When analyzing the reason for the failure of a welded connection, it's important to look in the right place. It is easy to become fixated on a cracked connection and assume that improving the weld itself will solve the problem. In this example, the designer's reluctance to look beyond the failure hampered his ability to discover and correct the fundamental problem.

A farm wagon, made to hold grain, was designed with a welded frame of square tubing. As designed, the frame itself was sufficient to support the intended load. However, without sides to the wagon, the frame alone was worthless. To support the grain (but not to add any further capacity to the wagon), the fabricator added sheets of 10-gauge steel to the bottom and sides of the frame, welding the gauge steel to the tubular steel.

Wagons built in this manner worked in service, meeting the expectations of the farmers who purchased the units. After filling the wagons with grain, the farmers would typically cover them with a tarp to keep the rain out and the grain in. Some farmers complained, however, that applying and removing the tarp was inconvenient. In response, the wagon manufacturer decided to create another model — one with a permanent top.

Since 10-gauge material was readily available (it had been used for the sides and bottom), the designer decided to add a roof that looked much like the sides and bottom. He stipulated that the roof be attached to the same tubular frame, with welds.

After the company manufactured several such units and they were put into service, cracks began to develop adjacent to the weld, on the sheet steel side of the connection. The manufacturer was perplexed, since the welds to the sides and bottom of the wagon had performed flaw-lessly for many years. So, why were these particular welds cracking?

Everyone scrutinized the "problem" welds, and the steel for the roof. All kinds of explanations were suggested: the rolling direction of the steel in the roof might be the problem, perhaps the welding procedure was deficient, or maybe the welder was at fault. However, the steel for the roof was the same as for the sides, the direction of rolling of the steel was the same, and the welding electrode was the same, as was the procedure. In fact, the same welder had made all of the welds, using the same equipment and spool of electrode.

All the focus was on the point of failure, but the cause and solution eluded the manufacturing team until it took a step back and looked at the whole wagon. The sides were strengthened with transverse and diagonal members. The bottom had transverse members. The top, however, was totally open, until the roof was put in place. Although the designer had viewed the roof as simply a permanent tarp that kept rain out and grain in, it had, in fact, become a structural member, tying the two sides together.

When the wagon was loaded, the top sheet was in compression. The critical buckling strength of the unsupported sheet was very low, causing it to buckle. This made the sheet crack adjacent to the connecting weld.

The bottom and sides of the wagon also experienced similar loadings, but the presence of the vertical and diagonal tubular members provided proper support for the sheet steel.

The designer decided to add stiffeners to the top sheet to reduce the unsupported distance of the sheet, thereby increasing its critical buckling strength. By eliminating the buckling, he solved the cracking problem as well.

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.