Let's calculate the capacity, per foot of joint, of this double sided, staggered intermittent fillet weld. With a length of 2 in. on 4 in. centers, there will be three such segments on each side of the joint, for a total of six segments per foot. Assuming the weld is made with E70 filler metal, each weld segment has a capacity as follows:

With six weld segments per foot of joint, the capacity is 89,000 lbs./ft.

Next, let's determine what size of continuous weld could be used to resist the same 89,000 lbs./ft.

To do so, we'll first find the load per inch of joint, as follows:

Next, based on this loading per inch, we calculate the required weld size:

Thus, a continuous ¼ in. fillet weld on each side of the joint would have the same strength.

From a welder's perspective, a continuous ¼ in. fillet weld would be much easier to make than an intermittent ½ in. fillet. But, what about the cost of welding — does the intermittent weld cost less?

We'll make the reasonable assumption that welding costs are proportional to the volume of weld metal deposited. Assuming ideal, flat faced fillet welds, for one foot of joint, the intermittent option requires the following volume of metal:

For the continuous weld:

The designer who specified the intermittent fillet welds “because they reduce the cost of welding” actually doubled the volume of weld metal required!

The amount of distortion will be roughly proportional to the volume of weld metal.

Therefore, in this particular situation, using intermittent fillet welds is likely to double the distortion.

Next month, we'll examine a few other details about intermittent fillet welds, and then we'll wrap it up with a reliable method for evaluating when intermittent fillet welds are the best answer.

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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.