Falling Body Impact Behavior of Fiberglass Stepladders With Plastic Knee Braces

Abstract

According to estimates reported in the U.S. Consumer Product Safety Commission’s National Electronic Injury Surveillance System, there were greater than 10,000 stepladder related injuries treated in hospital emergency rooms nationwide per year in the period from 2009 through 2013. Research and experience have sought to correlate specific stepladder damage patterns to the causes of some injuries involving stepladders. Prior studies have associated a specific damage pattern — inward deformation of the stepladder’s front side rails — with impact loading of a user’s body onto the lower portion of a front side rail following a fall from the stepladder. Those prior studies were conducted using stepladders with metal knee braces and with the ladder cap simply supported during impact testing. Currently sold fiberglass stepladders often have plastic rather than metal knee braces. In our study, side rail impact testing was performed in order to evaluate how a design change from metal knee braces to plastic knee braces affects impact damage patterns in fiberglass stepladders. Biomechanics simulations were used to inform the selection of the weights used for impact testing and allowed the test results to be evaluated in the context of potential body contact scenarios that could produce equivalent loading of the side rail. Our study demonstrates that depending on the weight of the impacting body, fiberglass ladders with plastic knee braces show different dynamic responses to impact loading than do their metal counterparts. Additionally, the test methods in this study incorporate realistic dynamics in that the weight impacted the lower portion of the stepladder’s front side rail while the stepladder was actively tipping with only two of its feet in contact with the ground and with the top cap unsupported. The results indicate that ladders with metal knee braces can permanently deform when impacted with loads less than that required to permanently deform the ladders tested with plastic knee braces. The absence of permanent side rail deformation in the plastic knee braced stepladders tested even after undergoing significant elastic deformation during testing gives rise to new questions about the potential for damage that is not observable based on a visual examination.