Abstract

Hospitals are post-disaster buildings designed to withstand seismic forces that are amplified with an importance factor of IE= 1.5. Their seismic force-resisting system (SFRS) should be designed with Rd> 2.0, while the interstorey drift at each floor is limited to 1.0%hs. Herein, Rd is the ductility-related force modification factor and hs is the storey height. Although the non-structural components and the hospital contents are not part of this research, they constitute a larger loss in the event of an earthquake. As such, both interstorey drifts and floor accelerations should be within the required limits.

Concentrically braced frames (CBFs) are frequently employed as earthquake-resistant systems due to their high stiffness and moderate ductility. However, this system has shown several drawbacks such as the concentration of damage within a floor and high floor accelerations, which may be critical for acceleration-sensitive non-structural components. Recent experimental studies revealed that even moderately ductile concentrically braced frames (MD-CBF) may undergo unintended failure modes due to the limited deformation capacity of brace-to-frame connections. To overcome this drawback, it is proposed to provide an 8tg elliptical clearance band in the brace-to-frame gusset plate instead of a linear 2tg clearance, which is recommended by the code. Herein, tg is the thickness of the gusset plate. The results pointed out that gusset plates with 8tg elliptical clearance require less thickness than that with 2tg linear clearance and provide larger rotation capacity. In consequence, the ductility of MD-CBF with brace-to-frame gusset plates detailed with 8tg elliptical clearance is improved. Furthermore, in order to mitigate the floor acceleration, braces of CBFs can be replaced with sliding friction braces (SF), where each SF brace is made of a friction damper installed in-line with an HSS brace. The proposed sliding friction braced frame (SF-BF) system behaves elastically as a traditional CBF before friction devices are activated and experience nonlinear response after that. Thus, in the case of SF-BF system, the input energy is dissipated by friction devices and all adjacent members such as braces, connections, beams, and columns of the CBF system are designed to remain in the elastic range. It is noted that SF-BF systems are prone to residual interstorey drift, which can be mitigated by: (i) using braced frame’s columns continuous over all floors or (ii) adding back-up moment-resisting frames designed to provide the elastic frame action. The main objective of this thesis was three folds: (i) to investigate the inelastic behaviour of MD-CBF systems with 8tg elliptical clearance gusset plate versus 2tg linear clearance band; (ii) to develop an accurate numerical model for braces equipped with friction dampers using the OpenSees software and (iii) to examine the seismic response of SF-BF systems.

To carry out this research, a detailed model of a 4-storey hospital located in Victoria, BC on Site Class C was developed in OpenSees and subjected to 10 historical ground motions for nonlinear time-history analysis. In this manner, a model replicating the MD-CBF with 2tg linear clearance band gusset plates for brace-to-frame connections and a model replicating the MD-CBF with 8tg elliptical clearance band for brace-to-frame gusset plate detail were developed and the nonlinear time-history responses expressed in terms of interstorey drift, residual interstorey drift and floor acceleration were compared. A force-based design method was applied to design the SF-BF system. By optimizing the slip length and slip force in the damper, the slip-lock phase exhibited due to the bearing of the pretensioned bolts can be postponed while maintaining the drift below the code limits. Dynamic instability may become an issue when dampers with large slip lengths are installed.

From this research it was found that small difference was observed in the response of MD-CBFs when brace-to-frame gusset plates with 8tg elliptical clearance was selected instead of 2tg linear band detail. When the SF-BF system was designed using the force based design method, the HSS brace was proportioned such that the compression resistance of brace to be equal or greater than 130% slip force. Then, capacity design was employed to design the beams and columns of braced frames. An OpenSees model was developed to simulate the behaviour of Pall friction damper and brace assembly. From nonlinear dynamic analysis, it was found that large residual interstorey drift was observed when columns of braced frame were continuous over two storeys, although the interstorey drift is within the code limit, which is 1.0%hs for a hospital building. To mitigate the residual drift, continuous columns over the building height were considered. However, it was concluded that SF-BFs are not recommended for hospitals located in high risk seismic zones unless back-up moment-resisting frames designed for 25% base shear are provided.