Evaluation of Formation of Plastic Hinge and Seismic Behavior of Steel Structures Due to Soil–Structure–Tunnel Interaction

The seismic design of the structures is carried out by technical regulations and codes in free-field conditions (regardless of underground cavities). With the availability of tunnels and the complex interaction between the tunnel and the aboveground structures, which may be contemplated wrongly, it could be dangerous for over ground buildings and structures. Consequently, the examination of the underground tunnels and their impact on the land surface and adjacent buildings seismic response seems to be significant. The present research focuses on formation of the plastic hinges in steel structures due to underground cavities and the soil–tunnel–structure interaction of underground structures. First, an existing model was verified by finite element method and the results were compared with a sample specimen. Thus, several effective parameters were considered and studied such as soil type, multi-story structures (4, 8 and 12 stories) and dynamic load type. Then the models were evaluated under real earthquake records. As a result, the seismic response of the structures and plastic conditions of plastic hinge conditions were obtained. The results indicate that the underground cavities have affected the formation of plastic hinges in the structure. They increased the input energy to the structure and had an impact on the total behavior of the structures. Also, the high-rise structures were much more vulnerable to underground tunnels. Therefore, the structures which are located above the underground cavities should be retrofitted and rehabilitated.