Slip resistance versus surface roughness of deck and other underfoot surfaces in ships

Journal of Occupational Accidents, 13 (1990) 291-302

291

Elsevier

Slip resistance versus surface roughness of deck and other underfoot surfaces in ships Raoul Grongvist, Jouko Roine, Eero Korhonen and Ahti Rahikainen Institute of Occupational Health, Department of Occupational Safety, t .aajaniityntie 1, SF-01620 Vantaa, Finland

(Received 20 March 1990 ; accepted 2 May 1990)

ABSTRACT Grongvist, R ., Brine, J ., Korhonen, E. and Rahikainen, A ., 1990- Slip resistance versus surface roughness of deck and other underfoot surfaces in ships . Journal of Occupational Accidents, 13 : 291-302 . The slip resistance of thirteen deck and other underfoot surfaces commonly used in ships was assessed by measuring the kinetic coefficient of friction between these floorings, contaminated with glycerol, and three types of safety footwear. The floorings were used on decks, stairs and passages as well as in engine-rooms, kitchens and other indoor and outdoor facilities . The kinetic coefficient of friction (Pk,) was measured with a prototype apparatus, which simulates the movements of a human foot and the forces applied to the underfoot surface during a sudden slip on the heel. The measured average pks of the assessed floorings varied from 0 .05 to 0 .64 . Four floorings were classified as very slip-resistant and one as slip-resistant . All the others were classified as slippery. The rough floorings were more slip-resistant than the smooth ones . The correlation between the arithmetical average roughness (R,) and the average )u,, of the assessed surfaces was very significant (p < 0.001) . When selecting deck and other underfoot surfaces and developing new flooring products, more attention should be paid to their surface texture . From the slip resistance point of view the adequate R, value was 7-9 um, so smoother surfaces should be avoided . A raised-pattern on floorings also improved the slip resistance compared to corresponding unpatterned floorings .

1 . INTRODUCTION

Walking on decks and stairs is connected with 25% of all accidents in shipwork, on average 2200 reported cases per year, in Finland (National Board of Labour Protection, 1982-1987) . The slipperiness of underfoot surfaces often causes these accidents . According to accident reports, deck and other underfoot surfaces are in general icy, wet, oily or otherwise slippery . The motion of a ship and the swelling of the sea also increase the risk of slipping in shipwork compared to other work, e .g., the horizontal acceleration of a ship can be as 0376-6349/90/$03 .50 © 1990-Elsevier Science Publishers B .V .

2 92

much as 5-20 m/s' (Wragge and Jung, 1986) . In addition, the inclination of stairs can be steep (35-70 deg) increasing the risk of slipping in shipwork (Anderson, 1983) . A half of all slipping and falling (on . the same level) accidents in shipwork happens to the ships' officers or the deck and engine-room crew (National Board of Labour Protection, 1982-1987) . The other half happens to the housekeeping staff (National Board of Labour Protection, 1986) . In addition to "slipping and falling on the same level" accidents, slips occur in many other accident groups as well, e.g., in falls to a lower level, striking against objects and overexertions . Slipping and falling accidents occur in the following phases of work (National Board of Labour Protection, 1986) - moving on decks, stairs and ladders and in passages (41% of all slipping and falling accidents), - kitchen work, serving and dining (20%), - cleaning of structures and handling of surfaces (17%), - mooring to a quay or towing, handling of a cargo (16%), - other work phases (6%) . The risk of slipping during walking is especially dependent on the frictional forces between the shoe and the underfoot surface. Human behaviour, environmental conditions and work tasks also affect the risk . The frictional forces are important, because usually a slipping accident cannot be predicted . A slip occurs unexpectedly and in most cases shortly after the heel of the shoe and the underfoot surface come into contact (Strandberg, 1983) . Many studies have been made about the slipperiness of floor materials, but with conflicting results (Brungraber, 1976) . Different evaluation methods seem to lead to quite opposite conclusions . Strandberg (1985) has on the basis of his slipping experiments pointed out two main problems when. measuring the slipperiness of shoes and floors : (1) the lack of valid slip resistance testers, (2) the difficulties in determining adequate criteria for safe friction during walking. The aim of this study was to assess the slipperiness of typical deck and other commonly used underfoot surfaces in ships, when the slip resistance was reduced by using a slippery contaminant . The assessments, based on kinetic friction measurements, were then compared to surface roughness measurements . The ability of the applied measurement method to distinguish floorings in relation to their slip resistance was discussed too . 2 . MATERIALS AND METHODS

2.1 Slip resistance measurements

The slipperiness of floorings were assessed with an apparatus, the stationary step simulator, and a method developed at the Institute of Occupational Health



293

(Grongvist et al ., 1989) . The apparatus simulates the movements of a human foot and the forces applied to the underfoot surface during an actual slip (Fig . 1.) The slip resistance classification is based on actual risk of slipping and requirements for the minimum friction during normal walking (Strandberg and Lanshammar, 1981 ; Grongvist et al., 1989) . The floorings were classified into five slip resistance classes according to Table 1 . A slip is unlikely to occur when the coefficient of kinetic friction, measured with the heel-slide gait pattern (µ k,) according to our method, is at least 0 .20 (Grongvist et al ., 1989) . During walking the most critical contact angle between the shoe sole and the underfoot surface is approximately six degrees, when a forward slip is likely to occur (Strandberg and Lanshammar, 1981) . In the present study the coefficient of kinetic friction was measured when the heel of the shoe, at an angle of five degrees, was sliding on the flooring . During the measurements the sliding velocity was 0 .4 m/s and the normal force 700 N . The time interval of each measurement started 0 .10 s and ended 0 .15 s after the heel strike, so the duration of the normal force application time was as close to normal walking as possible. Glycerol with a viscosity of 200 cP (89% by weight) at room temperature Computer Control Data logger Direction control valves/ Hydraulic hoses

Measurement foot/ Hydraulic cylinders Inclinatio forward/ backwar Lift/ Lower

Hydraulic pump

Push/ Position Pull transducer Force plate Inductive pr inity : sensors Amplifier

Amplifier

Wav form analyzer

Plotter

i

_J Output - frictional force - normal force - coefficient of friction - displacement, velocity

Fig . 1 . Schematic diagram of the measurement apparatus .

294 TABLE 1 Slip resistance classification of shoe-contaminant-flooring combinations according to the uk, value (Grimqvist et A, 1989) Kinetic friction coefficient Heel-slide gait pattern, Rk,

0 .30 020-0 .29 0 .15-0 .19 0 .05-0 .14 <0,05

Slip resistance class No .

Explanation

1 2 3 4 5

very slip-resistant slip-resistant unsure slippery very slippery

Fig. 2. Reference footwear types . Shoe type and size, sole type and manufacturer : (a) 5411 . 45, compact nitrile rubber, Urho Viljanmaa Oy ; (b) 903, 45, compact styrene rubber, Kalvian Kenka KY ; and (c) 43-52068-133,45, microcellular PU, Sievin Jalkine Oy .

29 5

No . 1 . Painted deck plate, Jotundeck; decks ; smooth 2.5 pro; Wartsila Oy/Tikkurila Oy .

No . 2 . Rough painted deck plate, Jotundeck; decks ; very rough 17 pm ; Wiirtsilii Oy/Tikkurila Oy .

No .3 . Painted deck plate with raised-pattern, Jotundeck ; engine-rooms ; smooth 2.5 pm; Wartsila Oy!_Tikkurila Oy.

No. 4. Plastic carpet with anti-slip surface, 582 ; indoors; rough 7 .5 pm ; Upofloor Oy.

No. 5 . Finnflex plastic tile, 1610 ; stairs, lobbies, indoors; very smooth 0 .5 pro ; Upofloor Oy .

No . 6 . Joustosport vinyl carpet, 565; sporting rooms; smooth and flexible 4 .0 pre ; Upofloor Oy .

Fig . 3. Assessed flooring types . Beneath each photograph find following information: Flooring type, typical applications, surface quality and R e roughness, manufacturer/supplier .

2 96

No . 7. Finnplano vinyl carpet, 32150; stairs, dressing rooms; very smooth 0 .5 pm ; Upofloor O y.

No. 8 . Scotch Clad anti-slip mass; outdoors, indoors ; very rough 22 pm ; Suomen 3M Oy.

No. 9. Safety Walk anti-slip carpet, general quality ; indoors, very rough 16 pm ; Suomen 3M Oy.

No. 10. Rubber anti-slip carpet ; indoors; very rough 18 pm ; Suomen 3M Oy .

No . 11 . Glazed tile with raised-patterned antislip surface, Top 200 ; washing rooms, kitchens ; rough 5 .5 ; }on Ostara-Laufen/IJ .J . Ceramics Ltd. Oy .

No . 12 . Glazed tile with anti-slip surface, Riff 2 ; washing rooms, kitchens ; rough 5 .5 pm ; Ostara-Laufen/U .J, Ceramics Ltd . Oy .

Fig. 3 . (Continued)

29 7

No . 13 . Glazed tile, 110 ; washing rooms, kitchens ; very smooth 1 .5 µm; Pukkila Oy . Fig . 3 . (Continued)

was used as a contaminant in all measurements . This contaminant simulates a critical environmental condition in shipwork from the viewpoint of slipping . Other contaminants, such as sea-water or water and detergent, would most likely be less dangerous, since they have a lower viscosity than glycerol . However, icy surfaces could be more slippery but it was not possible to use ice as a contaminant in our experiments . 2 .2 Reference footwear

The reference footwear chosen for the assessments were new Finnish safety shoes, typically used in shipwork (Fig . 2) . The soles were slightly worn off before the slip resistance tests . This "running-in" procedure was necessary for the repeatability of the measurements, since new shoes were used in the tests . 2.3 Assessed floorings

Thirteen different floorings used on decks, stairs and passages as well as in engine-rooms, kitchens and other indoor and outdoor facilities, were assessed (Fig . 3) . A piece of every flooring was attached to a piece of plywood (size 400 mm X 600 mm), which in turn was attached to the force platform of our measurement apparatus . The flooring surfaces were untreated during the tests, i .e ., no waxing or polishing was done . With each flooring-footwear combination ten repeated tests were done . As a final result for each flooring type the mean kinetic friction coefficient of all three shoe types was reported . 2 .4 Surface roughness measurements

The surface roughness of each flooring type was measured with a TaylorHobson Surtronic 10 R, stylus instrument . The result obtained is the mean of 20 consecutive measurements (see Fig . 3) . Surface roughness readings were



298

compared to measured average fik,s by assessing Pearson's product-moment and Spearman's rank correlation coefficients r x,, and r„ respectively . The probability of significance (p) for r,,, was calculated with Student's t-test and for r, it. was obtained from tables . The roughness readings obtained with floorings nos . 3 and 11 were omitted when assessing the correlations, since they were raised-patterned . However, the corresponding unpatterned floorings nos . 1 and 12 were included . 3 . RESULTS

The obtained mean kinetic friction coefficients and the slip resistance classifications of the assessed floorings are presented in Fig . 4 . The standard deviations of these measurement results with three types of footwear varied from 0.01 to 0 .06 . The following floorings were classified as very slip-resistant (class 1) : 3M's Safety Walk anti-slip carpet (no . 9) ; 3M's rubber anti-slip carpet (no . 10) ; 3M's Scotch Clad anti-slip mass (no. 8) and a rough painted deck plate, Jotundeck (no . 2) . Flooring no. 11, a glazed tile with raised-patterned anti-slip surface, was classified as slip-resistant (class 2) . All the others were classified as slippery (class 4) : A smooth painted deck plate (no . 1) ; Finnflex plastic tile (no . 5) ; a glazed tile with smooth surface (no . 13) ; Finnplano vinyl carpet (no . 7) ; a painted deck plate with raisedpattern (no . 3) ; Joustosport vinyl carpet (no . 6) ; a plastic carpet with antislip surface (no . 4) and a glazed tile with anti-slip surface (no . 12) .

Kinetic coefflclent of triCOon . t 0 .70

very slip-resisfanf

slip-resistant unsure ,I "PO 'Y ~BrY slippery 11 12

4

6

3

7

13

5 1

Fig. 4. Mean kinetic friction coefficients (pk ,) and slip resistance classifications of floorings contaminated with glycerol.



299 Kinetic coefficient of friction,

I

0 .7

0 .6 0 .5

0 .4 0 .3

0 .2 0 .1 00

I I I 5 10 15

Arithmetical average roughness, R„ (um)

Rank order of kinetic coefficient of friction 12

(b) 10

a 6

4

2 9 •

00

2

4

I 6

12

Rank order of surface roughness

Fig . 5 . Correlation between arithmetical average roughness (R,) and mean kinetic coefficient of friction Off,) : a) Pearson's product-moment correlation coefficient r,,,=0 .87 and b) Spearman's rank correlation coefficient r,=0 .86. The numbers in the figures indicate measured Flooring types .

The floor surface roughness was the most important anti-slip factor according to this study . Rough floorings were less slippery than smooth ones . Pearson's product-moment (rx,,) and Spearman's rank (ra) correlation coefficients between the measured arithmetical average roughness (R.) and the average tLkt value were 0 .87 and 0 .86, respectively (Fig . 5) . Both correlations were very significant (p < 0 .001) . The adequate Ra value from the slip resistance point of view was 7-9 µm, since a flooring is expected to be slip-resistant when the Ukr is at least 0 .20 (see Table 1) . Smoother surfaces should be avoided (Fig . 5a), because the /tk1 would be less than 0 .15, i.e ., slippery. The raised-pattern in floorings no . 3 and no . 11 also increased slip resistance .

800

Corresponding unpatterned floorings no . 1 (compared to no. 3) and no . 12 (compared to no . 11) showed clearly lower kinetic friction values (see Fig . 4) .

d . DISCUSSION

The risk of slipl seems to be high on some very typical floorings in ships, e.g., nos . I and 3, which were classified as slippery . Surfaces which were classified as very slip-resistant, i .e ., floorings nos. 9, 10, 8 and 2, can be considered as anti-slip materials in all conceivable environmental conditions . There is no danger for a slip to occur on these surfaces, since most contaminants (e .g., seawater or water and detergent) are most likely less slippery than the viscous glycerol used in this study . The slipperiness of viscous glycerol corresponds closely with mineral oil . lee could be more slippery, so in the future it is necessary to evaluate icy surfaces too. Previously, we have compared the slipperiness of glycerol (89% by weight) with the slipperiness of water and detergent on flooring no . 4 (Grongvist et al ., 1988) . As reference footwear we used the same three types of safety shoes as in the present study . We found that glycerol in this case was 40% more slippery contaminant than water and detergent (0,5% active content) . The selection of reference footwear of course greatly affect the results and hence, the conclusions . Another important thing is the wear of soles . Previously we found that new soles are in general more slippery than slightly used soles (Grongvist et al ., 1988), e.g . on flooring no . 4 the mean y), with the same three shoe types as here, was 0 .16 (i .e ., 35% higher than in this study) when the shoes had been in wear for two months by workers in a shipbuilding company. Manning et al. (1983, 1985) found similar results in field trials of boots for oily surfaces. Especially the slip resistance of microcellular PU-soles improved during their trials, which is in accordance with the results during previous tests with our apparatus . The very significant correlation between the slip resistance and the surface roughness of assessed floorings indicates that it may be possible to roughly evaluate the slipperiness of various unpatterned underfoot surfaces with a portable surface roughness meter . For the above-mentioned "very slip-resistant" surfaces the obtained R e values varied from 16pm to 22 ftm . However, one should note that a mere surface roughness reading might not adequately describe the quality and the slip resistance of a surface, since quite different. macro- and microscopic surface profiles can give the same roughness reading . Harris and Shaw (1988) found that subjective opinon ranking of floor slipperiness in wet conditions and surface roughness rank correlated significantly (r, = 0 .83 ) . They also found that knowledge of roughness and kinetic friction could give a useful indication of slip resistance of wet floors and that a peakto-peak roughness (R„ a ) of 8-10 pra (note! corresponding R,, reading would be

30 1

about 2 µm) was required to avoid slipping problems in the wet, but when liquids of higher viscosity are encountered, the minimum roughness level should be increased. Our present study confirms Harris and Shaw's (1988) findings concerning the importance of roughness and kinetic friction for the safety of walking on contaminated surfaces . Wragge and Jung (1986) have evaluated the slip resistance of 17 different dry and contaminated (with oil or seawater) floorings in ships with five safety shoe types . In their study the friction values varied between 0 .10 and 0 .29 with the most slippery contaminant, but they did not compare these results with surface roughness readings . The ability of measured friction values to predict actual risk of slipping seems to be highly dependent on the choice of measurement parameters in the tests, i.e ., contact area between sole and floor, sliding velocity, normal force time derivative and normal force application time before each measurement, etc . (Strandberg, 1985 ; Strandberg and Lanshammar, 1985) . Parameter values in slip resistance tests should be based on knowledge of human biomechanics in walking and slipping, and facts about the friction concept of elastomers and other materials in the presence of contaminants . Otherwise, measurement results can be seriously misleading . The validity of our measurement apparatus and method is discussed in detail in our previous study (Grongvist et al ., 1989) . Those results indicate that the apparatus and method are capable of determining the actual average level of friction during walking in slippery conditions . According to this study, the measurement method can also distinguish different floorings in relation to their slip resistance when the floor is contaminated . ACKNOWLEDGEMENTS

The authors wish to thank the Finnish National Board of Labour Protection for the financial support and the flooring and safety shoe manufacturers and suppliers for fruitful cooperation .

REFERENCES Anderson, D .M ., 1983 . From accident report to design problems - a study of accidents on board ship. Ergonomics, 26:43-50 . Brungraher, R .J ., 1976 . An overview of floor slip-resistance research with annotated bibliography . National Bureau of Standards. NBS Technical Note 895. U .S . Department of Commerce- National Technical Information Service PB-248 985 . January 1976, Washington, 145 pp . GrSngvist, R-, Rome, J . and Korhonen, E., 1988 . Suojajalkineiden pitavyys uusina ja kaytettyina (Slip resistance of new and used safety shoes) Tyo ja ihminen, 2 :149-158 (summary in English) . Grongvist . R., Rains, J ., Jarvinen, E. and Korhonen, E ., 1989 . An apparatus and a method for determining the slip resistance of shoes and floors by simulation of human foot motions . Ergonomics, 32 : 979-995.

30 2 Harris, G .W . and Shaw, S .R ., 1988 . Slip resistance of floors : Users' opinions, Tortus instrument readings and roughness measurement . J . Occup . Accid ., 9 : 287-298. Manning, D .P ., Jones, C . and Bruce, M„ 1983 . Improved slip-resistance on oil from surface roughness of footwear . Rubber Chem . Technol ., 56: 703-717 . Manning, D .P ., Jones, C . and Bruce, M ., 1985 . Boots for oily surfaces . Ergonomics, 28 :1010-1019 . National Board of Labour Protection, 1982-1987. Industrial accidents 1981-1986 . Official Statistics of Finland . XXVI : 33-38 (in Finnish) . National Board of Labour Protection, 1986 . Seafarers' occupational accidents in 1984 . Statistical note 1/86 . National Board of Labour Protection, Tampere, 51 pp . (in Finnish) . Strandberg, L ., 1983 . On accident analysis and slip-resistance measurement. Ergonomics, 26 : 1132 . Strandberg, L ., 1985 . The effect of conditions underfoot on falling and overexertion accidents . Ergonomics . 28 :131-147 . Strandberg, L . and Lanshammar, H ., 1981 . The dynamics of slipping accidents . J . Occup . Accid., 3 :153-162 . Strandberg, L . and Lanshammar, H ., 1985 . Walking slipperiness compared to data from friction meters. In: D .A . Winter, R .W . Norman, R.P . Wells. K .C . Hayes and A .E . Patla (Eds . ), Biomechanics IX-B . Human Kinetics Publishers . Champaign, IL, pp . 76-81 . Wragge, F . and Jung, K ., 1986 . Zur Frage der Gleitsicherheit and der Eignung von Schutzschuhwerk unter den Bedingungen der Seeschiffahrt . Die Berufsgenossenschaft, 566-571 .