Wen-Ruey Chang

The slip resistance of common footwear materials measured with two slipmeters

The slip resistance of 16 commonly used footwear materials was measured with the Brungraber Mark II and the English XL on 3 floor surfaces under surface conditions of dry, wet, oily and oily wet. Three samples were used for each material combination and surface condition. The results of a one way ANOVA analysis indicated that the differences among different samples were statistically significant for a large number of material combinations and surface conditions. The results indicated that the ranking of materials based on their slip resistance values depends highly on the slipmeters, floor surfaces and surface conditions. For contaminated surfaces including wet, oily and oily wet surfaces, the slip resistance obtained with the English XL was usually higher than that measured with the Brungraber Mark II. The correlation coefficients between the slip resistance obtained with these two slipmeters calculated for different surface conditions indicated a strong correlation with statistical significance.

Field evaluation of two commonly used slipmeters

A variety of slipmeters have been used to assess the slipperiness of floor surfaces. International standards for the operation of slipmeters describe the protocol for a single measurement. These standards usually do not cover some of the critical elements in safety assessment such as methods for the selection of measurement locations and the necessary number of repeated measurements at each location. Furthermore, most of the slipmeters were evaluated in laboratory settings with new floor surfaces and artificial contaminants. Two commonly used slipmeters, the Brungraber Mark II and the English XL, were evaluated at actual worksites in this experiment. Four floor tiles in each of four different work areas in the kitchens of 18 fast food restaurants were selected for repeated measurements with these two slipmeters. The results indicated that sanding of footwear materials has a significant effect on the outcomes of friction measurements, and the tile-to-tile variations in friction in the same areas of restaurants were also mostly statistically significant. Significant local variation in friction among tiles in the same area could potentially increase the chances of slip and fall incidents. Both slipmeters used in this experiment could potentially have problems in the areas with grease, such as grill and fryer areas, since the build-up of grease during repeated strikes could alter the outcome of friction measured.

The Effect of Transverse Shear Force on the Required Coefficient of Friction for Level Walking

Objective: An enhanced methodology to extract the required coefficient of friction (RCOF) value was used to investigate the effects of the transverse shear component of the ground reaction force (GRF) on the RCOF. Background: The RCOF is an important indicator for slip incidents. However, the extraction of the RCOF from GRF is not standardized. The transverse shear force is usually ignored in calculating the RCOF value. Method: For this study, 40 participants performed four walking conditions. The RCOF values both with (RCOF2) and without (RCOF1) the transverse shear force were identified from each strike by the use of an enhanced method and were compared. Results: A total of 24,851 strikes were collected. The transverse component increased the RCOF value by more than 10% in 7.2% of the strikes. In 10.4% of the strikes, the RCOF2 occurred at least 20 ms earlier and the RCOF value was on average 8.9% larger than RCOF1. Conclusion: With this method, we were able to successfully identify the RCOF in a significantly large number of strikes across 40 participants. In a portion of the strikes, the transverse shear force increased the RCOF significantly. In a significant portion of the strikes, the RCOF2 occurred much earlier than RCOF1. Application: Better estimates of the RCOF magnitude and instant of occurrence could potentially improve risk assessment and identification of critical instants in gait.

Visually based perceptions of slipperiness: Underlying cues, consistency and relationship to coefficient of friction

If walkers can anticipate surface conditions, they can adjust their gait to help reduce the risk of a slip. This study investigated visual cues to slipperiness. Thirty-one participants made visually based judgements about 37 different floor surfaces. These judgements included ratings of slipperiness, reflectiveness, texture, traction, light/dark, likelihood of slipping, cautious intent as well as relative slipperiness. Correlational and regression analyses indicated that while reflectiveness is the predominant visual cue in forming judgements of slipperiness (r = 0.73; p < 0.05), texture and traction were also highly correlated with perceived slipperiness. Furthermore, participants were consistent in slipperiness judgements over time (r = 0.76; p < 0.05) and response measures and a significant relationship was observed between visual cues and coefficient of friction (COF) (r ranged from 0.16 to 0.58; all p < 0.05). Subjective ratings and measured COF, taken as a composite measure of slipperiness, may provide safety professionals with an improved indicator of ‘higher risk’ surface conditions. The results indicate that people rely on visual cues to judge slipperiness, that they do so consistently and that subjective ratings are related to measured COF. These results have implications for the measurement of slipperiness as well as the design of floor surfaces to be protective against slips and falls.

Comparison of different methods to extract the required coefficient of friction for level walking.

The required coefficient of friction (RCOF) is an important predictor for slip incidents. Despite the wide use of the RCOF there is no standardised method for identifying the RCOF from ground reaction forces. This article presents a comparison of the outcomes from seven different methods, derived from those reported in the literature, for identifying the RCOF from the same data. While commonly used methods are based on a normal force threshold, percentage of stance phase or time from heel contact, a newly introduced hybrid method is based on a combination of normal force, time and direction of increase in coefficient of friction. Although no major differences were found with these methods in more than half the strikes, significant differences were found in a significant portion of strikes. Potential problems with some of these methods were identified and discussed and they appear to be overcome by the hybrid method. Practitioner Summary: No standard method exists for determining the required coefficient of friction (RCOF), an important predictor for slipping. In this study, RCOF values from a single data set, using various methods fromthe literature, differed considerably for a significant portion of strikes. A hybrid method may yield improved results.

Slipping of the foot on the floor when pulling a pallet truck.

Workers pulling pallet trucks are likely to slip when pulling and stepping on a low-friction floor. This study investigated the slipping of male participants when pulling a pallet truck, walking backward, and stepping on either a dry, wet, or glycerol-contaminated vinyl surface. The weight of the load on the truck was either low (0 kg), medium (295 kg), or high (568 kg). A motion-tracking system was used to collect the three-dimensional coordinates of the markers on the shoes. It was found that subjects might slip either upon landing of the leading foot on the toe (slip I) or before taking off of the lagging foot on the heel (slip II). The results indicated that the slip distances for both types of slip were significantly affected by the load and surface conditions and their interactions. Micro-slips (slips between 0.1 and 3 cm) and midi-slips (slips between 3 and 10 cm) were more common in slip I than in slip II. On glycerol-contaminated surfaces, the probabilities of a slide, or a slip more than 10 cm, for both slips I and II were over 40%. The implications of the results were discussed.

Stochastic distribution of the required coefficient of friction for level walking - an in-depth study

This study investigated the stochastic distribution of the required coefficient of friction (RCOF) which is a critical element for estimating slip probability. Fifty participants walked under four walking conditions. The results of the Kolmogorov–Smirnov two-sample test indicate that 76% of the RCOF data showed a difference in distribution between both feet for the same participant under each walking condition; the data from both feet were kept separate. The results of the Kolmogorov–Smirnov goodness-of-fit test indicate that most of the distribution of the RCOF appears to have a good match with the normal (85.5%), log-normal (84.5%) and Weibull distributions (81.5%). However, approximately 7.75% of the cases did not have a match with any of these distributions. It is reasonable to use the normal distribution for representation of the RCOF distribution due to its simplicity and familiarity, but each foot had a different distribution from the other foot in 76% of cases. Practitioner Summary: The stochastic distribution of the required coefficient of friction (RCOF) was investigated for use in a statistical model to improve the estimate of slip probability in risk assessment. The results indicate that 85.5% of the distribution of the RCOF appears to have a good match with the normal distribution.

The effect of contact area on friction measured with the portable inclinable articulated strut slip tester (PIAST)

A portable inclinable articulated strut slip tester (PIAST) measures friction at the shoe and floor interface. The squeeze-film effect with the PIAST is excessive in representing a human strike. The goal of this study was to investigate the effect of the contact area size on friction for reducing the squeeze-film effect. The footwear pad area of this slip meter was sequentially reduced from 7.62 cm square to 2.54 cm square. Five walkways were constructed. Friction was measured on each walkway under three surface conditions. Thirty-five participants rated their perceptions of slipperiness. The results indicated that the friction increased and plateaued as the size of the contact area was reduced. The effect of the pad size on the friction coefficient was statistically significant. The correlation coefficients between the friction and perception rating did not give a clear indication of what pad size might have a better correlation with human perception. Friction measurement at the shoe and floor interface is a critical issue in assessing potential interventions and identifying potentially dangerous locations for slip and fall incidents. This paper addresses a potential improvement in measuring friction on liquid-contaminated floor surfaces.