Experience and experimentation have shown that the orientation of a fillet weld with respect to the direction of loading affects the joint's strength. To understand why, consider fillet welds applied to a tee joint, as shown in Figure 1. Because the applied load is parallel to the longitudinal axis of the weld, this is called parallel loading. Transverse loading occurs when the loading is applied perpendicular to the longitudinal axis of the fillet weld, as in Figure 2.

When parallel loading is involved, it is easy to visualize the loading on the welded connection. Three possible failure planes exist. Figure 3 illustrates the potential for the base metal to fail through shear. This can occur when the thickness of the vertical member is very thin, or when it is made of low strength material. Shear on this plane typically is limited to 40 percent of the base metal yield strength.

Figure 4 represents another potential failure plane where the shear would occur on the fusion plane between the deposited weld metal and the base metal. For an equal legged fillet weld in 90-degree, tee-joints, this plane is always 41 percent greater than the weld throat. Therefore, it is never a controlling failure mode for steel applications when the weld metal and base metal are "matched" in strength; that is, when the minimum specified tensile strength properties are essentially the same. Note that aluminum applications represent an exception.

The third failure mode and the one that typically controls in fillet weld design is failure by shear through the weld throat, as shown in Figure 5. It is easy to see why failure would occur along this 45-degree plane, since this is shortest distance from the joint root to the weld face, as represented by the largest triangle that can be inscribed inside the fillet weld.

When the same connection is loaded transversely, the potential failure modes include failure through the base metal as shown in Figure 6. In tension, this is typically limited to 60 percent of the base metal yield strength. Figure 7 illustrates the failure mode of shear in the fusion zone; just as in parallel loading, this is not a controlling mode since it is larger than the weld throat.

Transverse loading results in a different weld throat failure mode than was the case for parallel loading. Rather than occurring on a 45-degree plane, as was the case for parallel loading, the shear plane occurs on a 67.5-degree plane when the connection is loaded transverse to the fillet weld axis, as shown in Figure 8. The maximum shear stress occurs along this plane.

Another potential failure plane for transversely loaded fillet welds is illustrated in Figure 9 and consists of the maximum tensile loading plane in the fillet weld, which occurs at a 22.5-degree angle from horizontal. The size of the failure planes shown in Figures 8 and 9 are identical, but in the former behavior is limited by the shear strength, whereas in the latter, the limit is the tensile strength of the material. Because the shear strength of the weld metal is between two-thirds and three-quarters of the tensile strength of the deposited weld metal (for steel), failure does not occur along the 22.5-degree plane.

The final potential failure plane is shown in Figure 10 and represents a tensile failure in the fusion zone. Again, since the fusion zone is larger than the weld throat, this mode does not control in steel applications where matching strength filler metal is used.

For fillet welds loaded transversely, the failure plane is larger and the loading along the weld length is more uniform. The effect of these and other, lesser factors is that transverse fillet welds are about one-third stronger than parallel fillet welds.

For the lap joint shown in Figure 11, the designer can select either a pair of fillet welds that are transverse to the applied load, or can orient the same welds parallel to the applied load. While the differences in loading and their effect on the weld throat may be easier to visualize in a tee joint, the same principles apply in a lap joint. Again, the transverse weld will be approximately one-third stronger than the parallel weld. Or, in more practical terms, the transverse weld can be approximately 25 percent smaller than a corresponding parallel weld, yet the two will have similar strengths. In most circumstances, the transverse weld can be at least one standard size smaller than the parallel weld, yet carry similar loads. The transverse weld orientation will reduce production costs, compared with a weld with parallel loading.

Omer W. Blodgett, Sc.D., P.E., senior design consultant with The Lincoln Electric Co., struck his first arc on his grandfather's welder at the age of ten. He is the author of Design of Welded Structures and Design of Weldments, and an internationally recognized expert in the field of weld design. In 1999, Blodgett was named one of the "Top 125 People of the Past 125 Years" by Engineering News Record. Blodgett may be reached at (216) 383-2225.