Welds in steel structures typically have a larger cross-sectional area than the corresponding base material, leading to lower stress within the weld compared to the base material. This means that undermatching welds, which use a filler material that has a lower strength than the base material, can still be acceptable.
Additionally, when the weld joint is stressed, a three-dimensional redistribution of the stresses due to the plasticity of the joint can occur, often not affecting the total design. Furthermore, the allowable stress with respect to fatigue is lower than the yield strength and, in these cases, undermatching weld material is acceptable.