Abstract
There are a variety of consumer products designed to aid the hunter in ascending a tree and provide an elevated platform for spotting game and firing their weapon. One of the most widely used is the ladder-style hunting treestand. Such a stand combines up to a 20-foot ladder with a standing/seating platform to create a dual-purpose structure. Although these products perform the same function, ladder-style hunting treestand manufacturers do not observe the requirements of ANSI A14.2, American National Standard for Ladders - Portable Metal - Safety Requirements [1]. Instead, they adhere to the much less rigorous ASTM series of standards for treestands [2–5]. While the ANSI standard addresses the performance of metal ladders under foreseeable conditions of installation, including user error, the ASTM standards do not address the performance of ladder-style hunting treestands during the installation phase whatsoever. This phase is especially critical for such treestands, because they require the user to ascend the ladder to the platform at the top before the platform is secured to the tree by ratchet straps. Prior to ascending the ladder, all known commercially available designs require the user to affix an adjustable horizontal brace between the tree and the mid-point of the ladder. It has been demonstrated in the field that sudden release of this horizontal brace can occur for several reasons. If this happens during initial installation, the lack of a secure platform allows the structure to cascade down the tree, injuring the user. The scope of the problem is significant. Indeed, investigators have shown that falling from treestands is the number one source of medically attended injuries for hunters, out-pacing injuries related to firearms [6].
Better understanding of the role of the brace in these collapses is needed. Because the typical ladder-style treestand design is statically indeterminate, a static analysis is not possible. Moreover, testing conducted for this paper demonstrates that the load carried by the brace is relatively minimal, and the structure will support its rated load without the brace in place. How then, do such catastrophic collapses occur when the brace connections separate during use? This paper investigates the nature of the failure mode and identifies the relevant design and usage parameters that enable it.
It was found that due to the segmented design of the ladder, the tolerances allowed in the joints between segments, and the weak rail cross-sections employed, that small changes in brace length adjustment can affect the initial geometry of the assembly in ways that significantly impact its ability to arrest the momentum developed if the brace suddenly detaches under load. If the initial momentum is great enough, the ladder rails will yield in bending before equilibrium can be re-established. If the amount of yielding approaches a plastic hinge, the collapse will be comprehensive.
Finally, recommendations are made for improved designs that can preclude this mode of failure.
