Strength And Stability Of Reinforced Concrete Frame Column Considering Characters Of Nonlinear And Second-order Effect

The guidance of theoretic recognition to second-order effect about nonlinear strength and stability characteristics becomes more important for RC members in structural design. Although research on second-order effect for ordinary structural strength and stability problems has been made progress recently, there remains a series of important problems needed to clarify, in order to improve the design methods which are now ambiguous in conception and reply heavily on empirical experience. The research in this paper was driven by the revision of China Standard "Code for Design of Concrete Structure". From the analytical and testing results, this paper made some academic progress as follows.(1)Computer simulation of RC compressed members with different eccentricity was carried out, with strength and stability rules considering second-order effect taken into account. On this basis, the ultimate curvature corresponding to the maximal axial loading force as the main bearing capacity state is proposed. So it is necessary to add the check condition of axial loading force capacity for slender members. After identifying the rule from strength to stability failure of slender RC eccentricity compressed member, the ultimate bearing capacity check formula from strength to stability failure of such members is proposed, based on the nonlinear simulation results which were verified by testing results.(2) On the basis of analysis of nonlinear stability failure rule, the check system consisting of ultimate bearing capacity formula, the member's equivalent length, and the eccentricity modification factor Cn, is established for sway and no-sway frame's slender columns. According to the criteria of equivalent bearing capacity, the definite definitions of eccentricity modification factor Cn is proposed. The effectiveness of this method has been evaluated by a series of nonlinear finite element simulation analysis.(3)Based on the comprehensive analysis of second-order effect of sway and no-sway frame, the ultimate curvature adopted in the column's eccentricity curvature formula is not corresponding to the frame sway moment caused by the P effect in Europe and our country's RC structure code, In allusion to these problems, an improved suggestion of method adopting 77 curvature formula is presented. Verified by the nonlinear second-order structural analysis, the suggested method agrees well. In addition, it is also found for the first time that the equivalent length derived frommember isolation method is not in accordance with its actual second-order effect rule in some cases. Reasons of the discrepancy leading to inaccuracy of sway frames were identified and the improved methods were proposed.(4)Based on the member-by-member finite element analysis program formulated by our research group, the method that switches from arc-length methods to simple displacement control method was added to the program, so that the member's strength and stability failure characteristics can be simulated well. Comparison of the simulation results with both testing results of port frames under complex loading, and other eccentricity compressed column test results, shows that the simulation agrees well.(5)Equivalent elastic stiffness of the beams and columns corresponding to their nonlinear loading states was calculated one by one by using the nonlinear analysis program. And the rational uniform stiffness reduction factor used in elastic second-order analysis for China Standard "Code for Design of Concrete Structure" were proposed. These suggestions were adopted by the national code.