Investigation on the Role of Displacement Constraints in Seismic Performances of Floor Isolated Structures

Floor isolation technique is considered as a new approach in seismic design of structures in which the mass of building at each floor is isolated from the main structural system via seismic isolators. In this case, during earthquake actions, large relative movement between isolated floors and the main structural system at each floor level would be expected. In the current work, numerical studies have been carried out on a typical floor isolated building to investigate the role of displacement constraints (Stoppers) in limiting the gap between the floors and the structural system during seismic actions. A ten-story steel frame structural system located on stiff soil subjected to far-field earthquakes was selected for parametric study in this work. The structure has been proportioned in three different configurations, not isolated, floor isolated and floor isolated equipped with Stoppers. Since using floor isolation improves the structural performances of the system, a fourth configuration for the same structural system with 18% reduction in steel consumption is also taken into consideration. This structure was deliberately proportioned to provide the state of comparable seismic performances between the isolated structure and the non-isolated one. Direct time integration analyses using seven scaled bi-directional earthquake records have been carried out on the same structural system for all its configurations. According to the results of this study, floor isolation is quite effective in improving structural performances of the system. In fact, on average, floor isolation causes significant reduction on lateral displacement of the structural system (more than 50%) if it compares with the non-isolated one. The results also show the benefit of using floor isolation technique in design of structural system by decreasing the construction cost of the building (18% reduction in the weight of structural material) if a comparable seismic performance with the non-isolated structural system is required. It should be noted that, improving in seismic performances of the building or reduction in its construction cost comes in the expenses of large relative movement of isolated floors with respect to the main structural system. In the example used in this study such relative movement (average of seven earthquakes) reached to the level of 0.1 meter. However, in one of those earthquakes (Imperial Valley, 1979) this relative movement has soared up to 0.19 meters. Such large relative movement needs complicated non-structural detailing for the building assembly and expensive seismic isolators. To deal with this problem, displacement constraints have been provided for the floor movement using elastic Stoppers. The gap between Stoppers and the floor system is chosen at 0.1 meter to limit the Stopper’s functionality in structure only to the case when the system is subjected to large earthquakes. The results of parametric studies on the system with reduced structural weight (configuration fourth) shows a reasonable reduction in relative displacement between floors and the main structure in case of using Stoppers. These results were obtained using nonlinear time integration analyses on the structural system subjected to Imperial Valley earthquake record. According to these results, while Stoppers can reduce the relative movement between floors and the structural system to about 35%, they considerably add to the acceleration of the floor system (up to twice for the isolated structure at the roof level). In addition, the results also show using Stoppers may add to the inter story drift limit of the structure (up to 20%). These shortcomings in using Stoppers in this work can possibly be reduced using Stoppers with different arrangement and sophisticated characteristics.