Theoretical And Experimental Study On Fabricated T-shaped Buckling-restrained Braces

Buckling-Restrained Braces（BRB）can overcome the stability problem of common center braces,so it can fully utilize the hysteretic behavior of metals after yielding to dissipate energy.Under small earthquakes,BRB can provide enough lateral stiffness for the structure.Under medium or large earthquakes,the brace core can dissipate the seismic energy after yielding to reduce the seismic response of the structure and avoid greater damage to the main structure.However,the traditional BRB has generally low bearing capacity.In addition,because of the full section of the core member wrapped by the restrained component,it is harmful to rapid evaluate core damage after the earthquake.In view of the above problems,the cross-sections of the double T-shaped and H-shaped core were presented and the partial restraining mechanism was introduced in this paper.The experimental,numerical and theoretical studies were carried out.The main work are as follows:（1）By weakening the web in the middle of the H-shaped steel（opening the rectangular hole in the middle of the web）,a double T-shaped yield section was formed,and the double T core was restrained by the constraint component assembled by high-strength bolt to form a fabricated double T-shaped BRB.The quasi-static reciprocating loading tests on three T-shaped BRB were conducted to investigate the influence of the transition section and web height on the T-shaped BRB’s performance.Test results show that the transition section had little effect on the hysteretic behavior of the test piece before failure.The presence of the T-shaped core web can effectively reduce the local buckling of the flange and improve the hysteretic behavior of the buckling restrained support.（2）On the basis of the double-T-BRB test,the web of the double T-shaped core was integrated to form the H-shaped core yield section.In order to ensure that the middle part of the core is yielding and the end is always elastic,a stiffening plate parallel to the web was welded on both sides of the end web to form an H-shaped elastic section.The H-shaped core was restrained by the constraint component assembled by high-strength bolt to form a fabricated H-shaped BRB.On this basis,the pseudo-static reciprocating loading test of three H-shaped BRB with different constraint width ratios were carried out to study the effect of restraint ratio on the properties of BRB specimens.Test results show that when the constraint width is relatively large（in this paper,0.5）,the hysteresis curve of the specimen is full and the out-of-plane deformation of the unconstrained section inside the flange is small.When the inner surface of the core flange was unconstrained,severe local buckling occurred at the end flange of the core yield section of the specimen.（3）The finite element software Abaqus was used to establish the entity model of the specimen.The numerical analysis showed that the simulated hysteresis curves and the deformation modes were in good agreement with the test results,indicating that the numerical model had a high accuracy.The larger normal contact force between the core and the restrained parts is the reason that the specimens had larger bearing capacity coefficient.The upper and lower T shapes of the double T core are two independent balancing bodies,and the T cores can be analyzed separately.For the double T-shaped BRB,if a certain structural measure is taken to reduce the stress concentration of the core,its hysteretic performance is comparable to that of the H-shaped BRB.（4）The effect of two key parameters,T-shaped core web height to thickness ratio h_w/t_w and width ratio2b_re/b_f,on the hysteretic behavior of T-shaped BRB was further studied by using the finite element software Abaqus.Results of numerical analyses show that when the height to thickness ratio of T-shaped core web is relatively small（1.33 in this paper）,the outer deformation of the core flange is too large,and when the height to thickness ratio of the web is larger（9.33 in this paper）,the core web will have serious local buckling.The larger restrained width ratio of the T-shaped core flange is,the larger equivalent viscous damping ratio of the T-shaped BRB is,and the smaller the outer deformation and maximum compressive strain are.Therefore,in order to ensure the stable hysteretic behavior of the T-shaped BRB,it is suggested that the height to thickness ratio of the web should be 6.67,and restrained width ratio of the flange should not be less than 0.4.（5）The core unrestrained segment with partial restraining braces was simplified to unidirectional uniform pressure plate with fixed support at the loading edge and simple support at the unloading edge.Based on the theory of small deflection and large deflection,the elastic critical buckling load and post-buckling load of the plate were derived,and the design formula for critical width to thickness ratio of the core restrained section was proposed by combining the simplified boundary conditions of the core unrestrained section.Using two plastic theory of Stowell and Bleich,the elastic buckling theory of the plate was extended to the elastic-plastic state,and the elastic-plastic critical buckling stressσ_cr of the plate was derived.The constitutive models for two materials of Bilinear and R-O were introduced to obtain the elastic-plastic critical width to thickness ratio of the plate.