| 1. IntroductionThe steel construction of high-rise building, the steel structure of large span and the lightweight steel construction has had rapid development during the past twenty years. The lightweight steel construction is widely used with the industrial factory building, storehouse and supermarket because of its specialty of economic, speediness and high efficiency. The portal frame is the most widely used in the lightweight steel construction. When designing the portal frame, designers usually design the members with the variable cross-section on the base of the variation of the members' moment. With the different use, the height of the web plate and the width of the flange can be designed to be the form of linear variation respectively or synchronously, as well as the thickness of the flanges can be taken with the different values. The form of variable cross-section, used mostly in the practical projects, just has the change of the linear variation on the height of the web plate while keep the width of the flange changeless.The displacement method and the method of the balance of the differential equation are usually used to get the elastic instability load. While these two methods are appropriate only for the elements of constant section. Researchers suggested many numerical methods convenient for the engineering, such as the Rayleigh-Ritz method, Galerkin method, finite difference method, finite integral method, finite element method ,etc. These methods are improper to be used in this thesis except the finite difference method, finite integral method.In the actual ((Code for steel structure design}) (GBJ 17-88) and for the ((Technical specification for steel structure of light-weight buildings with gabled frames)) (CECS 102 : 98), the calculation method for the stability of the members is: obtaining the axial force through the analysis of the linear-elastic inner force of the whole structrue, then checking the stability inside and outside the plane respectively on the foundation of the yielding rule of the edge. There is much illogicality with the formula for the calculation of internal plane in the actual modes for the normal steel and the lightweight steel.Besides the methods mentioned above, there are some applied methods overseas to calculate the stability of the steel compression bar. The elastic methods are beam-columns method, the pseudo load method, the pseudo lateral load method, the iterative gravity load method and the negative rigidity method. Researchers advanced the magnifying coefficient method easy to the engineering practice. The methods considering the elastoplasticity are the elastic-plastic hinge method and the pseudo load method.Based on the need of non-linear analysis, researchers brought forward nonlinear calculation methods to control the iterative constringency, such as the load controlling method, the displacement controlling method, the arc length controlling method and the pseudo load method.2. The basal theory for the stabilityThere are many non-linear manners when the elements losing the stability under static loads. There are many classifications based on different theories and different emphasis to the problem. Based on the usually used stability theory for the normal steel structure, the non-stability are divided into three sorts according to the performance after instability, as the balance fork instability, jump instability and extreme point instability. It is very important to distinguish the qualities of the structural instability, or it is impossible to rightly measure the structural bearing capacity.When the element studied lost the stability during the elastic-plastic period, the stability balance equation turn to the special coefficient differential equation, and the analytic value cannot be got. Therefore, it is needed to get the result through the numerical value method to most instances. The most used numerical value methods in the member analysis are divided into two sorts. One is to get the value directly. The other is to get the... |
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