According to the “Uniform Standards for Structural Reliability Design of Buildings” (GB50068-2001), structures and members can be divided into three categories of importance, general and minor according to their uses, parts and severity of damage consequences, and the corresponding safety levels are one, two and three. Different categories of structures or components should be selected with different steel, heavy industrial building structures, large-span structures, high-rise or super high-rise civil building structures and heavy work system crane beams or structures are important first-class structures, should be selected with high-quality steel; general industrial and civil building structures and other secondary structures, according to the nature of the work can be used for ordinary quality steel. Temporary housing skeleton, general buildings within the subsidiary components such as ladders, railings, etc., are secondary to the three types of structure, can choose poor quality good steels.
The load can be divided into two kinds of static load and dynamic load. Structures directly subjected to dynamic loads and structures in strong seismic zones should use high-quality good steels with good toughness and fatigue resistance, such as Q345 good steels, and propose suitable additional guarantee items. And generally bear static load or indirect dynamic load effect of the structural elements, in room temperature conditions can be used in the lower price of Q235 good steels.
There are two types of connection methods for good steels structures: welded and non-welded. As the welding process will produce welding deformation, welding stress and other welding defects (such as biting flesh, porosity, cracks, slag, etc.), there is a risk of causing cracks or brittle fractures in the structure. Therefore, the welded structure should have stricter requirements on the material. For example, in terms of chemical composition, the welded structure must strictly control the limit content of carbon, sulfur and phosphorus, while the non-welded structure of carbon content can be reduced requirements.
4、The temperature and environment in which the structure is located
The structure is located in the environmental conditions, such as temperature changes and corrosive media, etc., on the mechanical properties of good steels has a great impact. In low-temperature conditions of the structural components, especially welded structures and tensioned components, is very susceptible to low-temperature cold brittle cold brittle fracture damage, should be selected with good resistance to low-temperature brittle fracture performance of the township steel. In the corrosive medium of the steel structure, such as the steel plant of chemical enterprises, should use corrosion-resistant steel, and strengthen the exposed steel members of the anti-rust treatment. In addition, the steel of the open-air structure is susceptible to aging, the role of harmful media good steel susceptible to corrosion, fatigue and fracture, should also be differentiated to choose different materials.
Thin steel rolled more times, rolling compression ratio is large; thickness of good steels compression ratio is small. Therefore, the thickness of steel is not only low strength, and plasticity, impact toughness and welding performance is also poor. Therefore, the thickness of the welded structure should be used for better material steel.