What principles govern the classification of weld quality grades?

 In the general notes of steel structure construction drawings, have you ever written a line like this: “Drafted, stamped, and issued.” Everything seems to flow smoothly. “For this project, Grade 1 welds shall undergo 100% non-destructive testing, Grade 2 welds shall be subject to 20% sampling inspection, and Grade 3 welds shall undergo visual inspection.” However, if someone were to ask at this moment, “Why must this weld be Grade 1, while that one can be Grade 2? Who exactly determines this ‘grade’?” would your answer still flow so smoothly?

This question, though seemingly basic, represents the most fundamental—and yet most easily overlooked—underlying logic in steel structure welding design. The classification of weld quality grades is by no means arbitrary, nor is it a matter of the designer’s “whim.” It is underpinned by a clear, rigorous set of principles, proven through engineering practice, which directly determine the structural safety and service life of the structure.

The selection of weld quality grades is specified in detail in Clause 5.1.5 of the “Welding Code,” and the principles it follows are as follows:

(1) The stress state determines the grade; this is the fundamental starting point for weld classification. The quality grade of a weld is primarily related to its loading conditions. The quality grade of a weld is primarily related to its loading conditions. The quality grade of a weld is primarily related to its loading conditions. The quality grade of a tensile weld must be higher than that of a compressive or shear weld; the quality grade of a weld subjected to dynamic loads must be higher than that of a weld subjected to static loads. This is because tensile stress directly tears open microscopic defects within the weld, whereas compressive stress may actually close them; fatigue effects under dynamic loads are extremely sensitive to defects such as cracks.

(2) For all butt welds—except for partially penetrated welds treated as fillet welds—full penetration and equal strength with the base material are generally required; therefore, non-destructive testing (NDT) is necessary. The quality grade of butt welds should not be lower than Grade 2. This is because without internal defect inspection, it is impossible to ensure that the weld is truly “equal in strength” to the base material.

(3) In building steel structures, fillet welds are generally not subject to non-destructive testing, and their visual quality grade is typically Grade 3; however, in low-temperature environments, the quality grade should be Grade 2.

Only by understanding these principles can steel structure engineers truly transition from “mechanically applying tables” to “precise design based on the essence of stress.” This tells us: the number on the weld is not a designer’s arbitrary mark, but a precise reflection of structural safety requirements on the drawing.