Theoretical Analysis Of Axial Load Capacity Of RC Composite Circular Columns

The composite structure forms after the structure of the pier column under pressure isstrengthened with enclosed concrete.Composite compressive column is called in this paper because that its stress anddeformation have distinct characteristics of composite structure. RC composite columnunder centric pressure is one kind of structure form with good performance. It can beused to strengthen the RC column, which is known as the enlarging section method.And this provides a new selection for design of strengthening pier column. This methodhas been widely used in strengthening the substructure of the pile column in High WayBridge.Literature and document research indicate that most of them have certain limitation inthe existed theoretical analysis and experiment study on capacity of compositecompressive column. Especially the recognition to the mechanical mechanism ofsecondary loaded RC composite circular section column under centric pressure is stillnot overall and systematic. Therefore, it is necessary to consummate the calculation ofthe force, deformation, ultimate capacity, relative parameters etc. of RC compositecolumn and carry out further research in order to lay groundwork for the widely use ofsecondary loaded RC composite circular section column under centric pressure.This paper takes RC composite circular section column under centric pressurestrengthened with enlarging section method as the objection. The paper mainly studiesthe basic mechanical properties of RC composite circular section column under centricpressure strengthened with enclosed reinforced concrete. In addition, the theoreticalanalysis on the capacity of cross-section of column strengthened with enclosed RC andrelative aspects are also done in the paper.Main research work is as follows:(1) Analysis of ultimate stateAccording to the phenomena of "stress excess" in the core concrete and compressivesteel, "strain delay" in the enclosed concrete and steel, the regularity of the internalforce redistributing in the cross section of composite circular section column undercentric pressure is studied in the paper. Besides, the paper analyzes all kinds of ultimatestates that are possible for this structure and talks about the ultimate capacity.(2) Research on basic prerequisitesBasic prerequisites mainly include the basic condition that secondary loaded RCcomposite circular section column under centric pressure strengthened with enclosedconcrete should satisfy, research on minimum steel ratio of enclosed concrete, concreteconstitutive relation in calculating the capacity and etc. basic hypothesis.(3) Research on influence factors for capacityThe main factors which influence the capacity of secondary loaded RC compositecircular section column under centric pressure such as stress level of core concretecolumn, the area proportion of core concrete to enclosed concrete, the connection modeof composite surface and so on are studied according to analyzing the superpositionformula of calculating capacity.(4) Analysis of relative parametersThe parameters which is related to the ultimate capacity of secondary loaded RCcomposite circular section column under centric pressure such as improvement factorcaused by confinement of core concrete, using efficient of steel strength and enclosedconcrete, stable factor of composite circular section under pressure and so on are studiedin the paper.Main research conclusions are as follows:⑴ There have been essential differences between the columns strengthened withenclosed RC and the RC columns. Initial stress caused by the permanent loads exists inthe former ones, and these belong to secondary loaded structure. Also, the phenomenaof "stress excess" in the steel and "strain excess" in the concrete exist in the oldcolumns. However, the phenomena of "stress delay" in the steel and "strain delay" inthe concrete exist in the enclosed concrete.⑵ The factors that influence the ultimate capacity of composite circular sectioncolumn under centric pressure mainly include stress level index of core concrete, thearea proportion of old concrete and new concrete, steel ratio, stable factor and so on.⑶ Enclosed concrete of secondary loaded circular section composite column undercentric pressure should meet the basic conditions. Among these the minimum steel ratiothat ensures the confinement effect of core concrete to enclosed concrete is2sv,min 0.065 c(1 2 )yvff dρ = + δ⑷ The Hognestad model that takes the influence of creep into consideration is usedto describe the constitutive relation of core concrete and enclosed concrete whentheoretical analysis is carried out. And the constitutive relation of the steel uses obliqueline combination with horizontal line. Meanwhile, the confinement of enclosed concreteto core concrete, the tensile-compressive composite stress of enclosed concrete, thePoisson ration varying with the stress level and etc. is taken into account.⑸ Taking the ultimate state that the capacity of composite column is up to as thejudgment standard, composite RC column under centric pressure have three possibleultimate states according to the difference that the capacity after the peak compressivestrain of core concrete ε pr=0.002 may increase or decrease. Based on this the limitdiscriminant of enclosed concrete thickness is deduced.( )( )24ξ + 4ξ 2 ? 2 1 ? β ? β γ c f c 2 = 83.33 × 0.002 ??1 ? 1?β?? fc1⑹ The following method is suggested when analyzing the ultimate capacity of thecomposite column core concrete.In the second type ultimate state, the strength improvement of core concrete afterconsidering confinement effect σ cor = 4σr may be directly to the following formula.[ ]N ≤ ? αc 0 fc0Ac0+fy′0As′0+σcorAc0+αcfcAc+αsfy′As′Where the strength reduction factors isα co= 0.8333 + 0.1667 1 ? βIn the first type ultimate state, generally the strength improvement of core concreteafter considering confinement effect σ cor = 4σr and the design compressive strengthare both considered. The improvement factor of core concrete after consideringconfinement is defined as the ration of the stress calculated by the formulaσ cor = 4σr to the design compressive strength. And calculation is according to thefollowing:[ ]N ≤ ? γc 0 fc0Aco+fy′0As′0+αcfcAc+αsfy′As′⑺ The strength improvement of core concrete after considering confinementeffect can be deduced by transverse confinement stress according to the ultimate stateand considering the variational factor of Poisson ration.The strength improvement of core concrete after considering confinement effect isσ cor = 4σr.The transverse confinement stress of core concrete can be calculated by thefollowing formula.[ ( ) ( )]''212f 1 2 fc2vvvvprprσ r = c???? ???? εΔ ε????????? εΔ ε????????=???⑻ The strength improvement of core concrete after considering confinementeffect and the improvement factor considering confinement can also be deducedaccording to the maximum transverse confinement stress provided by the enclosedconcrete.The strength improvement valueσ cor = 4σ r=0.52δdfc2The improvement factor considering confinement121 10.52ccc ffdγ = +δ⑼ Using ratio of enclosed concrete and steel strengthIf the ultimate state belongs to the first type, then( )α c = 1 ?βε2γcα s = 50E0 s f2y′2?1?βIf the ultimate state belongs to the second type, thenα c=0.652222 0.002ysyprss fEfEα = ε?′=′The major innovations in the paper include:⑴ The concept of "composite compressive column" is used in the paper to describethe compressive column strengthened with enclosed concrete and reveal the mechanicalessential of this structure.⑵ The imagination of three ultimate states is presented. In addition, the theoreticaldeduce is carried out. Meanwhile, the relation between the initial stress level index andthe thickness of enclosed concrete is established.⑶ The calculation method of tensile strength, the compressive strength consideringthe tensile-compression composite effect is introduced. Based on this, the improvementfactor considering the confinement of core concrete is studied and the strength use ratioof the enclosed concrete is calculated.⑷ The calculation method of secant Poisson ration that takes the stress level intoconsideration is introduced. Especially this method makes the calculation of the strengthimprovement factor more scientific.The capacity of the members under centric pressure is the strength upper limit ofcompressive member. Also, it is an important index to estimate the section capacity andto check the strength of compressive members. Therefore, not only the significance ofthe analysis on the ultimate capacity of the column under centric pressure itself istremendous, but also this work will provide theoretical basis for deducing thecalculation formula of the ultimate capacity of eccentric compressive members,experiment and research on the composite column and the analysis and simulation usingFEM method.