Transverse butt welds or T-butt welds and fillet welds with stress perpendicular to the length direction of the weld are first-class tensile welds. For some requirements on the definition of weld grade, please refer to Chapter VII Connection Calculation in Code for Design of Steel Structures (GB500 17-2003). 7. 1 Weld connection 7. 1. 1 Weld shall be of different quality grades according to the importance of structure, load characteristics, weld form, working environment and stress state, etc. , according to the following principles: 1 In the components that need fatigue calculation, the butt weld should be completely penetrated, and the quality grade is 1) vertical force. 2) The longitudinal butt weld with stress parallel to the weld length direction shall be Grade II. 2. In the components that do not need to calculate fatigue, the butt weld with the same strength as the base metal should be fully penetrated, and its quality grade should be not lower than Grade II in tension and not lower than Grade II in compression. In the heavy-load working system and medium-load working system with hanging weight Q≥50t, it is required that the T-joint welds between the web and the upper flange of the crane beam and between the upper chord and the node of the crane truss must penetrate, and the weld forms are generally butt joint and fillet joint, and the quality grade should not be lower than the second grade. 4 fillet weld of T-joint or butt weld without penetration is not required, and the quality grade of fillet weld of lap joint is: 1) For the structure that directly bears dynamic load and needs fatigue checking, and the middle crane beam with crane lifting capacity equal to or greater than 50t, the quality standard of weld appearance should reach Grade II; 2) For other structures, the quality standard of weld appearance can be Grade III.