Development of A New Self-centring Low-damage Bracing System for Earthquake Resistant Structures Using Resilient Slip Friction Joints (RSFJs)
According to the state-of-the seismic design practice, there are two accepted principles to be obeyed when designing a structure against different levels of earthquakes. The first and the explicit one is that the life-safety of the inhabitants must be assured by sacrificing the fuse elements with the intention of dissipating the input earthquake energy at the time of Ultimate Limit State (ULS) earthquake. The second and the implicit one is that the collapse of the building must be avoided at the time of any event beyond this level (up until the Maximum Considerable Earthquake). Adoption of these principles may bring a number of post-event consequences for the buildings such as structural and non-skeletal damage spreading throughout the structure (such as permanent deformation and storey-drift, strength and stiffness deterioration/loss and so on). Dealing with all of those repercussions on the scale of an urban area or a city with many buildings will put extra burden and demand on the local and national economy in the aftermath of a severe earthquake, which is the most critical time. This may prolong the time of recovery and bring consequent societal and economic short- and long-term impacts. Low-damage structures, as implied by their name, is referred to those structures whose sacrificial elements can be easily and quickly replaced or repaired, thereby resolving a portion of the above-mentioned complications. However, the problem with the possible permanent drift and accumulation of structural damage may still exist in the low-damage structures. In general, if the permanent drift surpasses a certain limit (normally between 0.2% - 0.5%), complete demolition of the structures or retrofit/realignment program would be required. To tackle this issue, the self-centring low-damage structures seem to be remedial and desirable in terms of eliminating the residual drift in buildings. This would be very crucial for the structures with high importance level (IL) because they shall stay operational after a seismic event to keep providing service for the community. In this manner, low-damage self-centring braces have the potential to become one of the popular lateral load resisting systems. They can provide not only a large elastic stiffness to control the inter-storey drift when shaking with low-amplitude seismic events (serviceability limit state) but also the passive damping, energy dissipation and self-centring characteristics to meet the resiliency requirements when shaking with large-amplitude seismic events (Design level earthquake or beyond). This study develops a new self-centring low-damage brace using RSFJ dampers which will act both in tension and compression. As for any element subjected to compression, the main challenge for design is the considerations for lateral instability and quantification of the ultimate capacity. Thus, the main purpose of this study is that how the brace can be designed and detailed for an intended level of compressive force in a way that the desired performance of such self-centring brace is kept uninterrupted until the intended force and deflection. More specifically, a series of small, large and component-level experimental studies have been conducted in this research program on RSFJ self-centring brace and has shown that the performance of the RSFJ self-centring brace can be interrupted by different failure modes when working in compression. These failure modes are categorized into two groups namely: local and global instabilities. The local (localized) instability is associated with the damper being not able to transfer the axial compression while the global instability is referred to the whole brace assembly being not able to resist the axial compression perfectly. It was found that the local instability is greatly influenced by the boundary conditions of the damper (end support) while the global one is more sensitive to the damper and brace sectional and member properties. The global failure mode, itself, can be of two types, elastic and inelastic, depending on the length, geometry and characteristics of the brace and other components. In order to quantify and predict the mentioned failure modes, proper analytical frameworks, supported by numerical simulations as well as small- and large-scale experimental tests have been performed throughout the study. Classic structural stability analysis (second-order differential equations of equilibrium) and second-order simplified plastic analysis (which is referred to as simplified collapse analysis – SCMA – in the text) has been employed to quantify the elastic and inelastic global buckling capacity of the brace. After proposing the design guide, the seismic performance of a prototype building equipped with the proposed lateral load resisting system is studied. According to the findings, employing this type of system in a structure will contribute to restricting the displacement demands, reducing the base-shear and floor acceleration and bringing the structure back to its upright position.