Friction in Temporary Works - University of Birmingham 2003

HSE Health & Safety

Executive

Friction in temporary works

Prepared by the University of Birmingham

for the Health and Safety Executive 2003

RESEARCH REPORT 071

HSE Health & Safety

Executive

Friction in temporary works

Dr N J S Gorst, Dr S J Williamson, Eur Ing P F Pallett and Professor L A Clark

School of Engineering The University of Birmingham

Edgbaston Birmingham

B15 2TT United Kingdom

During initial assembly, temporary works often rely upon friction to provide lateral stability. Frictional resistance is also utilised in temporary works design as a means of transferring horizontal forces through falsework or formwork to points of restraint.

The results are presented of an investigation to verify existing claimed values of static coefficient of friction and to establish practical values of the coefficient for the latest commonly used materials in temporary works. Friction tests were undertaken on 260 combinations of different material faces used in temporary works, including both "dry" and saturated timber. The tests generated data for combinations for which no codified data exist and also generated data which could be compared with existing British and German codified data.

For material combinations for which codified data exist, the friction values obtained in the current research tended to lie between the maximum and minimum bound code values, but closer to the minimum values. Recommendations are made for code friction values for all material combinations. It is considered that further research is required to investigate the variation in some measured friction values.

This report and the work it describes were funded by the Health and Safety Executive. Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

HSE BOOKS

© Crown copyright 2003

First published 2003

ISBN 0 7176 2613 X

All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmitted inany form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.

Applications for reproduction should be made in writing to: Licensing Division, Her Majesty's Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ or by e-mail to [email protected]

ii

CONTENTS

CONTENTS ...............................................................................................................iii

1. INTRODUCTION........................................................................................................ 1

2. THEORY AND CURRENT INFORMATION .....................................................3

3. EXPERIMENTAL PROCEDURES .....................................................................7

4. RESULTS.............................................................................................................13

5. COMPARISON OF RESULTS WITH CURRENT INFORMATION..................19

6. CONCLUSIONS...................................................................................................21

7. RECOMMENDATIONS ......................................................................................23

8. ACKNOWLEDGEMENTS ....................................................................................... 25

9. REFERENCES ........................................................................................................... 27

ABBREVIATIONS ............................................................................................................... 29

APPENDIX A Friction Test Data ............................................................................ 31

APPENDIX B Saturation Test Data ........................................................................ 53

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iv

SUMMARY

Friction tests were undertaken on 260 combinations of different material faces used in temporary works, including both "dry" and saturated timber. The tests generated data for combinations for which no codified data exist and also generated data which could be compared with existing British and German codified data.

For material combinations for which codified data exist, the friction values obtained in the current research tended to lie between the maximum and minimum bound code values, but closer to the minimum values.

Recommendations are made for code friction values for all material combinations.

It is considered that further research is required to investigate the variation in some measured friction values.

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vi

1 INTRODUCTION Temporary works of falsework and soffit formwork include arrangements of multiple levels of bearers, beams and grillages. Often these members are seated on each other with little or no positive connection. Lateral stability is an important consideration in all temporary works structures and, during the initial assembly, temporary works often rely upon friction to provide such stability.

Frictional resistance is often used in temporary works design calculations as the means of transferring horizontal forces through the structure to points of suitable restraint.

This project was carried out as a result of a recommendation from The Health and Safety Executive (HSE) report "Falsework Design – Comparative Calculations" (Ref 1) which required that confidence be established in the existing proposed values for friction. The aim of the work was to verify existing claimed values of static coefficient of friction and to establish practical values of the coefficient for the latest commonly used materials in temporary works. The main experimental work was completed in December 1999. Following comments from industry it was decided to extend the experimental work to include a second phase, which would investigate friction on wet timber.

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2

2 THEORY AND CURRENT INFORMATION When two items are placed one on top of the other and are not in motion there is a certain value of lateral force which can be resisted across the interface. In theory this force is a constant ratio of the applied load, is dependent on the materials in contact and is independent of the contact area; the ratio is known as the coefficient of static friction. The coefficient of static friction is given by the expression (see Figure 1):

Ff W sinq m = = = tanq [1]

R W cos q

where: R is the reaction force normal to the surface (N) Ff is the limiting value of the frictional force (N) W is the vertically applied force (N) q is the minimum angle from the horizontal, for a particular pair of materials at which sliding will commence

W Ff

q

P R

Figure 1 Restraint provided by friction

In practice it has been found that measured values of the coefficient can vary widely. It has been suggested that the coefficient is in fact a function of the load and that it may be affected by the location of the member, i.e. whether it is the upper load bearing member or the lower load receiving member. Values of coefficient of static friction recommended in Table 19 of BS 5975:1996 (Ref 2), Table 1, indicate that the coefficient is independent of member location.

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Table 1: Minimum value of coefficient of static friction, BS 5975:1996 (Ref 2)

Lower load-accepting Upper load-accepting member member

Plain steel Painted steel Concrete Softwood Hardwood timber

Plain steel 0.15 0.1 0.1 0.2 0.1

Painted steel 0.1 0.0 0.0 0.2 0.0

Concrete 0.1 0.0 0.4 0.4 0.3

Softwood timber 0.2 0.2 0.4 0.4 0.3

Granular soil 0.3 0.3 0.4 0.3 0.3

Hardwood 0.1 0.0 0.3 0.3 0.1

The current UK Code of Practice, BS 5975:1996 (Ref 2) on falsework gives values for guidance on friction for a few materials only and friction values have remained unaltered since its first publication in 1982. It has not been possible to find the origin of these values.

In April 1997 the European Draft, prEN 12812 (Ref 3) on performance and general design of falsework was published for comment. Many of the diagrams and content are copied from the original Table 7 in German standard DIN 4421 (Ref 4). The German standard quotes minimum and maximum values of coefficient of static friction: these are reported in Table 2. It is understood that the DIN 4421 values were from research by Professor Mohler, at Karslruhe University. Comparison of the English and German data shows that the British values agree quite well with the German minimum values.

4

Table 2: Friction coefficients, m, from German Standard DIN 4421 (Ref 4) and prEN 12812 (Ref 3)

Building material combination Friction coefficient, m

Maximum Minimum

1 wood / wood (rubbing surfaces parallel to grain or at right 1.0 0.4 angles to grain)

2 wood / wood (one or both rubbing surfaces at right angles to 1.0 0.6 grain (cross cut) or end grain)

3 wood / steel 1.2 0.5

4 wood / concrete 1.0 0.8 wood / mortar bed

5 steel / steel 0.8 0.2

6 steel / concrete 0.4 0.3

7 steel / mortar bed 1.0 0.5

8 concrete / concrete 1.0 0.5

The committee drafting the European Standard (CEN/TC53/WG6, Falsework) expects to have published a European standard shortly, which will see the withdrawal in the UK of BS 5975 and any of the conflicting information, such as the table on friction coefficients.

The future design of falsework will almost certainly require a specific calculation for positional stability, and a detailed check for sideways restraint using friction will be a requirement for all falsework calculations.

If reliable friction values do not exist, and fixings between members are specified, they will involve both man-hours for assembly and dismantling, and the use of expendable items such as bolts or nails. Of greater concern is the likely reduction in quality and/or re-use potential of the equipment. Items with drilled holes for bolted connections will reduce their load carrying capacity, and, in the case of extensive nailing, may be reduced to scrap. The use of more accurate friction values will lead to a reduction in the number of positive connections required and hence reduced erection and dismantling times, lower labour costs and extended life of temporary works items.

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3 EXPERIMENTAL PROCEDURES The originally specified test combinations are presented in Table 3. It was agreed with the HSE that the tests with fresh concrete as one member would not be carried out, simply because of the extra variability in results which would be introduced by factors such as mix type, age and test method.

It was originally envisaged that each material combination would be tested three times at three load levels (0kg, 25kg, 50kg) and with members in both the upper and lower position. Once testing was underway, however, it appeared that three load levels and alternating positions were not necessary. This allowed the original programme to be reduced and permitted tests of extra combinations to be undertaken; the extent of this testing is indicated in Table 4.

Coefficient of friction was measured by placing the two materials on a tilting table, illustrated in Figure 2. The table was raised manually by winding the handle, which operated a jack situated below the table. In order to avoid inconsistencies caused by change of operator a constant winding speed of approximately 60 rpm (equivalent to about 34o per minute) was agreed after preliminary testing. The table was raised until the point where slip occurred was reached and the angle at slip was recorded. A list of the materials used and their sources can be found in Table 5.

Figure 2 Test rig

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Table 3: Coefficients of static friction (m) for originally specified test combinations

Lower load-accepting member Upper load-bearing member

Plain steel Galv. Steel Painted steel Aluminium Wet concrete Hard concrete Softwood Hardwood Prop. Timber Plywood Max Min Max Min Max Min Max Min Max Min Max Min Max Min Max Min Max Min Max Min

Plain steel 0.8 0.15 tba tba tba 0.1 tba tba 0.4 0.1 tba tba 1.2 0.2 tba 0.1 tba tba tba tba

Galvanised steel

Painted or oiled steel tba 0.1 tba tba 0.0 0.0 tba tba tba 0.0 tba tba tba tba tba 0.0 tba tba tba tba

Aluminium tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba

Softwood timber – rubbing surface 1.2 0.2 tba tba 1.2 0.2 tba tba 1.0 0.4 tba tba 1.0 0.4 tba 0.3 tba tba tba tba parallel to the grain

Softwood timber – rubbing surface 1.2 0.2 tba tba 1.2 0.2 tba tba 1.0 0.4 tba tba 1.0 0.4 tba 0.3 tba tba tba tba right angle to the grain or on end grain

Hardwood timber – rubbing surface tba 0.1 tba tba tba 0.0 tba tba 1.0 0.3 tba tba tba 0.3 tba 0.1 tba tba tba tba parallel to the grain

Hardwood timber – rubbing surface tba 0.1 tba tba tba 0.0 tba tba 1.0 0.3 tba tba tba 0.3 tba 0.1 tba tba tba tba right angle to the grain or on end grain

Proprietary timber tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba

Plywood tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba tba

Note: Values of coefficients taken from BS 5975 Table 19 (Ref 2) and prEN 12812 Table 7 (Ref 3) Where not known, i.e. to be determined in current study, shown as tba

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Table 4a: Material combinations tested

Lower Load-Accepting Upper Load-Bearing Member Member Steel Alum. Timber Plywood

Softwood Hardwood Prop. beam Good one

side Combi ply

faced

Film faced

Finnish

Film faced

quality

Plain Plain Galv. Prop. Prop. Dry Wet Dry Wet unrusted rusted painted waling

Par. Perp. Par. Perp. Par. Perp. Par. Perp. Reused New Dry Wet

Steel Plain unrusted · ¨ · · · · x · x + · · + · · + ¨

Plain rusted ¨ ¨ + + ¨

Galvanised · ¨ · · · · · · · · · ¨ Prop. painted · ¨ · · · · · + · · + · · + ¨

Alum. Prop. waling · ¨ · · · · · + · · + · · ¨

Timber Dry softwood · · · · · x · x · · · · ¨ Wet softwood + + + + Dry hardwood · ¨ · · · · · · · · · ¨ Wet hardwood + + Prop. beam

¨ – reused

Prop. beam – new · ¨ · · · · x · x · · · · ¨

· Tests required by original programme ¨ Additional tests required by modified programme

Tests repeated with ‘planed all round’ softwood + Additional test with saturated timber

9

X

X

Table 4b: Material combinations tested


Softwood Hardwood Proprietary beam

Good one side Combi

ply faced

Film faced

Finnish

Film faced

quality



Plywood dry good one side · ¨ · · · · x · x · · · · ¨ wet good one side + + Combi ply faced ¨ ¨ ¨ ¨ ¨ ¨ Film faced

¨ ¨ ¨ ¨ ¨ ¨ Finnish Film faced

¨ ¨ ¨ ¨ ¨ ¨ ¨ ¨ ¨ ¨ ¨ ¨ quality used film

¨ ¨ ¨ ¨ ¨ ¨ ¨ ¨ ¨ face Hardened Concrete

trowelled face · ¨ · · · · · · · ¨ · ·

cast face ¨ ¨ + ¨ ¨ ¨ + ¨ ¨ ¨

· Tests required by original programme ¨ Additional tests required by modified programme

Tests repeated with ‘planed all round’ softwood + Additional test with saturated timber

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----

--

--

--

--

--

Table 5: Materials used in the test programme

Material Trade Name Source

Plain unrusted steel University of Birmingham Plain rusted steel University of Birmingham Proprietary painted steel Multijoist RMD – Kwikform Ltd Galvanised steel University of Birmingham Aluminium Alform RMD – Kwikform Ltd Softwood – rough cut University of Birmingham Softwood – planed all round University of Birmingham Hardwood University of Birmingham Proprietary timber GT24 PERI Ltd Plywood – good one side University of Birmingham Plywood Beto film, Wisaform, Kymmene Schauman

Wisaform special Concrete University of Birmingham

On completion of the main test programme and consideration of the data with the HSE, it was deemed pertinent to carry out two further test programmes to investigate the effect of member position and the effects of time and/or bedding effects on friction values. These two additional test programmes utilised two material pairs: plain unrusted steel/ proprietary painted steel and aluminium/plywood.

The effect of time on coefficient of friction was investigated by performing a zero load test, leaving the test set-up for two days then repeating the test. The effect of bedding on coefficient of friction was investigated by performing a loaded test, leaving the test set-up for two days then repeating the test.

Following comments from industry it was decided to extend the experimental work to include a further phase, which would investigate friction on wet timber. In order to produce saturated timber specimens, timber test specimens were stored underwater and the level of surface saturation was monitored over time by taking three measurements of surface moisture content using a commercial moisture meter. A timber specimen was deemed to be saturated, and thus ready for friction testing, when the measurements of surface moisture content remained approximately constant over time. The saturated timber samples were stored in water between individual friction tests and were only removed from the water immediately before the start of a test.

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4 RESULTS

The main body of results is presented in Table 6a and Table 6b on pages 14 and 15. Each piece of data from the current study included in this table is the average of three tests; the raw data can be found in Appendix A. The second line of values present in some cells of Table 6 are coefficients measured using a planed softwood as one member rather than the rougher softwood used in other tests. The rough softwood is more representative of that found on site.

The raw data for the saturation phase of the experimental work is contained in Appendix B. It is emphasised that the tabulated data have not been corrected for timber species and are intended simply to demonstrate that "saturation" had been achieved.

It was observed that the value of coefficient of friction was generally independent of member position (upper or lower). Hence, tests on pairs of materials were not repeated with each member in both the upper and lower position. On examination of the final results, however, it appears that, of the thirty-six pairs which were tested with each member in both upper and lower positions, eight are affected by location. The affected pairs are presented in Table 7.

Table 7: Combinations where coefficient of friction was affected by member position

Member 1 Member 2 Member 1 – Upper Member 2 – Upper Member 2 - Lower Member 1 - Lower

Max Min Max Min

Plain unrusted steel Prop. painted steel 0.4 0.3 0.6 0.5

Plain rusted steel Galvanised steel 0.6 0.4 0.4 0.3

Plain unrusted steel Softwood 0.4 0.3 0.6 0.5

Prop. painted steel Hardwood 0.7 0.5 0.5 0.4

Plain rusted steel Plywood 0.6 0.4 0.4 0.3

Aluminium Prop. timber (new) 0.4 0.2 0.5 0.5

Aluminium Plywood 0.5 0.3 0.3 0.2

In order to investigate this behaviour, the further tests given in Table 8 were carried out. These measured the friction values for pairs of (a) plain unrusted steel and proprietary painted steel and (b) aluminium and plywood. In both cases the pair were tested on both faces and in both upper and lower position. The apparent location dependence of the friction value was not manifested in the further test data, although slightly different values were obtained depending on which face was used. The results of the further tests suggest that the initial variations may either have been simply due to the natural scatter in friction values, or that different specimen faces with slightly different surface qualities were used when members were in the upper and lower positions, or a combination of both these factors. Hence, any future test programme with more replicate specimens would improve the reliability of the friction values obtained.

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-- -- -- -- --

-- -- -- -- -- -- -- -- -- -- -- -- -- -- --

-- -- -- -- -- -- -- --

-- -- -- -- --

-- -- -- -- -- --

-- -- -- -- -- -- -- -- --

-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

-- -- -- -- -- -- -- --

-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

-- -- -- -- -- -- -- --

Table 6a – Coefficients of static function obtained from current study



side Combi ply

faced

Film faced

Finnish

Film faced

quality



Steel Plain unrusted

0.4 0.3

0.5 0.4

0.3 0.3

0.6 0.5

0.4 0.3

0.6 0.5

(0.5)

0.4 0.4

(0.3)

0.7 0.7

0.5 0.5

0.5 0.5

0.7 0.6

0.5 0.5

0.4 0.3

0.6 0.5

0.2 0.1

Plain rusted 0.5 0.4

0.4 0.3

0.8 0.8

0.8 0.8

0.4 0.3

Galvanised 0.4 0.3

0.6 0.4

0.3 0.2

0.5 0.5

0.4 0.2

0.5 0.4

0.5 0.5

0.5 0.5

0.5 0.5

0.4 0.4

0.2 0.2

0.1 0.1

Prop. 0.4 0.7 0.4 0.8 0.4 0.5 0.7 0.8 0.5 0.6 0.9 0.6 0.5 0.7 0.3 painted 0.3 0.6 0.4 0.7 0.4 0.4 0.4 0.7 0.4 0.5 0.9 0.5 0.5 0.7 0.2

Alum. Prop. 0.3 0.5 0.4 0.5 0.4 0.4 0.5 0.6 0.5 0.5 0.7 0.5 0.3 0.2 waling 0.2 0.3 0.2 0.4 0.2 0.4 0.4 0.6 0.4 0.3 0.6 0.5 0.2 0.1

Timber Dry softwood

0.4 0.3

0.5 0.4

0.5 0.5

0.4 0.4

0.7 0.6

(0.5)

0.6 0.5

(0.3)

0.5 0.4

0.5 0.4

0.6 0.5

0.3 0.3

0.2 0.2

Wet 1.1 0.9 0.8 1.0 softwood 0.9 0.9 0.8 0.7 Dry 0.5 0.6 0.5 0.7 0.4 0.5 0.4 0.5 0.5 0.5 0.3 0.2 hardwood 0.4 0.6 0.5 0.5 0.4 0.5 0.4 0.4 0.5 0.4 0.3 0.2 Wet 0.8 0.9 hardwood 0.8 0.8 Prop. beam 0.6 - reused 0.6

Prop. beam - new

0.6 0.5

0.5 0.4

0.5 0.4

0.6 0.5

0.4 0.2

0.5 0.4

(0.5)

0.4 0.3

(0.4)

0.4 0.4

0.4 0.4

0.5 0.5

0.3 0.3

0.2 0.2

KEY: 0.6 Maximum value 0.5 Minimum value

(0.6) Values for softwood planed all round

14

-- -- -- -- -- -- -- --

-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

-- -- -- -- -- -- -- -- -- -- -- -- -- --

-- -- -- -- -- -- -- -- -- -- -- -- -- --

-- -- -- -- -- -- -- --

-- -- -- -- -- -- -- -- -- -- --

-- -- -- -- -- -- -- --

-- -- -- -- -- -- -- -- -- --

Table 6b – Coefficients of static function obtained from current study



side Combi ply

faced

Film faced

Finnish

Film faced

quality



Plywood Dry good one side

0.4 0.3

0.6 0.4

0.2 0.2

0.4 0.4

0.5 0.3

0.3 0.2

(0.4)

0.4 0.3

(0.2)

0.3 0.3

0.4 0.3

0.4 0.3

0.5 0.3

0.3 0.2

Wet good 0.9 0.8 one side 0.8 0.7 Combi ply 0.2 0.2 0.3 0.3 0.2 0.3 faced 0.2 0.2 0.2 0.2 0.2 0.2 Film faced 0.2 0.2 0.4 0.3 0.4 0.3 Finnish 0.2 0.2 0.2 0.3 0.2 0.2 Film faced 0.1 0.2 0.2 0.3 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.2 quality 0.1 0.2 0.1 0.1 0.1 0.2 0.1 0.2 0.2 0.1 0.2 0.2 Used film 0.6 0.3 0.4 0.3 0.5 0.3 0.4 0.4 0.3 face 0.4 0.3 0.3 0.3 0.5 0.3 0.4 0.4 0.3

Hardened Trowelled 0.6 0.7 0.3 0.7 0.6 1.1 0.8 0.7 0.8 0.8 0.7 0.4 Concrete face 0.5 0.7 0.2 0.6 0.4 1.0 0.7 0.7 0.6 0.8 0.6 0.3

Cast face 0.8 0.8

0.7 0.7

0.9 0.8

0.6 0.5

0.7 0.7

0.4 0.3

0.7 0.6

0.3 0.3

0.3 0.3

0.2 0.2

KEY: 0.6 Maximum value 0.5 Minimum value

(0.6) Values for softwood planed all round

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Table 8 Investigation of effect of member position (upper/lower)

Lower Member Upper Member Coefficient of static friction m

0 kg 25 kg Plain unrusted steel (face 1) Prop. painted steel (face 1) 0.6 0.6 Prop. painted steel (face 1) Plain unrusted steel (face 1) 0.7 0.6 Plain unrusted steel (face 2) Prop. painted steel (face 2) 0.5 0.5 Prop. painted steel (face 2) Plain unrusted steel (face 2) 0.7 0.6

Aluminium (face 1) Plywood (face 1) 0.3 0.3 Plywood (face 1) Aluminium (face 1) 0.3 0.3

Aluminium (face 2) Plywood (face 2) 0.4 0.4 Plywood (face 2) Aluminium (face 2) 0.3 0.3

Prior to the commencement of testing it was anticipated that if loading had any effect on the value of coefficient of friction it would be to cause an increase, and this was in fact generally found to be the case. In certain cases, however, the measured friction value actually reduced with an increase in load. To check whether the reduction in friction was due to surface changes, such as polishing, the zero load test was repeated each time this occurred. Member combinations affected by this behaviour and the corresponding results are summarised in Table 9.

Table 9 Tests where measured coefficient of friction reduced with increasing load

Upper Member Lower Member Coefficient of static Comment friction, m

0 kg 25 kg 0 kg Aluminium Plain rusted steel 0.6 0.5 0.5 Returned to higher value on re

testing at zero load Plywood (used Plain rusted steel 0.6 0.4 0.4 Reduced from initial value to loaded

film faced) value on re-testing at zero load Plain rusted steel Galvanised steel 0.4 0.3 0.3 Reduced from initial value to loaded

value on re-testing at zero load Aluminium Galvanised steel 0.4 0.2 0.3 Increased but did not reach initial

value on re-testing at zero load Galvanised steel Aluminium 0.4 0.2 0.3 Increased but did not reach initial

value on re-testing at zero load Plywood (combi Aluminium 0.3 0.2 0.3 Returned to higher value on re

ply faced) testing at zero load Plywood (film Softwood perp. 0.4 0.2 0.2 Reduced from initial value to loaded faced Finnish) value on re-testing at zero load

Aluminium Hardwood par. 0.5 0.4 0.5 Returned to higher value on retesting at zero load

Aluminium Hardwood perp. 0.5 0.3 0.4 Increased but did not reach initial value on re-testing at zero load

Prop. painted Prop. timber 0.6 0.5 0.6 Reduced from initial value to loaded steel (new) value on re-testing at zero load

Plywood (good Plywood (film 0.3 0.2 0.3 Returned almost to initial values on one side) faced quality) re-testing

16

One possible explanation for this behaviour is the existence of a cohesive element of sliding resistance, which is only perceptible between certain member combinations. The limiting frictional force would then be expressed as:

Ff = c + mR [2]

where: R is the reaction force normal to the surface (N) c is the cohesive reaction force (N) Ff is the limiting value of the frictional force (N)

This relationship is illustrated in Figure 3. It is clear from the figure that if the behaviour is as represented in equation 2, but is assumed to be as represented in equation 1, then an increase in reaction force from say point A to point B on Figure 3 will result in an apparent reduction in the friction angle from q2 to q3, whereas the true friction angle remains constant at q1.

mRFf

c

Ff

Ff = mRq1

B A

R

q2 q3

= c +

tan q1 = true friction coefficient

tan q2, tan q3 = apparent friction coefficients calculated using F = mR

Figure 3 Friction behaviour with cohesive component

The effect of time on coefficient of friction was investigated by performing a zero load test, leaving the test set-up for two days then repeating the test. From the data, presented in Table 10, it appears that friction values are not affected by time.

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Table 10 Investigation of bedding/time effects

Lower Member Upper Member Coefficient of static friction, m

0 kg 25 kg

Plain unrusted steel (face 1) Prop. painted steel (face 1) 0.6 0.7

Plain unrustedsteel (face 1) Repeated after 48 hrs

Prop. painted steel (face 1) Repeated after 48 hrs 0.6 0.7

Aluminium (face 1) Plywood 0.3 0.3

Aluminium (face 1 Repeated after 48 hrs

Plywood (face 1) Repeated after 48 hrs 0.3 0.3

For material combinations for which experimental data exist for dry timber, the friction values obtained in the current research for saturated timber exceeded the corresponding values for dry timber. One possible explanation for the increase in frictional resistance is that the surface roughness of saturated wood is greater than that of "dry" wood and that this hypothesised increase in surface roughness outweighs the lubricating effect of surface moisture.

For material combinations for which codified data exists, the experimental values obtained in this research for saturated timber lie between the maximum and minimum values quoted in the codes, with one exception. In the case of wet softwood lying parallel to wet softwood the maximum experimental value in this study exceeded the maximum value of the coefficient of static friction quoted in prEN 12812 (Ref 3).

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5 COMPARISON OF RESULTS WITH CURRENT INFORMATION

The existing data are presented in Tables 1 and 2, and the data from the current study in Table 6a and Table 6b. Where both British and German data already exist, the data from the current study (Table 6a and Table 6b) lie between the existing maximum and minimum values (Tables 1 and 2) and are closer to the British values with one exception. In the case of wet softwood lying parallel to wet softwood the maximum experimental value in this study exceeded the maximum value of the coefficient of static friction quoted in prEN 12812 (Ref 3) for dry timber. The only cases where there are large discrepancies between data are where the existing British values appear rather low and correspond to friction angles of zero or around five degrees; see for example the data for the hardwood/plain steel combination. Absolute agreement with either set of existing data would not be expected as the coefficient of static friction is an inherently variable quantity and susceptible to variation in test method and the surface quality of material used in the test. Unfortunately, it has not been possible either to determine the quality of the surface finishes of the materials used to obtain the data reported in the British and German Standards, or to locate details of the test methods.

The observed level of agreement between existing data and that from the current study implies that the friction values for previously untested material combinations, presented in this report, can be used with confidence in temporary works calculations. A summary of the friction values recommended for use as a result of this investigation is presented in Table 11. The values in bold italics are minimum values of the coefficient of static friction contained in BS 5975: 1996 (Ref 2).

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-- --

-- -- -- -- -- -- -- --

-- --

-- -- -- --

-- -- --

--

--

-- -- --

-- --

-- -- -- -- --

-- -- --

-- -- -- -- -- -- -- --

-- -- -- -- -- -- -- --

-- --

-- --

-- -- --

Table 11 – Recommended Friction values

SURFACE 1

Steel Alum. Timber Plywood Concrete

SURFACE 2 Plain Unrusted

Plain rusted

Galv. Prop. painted

Prop. waling

Soft wood

Parallel Perp

Hard wood

Parallel Perp Proprietary

beam Good one side

Combi ply faced

Film faced Finnish

Film faced quality

Cast face

Plain unrusted 0.3 0.4 0.3 0.3 0.2 0.3 0.4 0.4 0.5 0.5 0.3 0.1 0.1

Plain rusted 0.4 0.4 0.3 0.6 0.3 0.6 0.4 0.3 0.2 0.2 0.2 Steel

Galvanised 0.3 0.3 0.2 0.4 0.2 0.4 0.5 0.5 0.5 0.4 0.2 0.1

Proprietary painted 0.3 0.6 0.4 0.7 0.4 0.4 0.4 0.4 0.5 0.5 0.4 0.2 0.2 0.1 0.0

Aluminium Proprietary waling 0.2 0.3 0.2 0.4 0.2 0.4 0.4 0.4 0.3 0.2 0.2 0.2 0.2 0.1

Parallel 0.3 0.4 0.4 0.4 0.6 0.5 0.4 0.4 0.4 0.2 0.2 0.3 0.2 0.8 Softwood

Perpendicular 0.4 0.5 0.4 0.4 0.5 0.4 0.3 0.3 0.2 0.2 0.1 0.7

Timber Hardwood Parallel 0.4 0.6 0.5 0.4 0.4 0.4 0.4 0.4 0.5 0.4 0.3 0.2 0.5

Perpendicular 0.5 0.5 0.5 0.3 0.4 0.5 0.4 0.3 0.2 0.7

Proprietary beam 0.5 0.4 0.4 0.5 0.2 0.4 0.3 0.4 0.4 0.5 0.3 0.1

Good one side 0.3 0.3 0.2 0.4 0.2 0.2 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.3

Combi ply faced 0.2 0.2 0.2 0.2 0.2 0.2 0.3 Plywood

Film faced Finnish 0.2 0.2 0.2 0.3 0.2 0.2 0.3

Film faced quality 0.1 0.2 0.1 0.1 0.1 0.2 0.1 0.2 0.2 0.1 0.2 0.2 0.2

Cast face 0.1 0.0 0.8 0.7 0.5 0.7 0.3 0.3 0.3 0.2 0.4 Hardened Concrete

Trowelled face 0.5 0.7 0.2 0.6 0.4 1.1 0.7 0.7 0.6 0.6 0.3 0.4

Soil Granular 0.3 0.3 0.3 0.3 0.3 0.3 0.4

20

6 CONCLUSIONS

1. The value of coefficient of static friction does not appear to be affected by member position, i.e. upper or lower. Further testing of member combinations, where initial data suggested that friction values may be a function of position, did not produce any pattern indicating that the variation was either due to natural scatter or the testing of different faces in different positions.

2. Application of load to the upper member generally results in a small increase in friction value. Subsequent increases in load do not, however, appear to affect the friction coefficient.

3. The sliding resistance between two materials with contacting surfaces may consist of a cohesive component in addition to the frictional resistance.

4. Friction values quoted in BS 5975 are similar to the minimum values quoted by DIN4421. Where friction values have been obtained in this research for material combinations already quoted in existing standards, the results (with one exception) lie between the maximum and minimum values of the existing data and tend to be closer to the minimum values. For use in temporary works the recommended values from this research may be used as lower bound values (Table 11).

5. The agreement between current minimum values of friction coefficient and those obtained in the current study suggests that friction values obtained for combinations of materials not previously tested are acceptable for use as lower bound values of friction coefficient.

6. Conclusions 4 and 5 imply that the use of current minimum values of friction coefficient does not have adverse safety implications.

21

22

7 RECOMMENDATIONS

The amount of scatter observed in a few of the tests which were repeated with each member in both upper and lower position suggests that more replicates are needed in future testing.

The possibility that sliding resistance consists of a cohesive as well as a frictional component requires further investigation.

The material combinations tested to date are representative of the head, i.e. soffit, level in temporary works. Material combinations also need to be tested which represent the various interfaces at the base level.

23

24

11 ACKNOWLEDGEMENTS

The following companies provided materials for the tests:

Mr T C Page Kymmene Schaumann (UK) Ltd Stags End House Hemel Hempstead Hertfordshire HP2 6HN Tel No. 01582 794661

Mr I Fryer Chief Engineer RMD - Kwikform Ltd Stubbers Green Road Aldridge Walsall West Midlands WS9 8BW Tel No. 01922 743743

Mr C Heathcote Chief Executive Peri Ltd Market Harborough Road Clifton-upon-Dunsmore Rugby Warwickshire CV23 OAN Tel No. 01788 861600

25

26

12 REFERENCES

1 HEALTH AND SAFETY EXECUTIVE Falsework Design - Comparative Calculations, File No. 617/DST/1004/1998, Report 300-207-R01, October 1998, 113pp.

2 BRITISH STANDARDS INSTITUTION, BS 5975: 1996: Code of Practice for Falsework, London, March, 1996, 134pp. ISBN 0 580 24949 2 including AMD 9289 December 1996.

3 BRITISH STANDARDS INSTITUTION, Draft prEN 12812 Falsework -Performance requirements and general design, Draft for Public Comment 97/102975DC, London, April 1997, 40pp.

4 DEUTSCHES INTSTITUT FUR NORMUNG, Falsework - Calculation, design and construction DIN 4421: 1982, Beuth Veriag GmbH, Berlin 30, August 1982, 20pp.

27

28

ABBREVIATIONS

alum. Aluminium BS British Standards Insitutution

CEN Comite Europeen de Normalisation

DIN Deutsches Institut fur Normung

galv. galvanised

HSE Health and Safety Executive

par. parallel

perp. perpendicular

prop. proprietary

29

30

APPENDIX A

Friction Test Data

31

Upper member: Plain unrusted steel

Lower Member Test Coefficient of friction

0 kg 25 kg 50 kg Plain unrusted steel 1 0.4 0.3 0.3

2 0.4 0.3 0.3 3 0.3 0.3 0.3

Plain rusted steel 1 2 3

Galvanised steel 1 0.3 0.3 0.0 2 0.4 0.3 0.0 3 0.4 0.3 0.0

Proprietary painted steel 1 0.3 0.5 0.4 2 0.3 0.3 0.4 3 0.3 0.4 0.4

Aluminium 1 0.3 0.3 0.2 2 0.3 0.2 0.2 3 0.3 0.2 0.2

Softwood (parallel) 1 0.3 0.4 2 0.3 0.4 3 0.4 0.4

Wet softwood (parallel) 1 2 3

Hardwood (parallel) 1 0.4 0.5 2 0.5 0.5 3 0.4 0.5

Wet hardwood (parallel) 1 2 3

Proprietary timber beam (old) 1 2 3

Proprietary timber beam (new) 1 0.5 0.6 0.5 2 0.5 0.6 0.5 3 0.5 0.5 0.5

Plywood – good one side 1 0.2 0.4 0.4 2 0.3 0.4 0.4 3 0.3 0.4 0.3

Wet plywood – good one side 1 2 3

Plywood – combi ply faced 1 2 3

Plywood –film faced Finnish 1 2 3

Plywood – film faced quality 1 0.1 0.1 0.1 2 0.1 0.1 0.1 3 0.1 0.1 0.1

Plywood – used phenol faced 1 2 3

Hardened concrete (trowelled face) 1 0.5 0.5 0.0 2 0.6 0.5 0.0 3 0.6 0.5 0.0

Hardened concrete (cast face) 1 2 3

Note: * indicates that the third set of tests was a repeat of the zero load test and not a 50 kg test.

32

Upper member: Plain rusted steel


0 kg 25 kg 50 kg Plain unrusted steel 1 0.5 0.4

2 0.5 0.4 3 0.4 0.4

Plain rusted steel 1 0.5 0.5 2 0.4 0.5 3 0.4 0.5

Galvanised steel 1 0.6 0.4 2 0.6 0.4 3 0.5 0.4

Proprietary painted steel 1 0.8 0.6 2 0.6 0.7 3 0.5 0.7

* Aluminium 1 0.5 0.4 0.5 2 0.6 0.3 0.5 3 0.5 0.4 0.5

Softwood (parallel) 1 2 3




Proprietary timber beam (old) 1 0.7 0.6 2 0.6 0.6 3 0.6 0.5

Proprietary timber beam (new) 1 0.4 0.5 2 0.4 0.5 3 0.5 0.5

Plywood – good one side 1 0.6 0.4 2 0.5 0.4 3 0.7 0.4


Plywood – combi ply faced 1 0.2 0.2 2 0.2 0.2 3 0.2 0.2

Plywood –film faced Finnish 1 0.2 0.2 2 0.2 0.2 3 0.2 0.2

Plywood – film faced quality 1 0.2 0.2 2 0.2 0.2 3 0.2 0.2

* Plywood – used phenol faced 1 0.6 0.4 0.4 2 0.6 0.4 0.4 3 0.6 0.4 0.5

Hardened concrete (trowelled face) 1 0.8 0.8 2 0.8 0.7 3 0.7 0.8



33

Upper member: Galvanised steel



2 0.3 0.2 3 0.3 0.3

* Plain rusted steel 1 0.4 0.3 0.3 2 0.4 0.2 0.3 3 0.4 0.2 0.3



* Aluminium 1 0.4 0.2 0.3 2 0.3 0.1 0.4 3 0.4 0.2 0.3












Plywood – used phenol faced 1 0.3 0.3 2 0.3 0.4 3 0.3 0.3




34

Upper member: Proprietary painted steel



2 0.5 0.6 0.6 3 0.5 0.7 0.6




Aluminium 1 0.5 0.5 0.5 2 0.5 0.4 0.5 3 0.5 0.4 0.4



Hardwood (parallel) 1 0.5 0.7 0.6 2 0.6 0.6 0.7 3 0.5 0.6 0.6













35

Upper member: Aluminium



2 0.3 0.3 0.3 3 0.4 0.3 0.3


* Galvanised steel 1 0.4 0.2 0.3 2 0.4 0.2 0.2 3 0.3 0.2 0.2


Aluminium 1 0.2 0.4 2 0.3 0.4 3 0.2 0.4









* Plywood – combi ply faced 1 0.2 0.2 0.3 2 0.3 0.2 0.3 3 0.3 0.2 0.3







36

Upper member: Softwood (parallel)



2 0.6 0.6 3 0.5 0.6




Aluminium 1 0.4 0.4 2 0.4 0.4 3 0.4 0.4














Hardened concrete (cast face) 1 0.8 0.8 2 0.8 0.8 3 0.7 0.8


37

Upper member: Softwood (perpendicular)



2 0.4 0.4 3 0.5 0.4




Aluminium 1 0.5 0.4 2 0.5 0.4 3 0.5 0.4










* Plywood –film faced Finnish 1 0.4 0.2 0.2 2 0.3 0.2 0.2 3 0.4 0.2 0.2






38

Upper member: Wet softwood (parallel)


0 kg 25 kg 50 kg Plain unrusted steel 1 0.7

2 0.7 3 0.7

Plain rusted steel 1 0.8 2 0.8 3 0.8

Galvanised steel 1 2 3

Proprietary painted steel 1 0.7 2 0.8 3 0.7

Aluminium 1 0.6 2 0.6 3 0.6


Wet softwood (parallel) 1 1.0 2 0.9 3 1.1

Hardwood (parallel) 1 2 3



Proprietary timber beam (new) 1 2 3

Plywood – good one side 1 2 3




Plywood – film faced quality 1 2 3


Hardened concrete (trowelled face) 1 2 3

Hardened concrete (cast face) 1 0.9 2 0.9 3 0.8


39

Upper member: Wet softwood (perpendicular)


0 kg 25 kg 50 kg Plain unrusted steel 1

2 3



Proprietary painted steel 1 2 3

Aluminium 1 2 3

Softwood (parallel) 1 0.9 2 0.9 3 0.9















40

Upper member: Hardwood (parallel)



2 0.5 0.5 3 0.5 0.5




* Aluminium 1 0.6 0.4 0.5 2 0.5 0.4 0.5 3 0.5 0.4 0.5
















41

Upper member: Hardwood (perpendicular)



2 0.5 0.5 3 0.5 0.5




* Aluminium 1 0.6 0.3 0.4 2 0.5 0.3 0.4 3 0.6 0.3 0.4
















42

Upper member: Wet hardwood (parallel)


0 kg 25 kg 50 kg Plain unrusted steel 1 0.7

2 0.6 3 0.6

Plain rusted steel 1 0.8 2 0.8 3 0.8



Aluminium 1 0.7 2 0.7 3 0.6




Wet hardwood (parallel) 1 0.8 2 0.8 3 0.8




Wet plywood – good one side 1 0.9 2 0.8 3 0.8








43

Upper member: Wet hardwood (perpendicular)



2 3




Aluminium 1 2 3




Wet hardwood (parallel) 1 0.9 2 0.8 3 0.8




Wet plywood – good one side 1 0.7 2 0.7 3 0.8








44

Upper member: Proprietary timber beam (old)



2 3




Aluminium 1 2 3
















45

Upper member: Proprietary timber beam (new)



2 0.5 0.6 0.5 3 0.5 0.5 0.5


Galvanised steel 1 0.4 0.4 0.0 2 0.4 0.4 0.0 3 0.4 0.4 0.0

* Proprietary painted steel 1 0.6 0.5 0.5 2 0.6 0.5 0.5 3 0.6 0.5 0.5

Aluminium 1 0.5 0.5 0.5 2 0.5 0.4 0.4 3 0.5 0.5 0.5
















46

Upper member: Plywood – good one side



2 0.3 0.4 0.3 3 0.3 0.3 0.3

Plain rusted steel 1 0.4 0.4 2 0.3 0.4 3 0.3 0.4



Aluminium 1 0.2 0.3 0.3 2 0.2 0.3 0.3 3 0.2 0.3 0.2
















47

Upper member: Wet plywood – good one side


0 kg 25 kg 50 kg

Plain unrusted steel 1 0.5 2 0.6 3 0.6




Aluminium 1 2 3














Hardened concrete (cast face) 1 0.7 2 0.6 3 0.7


48

Upper member: Plywood – combi ply faced



2 3




Aluminium 1 2 3
















49

Upper member: Plywood – film faced Finnish



2 3




Aluminium 1 2 3
















50

Upper member: Plywood – film faced quality



2 0.2 0.1 0.1 3 0.2 0.1 0.2




Aluminium 1 0.2 0.1 0.1 2 0.2 0.1 0.1 3 0.2 0.1 0.1







* Plywood – good one side 1 0.3 0.2 0.3 2 0.3 0.2 0.3 3 0.3 0.2 0.2







Hardened concrete (cast face) 1 0.2 0.2 0.0 2 0.3 0.2 0.0 3 0.2 0.2 0.0


51

Effect of Member Position

Lower Member Upper Member Test Coefficient of friction 0kg 25kg

Plain unrusted steel Prop. painted steel 1 0.5 0.5 (face 1) (face 1) 2 0.6 0.6

3 0.6 0.6 Prop. painted steel Plain unrusted steel 1 0.7 0.6

(face 1) (face 1) 2 0.7 0.6 3 0.7 0.6

Plain unrusted steel Prop. painted steel 1 0.4 0.4 (face 2) (face 2) 2 0.5 0.5

3 0.6 0.5 Prop. painted steel Plain unrusted steel 1 0.7 0.6

(face 2) (face 2) 2 0.7 0.6 3 0.7 0.6

Aluminium (face 1) Plywood (face 1) 1 0.3 0.2 2 0.3 0.3 3 0.3 0.3

Plywood (face 1) Aluminium (face 1) 1 0.3 0.3 2 0.3 0.3 3 0.4 0.2

Aluminium (face 2) Plywood (face 2) 1 0.4 0.4 2 0.4 0.3 3 0.4 0.4

Plywood (face 2) Aluminium (face 2) 1 0.4 0.3 2 0.3 0.4 3 0.3 0.4

Investigation of bedding/time effects

Lower Member Upper Member Test Coefficient of friction 0kg 25kg

Plain unrusted steel Prop. painted steel (face 1) 1 0.6 0.7 (face 1) 2 0.6 0.7

3 0.7 0.7 Plain unrusted steel Prop. painted steel (face 1) 1 0.6 0.7

(face 1) Repeated after 48 hrs 2 0.6 0.7 Repeated after 48 hrs 3 0.7 0.7 Aluminium (face 1) Plywood (face 1) 1 0.3 0.3

2 0.3 0.3 3 0.3 0.3

Aluminium (face 1) Plywood (face 1) 1 0.3 0.3 Repeated after 48 hrs Repeated after 48 hrs 2 0.3 0.3

3 0.3 0.3

52

APPENDIX B Saturation Test Data

53

Time (days) Specimen Moisture Content (% water) Reading 1 Reading 2 Reading 3 Average

Softwood 1 6 6 6 6.0 Softwood 2 6 6 6 6.0

0 Plywood 0 0 0 0.0 Hardwood 1 12 14 12 12.7 Hardwood 2 14 14 14 14.0 Softwood 1 25 25 23 24.3 Softwood 2 25 23 25 24.3




19 Plywood 26 26 26 26.0 Hardwood 1 26 28 26 26.7 Hardwood 2 28 26 26 26.7

Printed and published by the Health and Safety Executive C1.25 02/03

ISBN 0-7176-2613-X

RR 071

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Friction in Temporary Works - University of Birmingham 2003

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transferring

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german codified

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