John R. Yannaccone, P.E., Principal Mechanical Engineer
Case Summary: One afternoon, two women were working out together at a fitness center. As part of their workout, they were doing hanging leg/knee raises on a long bar affixed to the side of a monkey cage. They were both hanging from the bar with their legs elevated when the bar separated from the structure and they fell to the floor, landing on their buttocks. While one of the women reported she was not injured, the other women reportedly sustained lower back injuries from the landing impact. Numerous parties were involved in the case including the fitness center, the monkey cage seller, the company that delivered and installed the equipment, as well as the manufacturers.
Analysis: The investigation of this incident began with an inspection of the monkey cage and the separated bar. The bar was welded to a plate on both ends that would be used to affix the bar to the system. After the bar was welded to these mounting plates, a powder-coated finish was applied to the entire assembly. This bar assembly was shipped with the other components of the monkey cage to the company that would then transport the system to the fitness center for installation. As part of the final assembly and installation process, the pull-up bar assembly was bolted to the side of the main monkey cage.
Following the incident, an inspection of the system revealed that the bar separated from the end plates, used to mount the assembly to the side of the monkey cage, due to a failure of the welds. Detailed inspection of the welds revealed that, while they looked like quality welds on the outside, they did not penetrate the base metal of the mounting plate, resulting in a weak or low strength weld. Poor penetration of welds is the result of issues with the technique used to create the weld. Low quality welds are frequently the result of insufficient heat. If the heat does not effectively melt the base metal, the weld basically sits on top of the item being attached to the base. From the outside, low-strength welds can appear of good quality, especially when a powder-coated finish is applied, but are only capable of carrying a fraction of the load of the intended weld.
In the case of the pull-up bar, the welds appeared sufficient on the outside and were capable of carrying a reasonable load, so they did not fail immediately when the system was used following installation. However, when the dynamic load from two people was applied to the bar, the bar separated from the monkey cage and the women fell to the floor. The design materials for this bar assembly did note that the bar was intended for use by more than one person, and had a rated load well above the combined weights of the two women who fell.
Result: The inspection demonstrated that there would be no way for the installation company to have identified the problem with the welds, which lead to the failure. The inspection also confirmed that the company hired to install the equipment had properly assembled the system. With this expert analysis, the installation company was able to get the plaintiff to release them from the case, which then proceeded to move forward against the remaining defendants. With the specific cause of the failure identified as the poor quality welds identified at the expert’s inspection, this matter was later resolved with the remaining parties.Categories: John R. Yannaccone | Mechanical Engineering