Industrial Flooring [Jul 2004]
Most people give little thought to the flooring on which they work and walk each day, unless they slip, trip or fall or have feet, leg or lower back pain from standing.
Industrial Flooring
Most people give little thought to the flooring on which they work and walk each day, unless they slip, trip or fall or have feet, leg or lower back pain from standing.
Flooring, however, can be a critical component of workplace safety. Liquid contaminates on floors or subtle changes in elevation may contribute to slips, trips, or falls. Cleaning and maintenance are also important.
Mats, when designed and installed properly, may clean off the bottom of the footwear and help improve the slip-resistance of floors, especially when contaminants are present. Mats with anti-fatigue properties can improve comfort and prevent injuries when working on hard surfaces over a prolonged period of time. A strategy focusing on the design, selection and maintenance of flooring (including mats) can go a long way toward reducing safety problems.
This article will provide guidance on the development of such a strategy, first by describing some of the causes of slips, trips and falls, and second by identifying issues such as friction, surface roughness and the effects of contaminants that are critical to selecting and maintaining a safe walking surface. Specific guidelines for flooring selection, cleaning, entryway design, and matting are included.
Causes of slips, trips and falls
Slips, trips and falls represent either the highest or second highest type of workers' compensation claims for most industry groups in the United States1. Further, 11% of claim cases and 12% of claim costs related to low back pain are attributed to slips and falls2.
The causes for slips, trips and falls are complicated and involve several factors including (but not limited to) aging, biomechanics and tribology.
Aging
As one ages, reaction time to unexpected events begins to increase. When younger people perceive the beginning of a heel slip, they can react rapidly to arrest the potential fall. Older individuals are slower to react, making them less likely to prevent the potential fall. Another age-related issue involves decreased muscle strength. Maximum body strength is reached in the late 20s or early 30s; an adult over the age of 65 has at most 75% of the strength they possessed during their peak years of body strength. The lowered strength level of the muscles associated with slip recovery decreases the time period during which a fall recovery is possible3, 4.
Aging can also have a pronounced effect on obstacle detection and avoidance. As the eye ages, a decrease in brightness contrast detection can potentially lead to a reduction in the ability to discriminate variations in walkway surfaces. Further, presbyopia commonly begins after age 40; this condition results in an increased difficulty in focusing at close ranges. Multifocal lenses (e.g., bifocals) are usually prescribed to correct this, but these lenses could result in potential problems. Images viewed through the portion of the eyeglass lens designed for close focusing become distorted while walking, because the correction is designed for objects located at reading distance, not from the eye to the floor. Research has shown that wearers of multifocal glasses are more than twice as likely to fall as wearers of single vision lenses5.
Biomechanics
Biomechanics is the study of the mechanics of body activities (especially muscular) and is critical for understanding how we walk and interface with flooring. Much fall-related research in this area has focused on the study of human ambulation and gait as well as how gait patterns, walking speeds, and footwear are affected by increased age. Recent studies of the required or utilised coefficient of friction during the gait cycle and its relationship with the available friction between the shoe and floor surface help provide an increased understanding of slip and fall probability.
A trip occurs when the foot strikes a near ground obstacle that abruptly stops the forward movement of the foot while the body's center of mass remains in motion. When the center of mass moves out of the area of the body's support base (the planted foot), an imbalance occurs, and a fall is likely to follow. In tripping accidents, the most critical dimension of foot kinematics is often the "toe clearance" between the moving foot and the highest projection of the walking surface during the swing phase of gait. Research has shown that during the swing phase, a minimum clearance point is reached approximately 56% of the way through the movement when the forward velocity of the swinging foot is at its maximum6. At this point, the foot averages a clearance height above the ground of approximately 1.3 cm (slightly over 1/2 inch) with a standard deviation of 0.45 cm (0.17 inch). Based on the variability of this measure and assuming a normal distribution, this suggests that an obstacle height of 0.55 cm (0.22 inch) would likely be a tripping hazard for 5% of the population, while one of 0.71 cm (0.28 inch) would represent a potential hazard for 10%. Both of these heights assume that the individual does not detect the obstacle and modify gait accordingly.
Tribology
Tribology, the study of the interaction between sliding surfaces, is derived from the Greek "tribos," meaning rubbing. The field of tribology includes the analysis of friction, wear, lubrication, and the application of these principles to mechanical design, manufacturing processes, and machine operation. In analysing slips and falls, tribology helps describe causes and support preventative strategies.
Friction is the resistance to movement of one body over another (in this case the shoe sole and floor surface). It is a function of the interface between the floor and shoe sole, the wear of the shoe sole and floor surface material, and lubrication due to contaminates such as grease, water, and dirt.
The coefficient of friction (a dimensionless quantity: ?) between two contacting surfaces is defined as the ratio between friction (horizontal) force (F) to normal (vertical) force (FN) at the interface, which is mathematically expressed as:
A surface with a 0.50 in friction coefficient is generally considered as a "slip-resistant" surface in the US7. Other factors that may increase the likelihood of slips, trips and falls include poor lighting, unexpected changes in environment (including transitions from slip-resistant to non-slip-resistant surfaces) and transitions in height.
Walkway surface slip-resistance
There are a variety of conditions that may affect the coefficient of friction on floor surfaces, one of the most common being the presence of contaminants (often liquid). Lubrication of the contact area between the shoe and floor surface will, in most cases, substantially reduce its slip-resistance. The reduction in friction between dry and wet surfaces may range from as little as 2% to as much as 81%, depending on the combination of shoe and walking surface materials involved8. Almost all combinations of shoes and dry, clean walking surfaces exhibit a coefficient of friction in excess of 0.50, while only limited combinations of wet shoes and floor surfaces result in a friction coefficient greater than 0.409. No estimate of the reduction in slip-resistance caused by walking on wet surfaces can be made globally, but the following values excerpted from a report by the British Standards Institution10 as shown in Table 1 provide some perspective. It should be remembered, however, that for known slippery or wet conditions, walkers commonly alter their gait by decreasing stride length. This results in a reduced angle of incidence between the heel and walking surface, and thus a reduction in the forward directed force imparted when the heel strikes the ground. This in turn reduces the required level of slip-resistance to prevent slipping.
Potential interventions
When a slip and fall occurs, there are usually several behavior factors involved, including inattention of the person who slipped and fell or the one who failed to clean up a spill. Many slips and falls are, however, caused by poor planning, poor design, or poor management. Mistakes include installing the wrong floor for the expected environment, unforeseen transition issues, and not keeping the floor clear of obstacles. They may also include inadequate cleaning or repairing of defective floors, inadequate reporting of spills leading to lack of clean-up, limited training provided to employees, inadequate footwear, and inadequate lighting. Lower back and leg fatigue from prolonged standing on hard flooring can be a problem that mats with anti-fatigue properties can help.
Floor Design and Selection
There are many different types of flooring, including a variety of tiles, carpeting, epoxy floors, terrazzo and concrete. The selection of flooring should consider contaminants expected and transition areas. A transition from a carpeted floor or non-slippery floor to a glazed tile or more slippery walking surface could increase the likelihood of a slip and fall due to the individual's lack of detection of the transition (change in slip-resistance) and adjustment of gait accordingly. In general, flooring should have similar slip-resistance properties when transitioning between different types of flooring, especially when liquid contaminants may be present.
Surface roughness affects friction; selection of floor surfaces with adequate roughness characteristics may potentially reduce slip and fall accidents. A floor that will be used under mostly dry conditions offers more flexibility in terms of both selection and use, since most dry, clean floors are "slip-resistant" by design. If liquid contaminants are expected on the floor, potential interventions could include molded surface patterns or profiled surfaces at the macro-scale or surface roughness on nominally flat surfaces at the micro-scale. One of the selection criteria should be to choose floors with high values in particular surface roughness parameters. These surface roughness parameters have been shown to relate to increased friction under such conditions
Although a higher friction can be achieved by increasing the overall surface roughness level such an increase is not always desirable due to cleaning and other requirements. Alternatively, friction can be increased without an increase in the overall roughness level through the proper selection of floor surfaces incorporating a range of microscopic geometric features on nominally flat surfaces. In the U.K.11 and Australia12 surface roughness and friction are recommended as two discrete parameters, both of which should be evaluated when making a detailed assessment of slip resistance. Although surface roughness is important in determining slipperiness there is insufficient information to establish a safety criterion based solely upon this single parameter. It can, however, readily provide a relative comparison between alternative floor surfaces or treatments.
Although there are other surface roughness parameters that are good indicators of friction, Rpm, which represents the allowable volume of contaminant before the surface is fully covered, is the easiest to measure using a relatively inexpensive profilometer (an instrument designed to measure the degree of surface roughness in micrometers)13, 14. A surface with a larger void volume can contribute to a higher friction by allowing direct contact between the shoe and floor surfaces covered with liquid contaminants. An increase in surface irregularities at the peaks of the surfaces due to a large Rpm value also makes it more difficult to establish lubrication due to liquid contaminants at the shoe-floor interface. Even under conditions where a floor is completely covered with liquid contaminants, it is easier for the footwear surface to penetrate the contaminants and establish a direct solid-to-solid contact when the floor surface has a larger Rpm value. Background information regarding surface roughness measurements and the desired settings of a profilometer are also available in open literature13. A surface with a higher Rpm value (the lower drawing in Figure 1) is preferred compared to a surface which has a lower Rpm value (the top drawing in Figure 1).
Part of preventing trips requires keeping the floor free of potential obstacles but it must be remembered that the walking surface itself may be the tripping hazard, not just materials located upon it. In the US, most building codes and other safety regulations and recommendations define any change in elevation of over 0.64 cm (approximately 1/4 inch) as a potential "tripping hazard." Guidance for minimising the potential for trips and falls in normal working areas includes:
- Abrupt changes in elevation of approximately 0.64 cm (1/4 inch) or greater should be avoided
- Door sills and other similar obstacles including floor mats should have beveled edges
- Stair heights within a given flight should not vary by more than 1/4 inch
When elevation changes cannot be eliminated or mitigated through design, consideration should be given to providing visual cues (e.g. changes in color or brightness contrast) to draw attention to floor level obstacles. Care must be taken, however, to avoid unduly drawing attention away from the walking path itself.
Floor Cleaning & Maintenance
Dirty or defective floors are often the causes of slips, trips, and falls. Contaminants may accumulate on floor surfaces due to inadequate cleaning processes, resulting in the reduction of surface roughness as soil, grease, or other contaminants fill in the pores or valleys in the floor surface. The accumulation of contaminants alters these surface features and, consequently, reduces the uncontaminated floor's original friction characteristics. It is, therefore, very important to keep floors clean in order to maintain these desirable features of the walking surface.
A floor cleaning protocol must consider the floor type, the contaminants involved, and the selection of the type of cleaning solvent most suitable for both. For example, general recommendations to remove grease include scrubbing the floor briskly using a deck brush and detergent with 71°C (160° F) or hotter water, followed by use of a wet vacuum or squeegee before rinsing.
Major categories of cleaners include alkaline, acidic, and neutral pH. Alkaline cleaners react with fats and oils, converting them into soap (saponification), and must be thoroughly rinsed with clean, hot water to prevent polymerisation. Acidic cleaners use a process known as oxide reduction to remove rust, scale and oxides from floors. Neutral cleaners are typically used on floors with glossy finishes or those that can be dulled by the abrasive qualities of acidic or alkaline cleaners15. It is important to remember that many commercial cleaners (e.g. pine tar-based disinfectants) may themselves leave a residue on the floor. The American National Safety Council recommends that floors be rinsed with clear water to avoid leaving such residue on floors after drying.
A good housekeeping program should include written instructions regarding floor maintenance. Basic elements of an effective program should include the following:
- Identification of the specific contaminants and selection of a cleaner/chemical that effectively breaks each one down
- Establishment of a floor cleaning protocol for the removal of contaminants
- Provision of appropriate tools to clean the floor (e.g. mops, buckets, deck brushes, and squeegees). Designation of dedicated tools for specific areas is necessary in order to avoid cross-contamination (e.g. mops used in areas with grease should not be used in non-greasy areas)
- Implementation of a floor-cleaning schedule that is consistently followed, including the identification of responsible employees and the time of day cleaning should take place
- Establishment of a training program for persons responsible for inspection, maintenance and cleaning. This includes definition of cleaning requirements, cleaning procedures, safe handling and disposal of chemicals and solutions, emergency conditions and operations, and record keeping or reporting related to housekeeping and maintenance
- Routine inspection of all floor surfaces for wear, damage, debris and contaminants. Clear communication of any needed repairs to the facilities or maintenance department is critical
- Occasional testing of floor surfaces to monitor slip resistance levels and determine effectiveness of the floor cleaning protocol
Floor Mats
Mats, when correctly selected and installed, can reduce both the likelihood of slips and falls and lower extremity and lower back discomfort. Mats may be warranted when pedestrian walking or working surfaces do not meet slip-resistance requirements, such as when moisture, grease, oil, dirt or other contaminates are present. Examples of such instances include building entrances, water fountains, sinks, restaurant kitchens, machinery process areas, and anywhere else that spills, water, dirt, and grease are part of the "normal" environment. Some mats also have anti-fatigue properties and are useful for areas where prolonged standing work is required. Mats can also be used as a temporary solution until floor surface conditions can be addressed.
Entrance Mats
Entrance mats improve overall floor maintenance by scraping and absorbing outside soil particles and moisture from footwear, thus aiding in keeping the floor in a clean, dry condition and by protecting the floor finish from unnecessary wear. Mat surface selection depends on expected foot traffic level, and whether the mats are to be used in winter or wet climates. Since the primary purpose of the mat is to remove moisture and soil from the shoe, selection of an inferior or inappropriate mat will result in premature wear and inadequate moisture absorption. More durable entrance mats may cost more, but usually last longer and perform better.
Mat length is an important factor. The number of steps required to effectively scrape and wipe feet is a function of the climate; as the climate improves, the demands on floor matting become less intense. Under snowy conditions, a minimum of 10-12 walking steps is a good guide to the length needed, while rainy conditions call for about 8-10 steps, and dry conditions may merit about 6-8 steps16.
Multi-Purpose Mats
Multi-purpose mats absorb liquids, elevate workers above standing water, provide a slip resistant working or standing surface, and/or provide anti-fatigue properties. The absorption or retaining of spills is a common purpose for mats used in grocery produce areas and around water fountains and sinks. Mats selected for this purpose need to be evaluated in terms of their liquid absorption characteristics, ability to contain spills and debris, and durability to conditions such as wear produced by grocery carts and foot traffic. Mats with slip-resistant surfaces are useful for standing work areas where grease and oil are common, such as in manufacturing and food processing areas. These mats have durable slip resistant surfaces that are also easy to clean.
Anti-Fatigue Mats
Studies have shown that hard flooring may contribute to lower leg and lower back fatigue or discomfort after as little as 3 hours of standing work17. This effect is believed to be the result of venous pooling, rather than muscle fatigue. Anti-fatigue mats are common in work areas where long-term standing work is performed (e.g., retail cashiers, machine operators and packing workers). Subjective ratings of perceived fatigue have been shown to be better than more quantitative measures when comparing the physical benefits of anti-fatigue mats against possible alternatives. Studies have also shown that, in general, floor mats characterised by increased elasticity, decreased energy absorption and increased stiffness result in lower levels of both discomfort and fatigue17. No study has yet recommended a specific material for anti-fatigue matting so, during selection, some experimentation with different types should be performed. It is usually beneficial to involve the affected workers in the final selection.
When selecting mats, edges designed not to curl are highly recommended. Such mats often have a beveled or flat edge to reduce tripping exposure. All mats more than 0.64 cm (1/4 inch) thick should have tapered edges to reduce the potential for creating a tripping hazard.
Summary
Identification of the causes of slips, trips and falls can become quite complex but fall prevention begins with the design, selection and maintenance of flooring. Floors with a rough surface can increase friction between the shoe and floor, and thus reduce the likelihood of slips and resulting falls which is particularly true in liquid contaminated environments. Development and adherence to formal floor cleaning protocols and house keeping programs are also an important element of removing contaminates and improving the slip-resistance of flooring. Use of mats may offer a number of potential benefits, including preventing slips and falls, protection of floors from particulate soil and excessive wear, and anti-fatigue properties for long term standing on hard flooring.
Authors
Wayne S. Maynard CSP, CPE is a product director, ergonomics and tribology at the Liberty Mutual Research Institute for Safety, Hopkinton, MA USA. Email: wayne.maynard@libertymutual.com
Wen-Ruey Chang, Ph.D. is a research scientist at the Liberty Mutual Research Institute for Safety, Hopkinton, MA USA.Email: wen.chang@libertymutual.com
David G. Curry, Ph.D, CHFP is the director of human factors for Packer Engineering, a multidisciplinary engineering consulting firm in Naperville, IL USA. Email: dgc@packereng.com
References
1. Leamon, T.B., Murphy, P.L. (1995) Occupational slips and falls: more than a trivial problem, Ergonomics, 38 (3), pp. 487-498.
2. Murphy, P.L. and Courtney, T.K. (2000) Low back pain disability: relative costs by antecedent and industry group, American Journal of Industrial Medicine, 37, pp. 558-571.
3. Lockhart, T.E., Smith, J.L. (2000) Effects of musculoskeletal and sensory degradation due to aging on the biomechanics of slips and falls, Proceedings of the IEA/HFES Conference, Vol. 5, pp. 83-86.
4. Lockhart, T.E., Woldstad, J.C. and Smith, J.L. (2002) Assessment of slip severity among different age groups, Metrology of Pedestrian Locomotion and Slip-resistance, ASTM STP 1424, Marpet, M.I. and Sapienza, M.A. (Eds.), ASTM International, West Conshohocken, PA.
5. Lord, S.R., Julia Dayhew, and Amelia Howland (2002) Multifocal glasses impair edge-contrast sensitivity and depth perception and increase the risk of falls in older people, Journal of the American Geriatrics Society, 50, pp. 1760-1766.
6. Winter, D.A. (1992) Foot trajectory in human gait: a precise and multifactorial motor control task, Physical Therapy, 72(1), pp. 45-53.
7. Underwriters Laboratories, Inc. (1996) Standard for Slip Resistance of Floor Surface Materials, UL 410, Northbrook, IL.
8. Grandjean, E. (1973) Ergonomics of the Home. Taylor & Francis, London.
9. Miller, J.M. (1983) Slippery work surfaces: towards a performance definition and quantitative coefficient of friction criteria, Journal of Safety Research, 14, pp. 145-158.
10. British Standards Institution (1984) Stairs, Ladders and Walkways, BSI 5395. Milton Keynes, U.K.
11. HSE (Health Safety Executive) (1999) Preventing slips in the food and drink industries-a technical update on floor specifications, HSE information sheet: Food Sheet No. 22, HSE Books, Sudbury, Suffolk, United Kingdom.
12. Bowman, R. (1997) Overstepping the mark - practical difficulties in maintaining a slip resistance standard, The Proceedings of the 13th Triennial Congress of the International Ergonomics Association, Tampere, Finland, Vol. 3, pp. 365-367.
13. Chang, W. R. (2004) Preferred surface microscopic geometric features on floors as potential interventions for slip and fall accidents, Journal of Safety Research, 35 (1), 71-79.
14. Chang, W.R., Courtney, T.K., Grönqvist, R., Redfern, M.S. (Eds.). (2003) Measuring Slipperiness; Human Locomotion and Surface Factors , Taylor and Francis, London.
15. Di Pilla, S. (2003) Slip and Fall Prevention: A Practical Handbook, Lewis Publishers, Boca Raton, FL.
16. Wolf, D. (1998) Keep It Clean By Being Up Front, Professional Retail Store Maintenance, September.
17. Rakie, C., Redfern, M.S. (2001) Effect of flooring on standing comfort and fatigue, Human Factors, 43 (5), pp. 381-391.
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