Wen Chang

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Chang.W.2013.jpgMeasurement of Slipperiness

Measurement of slipperiness plays an important role in slips and falls because it can be used to identify dangerous locations where slip incidents might occur and to evaluate potential interventions.

  • In the Hopkinton conference on the measurement of slipperiness in 2000, critical issues on the measurement of slipperiness were discussed. As a part of the conference activities, Dr. Wen-Ruey Chang served as the leading editor of a book, entitled Measuring Slipperiness: Human Locomotion and Surface Factors, published by Taylor & Francis.
  • In a subsequent study, he evaluated whether local friction reduction could be a better indicator of slipperiness than the mean friction level as speculated in the literature by investigating floor slipperiness in fast food restaurants over 40 working areas using both objective and subjective measurements. Their results indicated that, although some friction reduction variables had a better correlation with the perception rating than mean friction value, the mean friction coefficient of an area was a major predictor of perception rating and remained a reasonably good indicator of slipperiness in practice.
  • Chang investigated the relationships among different methods of slipperiness measurement based on psychophysics, biomechanics and tribology with a goal to identify relationship among them by exposing the participants to 5 floor types under dry, wet and glycerol conditions. The results showed that the participants appeared to rely on the potential for foot slip to form their perceived slipperiness rating (PSR) under wet conditions.  Under glycerol conditions, some kinematic variables also became major predictors of PSR.  In a different analysis of the data, the analysis of variance (ANOVA) and cluster analysis were used to eliminate redundant kinematic variables.  Among 12 output measures, utilized coefficient of friction, PSR and heel contact angle were three variables identified to dominate the input and output measures of the responses, and could be evaluated further as a measurement of slipperiness.

These studies resulted in the identification of critical factors contributing to the measurements of slipperiness.

Surface Texture on Friction

It has been generally assumed that a rougher surface could lead to greater friction, but it is not clear what features of surface texture could be better indicators of friction.  A series of experiments were conducted to investigate the effects of microscopic floor surface roughness and waviness on friction between footwear and floor surfaces.  The results showed that indicators for surface friction should include the roughness parameters reflecting the surface characteristics of surface void volume, surface slope and peak to valley distance, depending on the material combination and contaminant at the interface.  Surface waviness parameters could potentially be better indicators of friction than commonly used surface roughness parameters when the viscosity of the liquid contaminant is high.  Potential interventions to reduce slip and fall accidents should include floor surfaces with these preferred features to help increase friction.

The results of these studies identified critical features of “slip resistant” floors.

Prediction of Slip Probability

Slip prediction plays an important role in evaluating the potential risk with existing environments or benefits of potential interventions.  When underfoot friction demand (required coefficient of friction or RCOF) measured with a force plate for an activity exceeds the available coefficient of friction (ACOF) at the shoe and floor interface measured with a slipmeter, a slip may happen.  The prediction of the potential risk largely depends on a logistic regression model empirically derived from the data obtained from laboratory experiments.  Dr. Chang introduced a statistical model comparing the stochastic distributions of ACOF and RCOF to calculate the probability that RCOF exceeds ACOF.  For RCOF distributions, the results showed that the distribution of the RCOF appeared to have a good match the normal distribution for most of the conditions in their experiment (85.5%), but each foot had a different distribution from the other foot under the same conditions in 76% of cases.  Dr. Chang and colleagues also investigated the stochastic distributions of the ACOF of five floor surfaces under dry, water and glycerol conditions.  They reported that the ACOF distributions could be represented with the normal or log-normal distributions or a constant in 12 out of 15 cases.

These studies provided the most general method for predicting slip probability, one that is less case-dependent.  Also, in order to improve the prediction, the friction demand (RCOF) data from both feet of the same individual should be kept separate.

Ladder Safety

Portable straight ladders are widely used, both in business and by the general public. Although several methods have been developed to help the users set up their ladders properly, serious injuries still occur. The purposes of these studies were to identify factors that can be used to reduce the injuries caused by the slip-out at the ladder base in a laboratory study and to identify whether ladder setup training helps prevent employees in the field from being injured from ladder incidents.

  • In a laboratory study, they measured friction needed at the ladder base to support human climbing and the friction that can be supported by the “ladder shoes” to resist sliding. They identified ladder inclined angle, climbing speed and ground condition as three factors to reduce incidents caused by ladder slip-out at the base.
  • In a field study, professional installers of a cable TV company were observed for portable straight ladder setup at their worksites. All the participants had received training in portable straight ladder setup as demonstrated through their answers to a questionnaire. Although the recommended inclined angle is 75.5°, the participants’ inclined angle averaged 67.3° with a standard deviation of 3.22°.  Only a few participants used the methods that they learned during training in their daily work.  Further improvement may be behavioral − employers could take steps to improve the company’s safety climate/culture and to motivate and encourage employees to have better safety behavior.  Participants were observed for their environments, work conditions and safety behavior involving portable ladders and their correlations with self-reported safety performance.  Proper setup on slippery surfaces, correct method for ladder inclination setup and ladder secured at the bottom had the lowest compliance with best practices and training guidelines.  The observation compliance score was found to have significant correlation with straight ladder inclined angle and employees’ self-reported safety participation.

These studies identified important gaps in our knowledge for reducing straight ladder incidents.

Slipmeter Evaluation and Protocol Improvement

Slipmeters are mechanical devices used to measure friction between shoe and floor and are widely used by safety professionals and researchers both in laboratories and field environments.  In the 2000 Hopkinton conference on the measurement of slipperiness, critical issues on the measurement of slipperiness were discussed and slipmeters used around the world were reviewed.  Although the protocols of major slipmeters used in US were supported by national standards such as ASTM and ANSI organizations, some critical details were missing in these standards.  Several studies were conducted by Dr. Chang to improve the protocols for major slipmeters used in USA and the performances of these slipmeters were compared both in the laboratories and field environments.

The results obtained by Dr. Chang have made a significant impact on the use of these slipmeters by safety professionals around the world.