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T-Joints with Poor Fitup

Under ideal circumstances, the two members that constitute the T-joint should be brought as closely into contact as possible before those members are joined with a fillet weld. Along the length of a T-joint, perfect fit is never possible, and so some small gaps will exist. Larger gaps may be tolerable in certain situations. However, as the size of the gap between the two members increases, and if the fillet weld leg size is kept the same, the actual weld throat decreases.

This is illustrated in Figure 1. Taken to the extreme, this gap could approach the same dimension as the fillet weld leg size, creating a “nothin’ weld.” Externally, the weld may look identical to that of a properly prepared joint. Figure 1 shows the increased stress level that results from the applied load on the decreasing throat size. It should cause little surprise when such welds fail in service.

The AWS D1.1-98 Structural Welding Code addresses the issue of fitup in paragraph 5.22.1, which states, “The parts to be joined by fillet welds shall be brought into as close contact as practicable… If the separation is greater than 1/16 in (1.6 mm), the leg of the fillet weld shall be increased by the amount of the root opening, or the contractor shall demonstrate that the required effective throat has been obtained.” This principle is illustrated in the final schematic of Figure 1, and as illustrated by the numbers in the Table, acceptable stress levels can be maintained when the appropriate compensation is made.

The most straightforward method to avoid this type of “nothin’ weld” is to obtain good fitup. When good fitup cannot be achieved, it is important to note those joints that contain areas of poor fitup so that compensation can be made for these conditions. This requires an effective visual inspection program that includes pre-welding inspection.

T-Joints with Poor Fitup

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