All too often, a designer will add stiffeners to a weldment haphazardly. This may give the designer a warm and fuzzy feeling while adding little or nothing structurally. In other situations, stiffeners can be critical to the performance of the product since a properly placed stiffener can increase the capacity of the member it supports. The reason for this increase in capacity may not be readily apparent, however.

Consider the assembly illustrated in Figure 1. The applied moment results in compressive forces in the flange, called F1 and F2 in the drawing. Simple statics require these two forces to be balanced by a third force. In the center of the width of the flange, the web permits the application of a component force, called FC.

Away from the web and at the outside edge of the flanges, there is no material available to permit the application of FC, and thus FC approaches a value of zero (Figure 2). Once again, statics require equilibrium; with FC equal to about zero, the forces F1 and F2 at this point must be close to zero. At this location, the flange is only minimally effective in transferring the load.

The net effect is that the forces in the flange are not uniform. Peak stresses occur near the location of the web, and decrease to nearly zero at the tips, as shown in Figure 3.

When a stiffener is added as shown in Figure 4, the resisting force FC can be applied to the whole width of the flange, including the tip, and in turn, F1 and F2 can be increased in magnitude. In other words, adding a stiffener increases the capacity of the member supported by that stiffener. The stress distribution is now more uniform, as depicted in Figure 5.

Real-Life Example
A manufacturer of oil pumping equipment called me a few years back to ask for help with what he assumed was a welding problem. The product was an oil jack pump walking beam (Figure 6) that was failing after several months of service. Figure 7 shows the universal socket and bracket assembly attached to the bottom of the beam. The universal socket eliminates the application of forces except those resulting from the vertical load. When the beam is horizontal, only vertical forces are applied to the beam. As the beam pivots, however, the vertical forces result in a longitudinal force in the beam. When the universal socket is pulled down, a tensile force is applied to the beam, and resisted by the pivot (Figure 8). Pushing the socket up creates the opposite condition.

The 90 degree intersection between the bracket assembly and the beam is similar to the situation illustrated in Figure 1. Without any stiffeners in the beam, the load distribution shown in Figure 3 exists in the beam flange as well. With enough load cycles, the beam of the jack pump eventually cracked, and the tip broke off.

Since the cracking occurred near the weld, the manufacturer decided the weld must be somehow deficient. However, my analysis showed that the primary cause was a design flaw. The design did not include a stiffener that would have made the flange more uniformly loaded.

Fortunately, the solution was straightforward. The manufacturer added a stiffener in the proper position (Figure 9), making the tips of the beam flange more effective and resulting in a more uniform stress distribution in the flange.

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.