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

The tongue-assembly design for a large truck trailer failed to consider the resulting torsional loading.

Torsion transferred through the beams of the tongue assembly caused the beams to bend horizontally at the weld ends.

This design offers significantly greater resistance to torsion. Here, a closed section is created out of the I-shaped members.

The torsional stiffness of the tube, and the corresponding loading on the welds, were not considered in the design of a garbage-truck tailgate.

Deleting a 6-in.-diameter tube from the tailgate solved the weld-cracking problem.

Designers, welding engineers, and company managers should be aware that not recognizing a major force resulting from a load can lead to serious problems in their firms' product designs and welds. The following applications show examples where torsion was initially overlooked and good designs that took the torsional loading into account.

Trailer
First, consider a large trailer. The design calls for two long beams, with cross members, and two tongue assemblies tied into the cross members that connect to the main beams.

Keep in mind that the torsional resistance of an open section is poor. Further compounding this problem, the trailer's tongue-assembly design over-looks torsion.

After the trailer was put in service, the welds immediately began to crack.

The load path can be visualized (right): The long beams are loaded due to the mass weight of the trailer and the payload. The tongue resists the downward force. The load between the tongue and the long beams is transferred through the cross members by torsion. Yet, the pair of I-shaped sections had little torsional resistance. With the open sections and the considerable length of cross members, torsion caused the beams to bend horizontally at the weld ends.

The problem is solved by boxing in the sections where the torque is applied.

Creating a closed section out of the two I-shaped members eliminates the bending action on the weld. In the original design, no amount of welding could have withstood the forces created by the torsion. If the welds had not broken, the members themselves would have.

Truck tailgate
Torsion was also neglected in the design of the tailgate for a garbage truck.

As garbage is dumped into the tail end of the truck, two hydraulic cylinders pull the gate forward, forcing garbage into the truck under pressure. Once in awhile, a solid object becomes jammed, halting the motion of one end of the frame. The other end continues to be pulled forward (and the hydraulic cylinders can exert approximately 30,000 lb of force) subjecting the frame to a twisting action.

The door is rather flexible with regard to torsion, consisting of a flat plate and a few stiffeners that run parallel to the sides of the door. However, it has a 6-in.-diameter tube that passes through each of the stiffeners. Any twisting is resisted almost entirely by the tube.

Welds between the stiffeners and the tube were required to transfer the load. However, this type of loading was not considered in the weld design, and the welds were too small for this purpose.

In service, the welds between the tube and plate broke repeatedly under the 80,000-psi shear stress developed by the twisting action. The easy solution to this tailgate problem was to delete the tube.

Deleting the tube reduced the door stiffness, but having a more flexible door did not degrade the garbage truck's serviceability.

In these two examples, the welds failed because the designers didn't recognize that torsional loads were being placed on the welded connections. Good design must always take into account the nature of the forces that are applied to the connections.

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.