Structural steel buildings are the new trend in reinforcement and renovation. With their widespread application in the construction industry, the selection of materials, the execution of construction projects, and safety in use cannot be overlooked.

The strength indicators of steel consist of the elastic limit e, yield limit y, and tensile limit u, with the design based on the yield strength of steel. High yield strength can reduce the self-weight of structures, save steel, and lower costs.

Ultimate tensile strength "u" refers to the maximum stress a steel can withstand before it fails. At this point, the structure loses its serviceability due to significant plastic deformation, but it does not collapse, meeting the requirements for resisting rare earthquakes. The value of u/y can serve as a parameter for the steel's strength reserve. The plasticity of steel generally refers to the ability to undergo noticeable plastic deformation without fracturing after the stress exceeds the yield point. The main indicators for measuring the plastic deformation capacity of steel are the elongation and the area reduction ratio.

The cold bending property of steel is a measure of its resistance to cracking when it undergoes plastic deformation during bending at room temperature. The cold bending property of steel is tested through cold bending tests to evaluate its bending deformation performance at specified bending degrees.

The weldability of steel refers to the ability to achieve a high-performance weld joint under certain welding processes. Weldability can be categorized into two types: the weldability during the welding process and the weldability during service. Weldability during the welding process refers to the sensitivity of the weld and the adjacent metal to produce thermal cracks or cold shrink cracks during cooling. Good weldability means that under certain welding processes, the weld metal and the adjacent base material will not crack.

The research on adding floors to steel structures initiated an early transformation of existing Chinese buildings. To accommodate the rapid development in structural reinforcement, renovations, and additions of steel structures, as well as to better complete the addition and reconstruction of old buildings, China established academic research groups such as the Housing Addition and Reconstruction Technology Research Committee and the Reinforcement Technology Committee. This has fostered an active academic discussion and exchange of experience for the addition and renovation projects of steel structures.