In Part I, we examined the behavior of weldments that are subject to torsion, and learned that under such conditions, the designer should use closed sections wherever possible. Unfortunately, not all weldments lend themselves to closed sections, and under such conditions, we must consider alternatives. Part II will explain and illustrate two additional design rules for dealing with torsion.

Consider a frame like the one shown in Figure 1. If we made a transverse cut across the frame, the cross-section would be C-shaped, that is, an open shape. When the frame is subjected to torsional loading, we would expect twisting along the length.

A somewhat inexperienced designer, in an initial attempt to make the frame resist such twisting, might add a series of transverse stiffeners, as shown in Figure 2. Unfortunately, installing these stiffeners is a nearly worthless exercise: The frame will be almost as flexible with the stiffeners as without.

To understand why this is the case, we have to break the frame down into its individual parts. Figure 3 shows the parts of the frame, as well as the stresses on the parts that result from the applied torque. Notice that there are two main stresses: Transverse shear stresses and longitudinal shear stresses.

On the top surface of the frame, it is easy to see how the two stresses combine to produce diagonal tensile stress, and diagonal compressive stress. Notice that these two stresses are at a maximum when oriented at a 45 degree angle to the longitudinal dimension of the frame.

The members of the frame that are longitudinal (the side members) and the transverse stiffeners are all rectangular in cross-section. When torque is applied, these members twist, offering little resistance. As a whole, the assembly also adds little resistance.

A seasoned designer would study the assembly and realize that, instead of the transverse stiffeners, he or she could add a pair of diagonal braces, as shown in Figure 4. The stiffeners, oriented at a 45 degree angle to the axis of twist, are in line with the maximum tensile or compressive force. Such forces will tend to cause the brace to bend, but will not result in twisting in the frame. Such an assembly is remarkably stiff.

This takes us to the second design rule for dealing with torsional loading: Use diagonal braces.

Two types of braces are possible: single braces and double braces. In all cases, the orientation of the brace should be at a 45-degree angle to the axis of twist. Table 1 illustrates the two kinds of braces and provides a value for the torsional resistance R. The angular twist on a frame-like structure can be calculated with the formulas given in Part 1 of this series, substituting the values for R as contained in the table.

The final design rule for torsional loading also deals with open shapes such as the channel in Figure 5. As an open shape, this configuration will offer little torsional resistance, as shown in part b of the drawing. Part c helps us to understand what is happening: Not only do the three members twist, but the flanges swing outwards, allowing for even more twist.

To minimize the overall rotation, the flanges can be locked in place relative to each other, to reduce the swinging effect. There are several methods that can be used to tie these flanges together in terms of relative movement.

Perhaps the easiest is, simply, to weld them to an end plate arrangement, as shown in part d of Figure 5. The end of the channel can be boxed in, as illustrated in part e, or a pair of diagonal braces can be used as shown in part f.

Regardless of which method is used, the angular twist is reduced by about 50 percent when the flanges are restricted from swinging. This takes us to our third and final design rule for torsional resistance: Make rigid end connections.

In summary, three basic rules provide the tools we need to control twisting in weldment design:

1. Use closed sections where possible.
2. Use diagonal braces to counteract twisting.
3. Make rigid end connections to keep flanges from swinging.

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.