Research On Mechanical Properties Of New All-steel Assembled Buckling Restrained Braces

Due to the advantages of light weight,convenient loading and unloading,and good seismic performance,all steel assembled buckling restrained brace has attracted more and more attention in the academic field.The design method of buckling restrained brace has become more and more mature,but there is no mature method for the bolt design of all steel assembly buckling restrained brace at present,so it is very important to deduce the design method of high-strength bolt.There is no uniform standard for the loading rate of buckling restrained brace in the laboratory,and the tensile test shows that the steel performance is greatly affected by the tensile rate.In this paper,the variable rate reciprocating loading test is carried out on the buckling restrained brace specimens,and the test data are analyzed.It is found that the loading rate has an effect on the brace performance.When the support is compressed,the surface wear of the core material often occurs,which is caused by the direct contact between the core element and the restraint element.The unbonded layer has a significant impact on the seismic performance of braces,but the existing research focuses on the flexible unbonded layer,mainly asphalt,rubber and other materials.The research on the influence of rigid unbonded layer on the support performance is rare,and there is no effective method to fasten the rigid unbonded layer and the restraint element.In order to solve the above problems,this paper mainly studies the following contents through the methods of theory,experiment and finite element(1)This paper summarizes and summarizes the design method of all steel assembly buckling restrained brace.Aiming at the problem that the current specification fails to elaborate the bolt design,the selection principle of high-strength connecting bolt is deduced by using the existing theory of overall stability and local stability of core element and the tension theory of T-shaped member.Considering the effect of axial buckling of core material on high-strength connecting bolt,the high-strength bolt is obtained The design method of the bolt under the joint action of tension and shear,and the condition of multi wave buckling of the core along the strong axis direction are deduced.(2)As the mechanical properties of steel are significantly affected by the tensile rate,it is speculated that the buckling restrained brace performance is also affected by the loading rate.In this paper,four buckling restrained braces of the same size and tonnage are designed,and the loading rate is gradually increasing after low cycle loading test.The analysis of the test data shows that the loading rate has a significant impact on the seismic performance of the brace.With the increase of loading rate,the unbalance coefficient of tension and compression increases,the stability of hysteresis curve decreases,and the fatigue performance decreases.It is suggested that the strain rate of the core element should not be greater than 0.0025,which is the upper limit of the steel tensile test code.(3)The difference between the compressive and tensile properties of braces mainly depends on the friction between the core and the restraint element,and the friction coefficient of the unbonded layer is an important factor affecting the friction.In this paper,the friction coefficient between steel plate and unbonded material,i.e.steel plate,steel plate asphalt self-adhesive waterproof roll,steel plate polytetrafluoroethylene plate,steel plate modified polytetrafluoroethylene plate contact surface,was measured before the overall performance of support was studied.The relationship among friction coefficient,relative displacement and positive pressure is analyzed.The test results show that the relative sliding between the contact surfaces with similar hardness and rough surface will make the roughness of the contact surface increase rapidly,and the wear resistance is poor,so this kind of situation should be avoided in support design.The above test data provide the material parameters for the numerical analysis of all steel assembled buckling restrained brace.(4)In order to further study the effect of unbonded layer on the performance of all steel assembled buckling restrained braces and improve the compression performance of braces,this paper attempts to use rigid unbonded materials and compare them with flexible unbonded materials.Four specimens were designed.The unbonded layer was composed of unbonded layer,asphalt self-adhesive waterproof membrane,polytetrafluoroethylene plate and modified polytetrafluoroethylene plate.The low cycle reciprocating test was carried out on the specimens.The effects of unbonded layer on the failure mode,hysteretic performance,skeleton curve,tension pressure imbalance coefficient,equivalent viscous damping ratio and fatigue performance were analyzed.The results show that the flexible and rigid unbonded layers can significantly improve the seismic performance of the support.For the rigid unbonded layer,the smaller the friction coefficient,the better the seismic performance of the support.In addition,it is reasonable to use countersunk screws to fasten the inner lining plate as a non binding layer,which provides a new choice for support design.(5)Based on the experiment,the finite element models of brb1,brb3 and brb4 are analyzed by using the finite element software ABAQUS,and the reliability of the numerical method is verified by using the equivalent length method and Euler formula.The results of numerical simulation are basically consistent with those of experiment,which shows that the results of experiment and numerical analysis are correct.Furthermore,the influence of friction coefficient,bolt spacing and other parameters on the support performance is explored.