Slip Resistance Standards: Sorting It All Out

BY BARRETT C. MILLER, MED, OHST                                                                                                                   <Resume>     

This article was originally published in the March 1999 issue of Safety and Health, the journal of the National Safety Council. A copyright to this version is held with NSC

.About 20 percent of injuries are the result of surface falls. In 1940, there were 22 deaths per 1000,000 from falls; today that number is 1.6. Reductions are due in part to improvement in products and materials. But slips still cause injuries and death and we can do more to understand their causes.It's hard to believe, but things are improving.

What Is Slippery?

The relationship of the foot to the surface is called traction or friction. Friction is the resistance to lateral movement caused by the contact between two surfaces. Dividing the horizontal force by vertical force, 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.

COF has become one of the common performance measurements for products like floor finishes. COF even defines a legal duty. The first formal definition of slipresistant surfaces is in the proceedings of the Department of Commerce in 1953. That definition, a static measure of .5, has become a working definition under most state laws, municipal ordinances and building codes. The Americans with Disabilities Act has raised the simple notion of slip resistance to a civil right. Despite many reservations, most codes recognize a static coefficient of friction of .5 as legal and enforceable for slip-resistant pedestrian walkways.

Current Standards Aren't Enough The American National Standards Institute is a clearinghouse for national consensus standards. None of ANSI's 4,500 standards is mandatory in itself; they simply imply a consensus of those concerned with its scope and provisions.

The American Society for Testing and Materials develops test methods for defining quality in many materials. ASTM standards require a test protocol to be reasonable and methodologically sound. A test must be "precise," which is less rigid than "accurate." At present, slip-resistance tests that are correctly performed under different consensus standards may produce different results.

Though ANSI disclaims legal standing, ANSI standards and ASTM test protocols are incorporated by reference in state laws and building codes. Consensus organizations may reject legal authority and assert voluntary participation, but all national consensus standards are enforceable by judicial recognition, and by administrative complaint from state and federal agencies.

At times, consensus standards are incomplete and inadequate. Presently, both OSHA and the ADA Compliance Board accept test methods which meet any recognized national consensus standard. This decision is not without controversy, however. Some methods are frequently criticized. The bathtub slip resistance standard is one example.

Many Ways to Measure

There are many test devices available for measuring slip resistance that comply with the requirements of the ADA. However, no machine or reference surface provides an absolute reference for the measurement of a given slip resistance. The purpose of testing a floor is to compare ob'ects tested on the same device, and to help isolate variables. The perfect test device has yet to be developed. Here are a few of the machines out there:

may produce different results.

*The Horizontal Drag Slip Meter: This machine, a drag sled, measures the point at which a horizontal force acting upon a resting object first causes the object to move. For consistent results, the horizontal pulling force is applied through a small motor connected by nylon cord. The machine is easy to explain to nontechnical people like Juries. ASTM protocol shows that it may be used on wet surfaces. The device does not claim to be statistically accurate at COF readings above .8 and below .2. In practice, however, readings in these ranges are not important.

The Horizontal Dynamome ter Pull-Meter Method: This device is another drag-sled tester. An 80-pound laboratory scale pulls a 50-pound block with a carefully defined Neoprene surface to establish a static coefficient of friction. The COF measurement is computed by dividing the horizontal force by the vertical force. Critics say that measurements made with a device pulled by hand produce wide variations. ASTM-listed measurements of precision show that one can expect standard deviation reading of .05 on wet surfaces. However, deviations of .05 to .07 are significant improvements over many test devices. This machine was designed to be inexpensive and shop-built. As a test protocol, it is accepted by American courts for testing both roadway surfaces and tile floors.

The James Machine: This is a laboratory machine sponsored for a test protocol by the association of floor-care product manufacturers in 1975. The standard, ASTM D2047, is the oldest continuously operating protocol and the most often cited by other standards. 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. It is a large, heavy laboratory device, not meant for field operation. Some people criticize the James Machine for its shortcomings in testing wet surfaces. The James Machine has been out of production for years and is difficult - if not impossible - to find on the market.

Portable Articulated strut machines approximate the operation of the James Machine. The best known is the Brungrabber device developed in the early 1970's. A strut, with a fixed angle, pushes a movable arm against a test surface. The point at which the test sample breaks loose from the surface is recorded by a pointer on a protractor. The user computes the tangent of the angle where the second arm releases to estimate the COF. This machine is easy to use, but may be conceptually difficult for the non-professional to understand. A new ASTM standard for this device permits its application on wet and rough surfaces but does not explain why the James Machine was not used on the same surfaces.

The Sigler Pendulum Tester: This device 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, onto a test surface. A calibrated scale is marked by a pointer on pendulum arm. 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 at least three ANSI standards that use pendulum testers to measure the COF, including testing of roadway surfaces in auto accidents.

Slip Resistance in Practice

Slip Resistance in Practice

In a court cast, a Louisiana engineer reconstructed a worker's fall on a boat. It was alleged that the gelcoat surface of the deck was slippery and made him fall. The engineer took the shoe of the victim, weighted it, wet the surface, then dragged the shoe with a spring scale. He found that the surface measured .4 wet and judged it to be slippery. He felt his job was done.

But was it? He proved that the identical combination of shoe and surface were slippery. In part, he eliminated the pedestrian's theoretical contribution to the accident. However, he proved nothing that demonstrated a duty of the boat manufacturer. To demonstrate liability on the part of anyone, that engineer would have to show more.

Was the walkway adequately lighted? 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?

Most same-surface fall incidents take place on wet surfaces. However, to say that a surface was wet doesn't automatically explain anything. People fall on wet surfaces for reasons that have nothing to do with slip resistance. 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. Can a single number provide a red light/green light test of liability?

A Sticky Subject

The confusion about the measurement of slip resistance, particularly on wet surfaces, benefits no one. The first thing to do is to establish if measurements are to be made wet or dry.  Progress in the development of materials is slowed by conflicts between and within industries and within standards organizations. Special interests lobby for particular machines or restrictions and exclusions that benefit their industry or a product they promote. 

    In obedience to its charge, ANSI or a federal administrative agency must establish a published minimum COF standard. By definition, an hour has sixty minutes and a meter one thousand millimeters. In the same way, single object or surface must be established as a given COF. This measurement would not favor a single machine or technique but would become a basic value for describing future standards.

    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 someone 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.