Figure 3

A bracket designed to carry a 500 lb [227 kg] air compressor unit was welded to the center sill of a piggy back railroad car, as shown in Figure 3. There were no interior diaphragms. The vertical force from the weight of the unit was transferred as a moment into the bracket, creating bending at the web. The two horizontal bending forces had to eventually transfer to the parallel flanges, but with an open box section, there were no ready pathways. The upshot was that the web flexed, and fatigue cracks occurred.

Two possible ways to correct the faulty design are illustrated at the bottom of Figure 3. The sketch on the left shows a stiffener (diaphragm) installed inside the center sill, opposite the bracket. This could only be done before the box section was fabricated. The stiffener would be required to be welded to both flanges and to one web, providing paths for the bending forces to reach the flanges. The drawing on the right suggests another way to correct the design. In this case, the bracket was shaped so that it could be welded directly to the sill flanges in new fabrications, or pieces could be added to the bracket on existing cars to accomplish the same outcome.