As we consider some options, three principles of weld design will be illustrated and in the process, we’ll learn how to get the most out of each pound of weld metal.

One pound of steel weld metal occupies 3.53 in3. Weld metal can be deposited in a variety of configurations. If the weld metal is used to make a perfectly shaped cube, one pound would make a cube with edge dimensions of slightly more than 1.5 in. If formed into a sphere, it would have a diameter of slightly less than 1.9 in.

A more realistic use for one pound of weld metal would be to make fillet or groove welds. The length of weld we can make with one pound of weld metal depends on the type and size of weld needed.

To keep things simple, we will consider welds of a perfect size and with no reinforcement. Figure 1 includes the means of determining weld volume. If the required weld is a 1 in fillet, then each foot of such a weld has a volume of 6 in3.

To calculate how many feet of 1 in fillet weld can be made with one pound of weld metal, 3.53 in3 is divided by 6 in3/ft, resulting in 0.59 ft of weld. Similar calculations were done for in and … in fillets and the lengths possible are shown in Table 1. Next, the process was repeated for 1 in, in and … in partial joint penetration (PJP) groove welds where a 45 degree included angle was assumed–see Figure 2. Notice that this resulted in the same values as were obtained for fillet welds of similar size.

Finally, similar computations were made for complete joint penetration (CJP) single bevel groove welds with a … in root opening and 45 degree included angle as shown in Figure 3. CJP groove welds with throat dimensions of 2 in, 1 in and in were compared.

Examining the lengths of the various possible welds can teach us something about the economics of welding.

In Table 1, the possible lengths of various welds–all made with one pound of weld metal–are compared. If we make 1 in fillet welds, we can only get 0.59 ft, whereas if we make in fillets, we get 2.35 ft. This is intuitive–a smaller weld yields more length per pound. But, notice that as the weld size is halved, the length of the weld doesn’t just double, it quadruples. And, if we make … in fillets, we’ll get 16 times as much length for the same amount of weld metal.

The PJP groove welds follow the same trend: the smaller throat size permits a longer weld to be made with the same one pound of weld metal.

The CJP portion of the table shows what a welder would know from experience: CJP groove welds require a lot more weld metal for their throat dimensions than PJP groove welds of equal size. For example, only 0.39 ft of 1 in CJP groove weld can be made, whereas 0.59 ft of 1 in PJP groove weld can be made.

Next, we’ll consider the strength of the various welds that can be made with one pound of weld metal. We will presume that in all cases, the connection was loaded in shear.

For all the welds shown, E70 weld metal (i.e., 70,000 psi minimum specified tensile strength) was assumed, as was Grade 50 base metal (i.e., a 50,000 psi minimum specified yield strength). The results are summarized in Table 2.

In the case of the CJP weld, the strength is controlled by the base metal. If 50 ksi steel is assumed, and the load is restricted to 40 percent of the minimum specified yield strength, the connection strengths for the CJP welds are obtained.

Some specific comparisons are educational: consider a 1 in fillet versus a in fillet. One pound of weld metal will produce enough 1 in fillet weld to resist 105 kips, or 105,000 lbs. The same one pound will produce enough in fillet weld to resist 210 kips–twice as much. Comparing the 1 in fillet to a … in fillet, the smaller fillet with the longer length will resist four times as much.

This illustrates a basic principle of weld design: For economical designs, make fillet welds smaller in size and longer in length.

Next, let’s look at fillet welds versus PJP groove welds. While a 1 in fillet can resist 105 kips, the 1 in PJP groove weld is good for 148 kips, or 41 percent more. This is true because the throat of a 1 in fillet is 0.707 in, whereas the throat for a 1 in PJP groove weld is 1 in.

This illustrates another basic principle of weld design: For the same quantity of weld metal, PJP groove welds are 41percent stronger than fillet welds.

To deposit a PJP, the groove to receive the weld must first be prepared, adding to the cost. For economy, the savings in welding cost must more than compensate for the additional cost of joint preparation. While costs will vary, a general rule of thumb is this: Fillet welds with a leg size of 1 in or smaller are usually more cost effective overall, whereas PJP groove welds are typically more economical when the required weld throat is greater than in.

It is also instructive to compare the capacity of PJP versus CJP groove welds with equal throat dimensions. The 1 in PJP groove weld made with one pound of weld metal can resist 148 kips. Using the same amount of weld metal, the 1 in. CJP groove weld made with the specific geometry outlined above is limited to 94 kips, or 36 percent less capacity.

If a butt joint requires a CJP, it is impossible to substitute a PJP. However, if the joint involved is a tee or corner, using a PJP and an additional fillet weld (in the case of a corner joint, to the inside corner only) can develop the strength of a CJP, but at a reduced cost.

This then takes us to the final design principle: When the joint and design permit the change, economy is achieved when CJP groove welds are replaced by PJP groove welds with reinforcing fillet welds.

By applying these three principles, we can get the most out of our weld metal, whether we are dealing with one pound or a thousand.

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.