Introduction and Investigation of a New Steel Brace with Self-Centering and Energy Dissipating Capabilities

Current design philosophy for conventional lateral resisting systems is that the frames should not collapse during major earthquakes, however significant structural damage in elements such as beams, braces and sometimes columns may occur. The presence of residual drift due to inelastic deformations may hinder building occupancy or functionality after major earthquakes, and may increase associated repair costs significantly. During last two decades, practicing engineers and researchers have tried to develop seismic resisting systems that can minimize and potentially eliminate residual drift due to earthquakes. Proposed structural systems utilize the so-called “self-centering” systems that can improve the seismic behavior, provide higher resiliency and overcome the significant residual drift of conventional systems. Self-centering (SC) seismic resistant systems, introduced in the literature are developed for both steel and concrete structures. For the steel structures, they may be categorized into three primary groups: SC moment frames, SC rocking systems and SC braced frames. The most important similarity between self-centering systems is that the lateral load resistance of the system has a flag-shaped hysteretic loop. That is the characteristic of systems that self-center after large lateral displacements. Considering the normal practices of construction industry in Iran, it is more feasible and favorable to use metal yielding dampers instead of viscous or friction dampers. Also considering the economic issues, self-centering mechanisms which use pretension tendons are more feasible compared to shape memory alloys. A yielding metallic damper called comb-teeth damper (CTD) provides energy dissipation mechanism. CTD is made of steel plates and includes a number of teeth that dissipate energy through in-plane flexural yielding. The new self centering brace (SCB) can substitute the conventional braces to provide desired seismic performance and to reduce residual deformations and repair costs. The proposed brace can be easily disassembled in the field which provides the possibility of inspection of the core after a large earthquake. Parameters of this system should be selected so that they can provide appropriate stiffness, strength and energy dissipating capacity. In this paper, initially the overall mechanical behavior of the device has been defined in terms of its internal components, based on an analytical approach. The mechanical equations for the SCB were decomposed into two portions, which are the pretension tendons that cause the self-centering behavior and the CTD links that support the energy dissipation mechanism. Also finite element analysis has been conducted to verify the hysteretic responses and mechanics of the proposed SCB. Based on the results, the characteristics of finite element responses have good similarity with the analytical results and show that either of the approaches are reasonable to predict the SCB behavior. Then a parametric finite element analysis has been conducted by varying the mechanical properties of steel elements to optimize the properties of the system. The results show that the desired self centering and energy dissipating capacities would be achieved using the new SCB. Lastly, nonlinear time history analyses have been performed to investigate the characteristics of some 6 story steel buildings equipped with the new SCBs. The results confirm the feasibility of using the new SCBs in braced frame structures.