Research On Response Modification Factor And Displacement Amplification Factor Of Concentrically Braced Steel Frames

The structures will behave elastoplastic under strong earthquake and have the ability to dissipate energy,so the base shear will be much smaller than that of the elastic response.The ability of energy dissipation and ductility are made use of in modern seismic design,and the Response Modification Factor(R) is introduced to reduce the elastic response under seismic fortification intensity.The reduced values are used as the seismic action for structural design.The Response Modification Factor(R) and Displacement Amplification Factor (C_d) of Concentrically Braced Steel Frames(CBSF) were researched in this paper. Firstly,the Pushover test of the experimental model was done.And then the Pushover Analysis and Incremental Dynamic Analysis(IDA) of 24 CBSFs were carried out after analyzing the influence of local buckling and verifying the finite element method(FEM) models.Lastly,four influence factors,such as damping ratio,slenderness ratio of braces, modification of unbalanced forces on beams and data handling methods,were investigated.The lateral rigid,ductility,limit beating capacity and failure mode were researched and the yielding and buckling phenomenon were observed through the pushover test. The test results indicated that the lateral rigid of the structure would decrease markedly after the compressive braces buckled,but the horizontal bearing capacity would not decrease markedly and maintained the level before buckling,and the ductility of CBSFs was good.Furthermore,the test results also verified the valid of the FEM models used in this paper.Based on Pushover analysis and modified Capacity Spectrum Method(CSM), researches were carded out on inverted V-CBSFs,V-CBSFs,SX-CBSFs and diagonal lined-CBSFs that were no more than 12 stories.The influences of model of applying lateral force,brace shape,number of stories,and number of spans on the values of R, Ductility Factor(R_μ),Overstrength Factor(R_Ω) and C_d of CBSFs were investigated detailedly.The analysis results indicated that the influence of higher mode was increased with the stories,and the influences can be accounted for by calculating the equivalent mode of the first three modes.The influences of the number of stories and spans on the values of R,R_μ,R_Ωand C_d was little.For these four types of CBSFs, the arrangement according to the descending order of the values of R was diagonal lined-CBSFs,V-CBSFs,SX-CBSFs and inverted V-CBSFs.Based on IDA,the IDA capacity curves were drawn using the results of polynomial fitting.Then the influences of brace shape,number of stories,and number of spans on the values of R,R_μ,R_Ωand C_d of CBSFs were investigated systematically.The analysis results indicated that the influences of the number of stories and spans on the values of R,R_μ,R_Ωand C_d was so little that the difference could be ignored.In the composition of the values of R,the values of R_Ωoccupied a much bigger proportion than the values of R_μ.Four influence factors were studied,which were damping ratio,slenderness ratio of braces,modification of unbalanced forces on beams and data handling methods.The analysis results indicated that the values of R and C_d increased with the increasing of damping ratio.With the increasing of slenderness ratio of braces in CBSFs,the values of R decreased gradually,but the values of C_d increased gradually.The residually compressive bearing capacity was 70%of the buckled bearing capacity after the compressive braces buckled,so the magnitude of the unbalanced forces on beams was the vertical composition of tensile bearing capacity in tensile braces minus 70%of the vertical composition of the buckled bearing capacity in the compressive braces. Using the curve fitting based on Artificial Neural Networks(ANN),the more reasonable IDA capacity curves could be obtained.From the fitting curves,it was obvious to conclude that the structures would yield under the seismic fortification intensity,and the structures would depend primarily on the ductility to resist earthquake action after buckled.On the basis of the tests and FEM analysis,some suggestions on seismic design of CBSFs were given.Referring to many foreign specifications,a new type of special CBSF with good ductility should be added in our codes.If the method for calculating the seismic action prescribed in the Code for Seismic Design of Buildings (GB50011-2001) was adopted,a ductility modified factor,μ,was introduced to account for the difference of ductility for different structures.If the C(here the values of C was the inverse of the values of R discussed in this paper.) in the General Rule for Performance-Based Seismic Design of Buildings(CECS 160:2004) was adopted,the C=0.25 and Cd=5.0 for CBSFs.