Readers Note: The following is excerpt from the 2003 edition of ADA advisory on surface conditions. The text of this document represents the most current information on slip resistance measurement as understood by the Access Board. While it disclaims its own legal imperative, the reader may find that it debunks many of the false presentations found in both marketing and in court proceedings regarding choice of test methods and acceptable results.
Taken as a whole, one can say that ADA and ASTM have a similar position. That position is:
Given the present state of the art, it is not possible to designate a single slip measurement device as a representation of absolute value. Slip resistance measurements are useful for comparison as long as the method is defined and results are properly reported.
BULLETIN #4: GROUND AND FLOOR SURFACES
The landmark Americans with Disabilities Act (ADA), enacted on July 26, 1990, provides comprehensive civil rights protections to individuals with disabilities in the areas of employment (title I), State and local government services (title II), public accommodations and commercial facilities (title III), and telecommunications (title IV). Both the Department of Justice and the Department of Transportation, in adopting standards for new construction and alterations of places of public accommodation and commercial facilities covered by title III and public transportation facilities covered by title II of the ADA, have issued implementing rules that incorporate the Americans with Disabilities Act Accessibility Guidelines (ADAAG), developed by the Access Board.
U N I T E D S T A T E S A C C E S S B O A R D
A FEDERAL AGENCY COMMITTED TO ACCESSIBLE DESIGN
Why are surface characteristics specified?
Over twenty-seven million Americans report some difficulty in walking. Of these, eight million have a severe limitation; one-fifth of this population is elderly. Ambulatory persons with mobility impairments-- especially those who use walking aids--are particularly at risk of slipping and falling even on level surfaces. Preliminary research conducted for the Access Board in 1990 through the Pennsylvania Transportation Institute at The Pennsylvania State University compared the slip-resistance needs of persons with mobility impairments and those without disabilities walking on level and ramped surfaces both indoors and out. Findings from this limited human-subject testing confirmed that individuals who have gait and mobility disabilities make greater demands on the walking surfaces of floors, ramps, and walkways. The information in this Bulletin was derived from this and other research in order to provide designers with an understanding of the variables that affect the measurement and performance of materials specified for use on walking surfaces.
What surface characteristics are required of an accessible route?
The Americans with Disabilities Act Accessibility Guidelines (ADAAG) requires only that newly-constructed or altered ground and floor surfaces of accessible routes on sites and in buildings and facilities be stable, firm, and slip-resistant . No standards or methods of measurement are specified in scoping or technical provisions, although the Appendix to ADAAG contains advisory recommendations for slip resistance values derived from Board-sponsored research. Because the sample size was small, the testing method unique, and the findings not yet corroborated by other research, the suggested values have not been included in the body of ADAAG and should not be construed, as part of the regulatory requirements for entities covered by titles II and III of the ADA.
However, other regulations. such as those imposed by OSHA in the interests of worker safety, or design and testing standards applied by state, local, or industry mandate, such as certain ASTM (American Society for Testing and Materials) procedures, may require specific values or ranges of slip resistance.
A stable surface is one that remains unchanged by contaminants or applied-force, so that when the contaminant or force is removed, the surface returns to its original condition. A firm surface resists deformation by either indentations or particles moving on its surface. A slip-resistant surface provides sufficient frictional counterforce to the forces exerted in walking to permit safe ambulation.
Because of the great number of variables that affect the performance of a given walking surface--its slope and cross-slope, its material, texture and finish, the presence of moisture or contaminants, the material that contacts it and the method of ambulation--no single set of technical specifications or measurement standards can encompass all criteria that contribute to the safety of a walking surface.
Only slip resistance has a commonly applied unit of measurement--the coefficient of friction, which may be measured as static (at rest) or dynamic (in motion). Its calculation is complex and the methods and equipment of its measurement vary. Affected industries--floor finishes, ceramic tile, plumbing fixtures--each employ a different testing methodology in designating the slip resistance of their products. The static coefficients of friction measured according to the four major ASTM-standard testing procedures have never been correlated by research, although a considerable body of data exists.
What is slip resistance?
In its simplest sense, a slip resistant surface is one that will permit an individual to walk across it without slipping. Contrary to popular belief, however, some slippage is in fact necessary for walking, especially for persons with restricted gaits who may drag their feet slightly. While increasing the slip-resistance of a surface is desirable within certain limits, a very high coefficient of friction may actually hinder safe and comfortable ambulation by persons with disabilities. In fact, a truly non-slip surface could not be negotiated.
While visual inspection can provide some Information about a surface such as its degree of cleanliness, whether It is wet or dry, and even the type or texture it exhibits, it cannot provide sufficiently accurate information about a surface to be used in design.
Even clean, dry surfaces with readily-apparent texture will not always be slip resistant. Materials which might be suitable for level surfaces may be inappropriate for sloping surfaces; materials specified for dry conditions may be unsafe when it rains; a leather shoe may perform poorly on smooth dry surfaces yet provide adequate traction when wet. The presence of moisture or other contaminants, the characteristics of the shoe sole or crutch tip making contact, the direction (uphill and downhill effects differ) and slope of travel all will affect the slip resistance of installed surfaces. It is this interaction of material characteristics and human responses which fully characterizes slip resistance.
How is slip resistance measured?
Measuring slip resistance involves the minimum tangential force necessary to initiate sliding of a body over the surface and the body gravity force. The coefficient of friction between the two surfaces is the ratio of the horizontal and vertical forces required to move one surface over another to the total force pressing the two surfaces together.
There are three critical stages in an individual's gait: 1) touchdown, 2) full load, and 3) push-off. In order to avoid slippage while walking, the horizontal and vertical forces applied by the individual must be resisted by forces acting against the foot as it contacts the walking surface. The definitive component of this resisting force, and the variable most subject to manipulation, is the coefficient of friction of the surface material. Consider, for example, an icy surface with a negligible coefficient of friction. A runner whose forward motion applies a substantial horizontal force will slip-and probably fall-on such a surface. A more careful pedestrian may be able to limit his horizontal force contribution so that it balances the available frictional resistance of the ice and thus cross it safely. Adding sand to the icy surface will increase its coefficient of friction and allow for a more standard gait. Once the ice has melted, the higher coefficient of friction of the newly-exposed surface will offer sufficient resisting force to permit the runner to speed across it without incident.
The dynamic coefficient of friction varies in a complex and non-uniform way. Although R can be calculated and modeled in the laboratory using sophisticated computer programs, the more straightforward measurement of the static coefficient of friction provides a reasonable approximation of the slip resistance of most surfaces and is the method most appropriate for evaluating surface materials and finishes.
A variety of devices are available for such measurements. The most common device, the James machine, was developed in the early 1940s and was the testing device specified by the Underwriters Laboratory (UL) shortly thereafter when it established--from laboratory test data corroborated by field experience--a minimum value of 0.5 for the static coefficient of friction for floor polish bearing the UL seal. Since then, 0.5 has become the commonly-accepted threshold for classifying slip resistance in products. Furthermore, the James machine is the recognized test method and the 0.5 value (when measured by this tester) is the recognized minimum criterion for slip- resistant walking surfaces in courts of law in the United States.
Measurement by the James machine, utilizing a leather sensor, is the only method appropriate for assessing surfaces and products against the 0.5 UL standard for static coefficient of friction. Using a different sensor material, even If measured by the James machine, will give a different reading for the same surface material.
This is a significant point. An informal comparison of data collected under three different research protocols, involving four different friction-testers and four different shoe sensor materials, all applied to the same 8-inch by 8-inch ceramic tile surface, resulted in thirty readings ranging from a low of .29 to a high of .99-for its static coefficient of friction. Even limiting values to those measured by the James machine but using both leather and Neolite sensor material resulted in a range of 0.57 (leather) to 0.79 (Neolite) for the same surface being tested.
It is impossible to correctly specify a slip-resistance rating without identifying the testing method, tester, and sensor material to be used in evaluating the specified product and equally invalid to compare values obtained through one methodology to those resulting from different testing protocols. Because a consensus test protocol has not yet been identified, the Access Board did not specify a value or testing method for determining the coefficient of friction along an accessible route.
The James machine continues to be a laboratory mainstay, but is not portable and thus cannot be used in field testing. In order to measure the slip-resistance of surfaces already in place, researchers at The Pennsylvania State University evaluated three portable testers: the NBS-Brungraber Tester (also known as the Mark I Slip Tester), the PTI (Pennsylvania Transportation Institute) Drag Sled Tester, and the Horizontal Pull Slipmeter.
Study criteria included relevance (the measuring results should correlate in a known and constant manner with human perception of the surface slipperiness); versatility (accurate measurements of slip resistance must be possible on various types of surfaces and under diverse conditions); sensitivity to measuring technique (the difference between measurements performed on the same surface and under the same conditions by different persons should be minimal), and repeatability (tests of the same surfaces under the same conditions should be consistent over time). In addition, the reliability and precision of the testers were assessed.
Based on the results of this study, the NBS-Brungraber Tester was recommended as the best portable device currently available for measuring slip resistance under dry conditions on all but carpeted surfaces. Easy to use, the NBS-Brungraber testing procedure can be mastered In 30 minutes. It measures the static coefficient of friction between a representative sample of shoe sole material and a flooring surface. The result from the recording shaft is converted into an equivalent value of static coefficient of friction by means of a calibration chart supplied with the tester.
The PTI Drag Sled Tester performed well in the tests but was not commercially available at the time of completion of the report. The Horizontal Pull Slipmeter, which proved to be an excellent device for laboratory measurements of slip resistance, did not produce satisfactory results in field measurements. Other portable testers that may be used to measure static coefficient of friction include the Mark II Slip Tester (available from the manufacturer of the NBS-Brungraber Tester) and the Model 80 Tester.
The slip resistance of indoor and outdoor walking surfaces already in place can be measured with one of the portable testers listed in this Bulletin in order to monitor the process of wear and polishing of walking surfaces. An initial reading of the coefficient of friction taken after flooring has been placed and finished will provide a baseline for future comparisons. However, do not attempt to compare such readings to the UL 0.5 coefficient of friction standard or to a manufacturer's slip resistance values unless the same testing methodology, machine, and sensor material was used in each instance.
What values are recommended for ground and floor surfaces along an accessible route?
The surfaces of the accessible route on a site or within a building or facility must be designed to provide slip-resistant locomotion for both level and inclined travel by persons with disabilities. Research findings suggest that such surfaces should have a slip resistance somewhat higher than might be provided for individuals without disabilities.
In the study sponsored by the Access Board, laboratory measurements from a Kistler force plate and computer analysis of the gaits of persons with mobility impairments (including crutch users and above- or below-knee amputees using artificial limbs) and persons without disabilities graphed the dynamic coefficients of friction necessary for safe ambulation. The m-shaped curves that resulted gave a range of values from touch-down to take-off (control group: 0.2- 0.3; persons with disabilities 0.7-1.0). Wheelchair users were tested through a full cycle of push and recovery (0.5-0.7).
Correlating these values with a single static coefficient of friction (the relationship is complex and non-linear) is inexact and involves some approximation in order to facilitate simplified field testing procedures. In the Access Board research, the static coefficients of friction for a variety of common indoor and outdoor surfacing materials were measured in place using the NBS-Brungraber Tester with a silastic sensor material. Although this machine operates on a principle similar to that of the James machine, the use of a non-standard silastic sensor (instead of the leather required by the protocol for the UL standard) results in significantly higher values for the coefficient of friction of the surfaces being measured. As no correlation was made to any other standards or methodologies in the research, the values for coefficient of friction cannot be compared.
Researchers' recommendations for a static coefficient of friction for surfaces along an accessible route, when measured by the NBS- Brungraber machine using a silastic sensor shoe, were approximately 0.6 for a level surface and 0.8 for ramps. These values are included in the advisory material in the Appendix to ADAAG, but are not in any way mandatory.
What materials may satisfy ADAAG requirements?
In new construction and alterations, surface materials must be specified to be slip-resistant. If there is a choice between flooring materials otherwise suitable for a particular application, we recommend choosing the material with the higher coefficient of friction, particularly for ramps.
Materials that might be appropriate for ramps and level surfaces include concrete wood float surfaces, asphalt, and some types of carpets and resilient tiles. Materials which might be expected to be satisfactory for level surfaces, but which might not be appropriate for ramps, include concrete metal trowelled surfaces, ceramic tile, hardwood and flagstone. These finishes, tested during the Access Board research project, yielded coefficients of friction that fell within the recommended ranges for accessible routes.
However, not all products of the type mentioned may provide the desired slip resistance and many other materials can be expected to be suitable even though they are not included here. For example, some types of materials for which the coefficient of friction is low, are available--or can be treated--with finishes that increase slip resistance.
Products or finishes applied to surfaces after installation are not covered by ADAAG. but may fall under the Department of Justice (DOJ) regulation governing the maintenance of accessible features. Moisture and debris contamination adversely affect the surface slip resistance of most installed finishes. While floor treatments are available that will increase the coefficient of friction of a walking surface, some products or furnishings, such as furniture wax overspray or loose throw rugs, may reduce slip resistance significantly. Others-- for example, walkoff mats placed on lobby floors during rainy weather--do much to reduce the chance of slipping on a wet floor. Such mats are not considered carpets within the meaning of ADAAG 4.5.3.
What other surface considerations affect wheelchair travel?
In addition to slip resistance requirements, wheelchair users are affected by the rolling resistance of the surface of the floor and--on exterior surfaces--by cross slope. If the rolling resistance of flooring is high, wheelchair users must avoid those areas or expend extra energy maneuvering across the surface. In a limited study of wheelchair rolling resistance, the force needed to traverse four different surfaces was measured: concrete, linoleum, low-pile carpet (loop, 0.1-inch pile height, 10 stitches/inch, 16-ounce face weight excluding backing and glue, on jute), and high-pile carpet (cut, 0.5-inch pile height, 10 stitches/inch, 40-ounce face weight excluding backing and glue, on ActionBac).
Although the study was not intended to be comprehensive, the results provide some guidance in selecting carpet. With the force needed to traverse bare concrete as a baseline, the increase in force needed to cross each surface was measured to be: +3% for linoleum; +20% for low-pile carpet, and +62% for high-pile carpet. From these results it appears that linoleum and concrete equally require minor effort; low-pile carpet requires a noticeable. though moderate, increase in effort; and high-pile carpeting requires a significant increase in effort. Although the slip resistance ratings of carpet fall within the recommended ranges for use on ramps, its rolling resistance makes most types an inappropriate finish for sloped surfaces.
Exterior ramps and walks will generally be constructed with a cross-slope (perpendicular to the direction-of-travel slope) in order to provide positive drainage. Because the effects of cross-slope are particularly difficult for persons using wheelchairs--particularly along a steep running slope--ADAAG provisions limit accessible routes to a 2% cross-slope.
What other considerations are significant for persons with disabilities?
Materials such as gravel, wood chips, or sand, often used for outdoor walkways, are neither firm nor stable, nor can they generally be considered slip-resistant. Thus, walks surfaced in these materials could not constitute an accessible route. However, some natural surfaces, such as compacted earth, soil treated with consolidants, or materials stabilized and retained by permanent or temporary geotextiles, gridforms, or similar construction may perform satisfactorily for persons using wheelchairs and walking aids.
ADAAG also contains provisions that limit surface discontinuities along an accessible route, including elevator cab leveling tolerances at landings, gaps between car and platform in transit facilities, the size and orientation of openings in walkway gratings, the profile of doorway thresholds, and the pile height and attachment of carpeting. ADAAG 4.5.3 specifies that carpet and carpet tile be securely attached. This provision does not require that each tile--or the entire carpet or pad--be adhered to the floor surface provided the method of securement results in a surface that is stable, firm, and slip-resistant and does not pose a tripping hazard.
This technical assistance is intended solely as informal guidance; it is not a determination of the legal rights or responsibilities of entities subject to the ADA.