MEASUREMENT OF SLIP RESISTANCE, a legal and practical perspective
Copyright Barrett C. Miller, MEd, OHST
Authors Note: this article has been updated about twenty times. However, at this time, I am working on a radical rewrite. Please check your dates on individual standards. At this time the only ASTM standard that is in effect and up to date is ASTM F609-05. We hop this document is useful for you as a historical document.
About twenty percent of injuries are the result of same surface falls. This is true in both the public and private sector. Its hard to believe, but things are improving. In 1940, there were 22 deaths per hundred thousand from falls; today that number has fallen to about 1.6. Reductions are due in part to improvement in products and materials. A growing understanding of floor surfaces has resulted in the elimination of surface materials and finishes once considered acceptable. We seldom see carnauba waxes or new terrazzo floors, for example. Our walking surfaces are just safer than they used to be.
We now understand the human limitations which contribute to falls. For example, we commonly quantify the relation of the foot to the surface.(1) . This relationship, called traction or friction, can be simply modeled and understood. Mathematically, we call the relationship the coefficient of static friction. The coefficient of friction, or COF, is an application of Newton's general theory of relativity (gravity). To establish a surface measurement, we begin by pushing, pulling or dragging an object on a test surface. The resulting effect is recorded..
Friction or traction is the resistance to lateral movement caused by
the contact between two surfaces. Slipperiness = Too Little Friction. Dividing
the horizontal force by vertical force (weight), we get a number called
the coefficient of friction. Concrete, with .8 COF, would have more traction,
and be less slippery, than ice with a COF of .3, for example. The concept
may be used to describe the friction relationship between many kinds of
objects. COF has become on of the common performance measurements for products
like floor finishes. The application of the concept of slip resistance
can be misleading unless it is paired with information on test method used
to make the measurement.
The Americans with Disabilities Act has raised the simple notion of slip resistance to a civil right.(3) Many disagree about how measurements are to be made, but for now, a national working definition of slip resistance, exists. It was adopted from an Underwriters Laboratory standard; who adopted it from an ASTM standard; who adopted it from a Department of Commerce administrative committee. It's the American way.
FLORIDA'S SECRET STANDARD
Florida pioneered a slip resistance standard but for years no one understood it. It produced confusion and was unenforceable. An insurance carrier asked the author to discover what coefficient of friction readings meant in Florida and how measurements were to be made. It seemed easy, but was not. State requirements were written and available. The Department of Community Affairs provided a few details about the test protocol which originated at the University of North Carolina. There was even a sketch of a test device.
After weeks of research, we found no answer and wrote to the chairman of the Engineering Department in North Carolina. Months later, there was a reply. On the face of our original letter, neatly penned with great care, the department chairman wrote,
The requirements of different states can be confusing. The positive effect of regulation can be lost because the adopted method of measurement isn't clear. Florida changed it's administrative law. Now, any system for testing the dry coefficient of friction at .5 if the method meets a recognized national consensus standard.
Machine no longer exists... there was only one...now dismantled...faculty groups disbanded...no one remembers who they were...sorry...
The American National Standards Institute is a clearing house for national consensus standards. ANSI combines the effort of numerous private organizations and industry groups.
None of the 4500 standards issued by the American National Standards Institute (ANSI) is mandatory in itself. ...An American National Standard implies a consensus of those substantially concerned with its scope and provisions.ANSI standards, and ASTM test protocols are incorporated by reference in the state law and the building codes of American communities, but, for policy reasons, ANSI disclaims legal standing. Legislatures and the courts use consensus standards in a different context. Original research, often developed as corporate work product, has attained quasi-legislative status. Consensus organizations may reject legal authority and assert voluntary participation, but all National Consensus Standards are enforceable by judicial recognition (notice), and by administrative complaint from state and Federal agencies. This is common in our democratic system.
American Society for Testing and Materials develop test methods for defining quality in many materials. ASTM standards require a test protocol to be reasonable and methodologically sound. The method must be consistent, and reasonably analyze the subject-matter being investigated. Standardization implies a comparison between things, not absolutes. A test must be precise, which implies a less rigid standard of proof than accuracy. In most cases, a test protocol must demonstrate bias, that is, the variation between a known value and the result of the test device. At present slip resistance testing correctly performed under different consensus standards may produce different results. Both disabled persons, and manufacturers of products deserve specific performance-based regulations.
At times, consensus standards are incomplete and inadequate. Some have a negative effect and keep field conditions artificially low. OSHA is forbidden in many cases, for example, to enforce a standards of behavior which is more rigorous than published national consensus organizations dictate, but OSHA slip resistance duties are based upon a laboratory standard; not usable in the field. Few OSHA accidents happen in a laboratory so proof is difficult. Other standards, like the one defining bathtub slip resistance, are criticized by special groups. They claim it is impossible to find a tub that will fail, though many are dangerous. Government action on slip resistance standards seems justified.
FLOOR TEST GADGETS
The patent office has issued more than fifty registrations for slip resistance test devices since 1985, but the list of accepted test protocols has remained about the same. There are some new standards which reinterpret old work, but not much is new. The process favors private interest groups which are organized because the cost of rallying support for a consensus standard is both expensive and burdensome. An interested group can control an ASTM committee, and define material standards which have the effect of law in our various communities.
The test devices discussed here does not attempt to arbitrarily choose
the best gadget. Instead of ranking them by quality, we will name and describe
them by their principal mode of operation. For now, any one of these systems
may be used to demonstrate good faith comply with the requirements of the
ADA. But, at this time, no machine or reference surface provides an absolute
reference for the measurement of a given slip resistance. Devices and methods
aren't that sophisticated. One must remember that the purpose of testing
a floor is to provide a comparison between objects tested on the same device,
and to help isolate variables. Both are important functions. The perfect
test device remains to be developed.
Devices That Pull
At this time, this drag sled, sponsored by Liberty Mutual is the only ASTM standard that has been fully accepted. This machine is a drag sled. It measures the point at which a horizontal force acting upon a resting object, first causes the object to move. No external computation is necessary since the scale face is adjusted for the base weight of the machine. For consistent results, the horizontal pulling force is applied through a small motor connected by nylon cord. The HPS is self calibrating and requires no external computations. The machine is easy to explain to non-technical people like juries. ASTM protocols shows that it may be used on wet surfaces. A separate standard provides comparisons to the James Machine, a laboratory instrument. A HPS may be used as a laboratory or field instrument.
The slip meter was developed by Charles Irvine at the Liberty Mutual Research Center. Liberty Mutual also supported its adoption as an ASTM standard(5). They created the machine because it was easy for their own field personnel to understand and because features, like the use of a pull motor, limit the accidental or intentional manipulation of results. Results are repeatable. Computations of standard deviation figures produce results that are impressive. Three independent laboratories, testing the same samples showed precision measurements repeatable within a range of .017. This was the best published result among standard machines.
The device is simple, rugged, and easy to explain. It sells for about a thousand dollars. A unique feature of this standard and the machine that comes from it is the ability to isolate the effect of shoe materials. The HPS comes with replaceable surfaces which can be quickly be modified to hold portions of a victim's shoe of other materials.
Critics fault the device because it does not claim to be statistically accurate at cof readings above .8, and below .2. In practice, readings in these ranges are not important. No one has shown that the device claiming to be accurate in these numerical ranges really produces meaningful data. Critics say that it does not compare readings to an absolute measure, but if such a measure existed, this machine could easily be calibrated to match those results.
Horizontal Dynamometer Pull-Meters ASTM 1028
This device, and its ASTM protocol, is another second generation drag
sled tester. A fifty pound block with a carefully defined Neoprene surface
is pulled with an 80 pound laboratory scale to establish a static coefficient
of friction. The point where the weight begins to move and a COF measurement
is computed dividing the horizontal force by the vertical force. Critics
say that measurements made with a device pulled by hand will produce wide
variations. ASTM listed measurements of precision show that one can expect
standard deviations readings of .05 on wet surfaces. While more consistent
readings are found in devices, readings of .05-.07 are significant improvements
over many test devices. This machine was designed to be inexpensive and
shop-built. Construction cost are modest price-that was its purpose. As
a test protocol, it is accepted by American courts for testing both roadway
surfaces and tile floors.
ASTM 1028 offers one welcome addition to the slip resistance field. The Tile Council of America, sponsor of the protocol, has included a standardized test surface to be used as a reference. This is unique and very important because there has never been a clear definition of the meaning of a slip resistance reading in terms of a physical object. This will begin to simplify the existence of calibrating different devices.(6)
Devices that Push
The James Machine .ASTM D2047
This is a laboratory machine sponsored for a test protocol by the association by floor care product manufacturers in 1975. The standard is oldest continuously operating protocol and the most often cited by other standards. Some new devices compare themselves to the James machine to plead their own accuracy and correct operation. The James machine uses an articulated arm to push a test pad sitting in contact with a representative surface. The coefficient of friction is computed by graphing the angle of the articulated foot at the moment the power of the machine causes the sample to slip. The ASTM standard provides no information about precision or bias.
Critics of the machine list its size and weight as a shortcoming. This
is a laboratory device, not meant for field operation. The lack of portability
creates special problems in litigation. Others criticize the James Machine
for its shortcomings in testing wet surfaces. Some floor finishes emulsify
when they get wet and become the cause of fall accidents. This leaves the
floor finish industry with a standard which cannot be used to test one
of the principal shortcomings of many products.
For years our criticism was far stronger. The James machine has been out of production for years. Two safety engineers tried to find one in two cities with populations over one million people. No one could find a James machine. No one reported ever seeing one. Now, within the last few months, two companies manufacture the James machine.
Portable Variable Incident Testers ASTM 1679-96 and ASTM D5859
Articulated strut devices represent a specialized second generation test machine. In principal, these devices approximate the operation of the James Machine. The best known is the Brungraber Mark I tester. A strut, with a fixed angle, is used to push a movable arm against a test surface. The point at which the test sample breaks loose is recorded by a pointer on a steel slide. The slide computes the tangent of the angle where the second arm releases to estimate the coefficient of friction. The Penn State study of slip resistance testing done for the US Department of Justice found the BMI the best of the best when testing surfaces wet, dry, and contaminated. It can be used to accurately measure a coefficient of friction levels as low as .04. After months of practice and use of the BMI, we are left with the conviction that the study of this device is impressive in every way.
The BMI was developed at the National Bureau of Standards under a grant from the US Consumer Product Safety Commission. After developing the device, Dr. Brungraber donated his patents to the American people. For three decades, the Mark I has provided the only acceptable standard for the evaluation of bathtub surfaces. The ASTM standard that standardizes the testing methodology, is among the most complete and contains adequate internal safeguards. While some argue that the measurement of frictional coefficient as low as .04 is folly. The snake oil salesman of the slip resistance industry are quick to say that the BMI can not be used on wet surfaces, but one should understand that these people often confuse salesmanship with science.
The Brungraber Mark I is easy to use, but may be conceptually difficult for the non professional, and even some of its owners, to understand. Unlike original articulated test devices, second generation machines can be calibrated to remove the computational error introduced by the slope of the tested surface. However, some, like the VIT (Englishxl) test machine, can not be used to made adequate test of materials like shoe soles. This limits their use somewhat as an investigative tool.
Two ASTM test methods mimic the characteristics of the Brungraber tester. The protocol, issued by a captive subcommittee, does not directly address the methodological questions usually included in test protocols. One machine is the functional equivalent to the Brungraber Mark II, except that it is powered by the air canister from a bb gun.. (7) It is also much lighter weight than the Brungraber tester. Many questions will be raised as this device is changed and imitated by competitors. For example, this standard permits its use on wet and rough surfaces but does not explain why this is possible when the James machine it copies was not adequate on the same surfaces. To make things more complicated, one maker describes his product with what a language that differs dramatically from the normal vocabulary used in the field.
This articulated strut tester, the Englishxl, has acquired a committed, even zealous, following. Data from one university study shows that precision and bias figures claimed by enthusiasts, exceed the tolerances for the materials used in the device itself. Leaders of this group ( William English) cannot even agree if the machine measures static or dynamic coefficient of friction, or what reference material should be used as a contact surface. There is no question, however, that an amazing number of people are willing to pay in excess of three thousand dollars for a machine which is mechanically simple and easy to manufacture. Be aware that not all purchasers are happy with their choice. The existence of ASTM F 1679, will encourage the production of numerous tribometers(sic). Controversy has always shadowed the VIT. The controversy may have less to do with the machine than with some industry representations. (Historical note: In 2007, after fifteen years without completion of the Englishxl standard (F1679) , the standard was canceled by ASTM.) Link
The ASTM standard for measuring slip resistance in tubs is often criticized.(8), detractors claim that it is without without real world value. Critics of the bathtub standard claim that no one except the courts, accepts this standard. The standard measure of slip resistant compliance is found in tub surfaces having a static coefficient of friction of .04 above a baseline. What does the device measure? Perhaps nothing but qualities of the test contaminant (soap). Dr. Steven Rosen, a critic of the bathtub standard says: The bathtub/shower slip resistance standard is absurd. He believed that it would be very difficult to find a tub or shower that does not meet this standard. He believes slip resistance of bathroom floors can only be validly tested under dry conditions.(9) We have found that a great many people pay attention to the standard (ASTM 463) and find it persuasive. We find that those who criticize the bathtub standard usually don't understand it. In fact, the Brungraber Mark I is ingenious and attention to details of construction of the machine are impressive.
Conservation Of Momentum
This test protocol is simple and elegant. It was developed by the National Bureau of Standards and improved by the British Roadway Institute and is the most frequently used device outside of the United States. The test arm looks like a large clock pendulum mounted on a metal frame. The arm swings, powered by gravity, on to a test surface. A calibrated scale is marked by a pointer on pendulum arm.
The operational theory is sound, and has been used for many years to test metal surfaces. The study of friction is similar to the study of conservation of energy. All of the other testers discussed copy the theoretical features of traditional friction sled. The pendulum tester measures the loss of momentum while stopping. It computes the COF from the stopping characteristics in a manner similar to the measurement of speed by analyzing the skid marks of an automobile. There are currently prototypes for other momentum based devices. Other conservation of momentum devices will soon be available.
The most common machine of this type is called the British Pendulum tester and was developed by the British Roadway Research Laboratory. There are at least three ANSI standards that use pendulum testers to measure the coefficient of friction including roadway surfaces in auto accidents. The mysterious Florida standard mentioned earlier was a pendulum tester. It reappeared in our research as a standard for measuring the coefficient of friction of roadway surfaces(11).
The pendulum tester measures the COF my measuring the reduction of velocity similar to methods used to measure the conservation of momentum in auto accidents.
WHERE THE RUBBER HITS THE ROAD
A Louisiana engineer reconstructed a fall accident on a boat. The allegation stated that the gel coat surface of the deck was slippery and made a plaintiff fall. The engineer took the shoe of the victim, weighted it, wet the surface, and then dragged the shoe with a spring scale to demonstrate a COF. He found that the surface measured .4 wet and judged it to be slippery. The engineer, believing that he had proved his case, stopped. But was he through? He had shown that the identical combination of shoe and surface were slippery-something which might be assumed. In part, he had eliminated the pedestrian's theoretical contribution to the accident, but, he had shown nothing that demonstrated a duty of the boat manufacturer. He had not isolated or evaluated the correct variables. In most cases, a host is not responsible for every conceivable shoe worn by a pedestrian, or for factors impossible to control. If the engineer was to demonstrate liability on the part of anyone, he will have to show us more.
While slip resistance is an important element in the investigation of accidents, it is only one variable. It is just as important to understand falls as part of a larger context and to develop a correct methodology. The Americans with Disabilities Act, as well as civil litigation, has intensified pressure for simple answers. Can a single number provide a red light/green light test of liability. The trend toward oversimplification has also fueled efforts to have one proprietary device or another serve as the basis for judgment.
In every case, an investigation begins with a hypothesis. It may be formal or informal-conscious or unconscious. The theory of cause and effect serves as a straw dog against which floor testing is a single variable. Is floor wax slippery or substandard? Was the walkway adequately lighted?(12) What about the surface of the shoe or its materials? Did a crowded condition contribute to an accident? Was it too dark for the victim to see a puddle or object. Was it too crowded to look down? Did the pedestrian just walk too fast? If he walked too fast, what standard of behavior would support conclusions on speed? (13) Each element must be solved by the application of sound reasoning.
A STICKY SUBJECT
Most same surface fall accidents happen on wet surfaces. We can look at two hypothetical pedestrians. Over the weekend, the author walked his dog on a public sidewalk and didn't fall. Another man walking on the same street slips when he comes to the place where a drain spout feeds water across the sidewalk. Sound theory requires one to account for the differences in these example. To say that the floor of a retail store was wet doesn't automatically explain anything. People fall on wet surfaces for a number of reasons; some reasons have nothing to do with slip resistance.
Our language is not always standard. We speak about surfaces wet with water, for example. Surfaces wet with other viscous materials like break fluid or motor oil are described as contaminated. Water can activate innocuous materials and turn them into dangerous conditions. In fact, it is usually water that activates some second process and create slipperiness. A floor might be wet with water, but contaminated by the floor finish which emulsified when it got wet. The duty of a party to civil litigation would be completely different depending which of these hypothetical conditions could be demonstrated.
Water has an unusual molecule. The electrons in the outer ring of the molecule make it combine easily. Water can act as an adhesive or a solvent. The composition of water itself is not the same in every case. There are a few wet surfaces falls in which the accident cause appears almost hydraulic, although these are rare. In the case of the man that fell, the iron, calcium, and other minerals deposited on the roof as dust had been displaced during a rainstorm. In this case, water from our drain activated a "typical" relationship, it created a slippery surface.
Testing wet surfaces is difficult for both theoretical and practical reasons. All existing floor test methods depend on the mathematical manipulation of a theoretical model; a shoe and a surface in equilibrium. When a surface is wet, test results sometime show elevated friction levels. This happens for reasons that have are peculiar to floor testing and not surface characteristics. A phenomenon, called covalent bonding, can bind free electrons in the valence ring of the water molecule. Some device manufacturers claim their equipment to be free of this bonding. They are not.(14) Shoe materials like leather might swell when wet, creating still further analytical questions. For this reason, most standards for products exclude wet measurements, or at best, accept them with caution. Paradoxically, though slip accidents usually happen on wet surfaces, most experts agree that standards must be based on the measurement of dry uncontaminated surfaces.
At present, ASTM standards contain a disclaimer on issues of safety , accuracy, and suitability. Authoritative sources disagree about tests conducted under different protocols. The confusion about the measurement of slip resistance on walking surfaces benefits no one. Progress in the development of materials is slowed by conflicting industry preferences and within standards organizations. Special interests continue to lobby for particular machines or restrictions and exclusions needed by their industry. In obedience to its charge, the American National Standards Institute, or alternatively, a Federal administrative agency must take the initiative in establishing a published minimum coefficient of friction standards. It must be defined in terms of a material. Arguments against such action have merit, but there is more to be gained by acting prudently than in continuing to flounder.
Continued evaluation of new materials still offers the most safety benefit. An hour has sixty minutes and a meter one thousand millimeters. Our watches are correct by comparison with a clock at the National Bureau of Standards in Colorado. That clock may be accurate according to astrophysical research but for most of us, it is correct by definition. A single object or surface must be established as a given coefficient of friction. This would not favor a single machine or technique but would become a basic value for describing future standards. The establishment of reference materials was recommended by the National Bureau of Standards in 1961(15)
. Only the Ceramic Tile institute has attempted to establish a standard reference surface against which all measurements are to be made. Their method has not been validated or completely accepted, but their method is sound.
The paradox is that we agree on most things. With a single exception, test protocols seem adequate. The friction sled, used in all basic physics classes, is accepted as a theoretical model by almost everyone. If one establishes a single statutory reference surface and meaningful perimeters for tolerance, the makers of all existing devices will find a way to calibrate their instruments.
1. Miller, Barrett. "Human Factors Issues in Slip and Fall Litigation". Continuing Legal Education Notebook, The Florida Bar NO6495, Mar 89.
2. D2047-75 Static Coefficient of Friction of Polish-Coated floor surfaces as measured by the James Machine. Par 3.1. 1975.
3. 28 CFR part 36, Revised July 1, 1994. Nondiscrimination on the Basis of Disability by Public Accommodations and in Commercial Facilities.
4.Irvine, C.H. "Measurement of Pedestrian Slip Resistance," Professional Safety, pp 30-33, Dec 1984.
5. Irvine, C.H. "Measurement of Pedestrian Slip Resistance," Professional Safety, pp 30-33, Dec 1984.
6. Calibration tiles are available from the Ceramic Tile of America, 100 Clemson Research Blvd, Anderson, SC 29625.
7. ASTM F1677-96, Scope Par 1.1.1
8. See ASTM F 1678.
9. Rosen, S. Slip and Fall Handbook. Hanrow Press, 1994, p19.
10. For a time the ADA accepted only tests using a single device. This was the result of an 1988 Penn State study. The ADA has since abandoned that position due to the conditions under which the study was made.. Present ADA obligations refer to the use of the James Machine, but allow testing by devices of various types.
11. ASTM E707-90. Skid Resistance Measurements using the North Carolina State University variable speed friction tester
12. Miller, Barrett. "In Plain View: Blind Spots and Visual Limits. Occupational Safety and Health, May 1994.
13. 41Sherman, Roger. "Preventing Slips Tat Result In Falls." Professional Safety, March 1992.
14. Irvine, Charles. "Evaluation of the Effect of Contact-Time when Measuring Floor Slip Resistance." Journal of Testing and Evaluation, 1986.
15. Task Group Recommendation 1961. A History of Walkway Slip-Resistance Research at the National Bureau of Standards. Library of Congress Catalog Card #79600179.
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