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

Figure 1. When random lengths of rod were butted and when intermittent welds joined the rods to the box, as well as to each other, cracks developed.

Figure 2. A crack initiated at the interruption of a steel backing bar, causing a boom to snap in two.

Figure 3. Pressure from a clamp on a rack subjected the main member to bending with tension on the upper portion. When a fillet weld bridged the interface between two rack lengths, a crack propagated into the main member.

It is sad but true that sometimes a well-intentioned effort to reinforce or strengthen a welded assembly ends in disaster. Ignore this edict at your peril: There are no secondary members in welded design! Even something as simple as interrupted backing bars can cause a main member to crack. Let s look at a few examples.

Log Roller
Figure 1 shows a box member used as the central portion of a device called a log roller used for washing gravel. The box was fabricated from four angles welded together. Because the assembly with its attached paddles functions by rolling in gravel, the corners were subjected to high rates of wear. Someone suggested welding reinforcing rods to each corner to take the brunt of the wear. This proved to work fine, as long as the rods extended the full length of the box. But when random lengths were butted and joined to the box member by intermittent welds, and when the weld also joined the butted reinforcing rods, cracks developed when the log roller was used. It turned out the space between abutting bars acted as a notch - a stress raiser. When put into service, a crack initiated at the stress raiser eventually broke the box in two. This example shows how a good idea, incorrectly implemented, can destroy a fundamentally good design.

Boom
Imagine the surprise of workers when the boom for a piece of earth-moving equipment actually snapped in two on a cold, sub-zero day as the assembly was moved to a construction site. The villain turned out to be random lengths of backing bars - or rather, whoever s idea it was to use them. As illustrated in Figure 2, the boom was fabricated using two formed channels to create a box section. When full-length backing bars were used under the welds, the design functioned perfectly. But when random lengths were butted without welding them together, a notch was created. The notch created a stress concentrator, the point of fracture initiation. The low temperature decreased the fracture resistance of the steel, causing the abrupt fracture on that cold day. But the problem actually was created when interrupted backing bars were installed in the equipment.

Rack for a Clamping System
To make laminated wooden arches, planks of wood must be glued and clamped together. Figure 3 shows a heavy duty clamping system used for such applications. The pressure is applied to the jaws by a hydraulic cylinder, which, in turn, is connected to a dog that engages a gear--like rack. The dog permits quick adjustment of the clamping system when arches of different depths are made.

The rack is welded to the supporting beam with a series of intermittent fillet welds. When loaded, the beam is subject to a bending moment, with the high-tension fibers on the topside of the beam. The designer of the clamping system determined undoubtedly the required stiffness and strength of the beam, but probably didn t consider the contribution of the rack. Yet, the rack adds to the strength and the stiffness of the beam.

Because the rack is made from expensive bar stock, it is not strange that a conscientious welder would decide to make one of these clamping systems using small segments of the rack left over from previous jobs. However, even though it wasn t considered in the design, the rack is a load-bearing member. All was fine until the rack was interrupted. A fillet weld across the interface between segments of the rack created an ideal metallurgical path for a crack to initiate (see Figure 3) and subsequently propagate into the beam itself.

One of two alternatives could have prevented this problem. The fillet weld could have been interrupted so it did not bridge the interface between two rack lengths, which would be acceptable under some loading conditions, or preferably, the length of the rack could have been continuous.

So whether you re responsible for the initial design, or you re just tempted to tack a little improvement onto a welded assembly, remember, there are no secondary members in welded design!

Omer W. Blodgett, Sc.D., P.E., senior design consultant with The Lincoln Electric Company, 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, Mr. Blodgett was named one of the Top 125 People of the Past 125 Years by Engineering News Record.. Mr. Blodgett may be reached at (216) 383-2225.