Seismic Reliability Assessment of Two Case-Study Tunnel Form Buildings Considering the Effect of Soil-Structure Interaction

There are proven reasons indicating that during seismic excitation, considering substrate flexibility, i.e. Soil-Structure Interaction (SSI) may intensify displacement, change in internal members’ forces and damage and even lead to the collapse of the construction. Conventional structural design methods neglect the SSI effects. The lesson learnt from past earthquakes revealed a significant effect of SSI phenomena on the dynamic response of tall, bulky and heavy structures resting on relatively soft soils, for example, nuclear power plants, high-rise buildings, and elevated RC water tanks on soft soil. Neglecting SSI is reasonable for light weight structures in relatively stiff soil such as low-rise buildings.
The tunnel form buildings are one of these heavy and stiff systems that considering SSI may be important in modeling for seismic loading. To introduce, this system is a modern constructing technique that is recently used in mass construction projects This system lacks structural beams and columns in which only the elements of slab and wall as vertical and lateral load-bearing elements are used. The wall and upper slab are concreted at the same time. It seems that considering SSI phenomenon for such buildings concerning high lateral stiffness and weight, especially under strong earthquake or soft soil ground is of great importance. Despite widespread usage, unfortunately in the current design codes, the system is not considered as an independent structural system. Although there are valuable researches carried out on tunnel form buildings, they are still limited in a literature survey.
A literature survey shows that the experimental and numerical study to evaluate the seismic behavior of tunnel form buildings considering SSI effect is very limited. Now, in many densely populated cities with a relatively high risk of occurring earthquakes, this system is used as mass housing projects. Since the rigidity of soil bed below the foundation in analysis and design of these structures is a common assumption among designers, this study attempts to assess this presumption in a structural reliability framework. Therefore, in this study, these structures are examined through the considering SSI in analytical modeling and its influences on their seismic response and behavior. First 5 and 10-story regular buildings were designed with and without SSI modeling based on the current revision of Iranian seismic code. After controlling performance levels and responses based on probabilistic approach, the overturning and sliding factors were examined under seismic intensity levels of 0.35 and 0.65 corresponding to seismic hazard level of DBE (Design Base Earthquake) and MCE (Maximum Considered Earthquake).
It is to be noted that in this study, the elastic behavior of soil was the basic premise assumption. The results show that, with increasing building height and intensity of earthquakes, the SSI phenomenon influenced the structural responses including shear and inter-story drifts and also commence of starting position of damages. The research results indicated that the first damage level probability could be increased up to 30% and the sliding and overturning
probability increased at least 10 percent considering SSI respect to non-SSI assumption. It may be concluded that, especially in areas with high seismicity and soft soils considering SSI can reduce the reliability of this type of structures to attain predetermined performance. Thus, in the case of areas with high seismicity, soft soil and for taller buildings, special attention to the phenomenon of SSI in the seismic assessment of this structural system is necessary.