The Research Of Second-order Effect And Design Method Of Reinforced Concrete Structure

The theoretic recognition of second-order effects in RC structures has important guidance for structural design. Although many achievements have been made recently in the research on second-order effects at home and abroad, there remains a series of important problems needed to be clarified, in order to improve the design methods which are now ambiguous in conception and rely 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)Due to the inadequate researches on the regular pattern of P -δeffect in frame at home and aboard, the rule of additional deform, additional force, the moment at the end of column and the distance of where additional moment equal zero have been studied thoroughly according to analytical and numerical method. The situation that the rule of P -δeffect in eccentrically hinged column was used instead of the rule in frame column had been changed, thus giving back the theoretical archetype to the rule of P -δeffect in frame. A suggestion of C mη? l0 method considering P -δeffect was proposed for the revision of China National Standard"Code for Design of Concrete Structures".(2)A simple, convenient and improvedη_s - l₀ method which employs curvature expression was proposed. In the method P-Δstory effect in sway frame and nonlinear curvature increment caused by sway moment were considered. Careful comparison show the accuracy of this method is as same as the elastic second analysis with stiffness reduction.(3)According to the second-order analysis, the rule of P-Δeffect in shear wall, frame-shear wall, frame-tube and tube-in-tube had been investigated at large. Based on theoretical analyses, the rule of P-Δeffect, internal force and deform in these structures were explained for the first time. The disadvantage of the integrity magnifying method that is employed in China National Code was pointed out, and an improved suggestion was proposed. Finally a judgment whetherη? l0method and floor magnifying factor method could be employed in the frame part of these tall building structures had been made.(4)According to the detailed examination on loading cases in variable bent-column and second-order analysis in design loading case, the regular pattern of second order effect in bent column was reviewed. The disadvantage of effective length in China Code which adopted the Soviet Union Code was pointed out, and a reasonable effective length was proposed. The inaccuracy in compression-bar stability analysis though initial-parameter method was found, and an improved initial-parameter method was proposed.(5)The simplified structural analysis and design method employing stiffness reduction of members, has been accepted by Chinese Standard, in which second-order effects have already been considered through elastic finite element analysis,and its accuracy mostly depends on the rationality of the stiffness reduction factor. In view of the absence of investigation in the stiffness reduction factor of bent frame, according to the non-linear finite element analyses of several single-story single-span bent frames, the stiffness reduction factor of bent-columns was suggested. And the rationality of this factor had been proved by comparison of the internal force in bent-columns and displacement of bent frames in second-order elasticity analysis and non-linear finite element analysis.