Understanding the Coefficient of Friction
by Lisa Turner, CSI, Arden Architectural Specialties
Safety and accessibility are critical components of a building’s design. The slipperiness of flooring is a major factor in both. Measuring the coefficient of friction of flooring is the most common way to determine the expected safety of the surface. Most manufacturers publish this data. Comparing data from different manufacturers isn’t as straightforward as it might seem, however.
There are two types of friction and friction coefficients; static and dynamic. The static coefficient of friction measures the amount of energy required to move a stationary object across a surface. The dynamic coefficient of friction measures the energy required to keep an object moving. Anybody who has moved furniture can tell the difference. Getting that bookcase moving and getting it across the room require different levels of energy. Both types of friction can be quantified as follows:
Friction Coefficient = Friction force opposing motion Load at right angles to surfaces
Most flooring product manufacturers measure the static coefficient of friction. Any two surfaces, like a rubber platform shoe and dry shag carpet, will have a relatively constant static coefficient of friction. Trade the platform for a wing tip, however, and you’ve got a different story. Likewise, different testing conditions used in the various test methods yield different results. The American Society for Testing and Materials (ASTM) has at least seven methods for testing coefficients of friction for floors and floor coatings.
There is significant debate as to which method is the “right” method. Well-meaning manufacturers are left to taking the advice of testing labs to select the method. ASTM D2047 measures friction on a dry surface with a leather friction pad equivalent to the heel of a wing tip. ASTM C1028-89 uses a 50-pound drag-weight and a Neolite® heel assembly. Wet testing yields widely varied results depending on the type of heel assembly. Leather soaks up water and becomes sticky resulting in a high coefficient of friction. Wearing sneakers? Be careful. In real-life situations, different weights also have a major affect. Kate Moss and Louie Anderson will not experience the same coefficient of friction, even if they are wearing the same shoes.
Most studies show that people can walk comfortably and safely on surfaces with a coefficient of friction greater than 0.4. The Occupational Safety and Health Administration (OSHA) recommends a flooring surface with a coefficient of friction greater than 0.5. The Americans with Disabilities Act recommends higher coefficients of friction for some types of application. Accessible routes should have a coefficient of friction of 0.6 and ramps should be 0.8. Underwriters Laboratories (UL) standard also requires a 0.5 coefficient of friction (UL 410).
Understanding the testing methods will help in comparing products and their performance against published standards.
For reference information used for this article, explanation of specific tests, or a list of additional sources of information regarding coefficients of friction, contact Lisa at firstname.lastname@example.org or call 651-631-1607.
© 2000 Lisa Turner
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