1、 Concrete classification of steel structure design instability

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            (1) The first problem or the stability problem with equilibrium bifurcation (also known as bifurcation point instability).In order to improve the axial compression buckling of straight bars and improve the buckling of plates under pressure, it belongs to this type.

            (2) The stability or disequilibrium bifurcation of the second problem (also known as extreme point instability).Eccentric compression members of building steel belong to this kind of instability capacity developed to a certain extent.

            (3) The instability of jumping is different from the above two.It has neither equilibrium bifurcation point nor extreme point.After losing the stable equilibrium, it jumps to another stable equilibrium state.

             It is very important to distinguish the nature of structural instability types in order to correctly estimate the stability bearing capacity of structures.It seems that the stability of the above classification is not enough.The component designed for axial compression is actually an initial bending point, and the point of application of load is inevitable.Therefore, in order to truly grasp the performance of this component, we must understand the impact of defects on it, and other components also have defects.On the other hand, the post buckling performance of the members is studied in depth.

2、 Difference between strength and stability of steel structure

             Strength problem refers to whether the maximum stress (or internal force) caused by the load of steel structure or single member is higher than the ultimate strength of building materials under stable equilibrium state, which is a stress problem.The value of ultimate strength depends on the properties of the material.The maximum strength of brittle materials such as concrete is the maximum, and the yield point of steel is often used.

             The stability problem is different from the strength problem.It is found that there is an unstable equilibrium state between the external load and the internal resistance of the structure, that is, the deformation begins to increase rapidly, so as to avoid entering the state. This is a deformation problem.For example, due to the instability of axial compression and lateral deformation, the bending moment in the column is larger, so the failure load of the column is far lower than its axial compressive strength.Obviously, axial compressive strength is not the main cause of column failure.