By Omer W. Blodgett, Sc.D., P.E.

Gussets are triangular stiffeners or brackets added to members that intersect at roughly right angles. For some designs, gussets are essential. But to make them effective, the designer must place them properly, and this can be tricky. Common errors fall into three categories: the stiffener may be misplaced, it may be unnecessary, or it may be harmful.

Misplaced gussets
A manufacturer of huge highway trail-ers created a case of misplaced gussets. The detail involved large, square tubular members. The first design involved tubes welded directly to each other. When the company put the trailer into service, the connection cracked.

In attempting to keep the assembly from cracking, the manufacturer had put the gusset in the wrong place. To make the gusset effective, designers should have attached it to the stiff part, so that it would transfer forces into the member that lay parallel. Ultimately, they put brackets on both the top and the bottom.

The manufacturer strengthened the connection by putting a gusset into the corner. After part went into service, it cracked at the end of the gusset. To under-stand what was going on, the company installed some strain gauges on the gusset, right where it joined the tube and where the cracking had occurred.

Company personnel loaded up the assembly to see what kind of stresses were present. To their surprise, the gauge did not register any significant strain. How could it be that a member without stress still cracked? The gauge on the gusset showed little strain since the gusset was pushing on the flexible side of the tube. This was like pushing on a spring mattress. It would not take much force to move the mattress surface right away, because it is flexible. Therefore, the strain gauge showed nothing.

This arrangement ensured the load transferred directly through the part, solving the cracking problem.

Unnecessary gussets
In the past, when manufacturers commonly made machinery parts of cast iron, correcting for a poor design was relatively simple. The part designer would adjust the pattern, making the part stronger where it had broken. Often, the designer would specify gussets or stiffeners to strengthen the part. If the amount of material involved was minimal, he could add a gusset for almost no additional cost.

Adding gussets or stiffeners to welded assemblies is an entirely different matter. It is expensive, and often, the value added is questionable.

For example, a machine tool component consisted of a vertical cylindrical tube tied down to the factory floor by eight anchor rods. The original component was designed as a casting. To stiffen the bottom circular ring, the designer added eight gussets, strengthening the part at little added expense.

Later, the manufacturer changed the component from a casting to a weldment. Without thinking through the purpose of the gussets, the designer at first simply duplicated the configuration of the cast design. The assembly added eight gussets, each with four welds. Along with the gussets came a significant increase in cost.

To reduce the cost of the assembly, the designer eliminated the gussets and the 32 welds, but then he found that the circular ring was too flexible. By increasing the thickness of the ring, he eliminated the problem of flexibility. The slight increase in the material cost was more than offset by the total elimination of the stiffeners, and the corresponding cost of welding them in place.

Harmful stiffeners
Designers commit further errors in weldment design when they overuse, or improperly use, stiffeners. Shop personnel fabricated the framing for a heavy press according to a design that specified welding in four 1 ¹ /2-in.-thick brackets with the use of a backing bar. During welding, cracks developed at the corner of the intersection. The brackets did not transfer any force, and actually caused restraint that was to blame for the cracking. The designer removed the brackets from the design, and with that simple step, eliminated the cracking problem while also reducing the cost of fabricating the press.

Properly placed, gussets may be critical to the performance of a weldment. Conversely, improperly placed gussets are worthless. In other situations, unnecessary gussets may actually introduce new problems. Before haphazardly inserting gussets throughout an assembly, designers should think through these consequences.

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.