# Seismic Performance of Modal Transfer Stations on Soft Clays
## 6 Conclusions  
The results gathered from this numerical study provide valuable insights, and lead to design and retrofitting recommendations for earthquake-resistant structures in densely populated areas, where underground (i.e., tunnels) and on-ground structures (i.e., bridges and buildings) are often close to each other. In comparison with previous studies, the current research introduces a robust methodology, employing 3D numerical simulations, considering a suite of motions with varying frequency content, seismic intensity, and duration, to evaluate the seismic response of bridge-building-tunnel systems during intraplate and interplate events. In addition, the study introduces the concept of interaction factor as a function of the frequency range in which the energy of the excitation is focused as a structural demand parameter. This provides a new metric for assessing the interaction effects in complex infrastructure systems. The main findings revealed that seismic interactions among the structures in soft-soil conditions are highly frequency-dependent. Thus, remarkable differences were observed between the seismic responses generated by intraplate and interplate events. For all the scenarios studied, interaction effects were strongest at high frequencies (>1 Hz), with wave scattering and dispersion as the main contributors to the interaction mechanism, particularly when the tunnel was located beneath the bridge or the building. Focusing on the bridge performance, it can be observed that the tunnel induces an amplification potential up to 20% when is located beneath the bridge; nevertheless, such an effect is exceeded by the presence of the building that generates an increase in the bridge column drift up to 60% when it is located one diameter away. The seismic demand on the pile foundation is also affected by the tunnel's proximity, leading to an increase in the shear and bending moment at the pile as the tunnel gets closer to the bridge. Regarding the seismic performance of the building, the presence of the tunnel extensively affects the seismic demand, with an increase of up to 10% in relative displacements and greater accelerations observed at the roof level. Such effects were most notable when the tunnel was beneath  
Figure 20: Bending moment at the instant of maximum horizontal displacement of the foundation for (**a**) CU17 and (**b**) CU85 earthquakes for TU cases. the building, and for the interplate earthquakes, which exhibited a frequency content with energy concentrated around the fundamental period of the structure (Tf = 1.0 s (1.0 Hz)) and the characteristic elastic site period of the soil. On the other hand, minor interaction effects were observed in the presence of the bridge. In fact, the change in the seismic demand was less than 6% in comparison to the case of the building alone. This was associated with the difference in mass of both structures, with the building having five times the mass of bridge. Another key factor is the actual distance between each component. Particularly, for the case studied, such interaction effects tend to increase when the system is subjected to high-frequency seismic ground motion (larger than or equal to 1 Hz) and when the structures are closer to each other. Thus, such key variables should be considered in densely populated areas for the analysis of earthquake-resistant structures, when similar conditions are presented, particularly in soft soils. Although this paper partially fills the gap in information regarding the seismic interaction among on-ground and underground structures, further research is needed as this problem will become increasingly common in future infrastructure projects in large, densely populated cities. While numerical modeling can provide a robust methodology for exploring a wide range of scenarios, physical experimental validation will always be required to enhance the predictive capabilities of the numerical models. To deal with these shortcomings, instrumentation of actual structures must be conducted.