Elastic And Plastic Analysis Of Combined Frame Considering Second-order Effect

Composite beams and columns working together form a new kind structure. That is composite frame. It holds a lot of outstanding merits because of using the two kinds of material's capability. It is being applied in various fields progressively and gets great development. Secure and economic design of composite frame is more and more important. And during design, the analysis of composite beam is the committed step.Characteristics are introduced in this paper including bending stiffness of composite beam in composite frame, reduced stiffness used in deflection caculation, equivalence stiffness used to caculate stiffness ratio of beam to column and stiffness changing point. Derivation of elastic and plastic element stiffness matrixes, geometric stiffness matrixes is discussed. Influence of axial force to section plastic bendingstrength of rectangle and H section is introduced. Based on this, M — P interactionequation of composite beam section in nagetive moment area is deduced. During elastic-plastic analysis, a stiffness parameter is inducted to considering plastic dvelopment around the maximum bending moment section and the geomitric imperfection is performed by further-reduced tangent modulus. So stuctrual analysis would be more precise and appropriate. Paying attention to their features, a program for analysing composite frame is generated considering the above factors. Results of various caculations are analysed and compared through examples. The used caculation ways include first-order and second-order caculations whether considering the contribution of concrete or not, whether considering the change of stiffness changing point or not, and whether considering the geomitric imperfection of members or not. And approximate second-order analysis method from Code for design of steel strcutures (GB50017-2003) is also included. The comparison of results shows that the affection of second-order effect and the change of stiffness changing point should be noticed when designing and analysing multistory composite frame. Program in this paper is able to carry out second-order analysis of composite frame considering the change of stiffness changing point and the geomitric imperfection of members, and it will have some help.