A new self-centering brace with zero secondary stiffness using elastic buckling

Highlights

• A new self-centring brace has been introduced using elastic buckling.

• The Post-elastic stiffness of the SC-ZSB is zero and it can be beneficial where there is a limit on base shear.

• SC-ZSB provides self-centring characteristics than can be used for retrofitting of the structures

Abstract

It has been shown that having a low post-elastic (secondary) stiffness for conventional structures can be beneficial in terms of putting a limit on base shear of the structure although it could result in having a residual and permanent drift. A well-recognized example can be Buckling-Restrained Braced Frames (BRBFs) offering stable, repeatable and reliable damping while suffering from residual displacements owing to low secondary stiffness. This paper introduces a new type of self-centring brace with a flag-shape behaviour possessing a zero post-elastic (secondary) stiffness, Self-Centring Zero-Stiffness Brace (SC-ZSB), bringing the benefit of systems with minimal secondary stiffness without their deficiency. Basically, the elastic damage-free buckling of the brace is combined with the friction damping to form the intended flag-shape behaviour with zero post-elastic stiffness. The introduced bracing system will recover and re-centre at the end of buckling without any inelastic deformation, strength and stiffness degradation meanwhile providing passive damping. The proposed SC-ZSB is experimentally validated using a scaled self-centring brace. Besides, a comparative study is performed using OpenSeeS software to illustrate the seismic performance of the SC-ZSB in comparison with a BRB Frame. The results demonstrate that SC-ZSB system efficiently limits the base shear of the structure with an almost similar inter-story drift of the BRB frame, while it can eliminate any residual drifts as well.

Introduction

One of the major benefits of having a positive secondary stiffness in the load-displacement response of structural systems is the reduction of the residual drift after seismic events [1]. MacRae [1] showed that the residual displacement is directly correlated with the secondary stiffness of structures. It has been shown that buildings with the zero or negative secondary stiffness will experience a larger residual drift compared to those with positive secondary stiffness. Accordingly, some techniques have been suggested to increase the secondary stiffness such as by the employment of an elastic secondary system, either stiff or flexible, which is regarded to be one of the most efficient ways [[2], [3], [4], [5]]. In fact, controlling the residual displacement could be a critical seismic design parameter when the economic losses such as cost for realignment and repair are taken into account. In this regard, Pampanin and Christopoulos [6,7] have proposed a performance-based design method considering the residual drift requirements allowing an engineer to achieve a desirable performance for the target building. A well-recognized example of a system with negligible post-yield stiffness is the Buckling-Restrained Braces (BRBs), which has been reported to have strain-hardening ratio of about 0.005–0.020 [[8], [9], [10], [11], [12]]. Another problem induced by the residual drift could be the building vulnerability to aftershocks given the collapse probability of the structure increases. A recent study [10] showed that those buildings constructed with BRBs could be 15% more probable to collapse if the aftershock events are considered given the residual drift experienced in the mainshock. As such, researchers and practitioners have been focusing on finding efficient ways to reduce the residual drift as a significant structural response parameter by increasing the secondary stiffness.

Apart from increasing the secondary stiffness, the application of a re-centering mechanism was shown to be highly effective in decreasing the residual displacement [13]. Such re-centering mechanism can be achieved by using a post-tensioned tendons [13] or pre-pressed springs [14] coupled with a damping source (friction [15], yielding [16] or viscous [17]). Utilization of the SMA material has been also studied to provide the self-centering response [18,19]. It is worth noting that some recent techniques have been used with the main intention of reducing the residual displacement. As an example for BRBs, it can be referred to [[20], [21], [22]] where SMA (Shape Memory Alloy) rods were used to assist the recentring of the brace. Guo et al. studied the application of friction damping and pre-pressed disc spring to form a new tension-only brace [23]. Numerous novel self-centering LLRS systems have been suggested and experimentally verified in the literature, and readers are referred to [19,24,25] to see a comprehensive review. Other researchers also studied the application of bare friction dampers to be applied in structures as a low-damage system [15,[26], [27], [28]] without any need for a secondary system for recentring.

Resilient Slip Friction Joint (RSFJ) was developed by Zarnani and Quenneville [29] with inherent self-centering and damping and have been used in different damage-free LLRSs including rocking shear walls, tension-only and tension-compression brace [[30], [31], [32], [33]]. This study introduces a new SC bracing system with zero secondary stiffness (SC-ZSB) using RSFJ as the friction damper. It should be noted that nearly most of current SC braces have a positive secondary stiffness while the proposed SC brace possesses a zero secondary stiffness owing to taking a controlled damage-free elastic buckling as a means to zero the post-elastic stiffness. It should be noted that this self-centring brace with zero post-elastic stiffness could be utilized in some practical situations where there are limitations on both base shear capacity and residual drift of the structure. Furthermore, experimental tests were performed on three scaled specimens to further illuminate the concept. At the final stage of this study, a comparative study is conducted through which the performance of a building with proposed braces is compared to that of a building using Buckling-restrained Braces (BRBs).