Product Stability -- Step Stool

Ms. BC wanted to hang a picture on the wall over a bookcase. She placed a recently purchased step stool in front of the bookcase so its long axis was parallel to the wall and climbed it. (Folding Step Stool: Light duty Type III, Max Load: 200 pounds)

Shortly after driving a nail into the wall, BC stated that she suddenly fell to the floor as a result of the step stool collapsing. Subsequent examination of the ladder found that its two front legs were bent towards one another. BC was 62 inches tall and weighed about 235 pounds.

HF Investigation: While the structural integrity of the step stool was not directly a human factors issue, the manner in which it was used and the adequacy of accompanying safety information were human factors concerns. I was retained to determine the likely sequence of events that could best account for both the pattern of injury sustained by BC and the damage to the step stool.

HF Investigation and Analysis: I was provided BC's deposition; medical documents and photographs of her injuries; photographs of the damaged step stool; an evaluation conducted by a mechanical engineer of the step stool's stability and ability to resist, without bending, criterion loads to its rails; and exemplar step stools. I constructed models of the step stool in Working Model 2D (Knowledge Revolution) in order to explore, through simulation, the range of conditions that could cause the step stool to topple. Further, making use of computer simulation, I attempted to recreate the direction of leg bending observed in the damaged step stool.

Next, 3D computer models of both the step stool and of BC were generated in Extreme 3D (Macromedia). This allowed for the systematic examination of postures that BC would have likely adopted in order to achieve the results obtained through simulation as well as account for the pattern of her injuries.

Where possible, the exemplar step stools were used to evaluate whether a person could adopt the postures modeled. The process was an iterative one with the intent of coming up with the "best fitting" model given the available information.

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