Study On N-M Curve Of Circular UHPC Filled Steel Tubular Beam-columns

With excellent mechanical performance,the UHPC filled steel tube(UFST)is gradually being applied to engineering.The author focused on two main aspects to work out the N-M correlation equation for the UHPC filled steel tubular beam-columns to guide engineering design.On one hand,this paper analyzes the failed mode,the mechanical performance of the UFST axial compression and pure bending member by static test and finite element analysis,then finally obtained simplified calculation method of the compressive strength bearing capacity Nu,the coefficient ? of axial compression stability bearing capacity and the bearing capacity Mu of bending member by combining with some relevant research.On the other hand,the paper focused on the eccentrically compressed members.To start with,the failed mode and the mechanical performance of 14 eccentrically compressed members were analyzed through static test.Then,based on the test,the finite element model of 20 groups of 180 UHPC filled steel tube eccentric specimens were established to analyze the influence of each factor on the N-M curve of the UHPC filled steel tubular beam-columns.The main conclusions and relevant results are as follows:(1)The axial compression bearing capacity of UFST is proportional to the steel ratio,UHPC core strength and steel yield strength,which is inversely proportional to the slenderness ratio.The study shows that the hoop coefficient cannot fully reflect the combined effect between the steel tube and the core UHPC.Combining the result of the experiment and finite element analysis with relevant researchs,the simplified method for calculating the bearing capacity of axial compression and the stable capacity coefficient can be obtained,which is proved appliable.(2)Increasing steel ratio,core UHPC strength and yield strength of steel can increase the bearing capacity of pure bending members.With the other parameters unchanged,the larger the hoop coefficient,the better the ductility of the UFST pure bending members.By comparing the experiment and finite element data with,calculated values of different formulas,it was found that the calculation method for the flexural bearing capacity(Mu)which was proposed by Han is still applicable and appliable to the UFST members.(3)The eccentric compression test results show that there is no obvious change in appearance before reaching the peak load,and both are compression and ductile failure.In the process of compression,the lateral deflection curve of the eccentriclly long columns specimen can be approximately half-wave sinusoidal curve.Furthermore,the hoop effect of the eccentriclly long column should be reduced in turn from the middle to the end of the column..(4)The bearing capacity of UFST is proportional to the steel ratio,UHPC strength and the yield strength of steel,which is inversely proportional to the slenderness ratio of the member.Furthermore,the hoop coefficient is not the dominate factor of the bearing capacity of UHPC filled steel tube eccentrically compressed members;the effect of the UHPC strength on the bearing capacity of the small eccentric compression member is greater than that of the large eccentric member.(5)With the increase of steel ratio,hoop coefficient and yield strength of steel,the critical eccentricity corresponding to the characteristic point B of the N-M curve(the critical point of compression failure and tensile failure)increases significantly,while the UHPC strength has little effect on it.With the increase of the slenderness ratio,the characteristic point B gradually disappears.Meanwhile,the N-M relation tends to linear.(6)On the basis of Nu and Mu,combined with the test and finite element analysis data of the UFST bending member,the N-M correlation equation of the UHPC filled steel tubular beam-columns was worked out.The calculation results are in coincidence with test results in the relevant literatures,and are generally meets the standard of safety.So it's suitable for engineering.