Timber in Excavations

Timber in excavations Third edition

7 RA DA

Produced for the Timber Research and Development Association, Stocking Lane, Hughenden Valley, High Wycombe, Buckinghamshire HP14 4ND, by Thomas Telford Ltd, Thomas Telford House, 1 Heron Quay, London El4 9XF First published 1981 Second edition 1984 Third edition 1990

British Library Cataloguing in Publication Data Timber in excavations. 1. Civil engineering. Excavation I. Timber Research and Development Association 624.152 ISBN

0 901348

81 3

(11 Timber Research and Development Association, (c-1This edition Thomas Telford Ltd, London, 1990

High Wycombe,

1981,

1984, 1990

All rights, including translation, reserved. Except for fair copying no part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the Timber Research and Development Association. Typeset by MHL Typesetting Limited, Coventry Printed in Great Britain by Inprint, Hitchin, Herts

Preface

Origins and acknowledgements

This publication stemmed from an initiative by TRADA in 1977 to establish a new committee for timber in temporary works, chaired by J.R. Illingworth, then of George Wimpey Ltd, now Consultant in construction methods and technology. The general brief of the committee was to: act in an advisory capacity to TRADA in all matters concerning timber and wood based materials used in the fabrication of formwork, falsework, and excavation supports. To collate existing information and define areas in which further research and development is required, recommending priorities for a programme of work to this end. The committee was selected to represent both timber trade and construction industry interests. Support for its proposal was sought and obtained from the Department of the Environment in 1978 with TRADA contributing both financially and in providing technical and administrative support. This document is one of several which arose from the projects steered by the above committees and owes much to the efforts of D.J. lrvine of Tarmac Construction Ltd and J.R. Withers of TRADA, chairman and secretary respectively of the working party responsible for its production. This third edition of Timber in excavations has been revised by J.R. Illingworth and D.J. Irvine. TRADA wishes to thank them, and all members of the original committees for their advice, expertise and help in the preparation of this publication. Since the second edition was published, TRADA has undertaken a research project, part funded by the CEC (Commission of the European Communities), entitled ‘More efficient use and re-use of timber in temporary works’. In view of the close links being developed by UK contractors within the European Community, a brief summary of the use of timber in the support of excavations has been included. It is based on an analysis of information obtained by surveys in sixteen West European countries, some of which were outside the EEC.

Contents

vii

List of figures List of tables

ix

Introduction

xi ...

Guidance

XIII

notes

Wide excavations Typical support methods Design methods Safety Engineering a deep and wide excavation Design methods for a soldier pile support

system

Trenches Use of poling boards in trenches Use of runners Use of trench sheeting Use of sheet piling Safety

22 22 26 29 29 29

Shafts Use of poling boards in shafts Use of runners in shafts Struts and walings Chicago method Safety

35 35 38 39 39 40

Headings Supervision Sizes of heading covered by these recommendations Materials encountered in driving the heading Timber to be used in the heading Dimensions and alignment Excavation and timbering Securing the heading Lighting and ventilation Workmanship Safety

41 41 41

Workmanship in excavations Materials Construction and use

50 50 50

42 43 43 44 44 49 49 49

Safety Legislation Responsibilities Check list

52 52 52 55

Support of excavations in Europe Legal requirements and available standards Methods of support used Re-use of timber in excavation support

56 56 58 58

Appendix 1 Timber Stress grading Moisture content Derivation of permissible stresses Duration of load Length and position of bearing Lateral support Depth factor Derivation of permissible stresses supports for trenches Timber requisition

59 59 61 61 61 64 64 64 for timber when used as 65 67

Appendix 2 Check lists for excavation Planning and design Trenching operations Appendix References

3 Glossary

of terms

support

used in timbering

68 68 69 71 75

List of figures

1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

2 Battered sided wide excavation 2 Horizontal props and walings 2 Very wide excavations 3 Soldier pile and skin wall 3 Tie back method 4 Raking props 6 Raking struts for soldier pile wall Attaching timbers to H-piles; timber tucked in behind face 13 flange Use of random length timbering with H-piles; clipping 13 method 14 Clipping system to cope with H-pile location errors 14 Detail of clip sizes 20 Alternative strut arrangement for soldier pile wall 20 Alternative strut arrangement for soldier pile wall Initial excavation using poling boards in the 22 middle board method Completed trench using poling boards and the middle 23 board method 23 Typical view of the middle board method 24 Poling boards used in the tucking frame method 25 Typical view of the tucking frame method An alternative use of the tucking frame method using 26 tucking boards Stage 1. Uprights, walings and struts ready for pitching 27 runners 27 Stage 2. Runners pitched ready for excavation Stage 3. Runners driven and trench excavated ready for next frame 28 Stage 4. New runners pitched ready for excavation within the trench supported by the first set of runners 28 New set of runners set inside trench supported by two 29 levels of poling boards used in the middle board method 35 Methods of side supports in shafts Shafts with maximum dimension up to 2.75 m 36 Shafts with maximum dimension up to 5 m 37 Shafts with maximum dimension up to 1.8 m square 38 Chicago method 39 Box setting 45 Poling setting 45 Piling setting 46 Installation of piling settings 46-49

vii

List of tables

1 2 3

4 5 6 7 a 9 10 11

12 13 14 15 16 17

ia 19 20

21

22 23 24 25 26

Flow chart of activities - trenches, wide excavations, shafts and headings Permissible axial load (kN) in struts of SC3 strength class timber Permissible axial load (kN) in struts of SC4 strength class timber Permissible axial load (kN) in struts of SC5 strength class timber Permissible axial load (kN) in struts of Douglas firlarch species Size of strut required for designs in Figures 7, 12 and 13 Summary of methods of side support for trenches Ground conditions and slopes Minimum sheeting requirements Minimum requirements for walings and struts - no water head Minimum requirements for walings and struts - sheets loaded by water head Timber supports for trenches - permissible stresses General duties required under the Health and Safety at Work etc Act Summary of Construction Regulations: excavations, shafts, earthworks and tunnels Frequency of use by percentage of the various support methods for narrow excavations in Western Europe Frequency of use of various support methods for wide excavations in Western Europe Re-uses obtained with components for excavations support Softwood species/grade combinations which satisfy the requirements for strength classes SC1 -SC5 Grade stresses and moduli of elasticity for strength classes: dry exposure condition Modification factor K, by which the geometrical properties of timber for the dry exposure condition should be multiplied to obtain values for the wet exposure condition Modification factor K2 by which dry stress and moduli should be multiplied to obtain wet stresses and moduli applicable to wet exposure conditions Load duration factor K3 Modification factor K4 for bearing stress Maximum depth to breadth ratio Stresses and moduli of elasticity for strength classes: wet exposure condition Timber supports for trenches - permissible stresses

xiv 16 17

18 19 21 30 31 32 33 34 34 53 54

57 57 58

62 63

63

63 63 64 64 66 66

ix

Introduction

Timbering is the term commonly used in relation to temporary works involved in the upholding of excavated faces of wide excavations, trenches, shafts and headings. Modern practice tends to use timber in association with other materials for temporary works, although timber alone is still used economically for smaller or special jobs. The purpose of this book is to provide site staff and planners with quick and convenient methods for assessing site conditions and choosing appropriate excavation supports for trenches, shafts and headings where proprietary systems are unsuitable or are not considered to be cost effective in the circumstances prevailing. Although wide excavations normally require specific design, typical methods of solution are shown, together with example design calculations adequate for planners and site staff to assess feasibility and cost effectiveness at tender stage. Suitable check lists for all methods are provided so that site staff can be sure that any scheme has been properly installed and is safe to use. All ground can be expected to show variations from place to place and it is essential to keep a close watch for any changes and to vary the support system to suit. Any support provided to an excavated face should be regarded as one part of a structure which uses soil as the other part. Arching within the soil tends to modify the forces predicted by classical soil mechanics theories and the support systems suggested for trenches, shafts and headings are those which have been proved by experience to give safe and reliable results where the ground is subject to only visual inspection backed by common sense. These solutions may be termed ‘deemed to satisfy’ and may not in ail cases give the most economic structures. Generally, special design solutions are only justified where adequate soil investigation has been carried out to give information on the engineering properties of the ground. For such cases permissible timber stresses are given. It is recommended that only stress graded timber should be used in excavation support and a brief explanation is given of the history and current situation concerning stress grading, together with a check list of the main information required when requisitioning timber.

xi

Guidance notes

It is important to be systematic in planning and constructing excavations. The flow chart (Table 1) gives the essential steps for each of the general types of excavation, with cross references to the relevant sections in the text. The main sections cover trenches, wide excavations, shafts and headings. They describe methods of support and give advice on the sizes of components or methods to be used for particular circumstances. Detailed consideration of the design of supports to wide excavations is beyond the scope of this book. However, the soldier (H) pile method is so widely used throughout the world that typical methods and details are included, together with an explanation of the basis of design and worked examples. The earth pressures have been derived in accordance with the recommendations of the Civil Engineering Code of Practice No 2 Earth retaining structures, 1951, while taking cognisance of design methods used in the United States of America and the Federal Republic of West Germany. The timber sheeting and struts are designed using the permissible stresses for timber (Table 7) in conjunction with the design method of BS 5268 The slructural use of timber Part 2 Code of practice for permissible stress design, materials and workmanship, 1988. At the time of going to press of this third edition of Timber in Excavations, the Civit Engineering Code of Practice No 2, 1951 (CP2) is in the process of revision. While the final contents of the revised Code are unknown, this document has been based on the existing Code and other international design methods. This is justified in the light of international experience in the use of H-piling and its record over many years. While the safety record may suggest over-design to the purist, it has to be remembered that this is a desirable feature in temporary works of this nature, where earth pressures cannot readily be calculated precisely. In the Trenching section, Table 8 describes the field identification of different types of ground and suggests safe temporary slopes which may be used. These slopes can be used as an alternative to supporting vertical faces in trenches, wide excavations and shafts when space permits and the method can be justified on practical and cost grounds. Where vertical trench sides are required then Tables 8, 9, 10 and 11 can be used for various ground conditions to select the appropriate sheeting arrangement and the sizes of walings and struts. Adequate clearance should be allowed for lowering between struts and generally the designer will first consider the length of pipe, etc to be lowered into the trench and select appropriate strut centres. The vertical centres will tend to be determined by the pipe diameter or by the clearance required for construction purposes. The sections on Shafts and Headings describe the methods of support and scantlings of members which have been found to be satisfactory in use. Where these types of excavation are in use for a long period the risk of wear and tear and accidental damage during service is increased and it may be necessary to add timbers as buffers and rubbing strips to prevent damage to the structural timbers during lifting and mucking out operations. The Workmanship and Safety sections apply to all types of excavation .. .

XIII

Table 1. Flow chart of activities - trenches, wide excavations, shafts and headings

Shafts & Headings

Wide Exca t&ions

Trench es

I

1 information

r

Assemble design data from: Design drawings; Conditions of Contract; Specification; See Appendix 2

etc.

I t

f

Ground information Ascertain ground information from: Trial pits, site inspections, adjacent workings etc. Take particular note of ground water levels and flows. See Appendix 2

I

Consider

the possibility

t

Y

and economics of using an open excavation with side slopes. See Table 8 @I_31)

I

I

,

.

I

1

Sheet@ Choose minimum sheeting required. See Table 9 (p. 32)

I

Wide excavations are usually fully sheeted.

Shafts are usually fully sheeted.

Select the method of construction. See Main Text

T i ’ ’

Choose convenient waling and strut centres. Select appropriate member sizes. See Tables 10 and 11 (pp 33 and 34)

7 Size of Componenls Carry out the design. See wide excavations

stage drawings if necessary. Schedule

xiv

I

Choose sizes of members. See Shafts and Headings

labour, plant and materials.

r

T

Construct

t the Work

Supervise and check workmanship and materials. Ensure safe working practice is used. See Workmanship (p. 50) and Safety (p. 52)

Check

the Work

The Excavation Works Coordinator must inspect the excavation support system before work commences in the excavated area. See Appendix 2

t Use the Works The Excavation

Y

Works coordinator must inspect the excavation at regular intervals and at critical stages. See Appendix 2

t

t Dismantle

support

1

the Works

The excavation support must be removed in a planned manner. The actual method will depend on a number of factors, in particular, the type of construction carried out within the excavated area. Intermediate propping may be necessary. This must be carried out in accordance with the procedures indicated above. i

and should be used in conjunction with the Check lists given in Appendix 2. Appendix 1 on Timber describes the changes which have taken place in the use of stress grading to ensure the engineering qualities of timber. It also gives the derivation of the table of permissible stresses for timber to be used in excavations and sufficient information is provided to allow the designer to modify the stresses for a particular design condition. Having decided that a particular strength class or species/grade is required it is essential to specify it exactly and use of the Timber requisition section in Appendix 1 is recommended.

xv

Wide excavations

A wide excavation is an excavation whose width exceeds 5m. In plan its shape can be infinitely variable from an extra wide trench in open ground to an irregular shape defined by adjacent buildings as in city centre developments. For a given depth, a wide excavation when supported by walings and struts will require heavier struts to the frames than a trench supported in a similar manner. For very wide excavations, ground anchors, tie rods or raking struts should be considered as they will probably be more economic. Wide excavations have to take into account so many site factors that standard solutions are difficult to provide. A design method for raking struts and timber sheeting for a soldier pile system is given. However, it is recommended that wide excavations are subjected to individual design procedures. These designs should be prepared by a suitably qualified engineer previously experienced in this field. Appropriate guidance is available in the following references: Sritish Standard BS 6031: 1981: Code of practice for earthworks British Standard Code of Practice CP 8004: 1986 Code ofpractice for foundations Trenching and shoring manual. Californian Department of Transportation, 1977 Foundation design and construction. Tomlinson, 1976

Typical support methods

Where space is available the most economical method is to slope the sides of the excavation to a point where they become stable. This is known as the battered sided method and is illustrated in Figure 1. For wide excavations where the sides have to be supported, typical methods are illustrated by Figures 2 to 6 (b). Depending on the nature of the site it may well prove economic to use two or more of the methods depicted within the same excavation.

Design methods

Although it is recommended that all wide excavation supports are designed by an experienced engineer, some limited design at an earlier stage is useful for checking purposes or for initial costing. A design method for determining the size of timber raking struts and for the horizontal timber members of a soldier pile support system follows. Any design process must check the stability of the bottom of the excavation against heave or boiling and walls founded in clay must be checked for overall stability.

Safety

All the hazards encountered in trenches are equally applicable to wide excavations. In addition, on congested sites with other buildings close to the edge of the excavation, the effect of a collapse on the stability of adjacent buildings and the resulting consequences must be carefully considered and all necessary precautions taken. See section on Safety (P. 52). 1

Figure 1. Battered sided wide excavation (suitable for open sites with convenient access)

Ground

Slopes dependent on soils

Figure 2. Horizontal props and walings (limited by column buckling of strut)

Note If steel s!ruts or wallngs used. all connectlow should be adequately ;Nelded

Timber struts and wallngs

Steel sheet p,llng

r--..r

excavations

King post

2

level

Figure 4. Soldier pile and skin wall

Steel soldier pile-driven. Penetration fixity as calculation

Timber horlzontal sheeting wedged

for wedged

to soldler

Figure 5. Tie back method

6 :.

::

blocks

3

Figure 6 (a). Raking props

Timber

or steel prop

Steel sheeting (N B Wall to be stable of raking

prop)

II

Figure 6 (b). Raking props

Timber sheeting or concrete

Solder pile Penetration for --+ flxity as calculation

\ IFyb

Engineering a deep and wide excavation

General Deep and wide excavations are frequently required for the construction of building basements, pumping stations and other structures (such as retaining walls). The excavation removes a mass of soil and water and often causes a local lowering of the ground water table. This total stress release within the ground and the change to the water table can result in movement in the surrounding soil. In particular the bottom of the excavation may heave or suffer loss of strength. The degree of movement will have a significant effect both on the loads to be resisted by the system used to support the sides of the excavation, and on the total stability of the ground beneath and adjacent to the excavation. A satisfactory solution to each excavation problem must consider the following general requirements: l

l l

4

the construction method and support system must be safe and economical. there should not be excessive movement of the ground. in some cases the movement must be further limited to avoid danger to surrounding structures and buried utilities.

Movements These (a) (b) (c) (d) (e) (f) (g) (h)

and loads

are affected

by:

the dimensions of the excavation the soil properties ground water control time the type of support system the excavation and bracing sequence the nearby structures and utilities transient surcharge loads.

The larger the excavation the greater will be the change of horizontal stress beneath the base. The degree of horizontal wall movement will depend on the stress changes and the horizontal wall movement will influence the settlement at ground level adjacent to the excavation. The flexibility of the wall has some influence on soil movement although experience suggests that the stiffnesses of ordinary steel sheet piles and soldier piles are not sufficient to have a significant effect. Much of the wall movement occurs below the bottom of the current excavation level and practically ceases when the bracing is installed. Therefore, in order to limit movement, the bracing must be placed quickly and with the minimum of excavation. The use of berms can help to limit (not eliminate) wall movement and improve temporary stability by extending the length of potential failure surface, and inhibiting swelling and softening in clay.

Design The interaction of the soil and the support structure is complex and the design method must take this into account. Usually empirical methods of design are used which make allowance for the interactive effects without requiring them to be analysed explicitly. Consequently there is scope for engineering judgement. The more theoretical methods of analysis suffer from the difficulty of reliably assessing input parameters which limits the accuracy of predictions and hence their use as a design tool. The design method set out in CP2 is derived from the trapezoidal envelopes of strut loads produced by Terzaghi and Peck and allows for the redistribution of force into the supports of multipropped walls. The likely wall movements can be estimated by reference to Peck’s records of monitoring actual excavations (see Tomlinson, 1976). Stability analyses should be done to check against failure of the excavation by heave or base instability. If excavations in clay get too deep relative to the strength of the clay, then the heave may become uncontrollable causing excessive settlements of the surrounding ground and possible collapse of the bracing. Excavations below the water table in sand may give rise to high seepage forces which destabilise the base of the excavation. The designer should also beware of excess hydrostatic head beneath a shallow, relatively impermeable, layer which may cause the bottom to blow.

Design methods for a soldier pile support system

This section gives the design method for raking struts and the timber sheeting for a soldier pile support system and outlines the principles involved. Although the method detailed is specific to a particular application, it can be adapted to cover timber in any strut situation and is suitable for feasibility studies or costing purposes at the tender stage. Strut tables are also included which enable a variety of strut arrangements to be selected and considered, so that the most economic solution is adopted.

5

Design of raking struts for a soldier pile wall Symbols

Unit ‘weight density’ of soil of internal friction 6 of wall friction C Cohesion Ka Coefficient of active earth pressure Pa Thrust due to active earth pressure l-i Depth of excavation Angle 2; Angle

Figure 7. Raking struts for

Surcharge

soldier pile wall (alternative strut arrangements are shown in Figures 12 and

35 KN/rn’

J

T4iIIr

Shear key

13) Compact sand (water table 8 m below ground level)

mrr

_

5500

mm.

Packlw PI 2

_I

I

’ Ground #---

7

slab

SoIdler piles (Ttmber sheeting not shown) Penetration for

J-v-

-cu-

flxlty as caiculatlon Sotl charactenstics 7 = 17.5 kN/m3 I$ = 35O 6 = cJ” c=o Ka = 0.27

Notes and references

1

1 No water pressure considered as the timber sheeting between soldier piles will allow free draining conditions and wfll prevent a building of surface water

Soil Pasoil = Ka. y .H

2 A unllormly distributed live load has been used. Concentrated (line or point] loads must be treated separately where they occur (see also Note 7). 3 The raking strut will be installed while a berm is left on the Inside face of the wall. When the top strut is in position and secure the remamder of the berm is excavated and the horwontal strut is installed Other variations may be used and in some cases the horizontal strut IS omitted and the soldier piles are founded sufficiently deep lor the toe to develop the required passive resistance.

4 Even wtth the arrangement shown it IS essential to prwda sufficient penetralion to resist the upward component of the raking strut thrust (see also Note 18 below). In cases where the ground below formation level IS good the bottom horizontal strut may be omitted and the passive earth pressure at the toe of the pile used to provide the bottom reactlon. When such an arrangement is used the active pressure is assumed to be distributed hydrostatvzally on the wall

Active

on wall

pressure

Surcharge Pasu,cha,ge = 35 x Ka

= 9.45 kN/m*

At ground level

ie equivalent surcharge

Pasoil = 0 At formation level Pasoil = 0.27 = 28.35

X

17.5

x

kN/m*

:. Total soil force /m run Pa,,

= y = 85.05

x

6

kN/m

6

he

=

!k!? 17.5

= 0.54m

height of

5 The quoted clause of CP2 ‘Earth reta,nmQ structures’ requires the eqwalent fluId pressure lo be consrdered for cohesrve sorls However, many engrneers consrder that the mrnrmum pressure should also be used for cohestonless sotls since the coeffictents of aclive pressure may underesfrmate earth pressure in some ctrcumstances

Check

6 Refer to CP2 ‘Earth retaining structures CI.f.4343. The pressure dtstnbutlon dtagram for strut toads IS a maxtmum envelope and allows for the varratton of strut loadmg caused by arching and other effects over a pertod of trme Dependmg on a fudgement of the nature of the retained ground. the loadrng and other factors, the pressure diagram may be reduced when destgnmg the soldrer p&?s and the horrzontal sheetrng. Thus JUdQem@nt should be made only by an experienced desrgner

:. Use soil force for strutted (see below)

Fluid

equivalent density

2

Distribution

(CP2 Cl. 1. 4311)

PaSO,,) excavations

of active pressure

pressure

on strutted

diagram

wall

I 02H 1 = 1200 mm

i

16 y (Pa,,,,+ Pa,.,r,h~& Hi+

pressure

= 4.71 kN/m3

Pa = l x 6’ x 4.71 = 84.78 kN/m (
(I)

forces

24.1

x 4.8 +

= 134.4 kNm

10

+ 7.1 x

Subtract

(2) from (I)

3.5 FA’

= 242.2

FA’

= 69.2 kN m

FB’

= 65.2kNm

(2)

Construct

BMD for soldier

Point of zero shear 69.21

C

+ 24.1

=

! 2 = 78.72 + 24.1~ =

. . X

beam

x 1.2 + 24.1

x x

2.lm

ie point of zero shear = 2.1 + 1.2 - 1.5 = 1.8m below FA’ BMAo

=

7.1 x y

+ 17 F

+ 0.3

x 2.5

= (7.98 + 7.8) x 2.5 = 39.4 kN m (hog)

=

BMc

7.1 x 1.5 x + 17 x

1.5 r + 1.81 + 17 x 0.3 x i 0.3 2 + 1.8

'1.2 'L? x ! + 0.3 + 1.8 2 3

1.a2 + 24.1 x ~ 2

- 69.21 x 1.8I x 2.5 = (27.16 + 9.33 + 25.5 = 58.88 kN m (sag)

B&L 17 The 203 x 203 UEP secl~on IS the smallest we usually consldered for soldw piles for pracllcal reasons. However. any suitable secl~an may be used and soIdlers can also be made up using channel seclions back to back wlh spacers. 18 The penelratlon of the soldler pile must be sufficient lo prevenl It moving upwards under the actjon of the raking strut. In cases where the embedded length 01 the soldier pile 1s required to prowde a lateral reactton. the soli should be checked to ensure an adequate factor of safety agamst passwe failure. 19 A rigorous design check would mclude a consideration of shear and web crushing and buckling However, experience shows that flmber struts are not llkeiy lo overload a steel soldwr rn lhls way If good ~~n~lru~l!on procedures are followed. Web sllffeners would be an mconvemence when inslallmg the horizontal members. Care should be taken lo ensure that the raker force IS applied centrally lo the soIdler and no torsional effect IS produced The strut loads determmed m Note 3 should be used m lhts check. Local flange bending should also be consldered where sheeling IS lucked behind the front flange 20 In wsw of the englneermg judgemenls which are necessary lo derive the earth pressure loads it is suggested that the steelwork should be deslgned using BS 449. which uses a permissible stress approach The use of BS 5950, which uses a llmil slate approach lo design, would demand an asses%nenl of the load and serviceabllily factors

= 24.1

x ;

kN m (hog)

203

x 203 Universal

Bearing

(2 = 489.2 cm2)

Allowable

where

i,,

bending

-

124.58)

x 2.5

x 2.5

= 30.13 .__

Assume

+ 39.04

stress

Pile

grade

43

at C

= radius of gyration = 84.68

:. From Table

3A BS 449

Pcb = 165 N/mm* Actual f =

bending

stress

at C

58.88 x 1000 x 1000

b

489.2 x 1000 = 120.35

:. Section

N/mm2

< 165 N/mm2

is adequate

11

Allowable

bending

D

1500

;I=

-=118

T

stress

4.96

‘YY

x 3 x 10

= 90.72 :. From Table P,,

at A

N/mm2

3A BS 449

= 165 N/mm2

Actual

bending

fb = 39.7

x

stress 1000

x

1000

489.2

x

1000

= 81 N/mm’ is adequate

:. Section

retatwely

21 Timber is flexible and tends to deflect under load which enhances the archmg action within the solI. In cases where slifl materials are used for sheeting (eg reinforced concrete) the relieving effect of arching would be

6 Design Allow 50%

reduction

effect

of arching.

32.13 x 0.5 = 16.07KN/m*

I

BM in sheeting

=

=

16.07

x 2.4: 8 11.6 kN m/m height

Try 225

um,a,II

x 150 timber

(Z = 843 cm3)

11.6

x lo3

om,g,ll

x 0.225

=

843 x = 3.1 N/mm2

Permissible =

stress

6.29

Om.adm.lI

=

6.29

5.39 N/mm2 :. 225 x

x

for SC3 timber

22 modify x

lo3

IO3

N/mm2 for 1 week

:. From Table

12

timber sheets

in load as relieving I-

.,

22 When Installing horizontal Umber sheets. care should be taken to avold overdigging. In part~cufar the ground immediately agalrwt the rear flange of the soldier pile should be left undMurbed so that the soil arch is mamtamed. The horizontal timbers can be fixed in a number of ways, Including wedging members behind the front flanges of the soldiers (Flgure 8) or by using special flxlng clips (Figures 9, 10, 11). More detail about the application of the method IS given I” Temporary works lhw role in construction by J R Mngworth, Chapter 7, pages 116-131.

of horizontal

ifz

stress

from Table

12

load duration for 6 week duration

= 5.39 N/mm2

> 3.1 N/mm2

150 SC3 Timber

adequate

in sheeting

225mm

Figure 8. Attaching timbers to H-piles; timber tucked in behind face flange

X 75mm t’mber boards L

5Ai4

PLAN

150mm

X IOOmm X 50~,rrl limberwedges

Figure 9. Use of random length timbering with H-piles; clipping method

4mm

20mln clla.rn~ldsteel bar fl l~/F>rnrrl lcirl(j

UCT/VLOFCLlPPlNG

x 64mm

x 6kg

f

SYSTtM

13

Figure 10. Clipping system to cope with H-pile location errors

2‘25mm X 75mm tlrnber boards Steel scaffold tube offcuts 375mm long

PLAN

Figure sizes

11. Detail of clip

I

14

B

c

D

190

120

165

50

38

210

145

165

50

38

230

C

A

38

173

165

50

Check shear Shear force = 16.07 x 0.225 x ‘$ = 4.34 kN Max shear stress #v,a.

=

4.34 x 1.5 x 1000 225 x 150

= 0.19 N/mm’

I

From Table 12 av,Q,

1

1.34 N/mm*

=

Modify for load duration then

%.adm.

L

= 1.34 x ifz

= 1.15 N/mm”

0.19 < 1.15 :. Section O.K. in shear Check bearing Assume minimum bearing length due to lack of fit = 25 mm Bearing (compression) 0 = c’a’’

perpendicular

to grain

4.34 x 1000 = 0.77 N/mm’ 225 x 25

As bearing occurs at ends of the timber the compressive stresses given in Table 12 do not apply From Table 23 the allowable compressive stress must be divided by 1 .14 and also multiplied by 1.2/1.4 to correct for load duration .‘. &a&,, ,(no wane) =

2.19 x 1.2 1.14 x 1.4

= 1.65 N/mm2

0.77 < 1.65 :. Timber adequate for bearing

7 Tabulated design of timber struts (Tables 2-5) Strut B in Figure 7 and the struts for the alternative arrangements shown in Figures 12 and 13 may be calculated from first principles as shown previously in the section headed ‘Design of strut’. However, for convenience the Strut load Tables 2-5 have been prepared which give the allowable loads in kN for struts of various sections and lengths. The tabulated loads have been calculated in the same manner as that employed when using first principles and include the same load duration factors, densities etc. The tables cover strength classes SC3, SC4 and SC5 and also Douglas fir/larch as large sections are often more easily obtained in this group. The tables are simple to use. The axial load to be carried by the strut is determined by the method shown in the example on page 8 and the length of the strut is obtained from the geometry of the arrangement selected (Figures 7, 12, 13). For the example used the axial strut load = 266.5 kN and the length = 6.5 m. Using Table 3 a load equal to or greater than 266.5 kN is selected from the strut column headed 6.5 and the required section found by extending to the left along the line of figures.

By inspection the smallest adequate strut load is 269 kN which corresponds with the preferred section of 300 x 300 mm, the section determined by first principle calculations. 15

s

Table 2. Permissible axial load (kN) in struts of SC3 strength class timber LOADING

CONDITION

SELFWElGHT= 590kg/r+

WET CONDITIONS

AXIAL LOAD-

BENDING STRESS PARALLEL TO GRAIN = 6.29N/mm2 COMPRESSION STRESS PARALLEL TO GRAIN = 5.94 N/mm2 LOAD DURATION FACTOR K3 = 1.2 MINIMUM MODULUS OF ELASTICITY (E) = 5010 N/mm’

SECTION SIZE

LENGTH 1.25

1.75

1,s

2.25

2. 0

2, 75

2.5

I

3*25

3.0

~

LENGTH t--

M E T K E S

N

3.75

3.5

&4;r.n_

4.25

4.0

4.75

4.5

5.75

5,25

5,O

5 +5

Cl. 2'5 h.75 7 ,';! -> 5 7 *7::a 6 .!I 6 ,5 T+() '7,5 8 t0

200x200

181 176 171 165 i60 154 147 141 134 127 120 113 106

01

7s

70

6s

60

56

200x250

226 220 214 208 201 194 186 178 170 161 153 144 136 127 119 112 104

98

Pi

85

79

74

200X350

318 310 301 292 283 273 263 252 241 229 218 206 195 183 172 162 152 142 133 125 117 109 1.0:~ 96

200x400

,I :,( / :I.o:;9;,< !;p 363 354 344 334 324 313 301 289 276 263 ':'SO237 224 211 I.98 I.86 1.75 I.64 1.54 144 I.55 1.27 I.:13j.1.;]

~5OXT50 L L

?90 '84 378 371 VOY 200 191 182 173 164 Ix!?:? 147 lJ9 131 1.23 Il.6 1.09 1.03 96 L L _ ." 265 258 250 243 235 376 ...L 318 _ L

250X300

348 341 334 326 319 310 302 293 :1'84274 264 254 244 233 222 212 201 191. 181. 171 162 1.53 144 1.36 1.29 I.':'1 I.:\4'I.(.,::!

250x350

406 398 390 381 372 363 353 343 332 321 310 298 286 274 262 250 238 226 31s 203 1Y i3 1. tS 2 l72 I6 3 .l 5 4 1. 45 .l 3 7 1.S 0

250x400

r!3 2 11 200 11i\Y'I. 7'i 3. 69 f6 0 I5 1 465 456 446 436 426 416 405 393 381 369 356 343 3:!Y 316 302 288 275 261 248 T"35 f!

300X300

')33 ._Lh_ ')':i? 424 417 410 402 394 386 378 369 360 351 341 331 321 310 299 :!88 277 266 255 L!44 ,_ "' 1. 3.20 1. 'I, 9 :t 1iii 2 17:: 1C? 3

300X350

495 487 479 470 461 452 442 432 422 411 400 389 377 365 353 340 327 315 302 209 277 264 252 240 229 218 '7()7I';‘.'

300x400

566 557 548 538 528 517 507 495 484 472 460 447 433 420 406 392 378 363 349 335 3E0 306 293 27 9 26 6 254 24:? 2 j 0

350x350

584 576 567 558 '549 540 531 531 511 500 489 478 466 454 442 429 416 403 390 376 36i.J349 336 32::'309 :'96 284 271

350X400

668 658 649 639 629 618 608 597 585 573 561 549 536 532 508 494 480 465 450 43'S 420 405 3YO 3.2:;360 346 331 31:

400X400

769 760 750 740 730 719 708 697 686 674 662 649 636 623 609 SC.'5581 566 551 535 5PO 504 4813