Last month, we learned that the AWS D1.1 Structural Welding Code — Steel permits fillet welds to be full length, stopped short, or have end returns…with some exceptions. This month, we'll look at the exceptions and the reasons behind them.
Many readers of this column, no doubt, are not obligated to follow the requirements of D1.1. However, the principles behind these requirements are not limited to structural applications, and in fact constitute good practice in general.
Four exceptions are listed in clauses 126.96.36.199 through 188.8.131.52 in the 2008 edition of D1.1. In each case, the exception requires that the weld be stopped short; the welds cannot be full length nor have end returns in these cases.
The four exceptions are justified either upon concerns for workmanship, or for flexibility in the connection.
The last example cited in the code will be our first illustration of a workmanship-based concern. This deals with welds on opposite sides of a common plane, as shown in Figure 1.
When making the transverse weld, it is easy to see that starting or stopping the arc on the edge of the part will be difficult. Further, if this assembly is made in the shop, the part probably will be rotated to permit the longitudinal fillet welds to be made in the horizontal position. Thus, it is unlikely that the three welds will be made at the same time.
If the welds are to be tied together, the welder will need to “dab” a bit of weld on the corner of the part, hopefully tying this “dab” into the ends of the other welds. The corner will likely be melted away, leaving a notch in the main member. Recognizing this, D1.1 does the obvious thing and requires the weld to be interrupted at the corner common to both welds.
The second workmanship-based exception involves the condition shown in Figure 2.
For lap joints where one part extends beyond the edge of a part subject to tension, D1.1 requires that the fillet weld terminate not less than the size of the weld from the edge. This is to prevent the highly probable situation of the weld melting away the edge, or creating a notch.
This lap joint condition occurs, for example, between the tee chord and the web member of a truss. The hold-back requirement is imposed only when the loading involved is tension, because notches that create stress concentrations are most damaging in applications that involve tensile stresses.
Welds are inherently rigid. Sometimes, a connection requires flexibility or “give.” In simple terms, welds cannot be placed in such locations.
An example of such a condition involves attachments where flexibility is expected, as is the case with simple clip angle connections shown in Figure 3.
Notice that the flanges are not welded — only the web is attached. When loaded, connections of this type are expected to flex, allowing for some rotation of the beam as compared to the column support.
To enable the expected flexibility, D1.1 requires that, if end returns are used, they shall not exceed four times the weld size — that is, a mandatory hold-back provision applies. AISC further restricts the end return to be no longer than half the width of the part (AISC Steel Specification J2.2b(2) ). That permits the angle to flex where it is unwelded.
In this case, more weld is less! When the weld is made too long, the angle will be fixed rigidly to the column and the expected rotation will be lost.
Another example where flexibility is required is when a fillet weld joins transverse stiffeners to the webs of plate girders, as shown in Figure 4.
For girders with thinner webs (≤ ¾ in. thick), and when the stiffeners are not welded to the flange (the typical case), D1.1 requires that the fillet welds joining the stiffeners to the webs be held back not less than four times nor greater than six times the web thickness.
The hold-back dimension is measured from the toe on the web of the web-to-flange weld. Experience has shown that if this distance is not maintained, cracking can occur at the end of the weld when the girders are shipped due to slight flexing of the web with respect to the flange. Providing this hold-back dimension gives the web some ability to flex and accommodate the strains imposed by shipping.
This distance need not be maintained for heavier webs, which are stiffer, nor for situations where the stiffener is welded to the flange because there is no relative movement of the web with respect to the flange. Excessive hold-back dimensions may result in localized buckling.
A practical reality
These four exception clauses describe situations in which welds should be held back from being full length, either due to likely workmanship problems, or because of the need for some local flexibility.
However, for some weldments, it is desirable to “weld all around” or “seal weld” the assembly. Concerns about corrosion may drive such requirements. In some sanitary applications, sealing of the joints may be required, especially when “wash down” conditions exist. The weld designer is then confronted with another challenge, namely, reconciling conflicting requirements.
Let's look at the easier one first — the workmanship-related concerns. While it may be more difficult to make full length welds or welds with end returns, it is not necessarily impossible to do so. Accordingly, when required by the application, welding all around the joint can be done, but should be approached with due caution.
Special welding techniques may be required, and welders must be trained to handle these unique challenges. Applicable code compliance issues must be resolved. Finally, such requirements should be carefully noted since weld inspectors, familiar with exceptions like those noted, are apt to mark such work as unacceptable.
Now to the more difficult question: How does one handle the welded details where flexibility is required and the joints are also required to be sealed? The unfortunate reality is that welding the full length of the end of the angle in Figure 3 will prohibit the connection from performing as designed by the engineer. This case requires an alternative design.
As to the example in Figure 4, a simple solution would be to weld the ends of the stiffener to the flanges. This will keep the web from rotating with respect to the flange, and the flexibility in shipping will no longer be required.
Omer W. Blodgett, Sc.D., P.E., senior design consultant with The Lincoln Electric Co. He is the author of Design of Welded Structures and Design of Weldments, and an internationally recognized expert in the field of weld design. Blodgett may be reached at (216) 383-2225.