300A

1985

\57 h

Amt.

9

(g

;

\

\

t.53

F00twear Fricticn Assessed by

Walking Experiments

Lennart StrandbergNational Board of Occupational Safety and Health

Lars Hildeskcg and Anna-Lisa OttosonSwedish Road and Traf c Research Institute

(db Vag-00/) Sta tens va'g- och trafikinstitut (VTI) - 58 1 o 1 Linkc'ping

'IIIStItUtBt Swedish Road and Traffic Research Institute - 8-581 0 1 Linkc'ping Sweden

300A 1985

F00twear Friction Assessed by

Walking Experiments

Lennart StrandbergNational Board of Occupational Safety and Health

Lars Hildeskog and Anna-Lisa OttosonSwedish Road and Traf c Research Institute

Vag-06/7 Statens va'g- och trafikinstitut (VT/l - 58 1 0 1 Linkb'ping

'l St/tlltet Swedish Road and Traffic Research Institute - 8 587 0 1 Linkc'ping Sweden

Klintland Grafis-ka, Linképing 1986

Footwear Friction Assessed by Walking Experiments

by Lennart StrandbergNational Board of Occupational Safety and Health (AV)5 171 84 SOLNA Sweden

Lars Hildeskog and Anna Lisa OttosonSwedish 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

Researcher, in tribology from Kent Gustafsson and Evert Ohlsson, Chief

Researchers. Linguistic checks and word processing tasks were

performed by Christina Ruthger.

VTI REPORT 300A

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 12

VTI REPORT 300A

Footwear Friction Assessed by Walking Experiments

by Lennart StrandbergNational Board of Occupational Safety and Health (AV)5 171 84 SOLNA Sweden

Lars Hildeskog and Anna-Lisa OttosonSwedish 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

other ToF varied between 9 s and 20 s.

VTI REPORT 300A

II

Skofriktion bestémd genom gé mgfo'rsok

av Lennart StrandbergArbetarskyddsstyrelsen (AV)171 84 SOLNA

Lars Hildeskog och Anna-Lisa OttossonStatens 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.

VTI REPORT 300A

III

Footwear Friction Assessed by Walking Experiments

by Lennart StrandbergNational Board of Occupational Safety and Health (AV)8 17184l SOLNA Sweden

Lars Hildeskog and Anna Lisa OttosonSwedish 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.

VTI REPORT 300A

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.

VTI REPORT 300A

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.

VTI REPORT 300A

VTI REPORT 300A

Figure

1 TheVTIwalking

laboratorypa

ths.

a)No.

lcoarseepoxy

and

glyc

erin

e,piezo

electricforce

plat

ein

thecorner.

b)No

.2smooth

stai

nles

sst

eelandsame

glyc

erin

e(v

isco

sity

abou

t20

0cP

).c)

No.

3un

glaz

edceramic

tile

and

glas

ssp

here

swi

thave

ragediameter

0.3mm.

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.

VTI REPORT 300A

Tablel

Characteristics

oftested

footwear.

Sole

patterns

inFigure

2.SBR:

styrene-

butadiene

rub

ber,EVA:

ethylene

vinyl

acetate,

PU:

polyUrethane,PVC:

poly

viny

l

chlo

ride

,NR:

natur

al

rubb

er,

NBR:

nitr

ile

rub

ber.

Material

statements

not

checked.Paths

l3according

toFigure

l.

VTI REPORT 300A

Type

FRICTION

VALUE

SHOE

SOLE

of

Mean

12

subjects

Weight(g)

Flexibility

Material

Pattern

Hardness

Texture

A

depth

0Ordinal

FOOtwear

TFU-z

-TFU

Size

Ordinal

as

no.

path

1path

2path

34O

43

scale

stated

(mm)

IRH

scale

.227i.017

.059i.005

.l9li.009

350

450

10w)

SBR

or

EVA

1(

84

0(smooth)

.l63i.lOl

.039i.004

.l70t.004

360

490

2(medium)

SBR

or

EVA

2(m

2(

7O

0(smooth)

67

0(smooth)

66

0(Smooth)

.217i.009

.l46t.Oll

.286t.015

420

450

.l76i.009

.llBi.Ol8

.33li.015

410

470

edium)

SBR

or

EVA

medium)

SBR

or

EVA

OO Q N

r4 m co m

Ienseg

0")

V

.218t.01o

.144i.013

.329t.017

210

240

2(medium

)PU

710(smooth)

.2551.008

.246i.017

.302i.011

180

220

2(medium)

EVA

352

0(smooth)

KO

410ds

l\

.243i.012

.143i.014

.314i.008

440

490

0(rigid)

Pvc

768

.2oot.01o

.089i.006

.310i.011

420

480

0(rigid)

pU

262

-267

t.00

9.1

53t.

005

.245

i.00

641

049

01(

1OW)

pU

0(smooth)

0(smooth)

00

SBOIQ

m

O\

62

l(orange

peel)

10.2

07i.

006

.112

i.01

o.2

83i.

014

800

959

3(hi

gh)

NR11

.197

i.00

5.1

15t.

006

.314

i.01

774

082

03(

high

)NR

12.2

16i.

005

.124

:.oo

7.2

82i.

013

800

950

3(hi

gh)

NR.l

68i.

007

.094

i.01

o.2

18i.

006

780

890

3(hi

gh)

NR

54

53

orange

peel)

coarse)

sqoog

mH

l( 2(

51

l(orange

peel)

54

0(

KOme

smooth)

.249t.006

.157t.014

.264t.012

450

500

.229t.008

.054t.005

.237t.011

560

570

2(medium)

EVA

N

45

0(smooth)

2(medium)

NBR

l60

0(smooth)

VLF)r4 H

stepues

+_ aoq Teens -~4

.242t.006

.093t.005

.29oi.017

670

730

2(medium

)SBR

455

.275t.008

.189t.012

.278t.011

610

670

2(medium)

NBR

461

.250i.008

.135t.013

.263t.016

470

500

2(coarse)

0(smooth)

2(medium)

PU

466

0(smooth)

w rs mH 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, thematerial recipies may be different. Therefore, it must be discouraged touse the pattern for connections between the results and marketedfootwear.VARNING: Aven om monstret air detsamma som pa vissa skor i marknaden, 551 kan testskornas materialrecept vara sa annorlunda att resultat-ja'mforelser ar helt missvisande.

VTI REPORT 300A

VTI REPORT 300A

TFU

1I

GLYCERINE

0NCOARSE

EPOXY

GLYC

ERIN

EON

SMOO

THST

EEL

3E]

GLAS

SSP

HERE

SON

SMOO

THCE

RAMI

CTI

LE

0.3-

0.2-

0.1%

17

14

93

47

18

12

11

51O

16

81

15

2TOF

Figu

re3

TFU

mean

bars

and

conf

iden

celi

nes

(see

text

)fo

rth

e18

Types

ofFootwear

inTable

1andFigure

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.

VTI REPORT 300A

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

Gillespie (1965) and will not be discussed here.

A soft sole and flexible shoe may increase draping, but if there are

insufficient bulk drainage channels in the sole and flooring pattern,

VTI REPORT 300A

FRICTION

USAGE

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

\ w /

\

v.

1..

r'

.

.

__ . . .- .3. _,

. . . . :I c _ .n. o -.. . n...""...- -"n\':".::::: .;;:.N 01" ' . n..;.'. .

. . - . .. - .. . -' . H" --. 1.001.". 2." ...'» z . - .-' '. . .I L. .- . _;_ .(. _,:\_..Ew'i 1114...... n o . . ... 3.. I-..._.$f : . 00.31.:Z.. ~ v, V ._ -. nu... . ' ,A ;'J .' t ' . ' . . ' "

o I ' 'o'oou..o

' . . . . . I. 04'."...'..0 . '

z' '- ....... ur'-- :.".. "Wham. - - . -..... ° 0 K'. -...,.:n.;:. |.."'2% ,, W M M u me -,,...,:;-u v w,

: .;: ... ..'::....l.." '~.o'ou..; :fo %::'o'-I:_.-"'?M .

a... o. .I 5"" .o-.p~o-.QQ.I ' '

_v -v 0. x'h

""A . A A A 4 A L A A A A A r

O N V 9 w0 O 0

TIME (FRAéTloN OF STANCE TIME)

Figure 4 Friction usage over time divided by stance duration from slidingsteps: 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.

VTI REPORT 300A

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, Skoinsti-

tutet, Statshalsan and VTI.

VTI REPORT 300A

12

REFERENCES

Andersson, R., and Lagerlof, E. (1983). Accident data in the

new Swedish information system on occupational injuries.Ergonomics 26, pp 33-42.

Andriacchi, T.P., Ogle, J.A. and Galante, 3.0. (1977). Walkingspeed as a basis for normal and abnormal gait measurements.Journal of Biomechanics 10; pp 261 168.

Brownlee, K.A. (1965). Statistical theory and methodology. JohnWiley (Sc Sons Inc., New York.

Gillespie, T.D. (1965). Pavement surface characteristics andtheir correlation with skid resistance. Joint Road Friction

Program, The Pennsylvania State University, Report No. 12.

Lanshammar, H., and Strandberg, L. (1985). Assessment offriction 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 ioktober 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.

Strandberg, L. (1983a). On accident analysis and slip-resistancemeasurements. Ergonomics 26, pp 11-32.

Strandberg, L. (1983b). Ergonomics applied to slippingaccidents. In: T.O. Kvalseth (ed.), Ergonomics of WorkstationDesign, pp 201-228, Butterworths, London.

Strandberg, L. (1985). The effect of conditions underfoot onfalling and overexertion accidents. Ergonomics 28; pp 131 147.

Strandberg, L., and Lanshammar, H. (1985). Walking slipperinesscompared to data from friction meters. In D. Winter, R.Norman, R. Wells, K. Hayes, (St A. Patla (Eds.), Biomechanics

IX-B; pp 76-81. Human Kinetics Publishers, Champaign, USA.

VTI REPORT 300A