Rearch Of Unified Strength Calculation Method Of Reinforced Concrete Members Under The Combined Loadings

Tests of reinforced concrete (RC) members under the combined loading indicatedthat various kinds of force strength interact each other. However, strength interactionsare not considered thoroughly in the current Concrete Structure Design Code GB50010-2002. Using bending–torsion interaction as a bridge to connect shear–torsionand bending–axial compression interaction, the strength interaction formula betweenaxial compression, shear, bending and torsion is researched in this thesis.It is pointed out that the Trilinear Model used for calculating the bearing capacityof reinforced concrete members under the combined action of shear and torsionoverstates the bearing capacity of concrete in GB50010-2002. Two modified formulasare given. Formula I modified the reduction factor of concrete torsional capacitybased on GB50010-2002. Formula II does not distinguish shear reinforcement withtorsion reinforcement, and has a definite physical meaning. The shear-torsioncalculation methods are compared between Chinese code, American code, modifiedformula I and II in the terms of comparison of test data, reliability analysis,reinforcement calculation and Linear Model checking. Results show that the Chinesecode formula cannot satisfy the reliability demands of Unified Standard forReliability Design of Building Structures GB50068-2001and cannot satisfy bearingcapacity demands based Linear Model. American code provides a much moreconservative model with a greater safety degree. The safety of Formula I and FormulaII based on Linear Model is between Chinese code formula and American codeformula, which can not only basically satisfy the demand of GB50068-2001but alsoensure engineering economy.In the current GB20010-2002, the Mu-Nucurve has a sharp point at the junctionof the large eccentric compression curve and the small one, and the small eccentriccompression curve could not be connected with the axial point. Moreover, thereinforcement calculation steps are tedious. A unified calculation method is derived toconnect axial compression, eccentric compression and pure bending. The unifiedformula has a definite physical meaning and fit the experimental data well. The stepof judging the type of eccentricity can be canceled and the calculated reinforcementarea can be returned to the unified formula for checking. The reinforcementcalculation of the unified formula is close to that of Chinese code and can meet the demand of GB50068-2001.The torsion and bending capacity of RC torsion-bending components arecalculated separately according to the methods of pure torsion and bending inGB50010-2002, which is contrary to test results and cannot reflect force state.Therefore, the computing model of the code formula is inaccurate, which causes lowreliability and the demand of GB50068-2001cannot be satisfied. Through analysis ofexisting test data, a modified formula based on1/4arc and its calculation method areproposed. Overall force correlation of torsion and bending is considered in thisformula, with a definite physical meaning. The calculated reinforcement area can bereturned to this formula for checking. Results show that the computing model of themodified formula is more accurate and its reliability can meet demand ofGB50068-2001.Using flexure-torsion interaction as a bridge to connect shear-torsion andflexure-compression interaction, a unified formula is proposed considering the overallinteractions of compression, shear, flexure and torsion. Every kind of intera ctionstrength formulas can be deduced from the unified formula and it can be easily used todetermine the security of the member under various loading states. Experimental dataare compared with the calculations of the proposed formula and good agreement isobtained. The unified formula can be applicable to strength calculation of membersunder all pure force state, axial compression-bending, shear-torsion, bending-torsion,bending-shear, axial compression-shear, bending-shear-torsion, axial compression-bending-shear and axial compression-bending-shear-torsion. Reinforcement obtainedby the unified formula is more reasonable and more consistent with the experimentalresults compared with GB50010-2002. Finally, the applicability of the proposedformula is illustrated using a calculation example of a planar frame, indicating thatthe unified formula can be used as unified failure criteria of RC components todetermine the security of the members under various loading states, and can be usedto calculate residual capacity of components under various kinds of combinedloading.