Research On Stiffness Of Reinforced Concrete And Pre-stressed Concrete Superposed Flexural Member

As we all know, concrete superposed structures have been caused by the combination of pre-cast and cast-in-situ. The superior structures are widely used, and the advantages of both assembly and cast-in structure are outstanding during especially the design of floor system. Remarkable economic effect had been proved by a great number of engineering cases, and further development and research of composite structures is up to engineering requirements.The formulas for calculating short-term stiffness of reinforced concrete or pre-stressed concrete superposed flexural members in Code for Design of Concrete Structures (GB50010-2002) are made up of two-stage expressions-pre-superposed and post-superposed. Being taken fully into account mechanical properties of composite members, the formulas give a clear concept and the calculation result is accurate. But deformation computation is divided two steps and two-stage stiffness and two-stage deformation are calculated,which make the process complicated. Because of without synthetic stiffness expression of reinforced concrete or pre-stressed concrete superposed flexural members, it is difficult to find the law of stiffness change of the flexural members during the second loading stage.Based on previous achievement and the characteristics of two-stage casting and two-stage loading, in this paper, concrete mechanical principles and strips method that realize numerical integral are applied to calculate the complete curve of load~ deformation of concrete superposed beams, and the calculation program is worked out. After further research the law of stiffness change is given, and synthetic stiffness analytical formulas of RC and PRC superposed flexural members are put forward in theory. Then the analytical formulas are simplified in order to easy application during the engineering design, and a checking method to the deformation under short-period loading is presented. Four problems have been mainly solved in the paper as the following:1 In this paper, a nonlinear finite element method with taking variable stiffness along length into account has been put forward, and a two-dimensional nonlinear finite element analysis program has been compiled successfully with C language. Load ~ deformation curves of RC or PRC superposed members have been demonstrated on the base of Matlab platform.2. Based on concrete mechanical, principles, synthetic stiffness analytical formulas of RC and PRC superposed flexural members have been deducted in theory.3 The analytical formulas have been simplified. The calculation result is accurate while the operation is simple and the calculation precision is up to engineering requirements.4 A checking method to the deformation of RC and PRC superposed flexural members under short-period loading has been presented with engineering examples.At last, some problems about this research worth to be studied further are put up.