Experimental Study On Buckling-Restrained Braces With Welding-Free Cross Section

As a new metallic energy dissipation member, buckling-restrained braces (BRBs) have advantages of stable hysteretic performance, large energy dissipation capacity and low fabrication cost. So they have been applied increasingly both in China and worldwide. This paper focuses on the issues which remained unsolved in terms of BRBs by analysis, and proposes some optimal design of BRBs to achieve the demand of high performance BRBs. This paper studies the low-cycle fatigue property and hysteresis energy dissipation capacity of different constructional BRBs in three series through component tests and makes a comparative analysis of the test results. The main contents of this paper are as follows:(1)The causes of friction in BRBs and the influence of the friction on the performance of BRBs are theoretically studied. In order to understand the development of the internal force of the core member of BRBs clearly, Fortran program is used in comparison with the experimental results. The simulation results show that the existence of friction has a significant influence on the performance of BRBs. The existence of friction will cause the non-uniform distribution of stress and strain of the core member of BRBs, and as a result, leading to the uneven distribution of energy dissipation (more at the middle and less at the end). So specimens mainly rupture at the middle of the core plate. Besides, the existence of friction is the main cause of the asymmetric behavior of tension and compression.(2)In order to reduce the friction of BRB, and to meet the demand of high performance BRB, the first series of BRBs with welding-free cross section are designed. The core plates of the specimens are casted to form cross section and the welding residual stress is avoided as a result. The results show that this type of BRBs has stable hysteretic performance, and the compression strength adjustment factor meets the requirement of AISC. The shortage of this type of BRB is that the low-cycle fatigue capacity is relatively lower and cannot reach the requirement of high performance BRB. The main reason for that is that the cast process cannot meet the design requirement. There is too much imperfection for the specimens, hence the low-cycle fatigue capacity is affected.(3)In consideration of lessons of the first batches of BRBs, the core plates of the second batches of BRBs are designed by splicing four angle steels to form welding-free cross section. Because the core plates are designed by using hot-rolled angle steels, the uncontrolled imperfections are avoided. The results show that this type of BRBs has stable hysteretic performance and the cumulative plastic deformation (CPD) can reach 2165,4176 and 1957 respectively before the first angle steel fractures. Meanwhile the other angle steels still maintain the energy dissipation ability after the failure of the first angle. The compression strength adjustment factory is lower than 1.3 which meets the requirement of AISC.(4)Three BRBs with channel steels covered on the surface of the core plate and a common BRB are designed, and tested with the low-cycle fatigue loading tests. The results show that the four BRBs have stable hysteretic performance, good low-cycle fatigue capacity and the CPD can reach more than 2000 which meet the requirement of high performance BRBs. For the common BRBs, because the core plate can develop multi-wave buckling easily and the unbounded material will crack in tension at the peak of the wave, the compression strength adjustment factory will exceed 1.3 under large strain amplitudes. For BRBs with channel steels, even if the unbounded material crack in tension, the compression strength adjustment factor is still lower than 1.3 as a result of the lower friction factor between steel and steel.(5)Through the simulation of the BRB components, the finite element program, ABAQUS, can simulate the process of the low-cycle fatigue loading of the BRBs. In addition, this paper analyzes the contact pressure in theory and simulates it by finite element program. The results show that there is no relationship between contact pressure and the width of the core plate and the contact pressure is mainly related with the thickness of the core plate, which means that the contact area increases with the increase of the thickness and as a result, the contact pressure decreases remarkable. Besides, the contact pressure is affected by the stiffness of the restrained member, but the influence degree is small. According to the simulation results, the simplified estimation formula of the maximum of the contact pressure is given in this paper, and the strength grade of the concrete or mortar can be chosen reasonably based on this formula.