Seismic Design and Behavior of Steel Braced Frame Buildings with Friction Dampers as Dissipative Floor Connectors

Field observations and comprehensive life-cycle building assessment suggest that buildings designed according to today's seismic standards meet the life-safety requirement. However, they are prone to economic losses due to repairs in the aftermath of earthquakes. In the case of buildings featuring steel concentrically braced frames (CBFs), which form the main focus of this thesis, life-cycle costs are dominated by repairs in acceleration-sensitive non-structural components. This is attributable to the high absolute acceleration demands that arise from the high lateral stiffness of the CBFs and the acceleration amplification caused by the higher mode effects during earthquake shaking. Seismic repairs in steel braces due to flexural buckling is a second contributor to losses in steel CBFs. Due to the highly assymetric behavior of steel braces, steel CBF buildings are prone to the formation of local story collapse mechanisms. The above suggests that new construction concepts should be exploited to minimize widespread damage in steel CBF buildings. Within such a context, this doctoral thesis explores the use of sliding friction dampers as dissipative floor connectors for enhancing the seismic performance of steel CBF buildings. The dampers connect each floor of the steel CBF system to the diaphragms of the gravity framing system and they allow for controlled relative movement between them. They are employed as force capping mechanisms in order to (i) limit the magnitude of the absolute floor acceleration demands along the height of the building, (ii) mitigate higher mode effects, and (iii) minimize damage on the structural and non-structural components of the building under earthquake loading. In the context of this thesis, a full-scale sliding friction damper prototype is first developed and tested physically at EPFL's structures laboratory in order to identify a number of non-metallic composite friction pads that can be potentially used in supplemental damping devices. Subsequently, non-linear static and response history analyses are conduced in order to investigate the benefits of employing sliding friction dampers as dissipative floor connectors within multi-story steel CBF buildings. In this respect, a design methodology is proposed to determine the activation force of the dampers so as to ensure damage-free seismic performance in the steel CBF and the floor diaphragms of the building. The influence of the gravity framing system and damper activation forces on the seismic behavior of steel CBF buildings is also investigated. To this end, a general nonlinear modelling approach is developed to simulate the hysteretic response of four partially restrained gravity connections, which are commonly used worldwide. Furthermore, a simplified method is proposed to evaluate the effectiveness of the damper activation forces for controlling relevant engineering demand parameters of interest for damage control of steel CBF buildings. The simulation results suggest that the activation forces determined according to the proposed methodology are effective in mitigating higher mode effects and in preventing the CBF and the floor diaphragms from experiencing inelastic behavior under earthquake loading. Moreover, it is shown that the seismic behavior of steel CBF buildings with dissipative floor connectors becomes practically insensitive to record-to-record variability.