The strength problem of space frame steel structure refers to whether the maximum stress (or internal force) caused by the load of the structure or a single component exceeds the ultimate strength of the building material in a stable equilibrium state, so it is a stress problem. The value of ultimate strength depends on the characteristics of the material. For brittle materials such as concrete, its maximum strength can be taken, and for steel, its yield point is often taken.
The stability problem is different from the strength problem. It is mainly to find the unstable equilibrium state between the external load and the internal resistance of the structure, that is, the state where deformation begins to increase rapidly, so as to try to avoid entering this state. Therefore, it is a deformation problem. For example, for axial compression columns, due to instability and lateral deflection, a large amount of bending moment is added to the column, so the failure load of the column can be far lower than its axial compression strength. Obviously, axial compressive strength is not the main cause of column failure.
Classification of instability of space frame steel structures
(1) The first type of stability problem or stability problem with equilibrium bifurcation (also called branch point instability). The buckling of a perfect straight rod when its axis is under compression and the buckling of a perfect flat plate when its mid-plane is under compression both fall into this category.
(2) The second type of stability problem or stability problem without equilibrium bifurcation (also called extreme point instability). Eccentric compression members made of construction steel, which lose their ability to stabilize when their plasticity develops to a certain extent, fall into this category.
(3) Jump instability is different from the above two types. It has neither equilibrium bifurcation point nor extreme point. It jumps to another stable equilibrium state after losing stable equilibrium.
It is important to distinguish the nature of the instability types of steel structures so that it is possible to correctly estimate the stable bearing capacity of the structure. As the research on stability issues gradually deepens, the above classification seems to be insufficient. A component designed to be axially compressed will inevitably have some initial bending, and the point of application of the load will inevitably be eccentric. Therefore, if we want to truly grasp the performance of this component, we must understand the impact of defects on it. Other components also have defects that affect it.
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