Ships have always fascinated me.

My grandfather owned 23 ships that transported limestone, salt and pulpwood on the Great Lakes. They were wooden ships, typically powered by steam.

Then, in World War II, I had the opportunity to work as welding superintendent at Globe Shipbuilding in Duluth, Minnesota. We built 29 ocean-going ships for the war effort.

In my grandfather's day, a powered ship might tow one or more barges as shown in Figure 1.

Notice that the barges were equipped with masts. While not shown in this photograph, sails could be attached to the masts, enabling wind power to lighten the load of the tugboat's steam engine.

These tugs came to mind when I was thinking about the problem of welding a “bitt” to the deck of a steel ship. Sometimes called a chock or cleat, the bitt is the fitting used to attach the cables that are needed to tow barges. (See Figure 2.)

The problem appeared to be simple: A customer wanted to weld the bitt to the deck instead of using bolts. Since the bitt was cast steel (not cast iron), there was no problem with weldability.

The concept was to cast the preparation for a partial joint penetration groove weld bevel into the bitt. The engineer responsible for the project wanted to know how deep the bevel groove should be. In other words, he needed to know what weld size was required.

As is often the case, no one knew what loads would be created by the towed barge. However, the steel cables were known to be 1½ in. in diameter. For standard wire rope, the extra improved plow steel (or EIPS) grade has a breaking strength of 114 tons.

My thinking was simple: The weld attaching the bitt to the deck should have a capacity that exceeded the breaking strength of the rope. I assumed the maximum force applied to the bitt would never exceed 250,000 lbs, and used this value to determine the weld size.

Figure 4 (below) shows the basic arrangement of the bitt and the towing cable. The cable force created a bending moment of 2.5 million in.-lbs. This moment was in turn resisted by the weld on the 12 in. by 12 in. base of the bitt casting. Using the method of “treating the weld as a line” the property of the welded connection was determined as follows:

Since b = d = 12 in. in this situation, Sù = 1.33 b² or 192 in.²

Thus, the force on the weld treated as a line was determined as follows:

We found the required weld size by taking the force on the weld treated as a line and dividing it by the allowable force per length. Assuming the use of E70 filler metal and the AWS D1.1 allowable of 30 percent of the electrode classification strength, the following was obtained:

Notice that t_ω is the weld throat size, and it was rounded up to a standard size of ⅝ in.

I had the answer to the customer's question: A ⅝ in. deep bevel should be incorporated into the cast steel bitt. When a PJP groove weld was applied, the weld would be able to resist loads that would likely break the cable.

I delivered this information to the customer, along with my standard caveat to double check my work.

The customer came back to me with a concern. My analysis had assumed that all the loads would be transferred through the welds.

The customer was concerned that some of the compressional load might be transferred from the bitt to the deck in bearing. If this occurred, then the neutral axis of the resisting forces would be shifted to the right. That, in turn, would increase the stress on the tension side welds, and perhaps lead to an overload condition.

Figures 5 and 6 (next page) are useful for understanding the customer's concern.