1. From the load change, the extended service, the specification and the procedure change causes the structure bearing capacity insufficiency;
2. Due to all kinds of accidental deformation, distortion, disability, depression, etc., resulting in a weakened component cross-section, rod warping, connection cracking, etc.
3. The deformation, cracking and warping of the component or connection under the action of temperature difference;
4. Corrosion caused by chemical erosion and electrochemical corrosion caused the cross-section of steel structure to weaken;
5. Other errors including design, production and construction, and the use and operation during the service period.