By OMER W. BLODGETT, ScD., P.E.

Paying attention to your groove weld root details is important. When a butt joint requires a full strength connection, complete joint penetration (CJP) groove welds are typically specified. Unless the material involved is relatively thin (say, less than ¹ /4-in.), some type of cavity or groove is usually called for, to ensure that the full cross section of the joint can be fused by the welding process. The cavity may take on any of several forms, as illustrated in Figure 1. While double-sided joints are possible, only the single-sided options are shown here.

The gap between the two plates, called the root opening, usually is designated as "R." The total angle between the two inclined surfaces of vee and bevel groove preparations is known as the included angle, and is indicated as "". The combination of the root opening and the included angle must be such that the welding process is capable of achieving uniform and consistent fusion to the root of the joint.

Figure 2 shows four acceptable combinations of these two variables. Notice that narrow included angles are possible, when a suitable root opening is provided. On the other hand, with a large enough included angle, smaller root openings are possible.

The open root joint shown in Figure 2a has a narrow root opening with no backing. It can be welded successfully only if the root opening is relatively small. When the root opening is too large (Figure 3a), melt-through will occur (Figure 3b). Open root joints often employ a root face dimension, also called a "land," (Figure 3c) that can help limit melt-through.

Larger root openings typically require backing, as shown in Figures 2b, 2c and 2d. This detail can be advantageous in cases in which the gap between the two members is variable.

Selecting the optimum combination of root opening and included angle entails the consideration of many factors. Fusion to the root is required, as has been mentioned. Narrow root openings and tight included angles (Figure 4a) might inhibit good fusion (Figure 4b). An improper width-to-depth ratio will result in centerline cracking, and the groove root configuration can directly lead to this condition (see Figure 4c). The need for both fusion and crack free welds require details with larger included angles and larger root openings.

The configurations that are ideal for fusion and sound welds require more volume of weld material to be deposited, increasing welding costs. This results in an economic incentive to avoid details that require unnecessary additions of weld metal. Furthermore, such welds will result in additional distortion. So achieving a balance is necessary.

While the necessity to balance root opening and included angle dimensions has been known for many years for plate welding applications, tubular construction as shown in Figure 5 reinforces this in a new way. The intersecting orientation, known as the dihedral angle, can be (as shown in Figure 6) acute, 90 degrees, or obtuse in tubular construction. However, the welds that join the tubes often are CJP groove welds, made from one side. The root openings are held usually to small dimensions, and the dihedral angle becomes the included angle of the groove weld. AWS D1.1 Structural Welding Code-Steel has established a series of pre-qualified joint details for tubular connections, as shown in Table 3.6. As the dihedral angle decreases from 90 degrees, the likely depth of fusion also decreases, a factor that must be considered in the design — see Figure 7. In some cases, weld metal may be deposited in the root, but good fusion is unlikely. This is termed a "backup" weld, and it is not capable of transferring loads. The lack of fusion in the root is called a "Z-loss" factor, and tabular values are contained in D1.1, dependent on the weld process, dihedral angle and welding process.

Although AWS D1.1 may not specifically govern your work, using the pre-qualified joint details as described and illustrated in Section 3 of the Code can be helpful in ensuring weld quality in the root. Alternately, you can make test samples to evaluate the quality of the weld root. In all cases, it pays to "remember your roots."

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.