Stability Calculating Of Steel Frames With Semi-rigid Connections

Connections are very important in the analysis of steel frames. Connections not only have relations with themselves, but also have relations with the beams and columns that are connected with the connections. In general, fully rigid and pinned are always assumed. But a lot of theories and practices have proved that nodes of beam-columns should be semi-rigid.In the paper, during the traditional stability analysis of steel frames, semi-rigid connections are considered. Non-linear model of and elastic-plastic are still considered. In the first analysis of structural stability, the work is to find how semi-rigid connections affect the buckling load of structure. While in the second analysis of structural stability, we will get the degree which semi-rigid connections affect the ultimate bearing capacity of structure. In the determination of rotational stiffness at the semi-rigid nodes, iterative technique is used. When the method of incremental variable stiffness is used in the plastic analysis, the loads are applied sequentially. Once the plastic hinge is formed, the loads are increased. So the whole process is divided into many stages.Two methods are used to judge whether the structure is destroyed. The first method is that the structure cannot bear any load because of too many plastic hinges. Only using this method cannot accurately reflect the influence of semi-rigid connections on stability. Before a plastic hinge is formed at the node with semi-rigid connections, the rotational displacement at this node has been too large. It is not fact. So the second method is necessary. The second method is that once a large displacement is happened at any node, it can be said that the structure has been destroyed. Using these two methods the function of semi-rigid connections on the structural stability can be known accurately.How to know a structure cannot bear any loads? A new method is adopted. This method is more suitable for numerical calculating. Firstly, it is supposed that every element can move freely. Write down the element geometric restrictive matrixes, then integrate the whole geometric restrictive matrix. The rank of the whole geometric restrictive matrix can be computed. The rank can help us know whether the structure can bear more loads. In the paper, there are some examples. From these examples, we can see the influence of semi-rigid connections on the steel frames.