In order to ensure the bearing capacity of the load-bearing structure and prevent brittle failure under certain conditions, the appropriate steel grade and material should be selected according to the importance of the structure, load characteristics, structural form, stress state, connection method, steel thickness and working environment.Q235 steel, Q345 steel, Q390 steel and Q420 steel should be used for the steel of bearing structure, and their quality should meet the current national standards.When other grades of steel are used, they shall meet the requirements of relevant standards.For Q235 steel, killed steel or semi killed steel should be selected.

         The steel of load-bearing structure shall have the qualification assurance of tensile strength, elongation, yield strength, sulfur and phosphorus content, and the qualification assurance of carbon content for welded structure.The steel of welded load-bearing structure and important non welded load-bearing structure should also have the qualification guarantee of cold bending test.

        For the steel of welded structure that needs to be checked for fatigue, it shall have the qualification guarantee of normal temperature impact toughness.When the working temperature of the structure is equal to or lower than 0 ℃ but higher than – 20 ℃, Q235 steel and Q345 steel shall have the qualification assurance of 0 ℃ C impact toughness; Q390 steel and Q420 steel shall have the qualification assurance of – 20 ℃ impact toughness.When the working temperature of the structure is equal to or lower than – 20 ℃, Q235 steel and Q345 steel should be qualified with – 20 ℃ impact toughness; Q390 steel and Q420 steel should be qualified with – 40 ℃ impact toughness assurance.For the non welded structure steel which needs to check the fatigue, it should also have the qualified guarantee of normal temperature impact toughness. When the working temperature of the structure is equal to or lower than – 20 ℃, the qualified guarantee of 0 ℃ impact toughness should be provided for Q235 steel and Q345 steel, and – 20 ℃ impact toughness guarantee should be provided for Q390 steel and Q420 steel.When the z-direction steel is used for welding load-bearing structure to prevent the lamellar tearing of steel, its material shall comply with the provisions of the current national standard steel plate with performance in thickness direction (GB / T 5313).The design value of steel strength (standard value of material strength divided by partial coefficient of resistance) shall be adopted according to table 2-77 according to steel thickness or diameter.The strength design value of steel castings shall be adopted according to table 2-78.The strength design value of the connection shall be adopted according to table 2-79 to table 2-81.