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53
F00twear Fricticn Assessed by
Walking Experiments
Lennart Strandberg
National Board of Occupational Safety and Health
Lars Hildeskcg and Anna-Lisa Ottoson
Swedish Road and Traf c Research Institute
(db
Vag-00/)
Sta tens va'g- och trafikinstitut (VTI) - 58 1 o 1 Linkc'ping
300A
1985
F00twear Friction Assessed by
Walking Experiments
Lennart Strandberg
National Board of Occupational Safety and Health
Lars Hildeskog and Anna-Lisa Ottoson
Swedish Road and Traf c Research Institute
Vag-06/7
Statens va'g- och trafikinstitut (VT/l - 58 1 0 1 Linkb'ping
Footwear Friction Assessed by Walking Experiments by Lennart Strandberg
National Board of Occupational Safety and Health (AV) 5 171 84 SOLNA Sweden
Lars Hildeskog and Anna Lisa Ottoson
Swedish Road and Traffic Research Institute (VTI)
5-581 01 LINKOPING Sweden
PREFACE
This study was carried through with equipment and methods, originally developed at the AV research department in cooperation between the Associate Professors Anders Kjellberg, Hakan Lanshammar and Lennart Strandberg. Since August 1983, Lennart Strandberg has been stationed at the VTI, leading this and other accident research projects within the occupational safety area.
Lars Hildeskog, now completing his studies for a Physician's degree, was responsible for the organization and operation of the walking experi-ments. Anna Lisa Ottosson, Research Engineer, contributed to solutions of various problems within physics and engineering.
Adoption of Slipping and Falling research and development into the VTI program became possible, thanks to the support from Professor Kare Rumar during the planning and former stages of the project, which will continue. Funds for another test series have been granted by the main sponsor. The move, improvements, and reinstallation of laboratory equipment (from the AV in Solna/Stockholm to the VTI in Linkoeping) have beengoverned by the VTI Chief Designers, Matts Mattsson and Rolf Svensson. The same tasks for the computer software were completed by Electronics engineers Sten Lundstrom at the AV and Uno Ytterbom at
the VTI.
Various scientific problems have been solved with support from VTI personnel, e.g. in mathematical statistics from Stig Danielsson, Chief
TABLE OF CONTENTS
Page
ABSTRACT I REFERAT 11 SUMMARY III 1 BACKGROUND 1 2 METHOD 1 2 3 RESULTS 8 4 CONCLUSIONS 9 ACKNOWLEDGEMENTS 11 REFERENCES 12Footwear Friction Assessed by Walking Experiments by Lennart Strandberg
National Board of Occupational Safety and Health (AV)
5 171 84 SOLNA Sweden
Lars Hildeskog and Anna-Lisa Ottoson
Swedish Road and Traffic Research Institute (VTI) 5-581 01 LINKOPING Sweden
ABSTRACT
The slip-resistance of 18 types of footwear (ToF) on three contaminated floorings was assessed by experiments with 12 well-trained subjects, walking in a triangular closed path as fast as possible.
From the lap _t_ime the average value of the friction utilization (TFU) was
computed. Ground reaction force measurements from individual steps with sliding motions confirmed that the TFU was close to the actual
coefficient of friction.
On the most slippery surface the average time over five laps ranged between 7 s and 20 s for the 18 ToF. The lap time varied least between the three surfaces for the most slip-resistant ToF, from 7 s to 8 5, while
II
Skofriktion bestémd genom gé mgfo'rsok av Lennart Strandberg
Arbetarskyddsstyrelsen (AV)
171 84 SOLNA
Lars Hildeskog och Anna-Lisa Ottosson
Statens véig- och trafikinstitut (VTI)
581 01 LINKGPING REFERAT
Halkmotsténdet for 18 skotyper (med olika ovandel, sulmaterial eller
-monster) bestémdes pé tre golvbeléiggningar téckta med halkmedel. Tolv véiltréinade forso kspersoner gick snabbast mojligt runt respektive bana. Skador forhindrades med fallskyddsselar. Fri-in varvliderna beréiknades medelvéirdet av iriktionsgtnyttjandet (TFU). Kraftmé itningar fré m enstaka steg med glidrorelser bekréftade att TFU och det aktuella friktionstalet overensstéimde véil med varandra. P51 det halaste under-laget varierade medelvarvtiden mellan 7 och 20 s for de 18 skotyperna 1
ca 10 signifikant étskilda TFU nivéer frén 0,25 till 0,04 respektive.
Skotypen med béist grepp var ocksé minst kénslig for variationer i
underlagets textur (TFU : 0,25 0,26), medan andra skotyper gav riska
belt stora skillnader (TFU : 0,05 0,23) mellan sléitt och skrovligt golv.
III
Footwear Friction Assessed by Walking Experiments by Lennart Strandberg
National Board of Occupational Safety and Health (AV)
8 17184l SOLNA Sweden
Lars Hildeskog and Anna Lisa Ottoson
Swedish Road and Traffic Research Institute (VTI)
5-581 01 LINKOPING Sweden
SUMMARY
Analyses of accidents and gait biomechanics by Strandberg (1983) point
at the substantial influence on human safety from the walking friction on contaminated surfaces. However, friction meter data often lacks validity according to a past interlaboratory comparison. Therefore, the slip resistance of 18 types of footwear (ToF) on three contaminated floorings was assessed by experiments with 12 well trained subjects, walking in a triangular closed path as fast as possible without slipping and falling into the safety harness.
From the lap time the average value of the _f_riction utilization (TFU) was computed with a model by Lanshammar and Strandberg (1985). Ground
reaction force measurements from individual steps with sliding motions
confirmed that the TFU was close to the actual coefficient of friction.
On the most slippery surface the average time over five laps ranged between about 7 s and 20 s for the 18 TOP. The corresponding TFU means were about 0.3 and 0.04. The 18 TOP could be separated at about ten significantly different TFU levels. On the other two surfaces, the lap time variation was less pronounced and the rank order was different for
certain ToF.
The lap time varied least between the three surfaces for the most slip
resistant ToF: from 7 s to 8 5; while other ToF varied between 9 s and 20
s. The results elucidate the influence from flooring and footwear parameters such as pattern, hysteresis, hardness and stiffness.
1 BACKGROUND
Accident analyses and biomechanical measurements, see Strandberg
(1983a), as well as tribological considerations by Rabinowicz (1956) and
Moore (1972) point at the substantial influence on human safety from the dynamic slip resistance of shoes on contaminated surfaces. According to the Official Statistics of Sweden, about twice as many fatalities ocCur in falling accidents as in motor vehicle accidents. However, it is unknown how may of these fatal falls that are intiated by slipping.
In a study of traffic injury victims seeking emergency aid at various hospitals in Sweden, Nilsson and Thulin (1983) found slipping pedestrians to be the greatest group. Though many falling accidents occur without a preceeding slip, there are also many injuries from slipping without falling. Andersson and Lagerl'o'f (1983) found that falling occured in 63% of the M000 occupational accidents with slipping in Sweden in 1979. Though a great number of various friction meters were found in the literature by Strandberg (1983b), no apparatus should be considered perfectly valid, according to an interlaboratory comparison reported by
Strandberg (1985) and by Strandberg and Lanshammar (1985). Their
method for assessing reference values hasnow been modified and used in shoe testing.
While Andriacchi et a1. (1977) have used walking speed and ground reaction _f_orce (GRF) measurements for other purposes, this study applies a model from Lanshammar and Strandberg (1985), where the lap _t_ime in
a triangular closed path yields an average value of the _f_riction
utiliza-tion (TFU). Since the subjects are instructed to walk as fast as possible without slipping and falling into the safety harness, the TFU value can be considered close to the coefficient of friction of the actual shoe. This assumption was confirmed by GRF recordings from individual steps, where sliding motions were detected visually.
2 METHOD
The slip-resistance of 18 various types of footwear (19E) was assessed in 648 walking tests with 12 well-trained normal subjects (six male and six female) on three contaminated floorings, each forming a triangular (90
135 135 degrees) closed path with 12 m circumference. One test comprised five laps to be walked as fast as possible without slipping and falling into the safety harness.
From the lap Times the test average value of the Friction Utilization
(TFU) was computed with a model by Lanshammar and Strandberg (1985).
Assuming constant vertical force and constant acceleration/deceleration along every side of the triangle, Newtonian mechanics yield:
k
where k=l§ when the circumference of the triangular path is 12 m as in
our case.
This model yielded a correlation coefficient of 0.99 and a linear regression coefficient of 1.05 when considering the TFU a function of "the _f_orce plate recorded friction utilization (time average over one
stance phase)", FFU, from five steps in the 90-degree corner. Hence, the
TFU has been considered an appropriate measure of the practically available friction. Complete force plate data were recorded at 500 Hz in the present study, as well, together with visual observations on sliding motions for each step in the 90 degree corner of path no. 1.
The path no. 1 coarse flooring consisted of quartz sand (particle
diameter 1.1 1.8 mm) sealed with an epoxy. resin layer, which was
covered with 90% glycerine (viscosity varying between 140 and 300 cP,
depending on relative humidity and temperature). Path no. 2 was covered
with smooth stainless steel and the same 90% glycerine. Path no. 3 had its smooth unglazed ceramic tile flooring partly covered with glass
spheres (diametre median: 0.3 mm). See Figure 1.
Fi gur e 1 Th e VT I wa lk in g la bo ra to ry pa th s. a) No . l co ar se ep oxy an d gl yc er in e, pi ezo el ec tr ic fo rc e pl at e in th e co rn er . b) No . 2 sm oo th st ai nl es s st ee l an d sa me gl yc er in e (vi sc os it y ab out 20 0 cP ). c) No . 3 un gl aze d ce ra mi c ti lean d gl as s sp he re s wi th ave ra ge di am et er 0. 3 mm .
The 18 ToF evaluated in this study (Table 1, Figure 2) were distributed systematically between and within sessions in 12 different permutations (one per subject) to neutralize learning, fatigue and other order effects. Each subject participated in six three hour sessions over about two
months.
One session occupied three subjects, testing three ToF each (nine ToF per session to minimize influence between subjects). While one subject was performing the five lap test, the other two changed path and ToF, walked a few pre test laps, smoothed the lubricant or rested. The tests
were recorded by video (Figure lb), photo-cell and piezoelectric force
plate (only in path 1, Figure la) equipment.
Confidence intervals for the TFU-means over 12 subjects have been indicated by lines in Figure 3 being 25 tall on both sides of the TFU mean level, where 6 is the standard deviation of the mean. According to the t distribution these lines will approximately cover a 92% confidence interval for the ToF and TFU-mean in question. Considering two ToF with similar (max 200% difference) o magnitudes, simple calculus shows that the probability of reversed TFU mean order is less than 1%, if no
overlap exists between the 25-lines. See e.g.- Brownlee (1965). The
mentioned probability is less than 10% if the longer line does not reach
the other mean value.
Ta bl el Ch ar ac te ri st ic s of te st ed fo ot we ar . So le pa tt er ns in Fi gur e 2. SB R: st yr en e-but ad ie ne rub be r, EV A: et hyl en e vi nyl ac et at e, PU : po lyU re th an e, PV C: po lyvi nyl ch lo ri de , NR : na tur al rub be r, NB R: ni tr il e rub be r. Ma te ri al st at em en ts no t ch ec ke d. Pa th s l3 ac co rd in g to Fi gur e l. T yp e F R I C T I O N V A L U E S H O E S O L E of M e a n 12 s ubj e c t s We ig ht (g ) F l e xi b i l i t y M a t e r i a l P a t t e r n H a r d n e s s T e xt ur e A de p t h 0 O r d i n a l F O O t we a r T F U -z -T F U S i ze O r d i n a l a s no . p a t h 1 p a t h 2 p a t h 3 4O 43 sc al e s t a t e d (m m) IR H sc al e .2 27 i. 01 7 .0 59 i. 00 5 .l 9l i. 00 9 35 0 45 0 10 w) S B R or E V A 1( 84 0( sm oo th ) . l 6 3i . l O l . 0 3 9 i . 0 0 4 . l 7 0 t . 0 0 4 3 6 0 4 9 0 2 ( m e d i um ) S B R o r E V A 2 (m 2( 7O 0( sm ooth ) 67 0(sm ooth ) 66 0( Smoo th ) .2 17 i. 00 9 .l 46 t. Ol l .2 86 t. 01 5 42 0 45 0 .l 76 i. 00 9 .l lB i. Ol 8 .3 3l i.01 5 41 0 47 0 e d i um ) S B R o r E V A m e d i um ) S B R o r E V A OO Q N r4 m co m Ienseg 0") V .2 18 t. 01o .1 44 i. 01 3 .3 29t. 01 7 21 0 24 0 2( me di um ) PU 71 0( sm oo th ) .2 55 1. 00 8 .2 46 i. 01 7 .3 02 i. 011 18 0 22 0 2( medi um ) EV A 3 52 0( sm oo th ) KO 410ds l\ .2 43 i. 012 .1 43 i. 01 4.3 14 i. 00 8 44 0 49 0 0( ri gi d) Pvc 7 68 .2 oo t. 01 o .0 89 i. 00 6 .3 10 i. 01 1 42 0 48 0 0( ri gi d) pU 2 62 -267 t. 00 9 .1 53 t. 00 5.2 45 i. 00 6 41 0 490 1( 1O W) pU 0 ( s m o o t h ) 0 ( s m o o t h ) 00 SBOIQ m O\ 62 l ( o ran g e pe el ) 10 .2 07 i. 00 6 .1 12 i. 01o .2 83 i. 01 4 80 0 959 3( hi gh ) NR 11 .1 97 i. 00 5 .115 t. 00 6 .3 14 i. 01 7 74 0 820 3( hi gh ) NR 12 .2 16 i. 00 5.1 24 :. oo 7 .2 82 i. 01 3 80 0 95 0 3( hi gh ) NR .l 68 i. 00 7.0 94 i. 01 o .2 18 i. 006 78 0 89 0 3( high ) NR 54 53 o r a n g e pe el ) c o a r s e ) sqoog m H l( 2( 51 l ( ora n g e pe el) 54 0( KOme s m o o t h ) .2 49 t. 00 6 .1 57 t. 01 4 .2 64 t. 01 2 45 0 50 0 .2 29 t. 00 8 .0 54 t. 00 5 .2 37 t. 01 1 56 0 57 0 2 ( m e d i um ) E V A N 4 5 0 ( s mo o t h ) 2 ( m e d i um ) NB R l 60 0 ( s m o o t h ) VLF) r4 H stepues +_ aoq Teens -~4 .2 42 t. 00 6 .0 93 t. 005 .2 9o i. 01 7 67 073 0 2( me di um ) SB R 4 55 .2 75 t.00 8 .1 89 t. 012 .2 78 t. 01 1 610 67 0 2(me di um ) NBR 4 61 .2 50 i. 00 8 .1 35 t. 01 3 .2 63 t. 01 6 47 050 0 2 ( c o a r s e ) 0 ( s m o o t h ) 2( me di um) PU 4 66 0( sm oo th ) w rs m H r4 H -xIOM
Figure 2 Sole patterns for footwear in Table l.
CAUTION: Though the pattern here (and other parameters in Table l) are the same as those of footwear recognized on the market, the material recipies may be different. Therefore, it must be discouraged to use the pattern for connections between the results and marketed footwear.
VARNING: Aven om monstret air detsamma som pa vissa skor i markna den, 551 kan testskornas materialrecept vara sa annorlunda att resultat-ja'mforelser ar helt missvisande.
VTI REPORT 300A T F U 1 I GL YCER IN E 0N CO AR SE EP OXY GL YCER IN E ON SMOOTH ST EE L 3 E] GL AS S SP HE RE SON SMO OTH CE RA MI C TI LE 0. 3-0. 2-0.1% 17 1 4 9 3 4 7 1 8 1 2 11 5 1 O 1 6 8 1 1 5 2 T O F Fi gur e 3 TF Um ea n ba rs an d co nf id en ce li ne s (s ee te xt ) fo r the 18 Typ es of Fo ot we ar in Ta bl e 1 an d Fi gur e 2.
3 RESULTS
Based on each subject's TFU values on the steel path (no. 2 in Figure 3),
a discrete rank number was assessed to each of the 18 ToF. The 18 means over the 12 subjects of these ToF-ranks have been used in Figure 3 to order the bars. The three bars to the very left represent the TOP (no. 6) with the highest rank mean and the greatest slip-resistance on the steel path, while the most slippery ToF (no. 2) can be found to the very
right. The TFU mean orders of the 18 ToF vary considerably between the
three paths, however. The TFU values are also listed in Table l.
4 CONCLUSIONS
Since normal straight walking with constant speed required FFU-values
between 0.1 and 0.2 (Strandberg, 1983a), the choice of ToF appears more
important for safety on floors with a viscous contaminant (paths 1 and 2)
than with a solid one (path 3). See Figure 3, also showing the substantial influence from. flooring roughness: the coarse (but polished) texture of flooring no. 1 made the TFU on glycerine less sensitive to ToF than the
smooth Steel on path 2. The TFU variation on the coarse flooring (path 1)
is therefore of less importance to safety than on the smooth path 2, which also exhibits smaller TFU values in general when compared to path 1.
The relations between results (Figures 3 and 4) and ToF characteristics
(Table 1, Figure 2) may be enlightened by three phenomena, decisive for walking friction on contaminated surfaces: Drainage, Draping, and Damping.
Without sole pattern (ToF l, 2, 13) or with closed cavities in the pattern (ToF 5, 8, 15), poor bulk contaminant drainage prevented efficient sole draping and molecular contact with the flooring asperity tips on paths 1 and 2.. On path 3, however, the glass sphere layer was probably thin enough to avoid filling the cavities of ToF 5 and 8. Drainage of very viscous or solid (glass spheres, path 3) contaminants requires an edge-like pattern and a particularly small contact area. On the other hand, such ToF offer too small draping areas to be competitive on coarse floorings. Hence, paired comparisons of ToF 5, 11, and 8 versus 3, 12, and 9 respectively, exhibit great versus small TFU-means on path 3, and vice versa on path 1.
On the smooth Steel path (no. 2) the coarse sole texture of ToF 16 may have caused a small effective draping area, contributing to its compara tively small TFU. The effect from normal pressure on actual contact area and on the coefficient of friction has been elaborated on by
F R I C T I O N U S A G E 10
elastohydrodynamic separation (Moore, 1972) may cause the draping time to be longer than the stance time, see Figure 4. (The stanCe phase starts with heel strike and ends with toe-off.) The significant TFU differences between ToF 1 and 2 may be due to this phenomenon.
Test: 245 Laps: 2-5 ToF: 17 761 Laps: 1 5 ToF:
FFU: 0.27 Mean stance time:
r
Test: 2 f
0.51 FFU: 0.15 Mean stance time: 0.67s 1.0 .6 .4
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_v -v 0. x'h
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0 O 0
TIME (FRAéTloN OF STANCE TIME)
Figure 4 Friction usage over time divided by stance duration from sliding
steps: four with ToF 17 (TFU: 0.28, FFU: 0.27, stance duration: 0.51 3);
five with ToF 2 (TFU: 0.12, FFU: 0.15, stance duration: 0.67 5). Same female subject in her 3rd and 6th session respectively.
Coarse epoxy flooring with glycerine (path 1).
After drainage and draping, the damping or hysteresis (Moore, 1972) properties of the sole material play an important role. Though EVA and PVC materials appear more favourable in this respect, the great TFU-values of ToF 17 indicate that high damping can be achieved also with certain NBR-recipies. The great and path independent TFU~va1ues of ToF 6 demonstrate a well-balanced and probably the safest compromise between these slip resistant properties.
11
ACKNOWLEDGEMENTS
This paper is an extended preprint from BIOMECHANICS X (to be
published in 1986 or 1987) with kind permission from Human Kinetics
Publishers, Champaign, USA.
The study was sponsored by the Work Environment Fund (Arbetarskydds
fonden). The National Board of Occupational Safety and Health (Arbetar
skyddsverket, AV) provided equipment and project manager.
The subjects participated during working hours with salary from their
employers: Postverket, Cloetta AB, Farmek AB, Arla AB.
The laboratory flooring materials were provided without cost from:
Partek Byggvaror AB (ceramic tile), Perstorp AB (epoxy resin), Platsla-geriernas Riksforbund (stainless steel), 3M Svenska AB (Safety walk
adhesives beneath the triangular paths).
The authors wish to express their gratitude also to the members of the Advisory Group for their numerous contributions. This group consists of
people from Arbetarskyddsn amnden, AV, Branschforeningen for Personlig
Skyddsutrustning, Foreningen for Teknisk F'o'retagsha'lsovard,
12
REFERENCES
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new Swedish information system on occupational injuries. Ergonomics 26, pp 33-42.
Andriacchi, T.P., Ogle, J.A. and Galante, 3.0. (1977). Walking speed as a basis for normal and abnormal gait measurements. Journal of Biomechanics 10; pp 261 168.
Brownlee, K.A. (1965). Statistical theory and methodology. John
Wiley (Sc Sons Inc., New York.
Gillespie, T.D. (1965). Pavement surface characteristics and
their correlation with skid resistance. Joint Road Friction Program, The Pennsylvania State University, Report No. 12.
Lanshammar, H., and Strandberg, L. (1985). Assessment of friction by speed measurement during walking in a closed path. In D. Winter, R. Norman, R. Wells, K. Hayes, (St A. Patla (eds.), Biomechanics IX-B, pp 72-75. Human Kinetics Publishers, Champaign, USA.
Moore, D.F. (1972). The friction and lubrication of elastomers.
Pergamon Press, Oxford.
Nilsson, G., and Thulin, H. (1983). Trafiko1yckor och trafik-skadade i Ostergotland under tva veckor i mars och tvé veckor i oktober 1982. Undersokning av olika rapporterings- och regi-streringsrutiner av trafikolyckor och trafikskadade.
VTI Meddelande nr 348.
Rabinowicz, E. (1956). Stick and slip. Scientific American, 194; pp 109 118.
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Design, pp 201-228, Butterworths, London.
Strandberg, L. (1985). The effect of conditions underfoot on
falling and overexertion accidents. Ergonomics 28; pp 131 147. Strandberg, L., and Lanshammar, H. (1985). Walking slipperiness compared to data from friction meters. In D. Winter, R.
Norman, R. Wells, K. Hayes, (St A. Patla (Eds.), Biomechanics
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