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E1.1 The Contribution of Dry Contaminants to Slip Potential-Thorpe SC, Lemon PW, Jefferies SL

The majority of the research into pedestrian slipping to date has concentrated on the effects of the presence of wet contamination at the shoe/floor interface.

Although site-based experience, and Health and Safety Executive (HSE) statistics, suggest that the majority of workplace slip accidents occur as a result of wet contamination, it is well known that the presence of dry contaminants present a significant risk of pedestrian slipping. We have established, through simple observation, that dry contaminants can present a very real problem in certain industrial sectors, including food processing and production, laundries and construction.

This presentation will outline preliminary findings from ongoing research into the effects of dry contamination on pedestrian slip potential. We will study a number of popular portable coefficient of friction tests with a view to identifying which, if any, are capable of making accurate assessments of the effects of dry contaminants on a range of flooring types commonly encountered in industrial and commercial situations. We hope to be able to identify a useful site test and to recommend a testing regime for the use of that test.

In the latter stages of the study we propose to visit industrial sites in the food and non food sectors. This will allow us to evaluate the role of dry contaminants in real situations and to relate the results generated using our preferred test to incident records.

 

E1.2 Comparison of Coefficient of Friction Requirements During Gait to Tribometry Measurements: Evaluation of Shod and Barefoot Conditions at Various Walking Speeds-Powers CM, Burnfield JM, Brault JR, Flynn JE

Introduction: The Variable Incidence Tribometer (VIT) and the Portable Inclineable Articulated Strut Tribometer (PIAST) are frequently used to characterize walkway slipperiness, however the extent to which these devices reasonably emulate the dynamics of human gait has not been explored. The purpose of this study was to compare the utilized coefficient of friction (COF) during non-slip pedestrian gait to the COF obtained from the VIT and PIAST (as measured using a force plate).

Methods: Twenty healthy adults participated. Subjects walked barefoot and shod over a dry surface (smooth vinyl composition tile) at slow (57.3 m/min), medium (87.3 m/min) and fast (131.6 m/min) speeds with kinematic (VICON) and kinetic (AMTI force plate) data being recorded simultaneously. The VIT and PIAST tribometers were tested on the same force plate at angles equal to the average impact angle recorded across subjects for each condition tested.

Results: Utilized COF increased with higher walking speeds (slow, 0.22; medium, 0.24; fast, 0.26). When collapsed across walking speeds, the utilized COF during barefoot trials was greater than shod trials (0.25 vs. 0.22; p=.002). At similar inclination angles (averaged across shoe conditions and walking speeds), peak COF values recorded for the VIT and PIAST were higher (30% and 50% respectively) than those recorded during walking trials.

Discussion: At comparable impact angles, both the PIAST and the VIT overestimated the utilized COF during gait. These findings suggest that the PIAST and the VIT do not imitate the dynamics of human gait at impact. We believe that the differences between the utilized COF during walking and the measured COF from the PIAST and VIT is most likely related to the fact that the tribometer test feet do not have the same vertical and horizontal accelerations of the pedestrian's lower leg at heel strike.

 

E1.3 Measuring the Exposure to Slipping Hazards: A Novel Test Device-Grönqvist R, Hirvonen M, Rajamäki E, Polvi P, Chang WR

A prototype test device and methods of measurement have been developed for determining the exposure to slipping hazards. The test device is suited for both field and laboratory use. It can be used to measure static, transitional kinetic, and steady-state kinetic friction properties between floor surfaces and shoe material samples. Two optional modes of operation, impact and non-impact testing, can be utilized.

The current prototype has two optional pneumatic test wheels, a complete rubber wheel with tread and another test wheel constituting of six separate slider sensor units. During the impact mode, the test wheel is rotated freely at a desired constant speed (0-0.6 m/s). The normal force is produced by inflating the wheel instantaneously. The applied contact time between slider and floor is monitored and the interfacial shear (friction) force is measured using a built-in torque transducer (Vibro-Meter TM208) and a measurement software (HP VEE 5.0). During the non-impact mode, the test wheel is operated from an initial, stationary position with respect to floor surface. A laptop computer equipped with a PC card (DT 7101) for analog-to-digital conversion of measurement data is used to compute static, transitional and steady-state kinetic friction indexes. Frictional variations over contact time, apparent normal pressure, and sliding velocity comprise the basic safety criteria for determining the exposure to slipping hazards. Hazard prediction models for various applications of the method are currently examined.

The project continues with test methods development, including selection of slider sensor materials for testing purposes and selection of reference floorings for calibration of the instrument. The remaining questions are: 1) how to simplify the device for actual field use, and 2) how to accurately control the normal force on different types of floor surfaces.

 

E1.4 Validating Slipmeters: Aspects of Conformity Between Test Feet and Floor Surfaces-Bowman R, Angelopoulos TA

A wide range of slipmeters has been developed to indicate the available slip resistance of pedestrian surface materials. The validation of such devices is largely based on measurements of the coefficient of friction (cof) using force platforms. While slipmeters may demonstrate high degrees of repeatability and low bias, they may also measure different slip resistance values for the same surface. One of many reasons may be poor conformity between the test foot and the floor surface.

Pedestrians are most likely to slip during heel strike. Contact area measurements at heel strike led to a 9 mm diameter test foot being used in the Tortus. Other slipmeters use larger feet: for example, ASTM C1028 uses a 76.2 mm square slider, and the Pendulum a 75 mm wide test foot.

Pendulum measurements were made on different coloured glazed (orange peel texture) 100 mm tiles from one product range. The wet cof varied from 0.23 to 0.57, but there was no correlation with Rtm roughness (range of 12.4 to 20.6 mm, 0.8 mm cut-off). A 75 mm wide paint roller was made from a rubber of similar hardness to Four S rubber. When paint was applied to the tiles, a lack of planarity was evident. When the tiles were tested according to SATRA test method PM 144 using a 25 mm wide slider, the cof was consistently higher. The cof gave reasonable correlation with 60 degree gloss measurements.

While planarity deviations may affect a test result, they will not affect the friction that is available to the pedestrian. In order not to underestimate the available traction, it may be necessary to determine the degree of effective contact.

All slipmeters have potential limitations. When validating slipmeters a wide range of products should be evaluated and the results compared with subjective experiments using humans.

    

 

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Page last updated: March 2001
Page last reviewed: March 2001
Content Source: National Institute for Occupational Safety and Health (NIOSH) Division of Safety Research