CYCLIC LOADING TESTS OF BUCKLING-RESTRAINED BRACES WITH POST-TENSIONED CABLES

Abstract

 Buckling-restrained braces (BRBs) pose predictable and stable hysteretic behavior with excellent energy dissipation capacity. However, when such BRBs are used in steel frames with pinned connections, damage concentration and large residual deformations may occur due to their low post-yield stiffness. In overseas, post-tensioned cables are added on the BRBs to provide the self-centering force and eliminate residual deformations following a strong earthquake. Hysteretic curves of BRBs with post-tensioned cables can be defined with two parameters, namely post-yield stiffness ratio (α) and energy dissipation ratio (β). Previous studies have aimed to change β from the bilinear (β=2.0) to flag-shaped (β=1.0) hysteresis to have zero residual deformations. However, the flag-shaped behavior may require a significant post-tensioning force due to strain and compressive hardening. As such, limited energy dissipation capacity increase the peak displacement responses. Although previous numerical studies have found that residual deformations are small enough even hysteresis is not flag-shaped, an optimal range of α and β is not well studied. A newly developed BRB named as PT-BRB is introduced in this paper to provide self-centering force by adding post-tensioned cables while the BRB core dissipates input energy. Carbon Fiber Composite Cables (CFCCs) are used as post-tensioned cables and performance of the brace is evaluated experimentally. Section 2 presents the developed PT-BRB configuration and mechanism, and discusses the appropriate α and β ranges by conducting some numerical analysis. In Section 3, a 1/3 scale specimen of the PT-BRBs are actually manufactured, and cyclic loading tests with various post-tensioning forces are performed to confirm the hysteretic properties and deformation capacities. A numerical model that reproduces the behavior of each part of PT-BRB for application to the 3D model is proposed in Section 4. It is found that numerical values agree very well with the experimental results. Note this paper presents the results of a joint research between Japan and Turkey, and the contents conform to the US standards and design guidelines.

 

 In summary, the following results were obtained:

 1) In the SDOF model, the increase of α is effective in reducing residual deformations in the range of α<10%, but the slope of decrease is small when α is higher than 10%. Peak acceleration and brace axial force increases when α increases. Additionally, when the energy dissipation ratio β is decreased from β=2.0 to β=1.5, the residual deformations decrease significantly.

 2) In the 6-story MDOF model, it was shown that partially self-centering (α=5.6%, β=1.8) can minimize the residual deformation even though the maximum deformation and brace axial force are the same as bilinear case. It was also confirmed that a uniform story drift distribution can be expected by increasing the restoring force for this model.

 3) Partially self-centering behaviors (1.0<β<2.0) are obtained from the cyclic loading tests and, all PT-BRBs showed stable hysteresis. PT-BRB-16 with the smallest post-tensioning force reached up to 3% story drift. In all specimens, CFCCs remained elastic until the core plate has fractured.

 4) A numerical model of PT-BRB is constructed by connecting the Core, Inner Tube, Outer Tube and CFCC as individual element with contact element. This model accurately captures the experimental results and track the actual hysteretic behavior.