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FORENSIC CLUES # 29 - "Step Stool Accidents" by John L. Ryan

FORENSIC CLUES -A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.

Volume 29 December/January 2008

“Step Stool Accidents”

By John L. Ryan

Step Stool Accidents

Ladders of all types result in over 160,000 injuries each year. This staggering statistic includes stepladders, extension ladders, articulated ladders, and step stools. Step stools are short ladder type devices that allow users to reach elevations that are only a few feet off of the ground. Most step stools have one, two, or three steps on them.

Step Stool Structural Failure

Accidents involving these types of ladders range from falls due to step stool collapse to falls resulting from loss of balance. Step stools may collapse under load, sending the product user crashing to the ground. Some mechanisms of failure resemble that seen on stepladders. Failures such as the lower leg bending or twisting can occur, indicating premature product failure.

Stepladders and step stools rarely fail when the user is positioned squarely over the ladder feet and is not moving. Movement is necessary to perform tasks. With movement comes dynamic loading. Dynamic loading of ladders occurs when the mass of a person moves due to acceleration and deceleration generated by muscle movement. Normal movement that occurs when climbing and descending ladders, or working from ladders, can result in acceleration of up to 6 g’s. One “g” represents the acceleration of gravity, so 6 g’s is six times the acceleration of gravity, which results in a load that is 6 times greater than the person’s weight.

A ladder can fail from these simple dynamic forces when the ladder is not designed to withstand the true magnitude of forces that a person can generate. Besides simple vertical motion, a ladder user can generate sideways and twisting movement. If a ladder isn’t designed properly, these twisting forces can cause structural failure. Some manufacturers lack the personnel with engineering expertise to design a ladder that is strong enough, or sometimes the corporate bureaucracy steps in and cuts costs, leaving a ladder without proper support.

Ladder failure can also occur due to material or manufacturing defects. We see more and more cases of failed step stools and ladders due to the material not being up to design specifications. Most ladder companies in the United States have their ladders manufactured overseas. The companies manufacturing the ladders are often required to do some sort of quality control, testing a few ladders to make sure they meet applicable standards, and testing the material properties of some of the ladders. Problems occur when a few samples are tested, and tens of thousands of the ladder are produced without further checks. Heat treating processes that are crucial to the proper strengthening of the aluminum alloys are not performed correctly, or are simply not done at all. The end result is very soft, weak material that will fail under normal, everyday use. When the leg of a ladder or step stool fails during use, whoever is using the ladder will likely fall to the ground.

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Typical step stool

Falls From Step Stools

Step stools also can cause accidents without structural failure. Falls from step stools or stepladders can occur when a person loses her or his balance on the ladder. The problem with some step stools, though, is that there is little to lean against or grab hold of to assist in balancing.

People who climb traditional stepladders can lean against the ladder rails and/or grab the rails for stability. Unless the user is at the second to stop step, he or she will be much more stable on a stepladder since the user can rest their upper legs or body against the ladder. Without something to lean against or hold on to, it’s only a matter of time before anyone could lose their balance. Manufacturers like to put the blame of people losing their balance and falling from stools on the user. The problem is that the average person who uses a step stool can easily lose their balance without something to lean against or hold on to. This is a classic example of failing to design for the capabilities of the product user. Engineers must design products with the end user in mind. How the user interacts with the product and what the users’ limitations are must be designed for.

Other design variables that can have a dramatic effect on the user’s stability include the angle of the step stool and the depth of the steps. The angle of the rails of the step stool affect the angle at which the user must place his or her legs to lean against whatever steps or top cap there is to lean against. The close proximity of the top cap to the feet prevents the user from completely stabilizing their center of gravity, but the steps and top cap can provide something for the user’s legs to lean against. The steeper the angle of the rails, the more the next step or top cap of the step stool will overhang the lower step.

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Testing a step ladder type stool

Overhanging steps reduce the area that the user can place their feet on, and must be balanced with the need to provide a surface to rest the users’ legs against.

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Testing a step ladder type stool

Without something to lean against or hold on to, it’s only a matter of time before anyone could lose their balance. Manufacturers like to put the blame of people losing their balance and falling from stools on the user. The problem is that the average person who uses a step stool can easily lose their balance without something to lean against or hold on to. This is a classic example of failing to design for the capabilities of the product user. Engineers must design products with the end user in mind. How the user interacts with the product and what the users’ limitations are must be designed for.

Other design variables that can have a dramatic effect on the user’s stability include the angle of the step stool and the depth of the steps. The angle of the rails of the step stool affect the angle at which the user must place his or her legs to lean against whatever steps or top cap there is to lean against. The close proximity of the top cap to the feet prevents the user from completely stabilizing their center of gravity, but the steps and top cap can provide something for the user’s legs to lean against.

Standards

Step stools currently are grouped in ANSI A14 portable ladder standards, although many of the tests that stepladders are subjected to are not required for step stools. A committee of the CPSC began the framework for a step stool standard in the 1980’s. This standard was passed among various different organizations, including ASTM, but was never finalized and adopted. The reasons for this are unknown at this time. There is supposedly an ANSI standard in the works that will cover step stools, dubbed ANSI A14.11.

Important Design Elements

The depth of step stool steps is a very important factor. Step stools that use platform type steps that can accommodate the average user’s entire foot will make it much easier for the user to keep his or her balance. Step ladder style step stools use steps that are similar to stepladders. Studies have shown that these types of step stools are considered by users to be easier to lose one’s balance on. Other important design elements include the angle of the stepladder, the locking mechanism that keeps the ladder from collapsing, structural member sizing and bracing, material properties, and other factors.

Perhaps the single most important design element of step stools is the inclusion of a hand rail that extends vertically from the last useable step of the stool. This hand rail should extend high enough for the average user to comfortably use when standing at the highest level on the step stool. Quality step stools incorporate these design elements.

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Step stool with handle

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