The influence of footwear tread design on available friction and slip risk

Slips are a common cause of injuries in society. As such, there is a need for slip resistant footwear. The purpose of slip resistant footwear is to deter slipping by increasing the available friction (COF) at the shoe-floor interface. The SATRA STM 603 whole shoe tester is the most common device used to assess the available friction of footwear. Since the SATRA STM 603 is widely used to establish the slip resistance of footwear, it is essential to determine whether the results of such testing predict slip potential. Furthermore, while previous studies have identified several components of footwear outsoles that influence available friction, the tread groove parameters of width, depth and orientation have not been studied using a whole shoe model. Although tread groove parameters have been investigated with respect to their influence on available friction, it is unknown how these parameters affect slip potential. The overall purpose of this dissertation was to evaluate the ability of a whole shoe tester to assess available friction and predict slip potential and to determine how shoe tread groove parameters affect available friction and consequently slip outcome and severity. In Chapter III, the ability of the SATRA STM 603 to predict slip potential was evaluated using the published test method EN ISO 13287. A secondary purpose was to determine whether altering EN ISO 13287 test parameters (normal force, sliding velocity and contact angle) improves slip prediction accuracy and assessment bias of the SATRA STM 603. Results revealed that the EN ISO 13287 test method predicted only 35% of slip events and exhibited positive COF assessment bias (overestimated available friction). However, modifying EN ISO 13287 test parameters decreased assessment bias (0.00) and increased slip prediction accuracy to 74% of slip events. The purpose of Chapter IV was to evaluate how various combinations of tread groove width, depth and orientation influence available friction as measured by the SATRA STM 603 whole shoe tester. More specifically, to determine which tread groove parameter has the greatest impact on COF. Results revealed that tread groove orientation had the greatest impact on COF, explaining 81% of the variance in COF. Our findings suggest that groove orientation may be the tread parameter of importance when designing footwear outsoles for slip resistance. The purpose of Chapter V was to determine if footwear tread groove orientation influences slip outcome and slip severity. Results revealed that individuals who wear shoes with a parallel groove orientation experienced a greater proportion of slips and more severe slips than persons wearing shoes with a perpendicular groove orientation. Overall, the results of this dissertation established that the SATRA STM 603, utilizing modified test parameters, has the ability to predict slips and assess how variations in tread parameters affect available friction. Furthermore, the results from this dissertation provide evidence that the COF testing performed with the SATRA STM 603 may be used to make inferences on real life slip events. Taken together, these findings create a paradigm which may serve as a guide in the development of slip resistant footwear.