Green Oak in Construction
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Green oak in construction Green oak in construction by Peter Ross, ARUP Christopher Mettem, TRADA Technology Andrew Holloway, The Green Oak Carpentry Company 2007 TRADA Technology Ltd Chiltern House Stocking Lane Hughenden Valley High Wycombe Buckinghamshire HP14 4ND t: +44 (0)1494 569600 f: +44 (0)1494 565487 e: [email protected] w: www.trada.co.uk Green oak in construction First published in Great Britain by TRADA Technology Ltd. 2007 Copyright of the contents of this document is owned by TRADA Technology Ltd, Ove Arup and Partners (International) Ltd, and The Green Oak Carpentry Company Ltd. © 2007 TRADA Technology Ltd, Ove Arup and Partners (International) Ltd, and The Green Oak Carpentry Company Ltd. All rights reserved. No copying or reproduction of the contents is permitted without the consent of TRADA Technology Ltd. ISBN 978-1900510-45-5 TRADA Technology and the Consortium of authoring organisations wish to thank the Forestry Commission, in partnership with Scottish Enterprise, for their support in the preparation of this book. The views expressed in this publication are those of the authors and do not necessarily represent those of the Forestry Commission or Scottish Enterprise. Building work involving green oak must comply with the relevant national Building Regulations and Standards. Whilst every effort has been made to ensure the accuracy of the advice given, the Publisher and the Authors, the Forestry Commission and Scottish Enterprise cannot accept liability for loss or damage arising from the information supplied. The assistance of Patrick Hislop, BA (Hons), RIBA, consultant architect, TRADA Technology Ltd as specialist contributor is also acknowledged. Note: Any further information issued in relation to this publication will be posted on www.trada.co.uk/greenoakinconstruction. Printed in England on paper with a 50% recycled content. 2 Green oak in construction 3 Green oak in construction The authors Peter Ross BSc, DIC, CEng, MICE Formerly an Associate Director of Ove Arup and now, on retirement, a consultant to the firm, he is a well-known timber designer of both new-build schemes and historic repairs, including the new roof at York Minster (Case Study 9.5). He is President of TRADA, a member of the BSI Timber Committee and was Technical Secretary of the 1994 edition of Eurocode 5. Christopher Mettem MSc, C Eng, FIWSc Chief Research Engineer, TRADA Technology, and BSI Technical Secretary Structural Timber Codes and Standards. He has led many government, European Commission and industry-funded research projects in the field of timber engineering. In recent years he has been focussing on information repackaging, knowledge transfer, teaching, technical writing and photo- graphic illustrating. Andrew Holloway BA(Hons) Founder and owner of The Green Oak Carpentry Company. He was the carpenter-builder of the multiple award-winning 2002 Weald and Downland Gridshell (Case Study 9.8) and more recently of the Savill Building, Windsor Great Park, winner of the 2006 Wood Awards Gold Award. Additionally the Company was awarded the 2006 Best Use of British Timber Award for the Savill Building and Chithurst Monastery (Figure 2.10). He is an experienced City and Guilds qualified timber frame carpenter, with wide-ranging crafts skills and key strengths in design. Photographs Cover photographs: main The Green Oak Carpentry Company; inset left to right 1, 2 The Green Oak Carpentry Company; 3 P Ross, 4 The Green Oak Carpentry Company. Frontispiece: The Green Oak Carpentry Company. Other photographs are acknowledged as they appear throughout the book. 4 Green oak in construction Contents Page Chapter 1 Introduction 7 Chapter 2 Green oak past and present 10 2.1 The medieval period 10 2.2 The post-medieval period 12 2.3 The present day 13 Chapter 3 The supply of green oak 16 3.1 Timber supply 16 3.2 Environmental issues 17 3.3 Costs 20 Chapter 4 The properties of oak 22 4.1 The living oak tree 22 4.2 Timber properties 23 4.3 Moisture content and drying movement 24 4.4 Durability 29 4.5 Strength properties 31 4.6 Creep deflection 33 4.7 Working qualities 33 4.8 Chemical properties 33 4.9 Behaviour in fire 34 Chapter 5 Design of green oak structures 35 5.1 The design of the frame 35 5.2 Performance of the structure 39 5.3 Historical forms as models for today 43 5.4 Drying movements 54 Chapter 6 The green oak framing process 56 6.1 The traditional approach 56 6.2 Automation 58 6.3 Selecting the timber 59 6.4 Appearance considerations 69 Chapter 7 Enclosing green oak structures 74 7.1 Design criteria and construction forms 74 7.2 Construction detailing 74 7.3 Drying movements and maintenance 77 7.4 Example details 77 Chapter 8 Exterior uses of green oak 90 8.1 General approach to durability in design 90 8.2 Appearance grading 95 8.3 External cladding 95 8.4 Decking 96 8.5 Bridges and other external structures 98 5 Green oak in construction Chapter 9 Case Studies 102 Historic reconstructions: 9.1 The Globe Theatre 104 9.2 Stirling Castle Roof 108 Traditional models: 9.3 Mill O’Braco house 112 9.4 Abingdon School Boathouse 114 Modern frames: 9.5 New roof to the South Transept of York Minster 118 9.6 Bedales School, Olivier Theatre 122 9.7 Darwin College, Cambridge 124 Innovative frames: 9.8 Weald and Downland Museum Gridshell 129 Exterior uses 9.9 The National Maritime Museum, Falmouth 133 9.10 Ealing Bridge 136 9.11 Polesden Lacey Bridge 139 Appendix I Specifying oak for traditional framing 141 AI.1 Defects 141 AI.2 Specification 143 Appendix II Green oak strength grading rules 148 AII.1 Scope 148 AII.2 The grades 148 AII.3 Definitions 149 AII.4 Measurement of features 150 AII.5 Grade limits 155 AII.6 Reference documentation 159 Appendix III Engineering design data 160 AIII.1 Strength design to BS 5268-2: 2002 160 AIII.2 Strength design to Eurocode 5 Part 1.1 161 AIII.3 Deflection of beams 161 AIII.4 Self weight of frames 163 AIII.5 Strength of pegged joints in tension 163 Appendix IV The green oak strength grading rules: Quick reference sheet 164 References and further reading 166 Glossary of terms 172 6 1 Introduction 1 Introduction Green oak can be defined simply as oak which has been freshly cut. Thus green oak construction is the craft of forming a building frame with members of green oak. It is not a new craft, but dates back many centuries and was used by generations of carpenters to make the historic barns, houses and church roofs which can be seen up and down the land. Neither is the craft confined to history – it is very much alive today, and has provided the stimulus for the production of this book. So what is the difference between working with green oak and modern timber construction? Today, most timber used for building is seasoned either Figure 1.1 Oak logs Photo: Forest Life Picture Library by kiln or air-drying. This dissipates the movements which occur as the timber gradually loses its excess moisture before it is used for fabrication. Why not simply do the same with oak? There are four basic problems with the use of seasoned oak for framing work, all of which relate to the properties of the material. In detail they are: x Hardness Even using power tools, seasoned oak is much more difficult to cut and shape than green. Anyone who has pruned roses will appreciate the difference between a green shoot and a dead branch. x Drying time Oak dries very slowly and typical frame members, say between 150 mm Figure 1.2 Prefabricated sections of green oak framing ready for delivery to site and 200 mm thick, will take some six to eight years of air-drying to lose Photo: C J Mettem excess moisture. Even drying the material in modern kilns would not achieve a reduction in this time for oak of such thickness. Since oak frames have always been individually designed there are no stocks of material and so a project would effectively be on hold for this period. x Drying movement Oak also has a large coefficient of drying shrinkage and so seasoned members would require a second cut after drying to true up the faces and to make the joints, which must fit accurately. x Fissures Seasoned oak has a marked tendency to fissure. Fissures complicate the cutting of joints and make edge moulding particularly difficult. The consequence of all this is that a frame in seasoned oak, whether medieval or modern, is simply not a commercial proposition. Green oak, in Photo not available in PDF edition contrast, makes great sense, giving advantages of programme – the timber can be used immediately, conversion is simpler – only one cut is required, and working is easier. However, it has to be borne in mind that the drying movements of the timber, which might otherwise have taken place in a kiln or timber yard, will now occur in service. It is an obvious requirement that the frame should never- Figure 1.3 The natural fissures which develop in a green oak frame as the timber theless remain stable, with no slackness developing in the joints, and that dries. the building envelope – the walls and the roof - should retain their integrity. 7 Green oak in construction Photos not available in PDF edition Figure 1.4 here and opposite page. Moffat House utilises a green oak structure for the upper floors on a steel framed storey below, combining modern and traditional materials and techniques to create a unique home.