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Designing Fillet Welds for Skewed T-joints

Detailing fillet welds for 90-degree T-joints is a fairly straightforward activity. Take the 90-degree T-joint and skew it—that is, rotate the upright member so as to create an acute and obtuse orientation, and the resultant geometry of the fillet welds becomes more complicated (see Figure 1). The greater the degree of rotation, the greater the difference as compared to the 90-degree counterpart.

A series of equations can be used to determine weld sizes for various angular orientations and required throat dimensions. Since the weld sizes on either side of the joint are not necessarily required to be of the same size, there are a variety of combinations that can be used to transfer the loads across the joint. While there are theoretical savings to be seen by optimizing the combinations of weld sizes, rarely do such efforts result in a change in fillet weld size of even one standard size.

Codes prescribe different methods of indicating the required weld size. These are summarized herein. When acute angles become smaller, the difficulty of achieving a quality weld in the root increases. The AWS D1.1 Structural Welding Code deals with this issue by requiring the consideration of a Z-loss factor. This edition of Design File addresses the situation where the end of the upright member in the skewed T-joint is parallel to the surface of the other member. A future Design File column will consider the situation in which the upright member has a square cut on the end, resulting in a gap on the obtuse side. Also to be addressed in the future are weld options other than fillet welds in skewed T-joints.

The Geometry
Figure 1 provides a visual representation of the issue. For the 90-degree orientation, the weld throat is 70.7% of the weld leg dimension. This relationship does not hold true for fillet welds in skewed joints. On the obtuse side, the weld throat is smaller than what would be expected for a fillet weld of a similar leg size in a 90-degree joint, and the opposite is the case for the acute side. These factors must be considered when the fillet weld leg size is determined and specified. Careful examination of the fillet welds on the skewed joint raises this question: What is the size of the fillet weld in a skewed joint?

Figure 1 illustrates the fillet weld leg size for a skewed Tjoint, and is designated by “?.” This, however, is inconsistent with AWS Terms and Definitions (AWS A3.0-94) which defines a “fillet weld leg” as “The distance from the joint root to the toe of the fillet weld.” According to this definition, and as shown in Figure 1, the fillet weld leg is dimension “b.” The dimension that is labeled “?” is the distance from a member to a parallel line extended from the bottom weld toe.While not technically correct according to AWS A3.0, it is the dimension and terminology used when fillet welds in skewed joints are discussed in the AWS D1.1 Structural Welding Code, as well as other AWS publications (i.e., The Welding Handbook, ninth edition, volume 1). Such terminology will be used here.

This raises an additional question: What would a weld inspector actually measure when dealing with a fillet weld in a skewed T-joint? Conventional fillet weld gauges could be used to measure the obtuse side’s fillet weld leg dimension “?” as shown in Figure 1. Dimension “b” would be difficult to measure directly since the location of the weld root cannot be easily determined.Welds on the acute side are impossible to measure using conventional fillet weld gauges. The face dimension “f,” however, offers an easy alternative: when this dimension is known for the weld size and the dihedral angle, the welder and inspector can easily determine what the actual size is by using a pair of dividers. Alternately, a series of simple gauges of various widths could be made to directly compare the requirements to the actual weld size. Thus, dimension “f” may be important for controlling weld sizes in skewed T-joints.

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