift up to 60% when it is located at a 1D distance. Similar findings have been obtained in previous research. Based on the shaking table experiments carried out by [PERSON] et al. [11], the seismic response of tunnel-soil-surface structures depends largely on the earthquake type, having less impact at lower frequencies.