Study On Performance And Design Method Of Buckling-restrained Braces And Gusset Plates Under Bi-directional Load

Buckling-Restrained Braces(term as BRBs)have the characteristics of yielding without buckling.It has the advantages of both ordinary braces and metal dampers.It can provide additional stiffness for the structure under frequent earthquakes,and yield to dissipate seismic energy under rare earthquakes.It is widely used in engineering.At present,the research on BRB,BRB structures and gusset plates mainly focuses on their in-plane performance;however,structures in an earthquake are always subjected to bi-directional seismic action in horizontal direction due to the randomness of the earthquake.The load transferred from the structure to BRBs and gusset plates are also bi-directional.However,there were few studies on the mechanical properties and design methods of BRB and gusset plates under bi-directional load,as well as the seismic performance of BRB structures under bi-directional load.To solve the above problems,this subject studied the loading mechanism,mechanical properties and stability of BRB under bi-directional load,the seismic performance of BRB subframe under bi-directional load,and the mechanical properties and design methods of gusset plates under bi-directional load by by theory,test and finite element analysis,and proposed the design method of BRB structure under bi-directional seism.The main researches are as follows:(1)Study on the loading mechanism BRB under bi-directional load.For rigid-ends BRB,the loading mechanism of BRB under bi-directional load was studied,and the internal force of BRB under bi-directional load was obtained,and the solution method of internal force was deduced.The yield mechanism of BRB under bi-directional load was studied.Then a method to calculate the initial yield force of BRB under bi-directional load was proposed according to the Von-mises fourth yield criterion.A BRB finite element model was modeled and a bi-directional load was exerted to it to compare theoretical solution and finite element analysis results.The results show that rigid-end BRB subjected to axial force,out-of-plane bending moment and out-of-plane shear force under bi-directional load.The difference between theoretical solution and finite element analysis results of the value of BRB internal force and initial yield force under bi-directional load was within 10%,which has high calculation accuracy.(2)Study on mechanical properties of BRB under bi-directional load.Two BRB specimens were subjected to unidirectional and bi-directional pseudo static loads respectively to compare their mechanical properties.BRB finite element models were modeled and unidirectional and bi-directional loads was exerted to them to compare their stress distribution,yield mechanism and high-order deformation characteristics.The results show that the stiffness and yield force was reduced and yield displacement was increased,the yield mechanism and high-order deformation characteristics were changed when BRB under bi-directional load,compared that under unidirectional load.(3)Study on seismic behavior of BRB subsystem frame(BRBF)under bi-directional load.Two BRBF specimens were subjected to unidirectional and bi-directional pseudo static loads respectively to compare their seismic performance and stability.BRBF finite element models were modeled and unidirectional and bi-directional loads was exerted to them to compare their stress distribution and yield mechanism.The results show that the seismic performance and stability were reduced,and the stress on the gusset plate was larger when BRBF under bi-directional load,compared that under unidirectional load..(4)Study on the stability of BRB.A BRB stability design method of restraining unit based on bending strength was proposed.According to this design method,the maximum bending moment of rigid-ends BRB under different loads(unidirectional and bi-directional load)was derived,and the checking calculation method of the bending moment strength of restraining unit was proposed.The rationality of the proposed design method was verified by finite element analysis of a BRB.The results show that under different loads,the theoretical solution of the maximum bending moment of the restraining unit calculated by the proposed design method was larger than the analytical solution of the finite element analysis,and the proposed design method of restraining unit based on bending strength was reasonable and conservative.(5)Study on mechanical properties and design method of welded gusset plates under unidirectional and bi-directional loads.Five welded gusset plate specimens were designed and applied with axial tension and axial compression respectively to study their tensile failure mechanism and compression buckling mechanism under in-plane load.According to the Von-mises fourth yielding criterion,yield prediction method of gusset plates under o unidirectional and bi-directional tensile loads was derived.A simplified mechanical model of equivalent column of gusset plate under compression was established.According to the principle of stationary of potential energy and the stability principle of compression bar,the buckling prediction method of gusset plate under unidirectional and bi-directional compressive loads derived.Several finite element models with different design parameters was modeled and unidirectional and bidirectional tensile and compression loads were exerted to them respectively to study the effects of different stiffener lengths,different cross section widths,different gusset plates sizes,and whether or not to set side stiffeners on the yield bearing capacity and buckling bearing capacity of gusset plates under unidirectional and bi-directional loads,and to verify the accuracy and conservatism of proposed yield and buckling prediction method.The results show that the proposed yield and buckling prediction methods for gusset plates under unidirectional and bi-directional loads can accurately identify whether the gusset plates yield and buckle,and have high accuracy and conservatism.Based on the research results,the design methods of welded gusset plates under unidirectional and bi-directional loads were proposed respectively.(6)Research on design method of BRB structures under bi-directional seismic action.The design method of BRB structures under bi-directional seismic action was proposed.Through a practical project,the BRB structures were designed according to the proposed design method.The results show that the proposed design method of BRB structures under bi-directional seismic action was reasonable and feasible,and can be used for practical engineering.