Licensed Professional Engineers
A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.
Issue 72: Winter 2018/2019
Tree Stand Accidents
By John L. Ryan, P.E.
© 2019 M.A.S.E. LLC
Hunting is a great American past time, with cultural roots in our ancestors who immigrated to America and had to rely on their hunting skills to survive, and of course even further back in human history. Modern technology has facilitated the ease of hunting in various ways, including tree stands. Tree stands come in a variety of flavors but all have the purpose of providing an elevated stance from which to see, and shoot, game. This issue of clues will examine briefly the different types of tree stands, and common accidents involving tree stands, as well as root causes of these accidents.
The basic hanging tree stand has a seat and foot rest platform, and must be carried or hauled to the location in the tree where it will be installed. Climbing aids are used to gain elevation, and a safety harness system prevents the user from falling while the stand is being secured to the tree and during use.
A climbing tree stand incorporates a method of ascending the tree, where there is a foot platform and a separate seat platform. The user weights one of these, while the other is slid upwards. A safety harness is used as a backup during climbing and use.
The ladder tree stand has a ladder as part of the tree stand system. This type of tree stand is erected against a tree, strapped from the ground, then ascended with a safety harness protecting the climb. The user secures the top with straps, and continues to use the safety harness as a backup while using the stand.
(image courtesy of wikipedia.com)
Behavioral Factors of Tree Stand Accidents
While the cause of a tree stand accident is often inadequate components or a defective design, the tree stand user who uses a safety harness attached to the tree can minimize the injuries incurred in an accident as a fall can be minimized assuming he or she is safely anchored to a tree, and no failures occur in the harness or anchoring system. Manufacturers recommend using a safety harness while ascending the tree, installing the tree stand, and using the tree stand. The reality of human behavior results in many users not using the safety harness to climb and set up the tree stand, or while using the tree stand. Engineering design principles rely on personal protective equipment and human behavior as last resorts for safety. Tree stand users expect the stand to be non-defective and strong. Injuries resulting from tree stand failure are caused by defective tree stands, not by the failure to wear and use a safety harness. Safety harnesses are also not infallible, especially the low-quality harnesses often seen included with tree stands. Another relevant factor here is expectations of the consumer to read all instructions and warnings, which is not possible if the instructions are missing, lost, or if someone purchases a used tree stand. Manufacturers direct users to take the stand down after every use, which tends to run contrary to human nature. The failure of manufacturers to accommodate true human behavior results in tree stand failures when users exhibit the predictable and common behavior of leaving the tree stand up exposed to elements.
Failure Mode 1: Strap Failure
Webbing straps are used to secure all types of tree stands. Nylon straps used are often low quality, with relatively low failure strength, unlike nylon strapping and webbing used for other life-sustaining activities like rock climbing, tree work, and working at height. Webbing straps that are not climbing-spec can have low failure strengths, 1000 lbs. or less in new condition. Add in the damage caused by ultraviolet radiation and other elements, even when the tree stand is brought down after every use, and strapping can break when any sort of dynamic loading occurs, especially with a heavier user. Webbing of safety harness can also fail in falls due to dynamic loading that can overload webbing.
Failure Mode 2: Cable Failure
Many tree stands utilize steel cable either to connect the tree stand to a tree, and/or as a structural element of the tree stand such as supporting the foot platform. While steel cable can be much stronger than nylon, corrosion can weaken steel cable quickly, especially if there are bending stresses on the cable that it is not truly intended to be subjected to. At connection points of cable tree stands can fail, often at points that are subject to rotation such as a collapsible foot platform, or if a cable is used to rap around the tree as it is subject to bending each time it is moved. Climbing tree stands can add the movement of climbing a tree to the bending of the cable, potentially causing premature failure.
Failure Mode 3: Structural Failure of Stand
Structural failure of tree stands occurs when the stand frame is overloaded, which can be due to dynamic forces, structural defects, material defects, excessive loading of the stand, or the structure being loaded in a direction that the designers did not intend.
Failure Mode 4: Falls
Falls are the most common mode of injury involved in tree stand accidents. Any of the failure modes above can result in a fall. Falls can also occur due to a loss of balance, or destabilization of the tree stand itself due to installation and tree configuration. Tree stands typically are sold with safety harnesses, which when used properly can prevent a fall regardless of the nature of the failure.
Lack of Factor of Safety
In today’s competitive market, manufacturers of any product must take care to cut costs when possible in product design. While this may be acceptable in non life-supporting products, any product that someone relies upon to prevent their serious injury or death must have adequate factors of safety, and take into account any weakening of components over time, due to exposure to elements, as well as relevant human behavior. Tree stands are often produced with inadequate or minimal factors of safety when the stand is brand new. Manufacturers fail to consider corrosion and weathering which will occur even if a tree stand is brought down and erected each time it is used. Without an adequate factor of safety, and a design that withstand elements, weakening of structural and safety components will occur in tree stands that result in tree stand failure.
My client left the tree stand up—isn’t that against manufacturer recommendations?
Tree stand manufacturers typically prohibit users from leaving their tree stands up in the tree, rather they expect users to bring the stand down after each day of use. The main logic behind this is that the elements can cause corrosion of metal, deterioration of nylon straps and result in failure of these components. Taking a stand up and down for each use adds risk in that the erection process can be difficult, and result in falls that would not occur if the tree stand was left in place, can result in an improperly secured tree stand, and the reality is that often hunters will simply leave their stand up once it is in place, which is a known behavior and should be considered in the design.
How We Can Help
At MASE, we can determine the cause of a tree stand accident, identify contributing defects, whether human behavior was a part of the accident, and what would have prevented the accident. We offer full service mechanical engineering expert witness services. Call us at (855) 627-6273 / email email@example.com
We offer full service mechanical engineering expert witness services from inspection to trial testimony. Call us at (855) 627-6273 or email us at firstname.lastname@example.org
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