The key point here is the allowable undersize of a fillet weld according to welding codes. For a 5/16 inch fillet weld, the specified nominal size is important in ensuring the weld's structural integrity. When assessing whether a weld is acceptable, specific rules define how much smaller a weld can be than its nominal size without requiring correction, particularly in the context of its length.
In this case, the notable guideline states that a weld may be undersized by a maximum of 1/8 inch provided that the undersize portion does not exceed 10 percent of the weld length. Therefore, for a 5/16 inch weld, this criterion stands: the maximum acceptable undersize is indeed 1/8 inch and is applicable when the weld's implications on strength and performance are still valid.
This recognition allows welders to understand the flexibility around weld sizes while ensuring compliance with safety and engineering standards. Since the options include varying undersized allowances, the selection of 1/8 inch emphasizes adherence to those specific thresholds defined by standards for acceptable fillet welds.