The effect (internal force and deformation) produced by various factors in steel structure design is not greater than the resistance and specified limit value determined by material properties and geometric factors of structure and link.Various factors affecting the structure function, such as the size of the load, the strength of the material, the size of the cross-section and the quality of the construction, are uncertain and random variables.Therefore, the steel structure should meet the following conditions in the design:

(1) It can withstand all kinds of functions that may occur in normal construction and use, and can still maintain the necessary overall stability in the event of accidents, that is, the safety of the building structure; 

(2) It has good working performance under normal conditions, that is, the applicability of building structure

(3) It has enough durability under normal maintenance.