NBSIR 74-613

Preliminary Study of tlie

Slipperiness of Flooring

A. Philip Cramp

Building Safety Section

and

Larry W. Masters

Materials and Composites Section

Center for Building Technology

National Bureau of Standards

Washington, D. C. 20234

July 1974

Final Report

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 74-613

PRELIMINARY STUDY OF THE

SLIPPERINESS OF FLOORING

A. Philip Cramp

Building Safety Section

and

Larry W. Masters

Materials and Composites Section

Center for Building Technology

National Bureau of Standards

Washington, D. C. 20234

July 1974

Final Report

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

PRELIMINARY STUDY OF THE SLIPPERINESS OF FLOORING

A. P. Cramp and L. W. Masters

The National Commission on Product Safety reported in 1970, that falls in the home each

year kill about 12,000 and injure 6,000,000 in the U.S.A. Slippery floors are listed as a

large contributor to these very high casualty figures. Although there are some standardized

test methods that are or may be made suitable for such standards, there are no slipperinessstandards for flooring. There is an immediate need for studies aimed at the development and

establishment of such standards. Consequently, a preliminary study of floor slipperinesswas sponsored by the Building Safety Section of the Center for Building Technology. The

study included a state-of-the-art investigation on flooring slipperiness research and a

laboratory evaluation of three existing test methods for measuring floor slipperiness.

Samples of the three most commonly used resilient flooring materials, vinyl asbestos, vinyl,

and linole\am were used in the study, ihe sliding material components for the frictionaltests were leather and a commonly used styrene butadiene sole and heel rubber. The testswere performed on both dry and wet surfaces. The results from this study were used in

planning a large comprehensive study for the development of accepted floor slipperinessstandards. This report contains the results of the preliminary study.

Key words : Floor slipperiness; resilient flooring; slipperiness standards; frictionaltests; slip tests; coefficient of friction; human perambulation.

SI CONVERSION UNITS

In view of the present accepted practice in this coiintry for building technology,common U.S. units of measurement have been used throughout this paper. In recognition ofthe position of the United States as a signatory to the General Conference on Weights andMeasures, which gave official status to the metric SI system of units in I96O, assistance is

given to the reader interested in making use of the coherent system of SI units by givingconversion factors applicable to U.S. units used in this paper..

Length

1 ft

1 in 0.025'+ meter (exactly)

O.3O48 meter (exactly)

Mass

1 lb ^53 . 59 grams

Temperature

°C = 5/9 (Temperature °F - 32)

ii

LIST OF FIGURES

1. James Static Friction Tester

2. British Portable Skid-Resistance Tester

3. Horizontal Pull Slipmeter

h. Bottom of Horizontal Piill 'Slipmeter Showing Leather Feet

iii

LIST OF TABLES

No. •

, ,"

, TITLE

1. Apparatus for Measuring Slip-Resistance

2. Static Coefficients of Friction of Vinyl Asbestos Tile Using the James StaticFriction Tester

3. Static Coefficients of Friction of Vinyl Tile Using the James Static FrictionTester

k. Static Coefficients of Friction of Linoleum Using the James Static Friction Tester

5., Static Coefficients of Friction of Resilient Flooring Using the James StaticFriction Tester

6. British Portable Test Number of Vinyl Asbestos Tile Using the British PortableSkid Resistance Tester

T. British Portable Test Number of Vinyl Tile Using the British Portable Skid Re-sistance Tester

8. British Portable Test Number of Linoleum Using the British Portable Skid Resis-tance Tester

9. British Portable Test Number of Resilient Flooring Using the British Portable SkidResistance Tester

10. ' British Portable Test Number of Vinyl Asbestos Tile Using the British PortableSkid Resistance Tester

11. British Portable Test Number of Vinyl Tile Using the British Portable Skid Re-sistance Tester

12. British Portable Test Nmber of Vinyl Asbestos and Vinyl Tile Using the BritishPortable Skid Resistance Tester

13. Coefficient of Friction of Vinyl Asbestos Tile Using the Horizontal Pull Slipmeter

ik. Coefficient of Friction of Vinyl Tile Using the Horizontal Pull Slipmeter

15. Coefficient of Friction of Linoleum Using the Horizontal Pull Slipmeter

16. Coefficient of Friction of Resilient Flooring Using the Horizontal Pull Slipmeter

IT. Comparison of Frictional Properties of Resilient Flooring

iv

TABLE OF COMTENTS

Page

Abstract i

SI Conversion Units

List of Figures iii

List of Tables '

1. Introduction ...... 1

1.1 Slipperiness Standards •1

1.2 Biomechanical Studies of Walking ;2

2. Current Methods for Determining Floor Slipperiness 2

3. Method of Conducting the Study 3

h. Equipment and Materials ^

k.l Equipment ^

k.2 Flooring Materials ^

k.3 Slider Shoe Materials h

5. Test Procedures k

5.1 General Test Procedures k

5.2 Detailed Test Procedures 5

5.2.1 James Static Friction Test 5

5.2.1.1 Tests of Dry Surfaces 5

5.2.1.2 Tests of Wet Surfaces 6

5.2.2 British Portable Skid Resistance Test 7

5.2.2.1 Tests of Dry Surfaces 7

5.2.2.2 Tests of Wet Surfaces 8

5.2.3 Horizontal P\ill Slipperiness Test 8

5.2.3.1 Tests of Dry Sixrfaces 8

5.2.3.2 Tests of Wet Surfaces 9

6. Results 9

6.1 James Static Friction Test 9

6.2 British Portable Skid Resistance Test 10

6.3 Horizontal Pull Slipperiness Test 11

7. Discussion of Results 11

8. Discussion of Possible Equipment Improvements, 12

9. Conclusions 13

Acknowledgment ik

Bibliography 15

V

1.0 INTRODUCTION

The final report of the National Commission on Product Safety, issued in June 1970,states that falls in the home each year kill about 12,000 people and injiire 6,000,000 in the

United States. The precise causes of the falls are uncertain, but slippery floors are

listed as a large contributor to these high casualty figxires . Consequently,- a preliminarystudy of floor slipperiness was sponsored by the Building Safety Section of the NationalBureau of Standards. The study was intended as a basis for a comprehensive fut'ore study of

the subject. This report contains the results of the preliminary study.

1.1 Slipperiness Standards

Because of the seriousness of the problems resulting from floor slipperiness and thelarge number of individuals involved, this subject has been the source of many scientificinvestigations by flooring, floor finish and shoe manufacturers; research laboratories; andalso insurance companies for many years. Nevertheless, no national standards for floorslipperiness have yet been established in the U.S.A. Thus, there is an immediate need for

the development of such standards. The problems involved in the development of slipperinessstandards are complicated. For meaningful standards to be developed, the test methods uponwhich the standards would be based should yield results which relate logically and directlyto results obtained from human perambulation tests.

A nimiber of laboratory test methods for determining the slipperiness, skid resistanceor coefficient of friction of flooring and pavement surfaces have been developed. However,little work has been devoted to the intercomparison of results from these test methods or totheir relationship with tests and investigations involving human perambiilation and reaction.

From contacts with the flooring and floor finishing industries, it has been determinedthat little, if any, work is presently being done in research on floor slipperiness by them.

Although such ASTM committees as F-6 on resilient floor coverings and D-21 on polisheshave worked on the floor slipperiness problem, they are not currently active in this area.By contrast, the footwear industry has recently become very active in the study of safety offootwear insofar as the relative slipperiness of sole and heel materials is concerned.

A new ASTM Committee, F-13, Safety in Footwear, was formed in October 1973. The presenttask of subcommittee F-13. 10, Traction in Footwear, is to determine whether or not twoestablished test methods for evaluating the frictional properties of pavements and floorfinishes, or modifications of these, might be suitable for adoption as standard tests forthe frictional properties of sole and heel materials.

The Committee hopes to have a standard test established for the evaluation of thefractional properties of sole and heel materials by the end of 197*+.

1

1.2 Biomechanical Studies of Walking

According to Carlet [l]—^, the study of the mechanics of walking can be traced back as

far as Aristotle. The earliest author to explain the process of walking in terms of leversand forces was Borelli [2] in 1679- In the nineteenth century, Marey [3] and Fischer [k]

used photography to further study the subject. A study of the dynamic anatomy of the foot

and leg in connection with the development of artificial limbs was made shortly after WorldWar II at the University of California [S].

Au extensive study of human locomotion and the interaction between walkways and footwearwas conducted by R. B. Hunter and later P. A. Sigler and coworkers at the National Bureau ofStandards. Moving pictures were taken of people walking to serve as a basis for theirconclusions about the mechanics of human perambulation. Instrumentation for the measurementof the relative slipperiness of walkways was also developed. This work was reported in a

series of publications [6], [T], [8], [9] and [lO].

In England, F. C. Harper conducted extensive studies on the mechanics of walking andthe forces applied by the foot to floors. This was reported in I96I [ll]. He suggestedthat the coefficient of friction between floor surfaces and shoe materials should be a

minimum of O.h for safe walking. However, he did not specify a test method to be used in

determining the coefficient of friction.

Using the apparatus developed by Hunter at NBS in 1930 [6], S. James [l2] established a

standard method which was originally used for testing the static coefficient friction of

shoe heels in the mid 1950's. The Jsanes Static Friction Test is described in Section 5-2. 1.

This test was subsequently established as the Standard ASTM Method D20U7 for measuring thecoefficient of friction of floor finishes. James believed that a coefficient of friction of

0.5, as determined by his test, should be established as the minimum requirement for frictionbetween sole and heel materials and floor surfaces to ensure safety against slipping.

When D. F. Ekkebus and W. Kelley [13] could find no published account as to why Jameshad decided that the 0.5 figure was an appropriate minimum for slipperiness safety, they setout to validate its use. They measured the leg and stride length of a number of subjects,and then calcuJ.ated how much larger each individual's stride wo\ild have to be before thehorizontal force would be more than half the vertical force, i.e., the ratio would exceed0.5- No normal stride was found to be nearly long enough for the horizontal force exertedto exceed half the vertical force. Consequently, the 0.5 coefficient of friction specifiedby James was concluded to be a very conservative level for slipperiness of floor finishes(polishes) and flooring surfaces.

2.0 Current Methods for Determining Floor Slipperiness

The need for experimental data relating to floor slipperiness has lead to the developmentof many types of devices for measuring slipperiness.

The most commonly used technique involves the determination of the sliding coefficientof friction, or some sort of "index of slipperiness" or "skid resistance index" related tothe coefficient of friction between flooring material and other materials such as footwearsole and heel material.

— Numbers in brackets refer to references in the Bibliography.

2

The coefficient of friction "between two surfaces is defined as the ratio of the force

required to move one surface over the other to the total force pressing the two surfaces

together . Thus

,

Horizontal Forcethe Coefficient of Friction =

Vertical Force

Two types of friction must be considered in measuring floor slipper iness , nainely,

static and dynamic (kinetic) friction. Static friction is the amount of friction "between

two surfaces at the precise -instant when one commences to move over the other. Dy^aamic

friction is the friction between two surfaces when movement is occurring without interruption.

The coefficient of static friction is usually equal to or greater than that for dynamic

friction.

The wide variety in types of machines for measuring floor slipperiness is probably best

catagorized by the principle by which the apparatus works. The two principles upon which

flooring or-pavement slipperiness testers on record are based, are either the horizontal

plane track or swinging penduliom. A niomber of such test methods are described in Federal

Construction Council Technical Report No. ^+3 [ih] . Table I, which was taken from reference

ih, contains pertinent information about these test methods.

Two tests being evaluated by the ASTM Safety in Footwear Committee are the existing

British Portable Skid-Resistance Test (Pavements) and the existing James Static Coefficient

Test (floor finishes).

The Committee is also investigating the use of an experimental test method for measuringfrictional properties which operates on the principle of a weighted glider shoe, with the

material under test for frictional properties being pulled horizontally at different rates

across standard flooring samples.

3.0 Method of Conducting the Study

The following tasks were performed in this study:

1) A literature survey.

2) Meetings with representatives of the flooring and footwear industries and appropriateASTM Committees to determine what technical developments had recently been made, or arenow in progress on the subject of floor slipperiness, and subjects related to it.

3) Selection of three test methods for this study, from the variety of surface frictionalproperty tests available, which appeared to hold most promise for the evaluation offloor' slipperiness.

k) Determination of the reproducibility of the three selected tests when determining thefrictional properties of three standard types of resilient flooring tile; vinyl asbestos,vinyl, and linoleum. The frictional properties for the flooring were determined withstandard shoe leather and sole and heel rubber as the sliding surfaces material. Thefrictional properties of the flooring were determined wet as well as dry.

5) Formulation of plans for a large comprehensive study intended to follow this preliminarystudy.

3

h.O Equipment and Materials

h

.

1 Equipment

The three test methods selected for evaluation and use in the preliminary study werethose which utilized the following apparatus:

a) The James Static Friction Tester (Fig. l)

,, b) The British Portable Skid Resistance Tester (Fig. 2) .

c) The Horizontal Pull Slipmeter (Fig. 3)_

Test methods utilizing these testing machines were selected for this study because theyappeared to hold the most promise of any surface frictional property tests available for usein floor slipperiness testing, i .

The James Machine is the basis for ASTM Method D20^T for Determining Static Coefficientof Friction of Polish-Coated Siirfaces and has been used by General Services Administration(GSA) and Underwriters' Laboratories Inc. for evaluating floor coatings, although no criteriaor standards have been developed. This method is only suitable for specimens of flooringwhich can be mounted on the machine, and is not a field method. The British Portable Testeris the basis of ASTM Method E 303 [15], which is intended for testing roadways. The HorizontalPull Slipmeter is much lighter and less bulky than the British Portable Tester and favorableresults have been reported in articles published in ASTM Standardization News.

k,2 Flooring Materials

The flooring materials selected for the study were the three most commonly used typesof resilient flooring. They were:

a) Vinyl asbestos tile, 9" by 9" (22.9 x 22.9 cm) samples.

b) Vinyl tile, 12" x 12" (30.5 x 30.5 cm) samples.,

c) Linoleum, 9" x 6" (22.9 x 15-2 cm) samples.

k.3 Slider Shoe Materials ',

" "

Two commonly used shoe sole and heel materials were selected for the sliding componentmaterials in the preliminary study. They were: . ,

a) 100^ cowhide, sanded to a smooth flat surface and

b) Styrene butadiene rubber meeting the requirements of ASTM D I63O Standard Methodof Test for Abrasion Resistance of Soles and Heels [16]

.

Materials from the same sheets were used in the three different test methods.

5.0 Test Procedures

5 . 1 General Test Procedures

The flooring samples were tested with dry surfaces and also with wetted surfaces usingdistilled water. Before testing, the flooring materials were washed with a mild soap andwater solution, rinsed with distilled water, and dried thoroughly at room temperature.

k

It was found that the first few determinations made on new flooring samples with boththe James and British Portable testers, yielded decreasing values. It was obvious that

during these determinations the flooring samples were being conditioned (i.e., possiblesuperficial asperities being removed) because after two to four determinations the decrease

stopped and the values obtained became approximately consistent with one another. Thus, the

res'olts of test runs which showed that surface conditioning was taking place were discarded.

No surface conditioning of the flooring appeared to take place in the Horizontal Slip-

periness Test. Possibly the effect of the conditioning on the test results may have beensmall, because of the comparatively small area of the feet of the testing device and its

slow speed.

At least five determinations were made on each of five samples of the three types offlooring being tested. This procedure was followed for all three test methods.

All of the samples were tested in the machine direction of the flooring. This is thelongitudinal direction of the flooring during the fabrication process and is the directionof the pattern of the flooring. In addition, some of the samples were also tested transverseto the machine direction. The transverse testing was curtailed for the James Test andHorizontal Pull Slipmeter, because, like the results from the British Portable Test, theyall agreed closely with those from the machine directional tests.

The ambient temperature of the room in which the tests were conducted was 7^ i 1°F(23.3°C) and the relative humidity, i+5 + 3^.

5.2 Detailed Test Procedures

The following are descriptions of the specific procedures followed in this study in

evaluating the James Static Friction Test, the British Portable Skid Resistance Test, andthe Horizontal Pull Slipperiness Test.

5.2.1 James Static Friction Test

The James Tester used in this study is similar to the Tester shown in Figure 1, exceptthat the machine used in this study has a motor-driven test table, making it more versatilethan the illustrated model, which is hand cranked.

The test device has a 3x3 in. (7.6 x 7.6 cm) steel shoe. The two shoe materialswhich were studied by placing them on the steel shoes were:

a) 100^ cowhide sole leather, 1/1+ in (0.6U cm) thick, sanded to a smooth flat surfacewith No. 400A carborundum paper and

b) Styrene butadiene rubber, meeting ASTM D l630 requirements.

5.2.1.1 Tests of Dry Sixrfaces

Preparation of Slider (Shoes)

Leather shoe. A 3 in. x 3 in. (7.6 x 7.6 cm) piece of leather was bonded with anadhesive to each of the steel shoes. The leather shoe was sanded by gently moving it fourtimes back and forth, about k in. (10.2 cm) over a sheet of kOOA carborundum paper, thenrepeating this proced\ire at an angle of 90°. The dust and loose material were then removedfrom the shoe siirface by rubbing it lightly with a clean cotton cloth.

This procedvire was repeated before each determination.

5

Ru"bber shoe . A 3 in. x 3 in. (7-6 x 7.6 cm) piece of test rubter was bonded adhesively

to each of the steel shoes used for the tests with rubber. The rubber shoe was dusted free

of any loose material prior to each determination.

'^ Test Procedure . The standard procedure described in ASTM D 20^+7 was followed in general,

except that the tests described in this report were of flooring instead of floor coatings.

The test device was leveled by placing shims under the legs as it was not

equipped with leveling screws. Three 25 lb. (11.3 kg) weights were placed on the weight

platform to give a total load of 80 lbs. (36.3 kg) on the shoe (j) (see figure l).

A copy of the James Tester Chart (c) was fastened to the chartboard (d) so that the

spring-fitted recording pencil would be at zero when the test started. This chart is

graduated directly in terms of the tangent of the angle assumed by the strut, or the

static coefficient of friction.

The table was then moved to the starting position. By means of the small wheel in

figure 1, the weighted column was raised, the flooring specimen placed on the test table

against the retaining bar (l), and the shoe (j) inserted in the yoke in such a position that

the strut was 90° to the flooring sample. The recording pencil (f) was released and checked

for zero position on the chart.

The speed of the test table was arbitrarily set at about 100 in cm) per min by

means of a speed control dial. Preliminary studies indicated that varying the test table

speed did not cause a significant change in results for the flooring tested.

The test was started by setting the test table in motion and the strut angle with the

flooring sample became progressively smaller. When the angle reached a critical size, whichis a function of the static coefficient of friction between the foot and the floor specimen,

the foot slipped, and the column dropped.

The line drawn by the recording pencil during the test was a smooth descending curveuntil the foot slipped, at which point there was a fairly sharp break, and the line droppedmore or less vertically. The coefficient of friction is the point at which the curve changessharply downward. The readings were taken to the nearest 0.01 units of static coefficientof friction.

If the foot did not slip, there was no break in the curve, and the coefficient offriction between the two surfaces was too high for the scope of the test device.

5.2.1.2 Tests of Wet Surfaces

The procedure for the wet tests was basically the same as for the dry tests, except for

the following modifications:

a) The leather shoe was not sanded before each test. Instead, it was soaked in

distilled water for at least one half hour before being used for testing.

b) The leather and rubber shoes and the flooring samples were thoroughly wetted withdistilled water prior to each test.

c) It was found that if the time period between the placing of the wet leather shoeon the wet flooring sample and the start of the test table movement was increased,t.he coefficient of friction value* increased. This effect did not appear to be as

pronounced between the wet rubber and the wet flooring. It was decided that inorder to minimize this effect on the test results, the period between the placementof the wet shoes on the wet flooring and the start of the test should be as short

. . as possible and constant. Fifteen seconds was established for this period, becauseit was just long enough to enable the operator to place the recording pencil inposition on the chart after the shoe had been placed on the flooring, withoutbeing unnecessarily hurried.

6

5.2.2 British Portable Skid Resistance Test

This test method, intended for measuring the frictional properties of surfaces, is

suited for laboratory as well as field use. The values obtained, British Portable Tester

Numbers, represent the frictional properties determined with the apparatus, and do not

necessarily agree or correlate with values from other slipperiness measuring equipment.

The tester (figure 2) is a dynamic pendulum impact type tester which measures the

energy loss when the edge of a slider attached to the bottom of the pendulum is propelled

over a test surface. D\iring its forward swing, the pendulum engages and pushes a drag

pointer to the highest point of its travel, at which time the pointer stops to indicate the

Test Number on a scale.

The pend\ilum with slider and slider mount weighs 3-31 + 0.07 lb (1500 +30 g). The

distance of the center of gravity of the pendulum from the center of oscillation is l6.2 +_

0.2 in {hll h mm). The tester is capable of vertical adjustment to provide a slider

contact path length of k 15/l6 + l/l6 in (125 ± 1.6 mm). The slider load applied to the

flooring under test is 5-51 ± 0.22 lb (2500 + 100 g). The slider load, however, is not

constant throughout the contact path because of the circiaar path of the pendulum. The

slider assembly consists of an aluminum backing plate to which is bonded a 1/k by 1 by 3 in

(6.35 by 25. i| by 76.2 mm) strip of the shoe sole-and-heel materials required for the study.

The leather and rubber materials used were the same type as described in Section 5.2.1.

A contact path gage consisting of a thin ruler suitably marked for measuring contact

path length between h 7/8 and 5-0 in [12. h and 12.7 cm) was used.

The instrument was levelled by means of levelling screws, and the pendulum adjusted as

described in ASTM E 303. The drag pointer was adjusted so that it would register zero as

required when the pendulum was released to swing with the slider not contacting a s\irface.

5.2.2.1 Tests of Dry Surfaces

Preparation of Sliders (Shoes)

A 1 X 3 in (25. i+ x 76.2 mm) piece of slider material was bonded to each of the aluminumshoes. The leather slider was then swung ten times across No. hOO A carborundum paper in

order to condition it for the tests. This broadened the line of contact. The rubber sliders

were conditioned in the same way as the leather slider, except that the abrasive surface

used for conditioning was No. 80 grade silicone carbide cloth.

NOTE: Wear on the striking edges of the sliders should not exceed 1/8 in (3.2 mm) in theplane of the slider or l/l6 (1.6 mm) vertical to it (15)-

Test Procedure . The flooring samples to be tested were fixed in position on an aluminumplatform on which the British Portable Tester was placed. They were dusted free of any dirtor grit. The pendulum was released and its height adjusted so that the slider had a contactpath length of k 7/8 in to 5.0 in (l2.i+ to 12.7 cm) on the flooring samples.

To perform tests, the pendulum arm was first locked in the horizontal position and thedrag pointer was rotated clockwise until it stopped against the plastic screw of the pendulimi

arm in the horizontal position. Normally the pointer would be in this position prior to

each release of the pendulum arm.

The test determinations were made by releasing the pendulum, letting it take its fullswing, and stopping it on its return swing as soon as possible (the slider should never beallowed to swing back over the test surface). The point on the scale at which the sliderstops was recorded.

7

Multiple determinations were made and the resiilts obtained for the first two to foiir

determinations were discarded. Subsequent readings obtained were in good agreement with one

another and were recorded. The slider and floor sample were dusted free of any grit or dirt

before each determination.

The determinations in which the results were decreasing were regarded as conditioningswings for the flooring samples. The values obtained following the conditioning swings were

recorded as valid test results. Six determinations were made on each flooring sample, in

the machine direction and at right angles to it (cross machine).

5.2.2.2 Tests of Wet Surfaces

The wet test procedure for testing the flooring was the same as that for the dry test

procedure, except that the sliders and the flooring samples were wetted thoroughly withdistilled water before each determination.

5.2.3 Horizontal Pull Slipperiness Test

This test method is designed for measuring the slip index of floor surfaces by means of

the Horizontal Pull Slipmeter. The slip index, according to the manufacturer's literature,is approximately ten times the coefficient of friction. The Slipmeter is able to measureboth static and dynamic coefficients of friction, but not necessarily both for all types of

surfaces.

This device measures the frictional properties of a flat surface as a function of theeffort required to pull a weighted meter fitted with shoes of a specific sole or heel materialacross the surface at a fixed rate of speed. The stress placed on the spring of the meter is

indicated on the meter dial as the slip index of the surface under test.

The Tester is a portable instrimient which utilizes a weighted meter resting on threefeet 1/2 in (1.2T cm) in diameter and 3/l6 in {0.h^ cm) to 1/k in (0.6*+ cm) thick, and a

battery-operated power unit with a cord which is used to pull the meter at a fixed rate of

3.93 in 9-98 cm) per minute across the surface being tested. Figure 3 shows the apparatusassembled and figure h shows the underside with the three round leather shoes installed.

The shoe sole and heel materials were the same as those used for the James and BritishPortable Tests.

5.2.3.1 Tests of Dry Surfaces

Preparation of Sliders (Shoes) . Pieces of leather and rubber, 1/2 in (1.27 cm) in

diameter, were bonded to the feet of the Tester. The leather shoes were sanded with cai'borun-

dum paper No. UOOA with four strokes in two directions at right angles to one another beforeeach determination. The shoes were wiped free of dust with tissue.

The rubber shoes were dusted free of any loose material before each determination.

Test Procedure . The flooring sample to be tested was placed on a table against a

retaining bar to prevent it from moving during the test. The slipmeter was then placed onthe flooring sample facing the retaining bar. The power unit was placed on the other sideof the retaining bar from the flooring sample with the pulley in line with the slipmeterhook. The cord was attached to the hook.

The meter-control switch was released, and the meter zeroed by moving the rim of themeter. The meter switch was then pushed to the rear position for registering the staticslip index, which is the highest reading attained just as the slipmeter starts forward.

8

The cord was held taut at the center of the piilley, and pushing down on the power unit

to keep it from moving, the power switch was depressed. As soon as the slipmeter moved, the

power was shut off. The static slip index was then read from the dial and the needle released

to return to zero by moving the control switch to the middle position. The control switch

did not always function properly, so that the true maximum reading was not always retained.

In order to determine the dynamic slip index, the control switch was left in the middle

position, and the slipmeter was. pulled forward by the power unit.

If the movement of the slipmeter was steady, the slip index recorded on the dial was

dynamic. Occassionally th-e slipmeter moved in a jerky or "stick-slip" manner indicating .

that both static and dynamic friction were being displayed.

Five slip index determinations were made for each flooring sample with the slipmeter in

different starting positions.

5.2.3.2 Tests of Wet Surfaces

The leather and rubber sliders which were used for the dry tests were used for the wet

tests. The leather slider was soaked in distilled water for one half hour before use and

was wetted before each determination. The leather slider was not sanded after wetting. The

rubber sliders were wetted before each determination using distilled water.

The wet test procedure for testing the flooring was the same as that used for the dryprocedure, except that the sliders and the flooring samples were thoroughly wetted prior to

the wet determinations.

6.0 Results

6.1 James Static Friction Test

The data obtained with the James Static Friction Tester for vinyl asbestos tile, vinyltile and linoleum are presented in tables 2, 3, and h, respectively. Each table includesfive recorded values of coefficient of static friction using both leather and rubber shoesin wet and dry conditions for each of five samples tested, the average and standard deviationof those five values, and the total average and standard deviation of all recorded valuesfor each type of flooring. The data in tables 2, 3 and k were obtained in the machine direc-tion of the flooring. Table 5 contains a summary of the data from tables 2, 3 and k.

The total averages and standard deviations, which represent a total of 25 coefficientof static" friction values, for the samples tested dry using leather shoes were 0.h2 +_ 0.02for vinyl asbestos tile, 0.60 + O.Oh for vinyl tile and 0.57 ± 0.02 for linoleum. Thevalues obtained for the wet tests were O.Ul +_ 0.02 for vinyl asbestos tile, 0.35 ±_ 0.02 forvinyl tile and O.i+5 +_ 0.02 for linoleum.

The results indicate that wetting the siirfaces of vinyl tile and linoleum lowers thecoefficient of static friction substantially, but that wetting has little if any effect onthe values obtained for vinyl asbestos tile. The results also indicate that coefficients ofstatic friction determined with the James Static Friction Tester yield reasonably goodreproducibility.

The friction between all three types of resilient flooring and the rubber shoes was sohigh in both the wet and dry tests that no slippage occurred in any of the determinations.Thus, the coefficient of friction could not be determined for any of the three types offlooring in contact with the rubber shoes.

9

6.2 British Portable Skid Resistance Test

The data obtained with the British Portable Skid Resistance Tester for vinyl asbestos

tile, vinyl tile and linoleum tested in the machine direction of the flooring are presented

in tables 6, 7 and 8, respectively. Tables 10 and 11 contain the data obtained for vinylasbestos tile and vinyl tile, respectively when tested in the across machine' direction of

the flooring.

Each of these tables includes six recorded British Portable Test Number values usingboth leather and rubber shoes in wet and dry conditions for each of five samples tested, the

average and standard deviation of those six values, and the total average and standarddeviation for all recorded values for each type of flooring. Table 9 is a summary of the

averages presented in tables 6, 7 and 8 while table 12 contains a summary of the data present-ed in tables 10 and 11.

The total averages and standard deviations for the samples tested in the machine direc-tion (table 9) with leather shoes were: 36.9 i 2.h for vinyl asbestos tile tested dry and

10.2 +_ O.k for vinyl asbestos tile tested wet, 19.^ ± 2.0 for vinyl tile tested dry and lO.ii

+_ 0.5 for vinyl tile tested wet and 57.8 + k.9 for linoleim tested dry and 26.3 ± 3.1 for

linoleum tested wet. Comparable values for samples tested in the machine direction (table 9)

with rubber shoes were 101. i+ i 5-2 for vinyl asbestos tile tested dry and 2k. 6 +_ 1.3 for

vinyl asbestos tile tested wet, 91.1 i. -'-•9 for vinyl tile tested dry and lk.2 +_ O.h for

vinyl tile tested wet, and 117.2 i 3.5 for linolem tested dry and ^1.3 ±_ 2.3 for linoleumtested wet.

The total averages and standard deviations for the samples tested in the across machinedirection (table 12) with leather shoes were: 37-^ i 2.5 for vinyl asbestos tile tested dry

and 10. k +_ 0.5 for vinyl asbestos tile tested wet and 19-6 +_ l.k for vinyl tile tested dryand 11.0 +_ 0.6 for vinyl tile tested wet. Comparable values for samples tested in theacross machine direction (table 12) with rubber shoes were: 99-7 i 5-3 for vinyl asbestostile tested dry and 25.3 ± 1.1 for vinyl asbestos tile tested wet and 93.6 +_ 2.6 for vinyltile tested dry and ih.O +^ 0.2 for vinyl tile tested wet. Linole\mi samples were not testedin the across machine direction because the samples were too narrow to obtain measurementsin that direction.

As can be seen the direction of test with respect to the direction of manufacture, hadlittle effect in the British Portable Tester resiilts. This was also the case for the othertwo testers.

The values obtained in the wet test condition were considerably lower in each case thanthose obtained in the dry condition. Wetting lowered the values for vinyl asbestos testedin the machine direction from 36.9 to 10.2 {12. k percent) with leather shoes and 101. i+ to2h.6 (75.7 percent) with rubber shoes. With vinyl tile, tested in the machine direction,wetting lowered the values from 19.^ to 10. i+ {h6.k percent) with leather shoes, and from91.1 to lk.2 {8k. k percent) with rubber shoes. Wetting lowered the values for linoleumtested in the machine direction from 57.8 to 26.3 (5^-5 percent) with leather shoes and from117.2 to kl.3 {6k. 8 percent) with rubiber shoes. Thus, the precentage change of the BritishPortable Test Number due to wetting varies substantially depending on the shoe material.The lower friction values in the wet test may be due to hydroplanning of the slider as it

passes over the flooring material.

Comparisons between values obtained in the two directions (tables 9 and 12) machine andacross machine, show that there is little if any effect from the test direction for bothvinyl asbestos and vinyl tile tested with both leather and rubber shoes and tested in bothwet and dry conditions.

.1:

The results also indicate that values obtained using the British Portable Skid ResistanceTester yield reasonably good reproducibility.

The manufacturer's literature states that the British Portable Test measures dynamic(or kinetic) friction. It did this in all the tests except the dry rubber slider determina-

10

tions on the vinyl asbestos tile, in which "stick-slip" (jerky, instead of smooth motion)

occurred. Where "stick-slip" occurs, the friction is a mixture of static and dynamic.

6.3 Horizontal Pull Slipperiness Test

The data obtained with the Horizontal Pull Slipperiness Tester for vinyl asbestos tile,

vinyl tile and linoleim tested in the machine direction of the flooring are presented in

tables 13, ih and 15, respectively. Each of these tables includes five recorded values of

coefficient of friction using both leather and rubber shoes in wet and dry conditions for

each of the five samples tested and the average and standard deviation for all recorded

values for each type of flooring. Table l6 contains a summary of the data presented in

tables 13, 1^ and 15.

The total averages and standard deviations for the samples tested with leather shoes

were: O.hh + 0.02 for vinyl asbestos tile tested dry and 0.66 + 0.02 for vinyl asbestos tile

tested wet, 0.60 + 0.0k for vinyl tile tested dry and 0.60 +_ 0.05 for vinyl tile tested wet,

and 0.6h +_ 0.02 for linoleum tested dry and 0.88 + 0.02 for linoleiun tested wet. The values

obtained for the three types of flooring tested with rubber shoes were 0.82 +_ 0.03 for vinyl

asbestos tested dry and 0.8l + 0.02 for vinyl asbestos tested wet, 0.9I+ + 0.03 for vinyl

tile tested dry and O.96 + 0.02 for vinyl tile tested wet, and O.8O + 0.03 for linoleumtested dry and 0.87 i 0.02 for linoleum tested wet.

This test method indicates the frictional properties for two surfaces in contact as a

"Slip Index". The Slip Index is reported by the manufacturer to be about ten times the

coefficient of friction. Using this factor the data as reported in the tables were converted

to coefficients of friction.

The Horizontal Pull Slipmeter is capable of determining both static and dynamic friction.However, in these tests, it measured only one or the other type for each type of slidingmaterial used. For example, for all the tests, both dry and wet, involving the leathershoes, the slipmeter motion was jerky. This motion is called "stick-slip", and the type offriction is static. The slipmeter moved smoothly throughout the tests with the rubbershoes, both dry and wet, the friction being dynamic. Thus, the results in the tables for

leather shoes are termed static and those with rubber shoes are termed dynamic . If themotion was smooth throughout a test, including the start, the static and dynamic coefficientswoiild be equal. Significant jerk indicates that static and dynamic coefficients are notequal and that there is too much capability for the storage of elastic energy in the testdevice, probably in the string.

The coefficient of friction results from the Horizontal Pull Slipmeter from all wettests were equal to or higher than those from the dry tests except for the vinyl asbestostile tested with rubber shoes. This was in direct contrast to the results from the BritishPortable Test, and the James Test with a leather foot (no slippage occurred with the rubberfoot in the James Test). In these two tests the dry test results were higher than thosefrom the wet tests.

The data indicate that values obtained using the Horizontal Pull Slipmeter yield reason-ably good reproducibility. However, according to the manufacturer, the reliability ofcoefficients of friction above 0.8 is rather doubtful because interpolation is requiredabove that point.

7.0 Discussion of Results

Table 17 contains a summary of all test data obtained in this preliminary study.

The static coefficients of friction obtained with the James Static Friction Tester andthe Horizontal Pull Slipmeter using leather shoes and dry test conditions show reasonablygood agreement for all xhree types of flooring tested. However, the results of these twomethods using leather shoes and wet conditions show no agreement for any of the three typesof flooring.

11

The difference between the values in dry and wet tests show no consistent trend when

comparing the results obtained using the three test methods. For example, the dry and wetresults of vinyl asbestos tile using leather shoes were: 0.U2 +_ 0.02 and O.hl + 0.02 usingthe James Tester, 36.9 ± 2.1+ and 10.2 + O.k using the British Portable Tester and 0.1+1+ +

0.02 and 0.66 + 0.02 using the Horizontal Pull Slipmeter.

As mentioned in Section 6.2, and as is evident in table IT, the wet values obtainedwith the British Portable Tester were considerably lower than the dry values for both leatherand rubber shoes and for all three tj^-pes 'of flooring. The lower values in the wet tests maybe due to hydroplanning of the strider as it passes over the flooring material. The othertwo test methods did not show a consistent trend for the three types of flooring.

With the James Tester and leather shoes, the vinyl tile had the highest coefficient of

friction in the dry test. However, with the British Portable Tester and the Horizontal PullSlipmeter and leather shoes, the linoleum had the highest value in the dry test. In the wet

tests with leather shoes, all three methods showed linolem to have the highest value.

•

: • • ^''^ Discussion of Possible Equipment Improvements

We believe that the apparatus used in all three tests could be improved by redesign or

modification. This was especially true of the Horizontal Pull Slipmeter.

Our suggestions for improvements in the test apparatus are as follows:

James Tester

,. a) The recording pencil should be replaced by a pen with a much stronger release

spring. The lines drawn by the pencil are very faint towards the end of thecurves.

b) The springs of the chart clamps are weak, and easily bent. Unless the charts arefixed by other means, such as adhesive tape, they frequently come loose d\rring thetest. Thus, improved chart clamps are needed.

c) Lock nuts are needed for some of the bolts of the Tester. The impact of the 80 lb.

weight on the shoe falling at the end of the test gradually loosens some of thebolts of the Tester and frequent tightening is necessary.

d) The Tester needs leveling bolts, and indicators. Shims under the legs are the•,

; best way of leveling the Tester at present. This method of leveling the Tester is

unsatisfactory because the Tester is free to slide on them during the tests.

e) The Tester utilizes an 80 lb. (3.63 kg) weight and a relatively large shoe contactarea. Studies are needed to determine if the shoe contact area and the weightcould be reduced substantially. Reducing the weight of the Tester could make it

portable, and therefore, suitable for field studies.

f) The shock absorber currently used is a rubber biscuit. This is not sufficient toprevent shaking due to the large weight applied to the shoe. It is not practicable

- to increase the thickness of the biscuit because that would restrict even further

;

the slide path of the shoe. If an 80 lb. (36.3 kg) weight is necessary, a springshock absorber may be a better alternative. If the weight can be reduced, asmaller absorber may be an appropriate alternate.

British Portable Tester

A method for conversion of the British Portable Test Niombers to coefficients of frictionshould be developed. This may not be possible, since this machine measures line contact

12

between the shoe on the pendulum and the flooring surface and the pressure on the shoe

during the penduliom swing is not constant over the path of contact. It is also highly-

receptive to hydroplanning of the slider in wet surfaces. This test method, therefore, has

inherent weaknesses resulting from the basic design of the equipment which, we believe, may

make it less acceptable for standard methods than the other two studied.

Horizontal Pull Slipmeter

The Horizontal Pull Slipmeter, in our opinion, would be more accurate and versatile if

the following modifications -were made:

a) The pull cord presently used is made of cotton string. It is fairly elastic, and

during the test, can introduce variations in test results. Consideration should

be given to replacing the cotton string with a polymeric cord or wire so that

stretching can be minimized. The cord, however, must be very flexible.

b) The power unit has only one speed, which is very slow. If the speed could be

varied, the apparatus would be more versatile for frictional characteristics

studies.

c) The present recording meter should be replaced by one which is more accurate. For

example, the indicator occasionally was observed to slip from the maximum reading

to a lower reading. Also, the accuracy of the recorder, as indicated by the manu-facturer, is poor in the higher slip index range. The results above a slip index

of 8.0 (coefficient of friction of O.8O) must be obtained by extrapolation.

d) The Slipmeter was observed to "stick-slip" frequently during the tests. Design

changes are needed to minimize this effect.

9-0 Conclusions

1. The reproducibility of the James Static Friction Test fitted with the leather shoe wasvery good. The friction between the styrene butadiene rubber shoe and all resilientflooring samples both dry and wet was too high to be determined by the James Tester.

2. The reproducibility of the British Portable Test was good, and it was capable of eval-uating the frictional properties between styrene butadiene rubber and all three types ofresilient flooring, dry and wet.

3. The Horizontal Pull Slipmeter Test had good reproducibility for leather in contact withall three types of resilient flooring, both dry and wet. However, this test is notaccurate for measuring coefficients of friction higher than 0.8, which is the region inwhich rubber and resilient flooring friction falls.

k. There was some correlation between results from the James Test and the Horizontal PullSlipmeter Test as discussed in Section 7 5 hut there was little correlation between theresults from the British Portable Test and those of the other tests. However, this is

understandable because the tests do not necessarily measure the same frictional proper-ties, largely because of the differences in speed and inter-surface pressures at whichthe tests are conducted.

5. To obtain a fuller understanding of the significance of the results of these tests andthe capabilities of the apparatus used in performing them, a much larger range offlooring samples than that used in our preliminary study should be investigated. Awider range of sole and heel materials should also be used in these tests. This isdiscussed in the Project Proposal entitled "Safety Criteria for Slipperiness of Floor-ing", in the Appendix.

6. There is a need for additional studies to improve existing floor slipperiness testmethods or develop new methods to adequately reflect human perambulation factors.

13

7. From the work done in the preliminary study, it is obvious that with some design modi-fications, the apparatus used in the three test methods evaluated could be improved.This is particularly true for the Horizontal Pull Slipmeter and the James Tester.

''

' Acknowledgement

Acknowledgement is hereby given to Mr. Jack Lee and Mr. Edward Embree for their sub-stantial efforts in obtaining the test data presented in this report.

Acknowledgement is also given to Dr. Robert J. Brungraber, Intergovernmental Personnel.

Act appointee to the Building Safety Section, National Bureau of Standards, for his assistancein editing this report.

11+

Bibliography

1. CARLET, G., Ann. Sci. Nat ., Zool l6 , 1 (i8T2).

2. BORELLI, G.A., De Motu antimatum (l679).

3. MAREY, E.J., "Animal Mechanism", New York (i8T^ ) ; "La Methode Graphique dans les sciences

experiment ales Paris" (1885).

h. FISCHER, 0., Der Gang des Menschen, Abh. K. Sachs. Ges. (Akad.), Wiss., I898.

5. "F\indamental Studies of Human Locomotion and other Information Relating to the Design

of Artificial Limbs", Report to U.S. National Research Council, University of California,

19hl.

6. HUNTER, R.B., "A Method for Measuring Frictional Coefficients of Walkway Materials",

Journal of Research, National B\ireau of Standards, Vol. 5, p. 329, 1930.

7. SIGLER, P. A., "Relative Slipperiness of Floor and Deck Surfaces", Building Materials

and Struct\ires Report, National Bureau of Standards, July 19^3, BMS 100.

8. SIGLER, P. A., GEIB, M.N. , BOONE, T.H. , "Measurement of the Slipperiness of Walkway

Surfaces", Journal of Research, National Bureau of STandards , Vol. kO , p. 339, 19^8.

9. "Electronic Slipmeter Reveals Mechanics of Walking", NBS Technical News Bulletin, Vol.

35, No. k, April 1951.

10. BOONE, T.H., AULD, C.W., Progress Report I, "Standard Measurements of Slipperiness on

Walkway and Roadway Surfaces", NBS Report 7510, National Bureau of Standards, Washington,

D.C. , 1962.

11. HARPER, F.C. et al.. Research Paper 32, "The Forces Applied to the Floor by the Foot in

Walking", National Building Studies, London, England, I96I. "

12. "Standard Method of Test for Static Coefficient of Friction of Polish Coated Floor Sur-

faces as Measured by the James Machine", ASTM D201+7 , American Society for Testing and

Materials, Philadelphia, Pa.

13. EKKEBUS, C.F., and KELLEY, W., "Validity of 0.5 Static Coefficient of Friction (James

Machine) as a Measure of Safe Walking Surfaces", Candy and Co., Inc., 1971, Revised,Jan. 20, 1972.

ik. "Causes and Measurement of Walkway Slipperiness, Present Status and Future Needs",National Academy of Sciences, National Research Council, Federal Construction Council,Technical Report No. U3, Publication 899, Washington, D.C. 20Ul8, 1961.

15. "Standard Method of Test for Measuring Surface Frictional Properties Using the BritishPortable Tester", ASTM E 303-69, American Society for Testing and Materials, Philadel-phia, Pa.

16. "Standard Method of Test for Abrasion Resistance of Soles and Heels", ASTM D-I63O,American Society for Testing and Materials, Philadelphia, Pa.

15

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Table 17

COMPARISON OF FRICTIONAL PROPERTIESOF RESILIENT FLOORING*

Test Method Type of Flooring Tested

Vinyl Asbestos Vinyl Tile Linoleum

(Average and standard deviations from 5

samples of each type of flooring.)

James Static FrictionTest

Leather

Dry

Wet

Static Coefficient of Friction

0.42 ± 0.02

0.41 ± 0.02

0.60 ± 0.04

0.35 ± 0.02

0.57 ± 0.02

0.45 ± 0.02

SB Rubber

Dry . No Slip No Slip No Slip

Wet 1 No Slip No Slip No Slip

British Portable Skid1

Resistance Test British Portable Test Number(B.P.T. Number)

Leather j

i

Dry 36.9 ± 2.4 19.4 ± 2.0 1

j

57.8 ± 4.9

Wet 10.2 ± 0.4 10.4 ± 0.51

26.3 ± 3.1

SB Rubber!

1

j

Dry 101.4 ± 5.2 91.1 ± 1.9 i

i

117,2 ± 3.5j

Wet 24.3 ± 1.3 14.2 ± 0.41

i

i.

41.3 ± 2.31

1

Horizontal PullSlipmeter

i

Leather (Static coefficient of friction)=^

1

DryI

0.44 ± 0.02

WetI

0.66 ± 0.02

Rubber (Dynamic coefficient (if friction)

Dry ! 0.82 ± 0.03

Wet 0.81 ± 0.02

0.60 ± 0.04

0.60 ± 0.05

0.94 ± 0.03

0.96 ± 0.02

0.64 ± 0.02

0.88 ± 0.02

0.80 ± 0.03

0.87 ± 0.02

*Slip in machine direction.

USCOMM-NBS-DC

BlBLICr Af--,:iC (1.VTA

s^irr r

1 I '1 I ^ 1 \\ VI 1 f > \ ilk' 1 1 1

*M 1 ;'

} V ( >t( I V I'l i" .ill''.* \.'\\ 1^1. 1 v'I\i .Nv'.

?;bs "(r

2. t f 1 1 V t A . ^ <. > 1 1 ifi

No.3. Kr; ;ii',nt's Ai. i. c s ;

,

1 1 1 1 . 1. A . P n I 1 1 1. !

.

Prelir.inary Study of the Slipperaness of Flooring

5. 1 'uhl U' .It i.xi D.itr-

6. i^ iiuftninp Of .in 1 /. .1 1 h . n t>;-

7. Ai; l)\0\<'>)

A. PhiliD Crr.rr; - Larrv V.'. ^riSters

8. Pc-rfornuim Oni.in. i-U :: .No.

9. FliKKOKMiNt-i OKc-AM/.A TiON NAMi-. AMI AHOKl-SS

HATIONAL BUREAU OF STAfJDARDS. -

'

^, DEPARTMENT OH COMMERCEWASHINGTON, D.C. 20234

10. Pru)d 1 '-'orl; ffi;; N.-.

A615200 - 461 .0311. (-oniract C>rant .\o.

12. Sponsoring Or^;,!;! ization Name acJ Complete AJJrcss (SCreet, City, Stutc, ZIP)

National Bureau of Standards. Departi^icnt of Conmerce

Washington, D. C. 20234

13. Type of Report & PeriodCovered

Tntprii'-. - .Tiilv IP?/;14. Sponsoring Agency C.'.ijc

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.)

The National Coinnission on Product Safety reported in 1970, that falls in thehome each year kill about 12,000 and injure 6,000,000 in the U.S.A. Slipperyfloors are listed as a large contributor to these very high casualty figures.Although there are sone standardi2:ed test r.othods that arc or right be suitablefor such standards, there are no slipperiness standards for flooring. Thus,there is an inniediate need for studies aiir.ed at the ^'jEvelop-icnt and establishmentof such standards. Consequently, a preliminary study of floor slipperinesswas sponsored by the Building Safety Section of the Center for Building Technology.The study included a state-of-the-art investigation on flooring slipperinessresearch and a laboratory evaluation of three existing test methods for measuringfloor slipperiness. Samples of the three most corr^only used resilient flooringmaterials, namely: vinyl asbestos, vinyl and linoleum vere used in the study.The sliding m.aterial components for the frictional tests were leather and acoiriEionly used styrene butadiene sole and heel rubber. The tests were performedboth dry and wet. The results from this study were used in planning a largecomprehensive study, which would lead to the development of accepted floor slipperinessstandards. This report contains the results of the preliminary study.

Floor S

17. KEY UORDS (si\ to tuelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper

name; separated by semicolons)

Floor Slipperiness; resilient flooring; slipperiness standards; frictional tests;slip tests; coefficient of friction; human peri.^^bulation.

18. AVAILABILITY Unlimited

( !For Official Distribution. Do Not Release to NTIS

1 1Order From Sup. of Doc, U.S. Government Printini; OfficeVlashiRKton. D.C. 20 402. SD Cat. N.>. < \ ^

Q"^ Order F rom .National T cclinical Inlorri at loii "Service (NTIS)Sprinf;licld, \ irtjiiua ^Jl^l

19. Sl-CURITY CLASS(THIS REPORT)

UNCLASSIFIED

21. NO. OF PAGES

20. SK( LRU Y CLASS(IHIS PAGE)

UNCLAS-^ii ii:d

22. Price