Technical Guide - Timber Bridges - How to ensure their durability
With the environmental concerns, the bridges owners rediscover timber construction and its cordial aspect.
In addition, the timber bridges are light and easy to erect: they thushave a strong potential of development, in particular for pedestrianbridges.
However, the durability of these structures is mainly based on thechoice of the right initial drawings who protect the wood. It is alsobased on the quality of maintenance.
With the environmental concerns, the bridges owners rediscover timber construction and its cordial aspect.
In addition, the timber bridges are light and easy to erect: they thushave a strong potential of development, in particular for pedestrianbridges.
However, the durability of these structures is mainly based on thechoice of the right initial drawings who protect the wood. It is alsobased on the quality of maintenance. Moreover, in the event ofdemolition of a bridge whose wood were treated with chemicalpreservatives, the bridge owners are responsible for the producedwastes.
After the history of timber bridges, the guide published by Sétra :
* recalls the essential concepts on the anatomy and themechanical and physical properties of this material;
* presents the wood attackers and the methods used to preservewood, underlining the disadvantages of certain chemicalmethods;
* exposes the principal engineering drawings to ensure theprotection of timber structures;
* provides recommendations to the project managers toformalize a good choice of wood, to choose the treatments ofpreservation and the engineering drawings.
Synopsis
1. - General Presentation..............................................................................................................................................8
1.1. - WOOD: A LITTLE KNOWN MATERIAL TODAY............................................................................................... 8
1.2. - HISTORY OF TIMBER BRIDGES................................................................................................................... 10
1.2.1. – The first bridges.................................................................................................................................... 10
1.2.2. - Evolution of bridge construction and maintenance.......................................................................................... 11
1.2.3. - Durability, a forgotten design criterion......................................................................................................... 12
1.2.4. - An exception: timber bridges of Switzerland................................................................................................. 16
1.2.5. – Timber bridges of North America............................................................................................................. 17
1.2.6. - The return of wood.................................................................................................................................. 18
2. – Wood - The material ...........................................................................................................................................21
2.1. - GENERAL.................................................................................................................................................. 21
2.1.1 – Occurrence and availability....................................................................................................................... 21
Metropolitan Woods (or indigenous Woodsof Europe).............................................................................................. 21
The particular case of French Guiana......................................................................................................................... 22
The Northern woods.................................................................................................................................................. 23
Tropical woods........................................................................................................................................................... 23
2.1.2. – Sawn and reconstituted products............................................................................................................... 23
Sawing........................................................................................................................................................................ 24
Glued-laminated wood ( Glulam or glue-lam )............................................................................................................. 24
Industrial products “LVL, LSL and PSL”.................................................................................................................... 27
Panels......................................................................................................................................................................... 27
2.2. - ANATOMY OF WOOD................................................................................................................................. 28
2.2.1. – Untreated wood : a natural, living material................................................................................................. 28
2.2.2. – From the macroscopic to the microscopic...................................................................................................... 29
Wood is strong in both compression and bending....................................................................................................... 29
2.2.3. - Observation of the log.............................................................................................................................. 30
2.2.4. - The ligneous plan................................................................................................................................... 32
The ligneous plan of the coniferous trees.................................................................................................................... 33
The ligneous plan of leafy trees................................................................................................................................... 34
Ligneous plan and properties of wood......................................................................................................................... 34
2.2.5. - Cellular structure and chemical composition.................................................................................................. 36
2.3. - PHYSICAL PROPERTIES.............................................................................................................................. 37
2.3.1. - Wood and water..................................................................................................................................... 37
Wood moisture........................................................................................................................................................... 37
Drying........................................................................................................................................................................ 38
An anisotropic shrinkage............................................................................................................................................. 40
Influence of water on mechanical properties............................................................................................................... 44
2.3.2. - Behavior with respect to fire...................................................................................................................... 44
Flammability, reaction to fire....................................................................................................................................... 44
Stability with fire......................................................................................................................................................... 45
2.4. - MECHANICAL PROPERTIES OF WOOD........................................................................................................ 46
2.4.1. - Mechanical properties.............................................................................................................................. 46
Density....................................................................................................................................................................... 46
Orthotropism............................................................................................................................................................. 46
Rheology of wood....................................................................................................................................................... 48
Factors influencing performance................................................................................................................................. 49
2.4.2. - Dynamic Damping................................................................................................................................. 49
2.4.3. - Classification of solid wood....................................................................................................................... 49
Methods..................................................................................................................................................................... 49
Singularities of wood................................................................................................................................................... 52
2.4.4. - Creep................................................................................................................................................... 53
2.5. - DURABILITY.............................................................................................................................................. 53
2.5.1. - Aggressors............................................................................................................................................ 53
Fungi.......................................................................................................................................................................... 53
Insects........................................................................................................................................................................ 55
Marine borers............................................................................................................................................................. 57
Sun and rain................................................................................................................................................................ 57
2.5.2. - Preservation........................................................................................................................................... 57
General principle........................................................................................................................................................ 57
Classes of employment (standard EN 335).................................................................................................................. 60
Natural durability according to wood type................................................................................................................... 60
Natural durability and class of employment................................................................................................................. 61
Impregnability............................................................................................................................................................. 62
2.5.3. – Preservation treatments........................................................................................................................... 63
Products..................................................................................................................................................................... 63
Implementation.......................................................................................................................................................... 64
Requirements of penetration and retention.................................................................................................................. 65
Guarantees.................................................................................................................................................................. 66
2.5.4. - Finishes................................................................................................................................................ 67
Protective coatings...................................................................................................................................................... 67
Varnishes and paint.............................................................................................................................................. 67
Other finishes............................................................................................................................................................. 68
3. - Use of chemical preservatives: regulations and management at end of life ................................................69
3.1. -RESPECT OF CONSTRAINTS RELATED TO HEALTH AND THE ENVIRONMENT.............................................. 69
3.2. – PRESERVATION TREATMENTS FOR CLASS OF EMPLOYMENT 2................................................................... 69
3.3. – PRESERVATION TREATMENTS FOR CLASSES OF EMPLOYMENT 3, 4 AND 5................................................. 69
3.4. - REGULATIONS APPLICABLE TO TREATED WOOD,CONCERNING THE USE OF TOXIC PRODUCTS................. 70
3.4.1. - General regulation context........................................................................................................................ 70
3.4.2. – Regulation situation of traditional chemical treatments................................................................................... 71
Arsenic salts (CCA)..................................................................................................................................................... 71
Pentachlorophenol (PCP)............................................................................................................................................ 71
Creosote..................................................................................................................................................................... 72
3.4.3. - Alternative treatments............................................................................................................................. 72
Woods treated at high temperature.............................................................................................................................. 72
Substitutes for CCA.................................................................................................................................................... 73
3.5. - REGULATIONS CONCERNING TREATED WOOD WASTE:.............................................................................. 73
3.5.1. - Demolition of old structures...................................................................................................................... 73
3.5.2. - Treated wood waste: classification and nomenclature....................................................................................... 73
Wood treated with CCA or creosote: a waste classified as dangerous......................................................................... 73
3.5.3. – Channels of waste treatment:.................................................................................................................... 74
Particular case of wood waste contaminated by xylophagous insects (Article 10 of the decree of October 2, 1992)............... 75
3.5.4. - Obligation of the building owner, producer of waste........................................................................................ 75
3.6. - CONCLUSION............................................................................................................................................ 77
4. - Design of engineering structures in wood..................................................................................78
4.1 - Types of structures adapted to wood..........................................................................................78
4.1.1. - Principles of use of wood in bridges ................................................................................................ 78
Timber bridges and heavy vehicle traffic................................................................................................................... 78
General design principles............................................................................................................................................ 78
Importance of association of wood and other materials.............................................................................................. 79
4.1.2. - Various structures.................................................................................................................................. 81
Arch bridges............................................................................................................................................................... 81
The composite timber-concrete bridges....................................................................................................................... 83
Farm bridges............................................................................................................................................................... 86
Lattice beam bridges................................................................................................................................................... 87
Strut frame bridges...................................................................................................................................................... 89
Suspension bridges or stayed...................................................................................................................................... 92
Composite timber- steel bridges................................................................................................................................. 93
4.1.3. - Examples of footbridges........................................................................................................................... 94
Vaires footbridge........................................................................................................................................................ 94
Ajoux footbridge......................................................................................................................................................... 94
Footbridge at Saint-Jorioz........................................................................................................................................... 95
Footbridge in Grigny.................................................................................................................................................. 95
Bridges with full side beams........................................................................................................................................ 97
4.2. – DISEASES AND CAUSES OF DAMAGE........................................................................................................ 100
4.2.1. – Lack of drainage and ventilation............................................................................................................ 100 6Sealing...................................................................................................................................................................... 100
Wood badly ventilated............................................................................................................................................... 100
Assembly trapping water........................................................................................................................................... 101
Flat surface............................................................................................................................................................... 102
4.2.2. - Solar Aggression and humidity gradient.................................................................................................... 102
4.2.3. - Other causes........................................................................................................................................ 103
4.2.4. - Maintenance........................................................................................................................................ 104
4.3. - CONSTRUCTIVE PROVISIONS................................................................................................................... 105
4.3.1. - General Rules...................................................................................................................................... 105
4.3.2. - Covered bridges.................................................................................................................................... 105
4.3.3. - Boarding............................................................................................................................................. 106
Boarding arrangements............................................................................................................................................. 107
Protective hoods....................................................................................................................................................... 108
Protection of the end wood............................................................................................................................. 110
Handrail.................................................................................................................................................................... 111
4.3.4. - Joints................................................................................................................................................. 113
Some rules about joints............................................................................................................................................. 113
Ventilation of wood.................................................................................................................................................. 116
Joining using supports............................................................................................................................................... 118
Water traps............................................................................................................................................................... 119
4.3.5. - Flooring decks..................................................................................................................................... 119
Wood flooring.......................................................................................................................................................... 120
Bituminous flooring.................................................................................................................................................. 120
4.4. - CHOICE OF WOODS................................................................................................................................. 122
4.4.1. - Wood in class of employment 2................................................................................................................ 122
Parts concerned........................................................................................................................................................ 122
Woods usable........................................................................................................................................................... 122
Treatments................................................................................................................................................................ 122
4.4.2. - Wood in class of employment 3................................................................................................................ 122
Introductory remark.................................................................................................................................................. 122
Parts of structure concerned...................................................................................................................................... 122
Woods usable........................................................................................................................................................... 122
4.4.3. – Special case of boarding......................................................................................................................... 123
Generalities............................................................................................................................................................... 123
Woods with sufficient natural durability.................................................................................................................... 123
Durability conferred by treatment............................................................................................................................. 123
4.4.4. - Wood in class of employment 4 and 5....................................................................................................... 124
Parts concerned........................................................................................................................................................ 124
Types of wood usable............................................................................................................................................... 124
4.4.5. – Summary table of choice of woods............................................................................................................ 125
5. – Help for writing order...................................................................................................................................... 126
5.1. - DEFINITION OF THE ORDER.................................................................................................................... 126
5.1.1. - The program of the structure................................................................................................................... 126
5.1.2. - Qualification of the company................................................................................................................... 127
5.1.3. – Project Management............................................................................................................................. 127
5.1.4. - External Control................................................................................................................................. 127
5.2.1. - Documents to be supplied by the contractor................................................................................................. 128
5.2.2. - Plan of quality assurance (PAQ)............................................................................................................ 128
5.2.3. – Execution procedures............................................................................................................................ 130
5.2.4. - Constructive Provisions.......................................................................................................................... 130
5.2.5. – Regulation texts and calculations............................................................................................................ 131
5.2.6. - Forces, stresses, justifications................................................................................................................... 131
5.3. - SOURCE, QUALITY AND PREPARATION OF MATERIALS............................................................................. 131
5.3.1. – Wood material.................................................................................................................................... 131
Types of wood.......................................................................................................................................................... 131
Wood humidity......................................................................................................................................................... 132
Mechanical classification........................................................................................................................................... 132
Section of woods and tolerances...............................................................................................................................132
Adhesive................................................................................................................................................................... 133
Tropical woods......................................................................................................................................................... 133
Chemical interaction with metal................................................................................................................................ 133
Receiving.................................................................................................................................................................. 133
Requirements, implementation.................................................................................................................................. 134
7Certificates, attestations............................................................................................................................................. 134
Finish....................................................................................................................................................................... 134
5.4. – EXECUTION OF THE WORK..................................................................................................................... 135
5.4.1. - Execution and assembly of the wooden structure.......................................................................................... 135
5.4.2. - Tests of the structure............................................................................................................................. 135
5.4.3. - Internal control.................................................................................................................................... 135
5.4.4. - External control................................................................................................................................... 135
5.5. - THE UNIT AND CONTRACT PRICE SCHEDULE (CPS)................................................................................. 135
5.5.1. - Price of framework................................................................................................................................136
5.5.2. - Price of boarding.................................................................................................................................. 136
5.5.3. - Price of pedestrian flooring...................................................................................................................... 136
5.5.4. - Price of on-site assembly......................................................................................................................... 136
5.5.5. - Steel Price for assemblies........................................................................................................................ 137
5.5.6. - Price of protective coating........................................................................................................................ 137
5.5.7. - Price of tests........................................................................................................................................ 137
5.6. - FOLLOW-UP OF THE STRUCTURE............................................................................................................. 137
5.7. - SUMMARY: WHO DOES WHAT?................................................................................................................. 139
6. - Appendices ......................................................................................................................................................... 140
6.1. - LEXICON................................................................................................................................................. 140
6.2. - CLASSIFICATION PROCEDURE OF THE AUTHORIZED SUBSTANCES........................................................... 143
6.3. - EUROPEAN AND FRENCH REGULATIONS: HEALTH, ENVIRONMENT AND BIOCIDES DIRECTIVE............. 144
6.4. - PRINCIPAL PRODUCTS USED FOR WOOD PRESERVATION IN FRANCE, AND REGULATIONS........................ 147
6.5. – WASTE CLASSIFICATION PROCEDURE..................................................................................................... 148
Structure of the classification of waste....................................................................................................................... 148
6.6. - DANGEROUS WASTE...............................................................................................................................151
Components which make waste dangerous ................................................................................................ 151
Properties that make waste dangerous....................................................................................................... 152
6.7. - HOW TO FILL IN AN INDUSTRIAL WASTE FOLLOW-UP FORM (BSDI)......................................................... 154
6.8. - REGULATION RELATING TO WASTE......................................................................................................... 156
6.9 – STANDARD SECTIONS.............................................................................................................................. 157
6.10 - BIBLIOGRAPHICAL REFERENCES............................................................................................................. 158
PRINCIPAL STANDARDS.................................................................................................................................... 158
Vocabulary.................................................................................................................................................... 158
Safeguarding of wood........................................................................................................................................ 158
Classification.................................................................................................................................................. 160
Manufacture and tolerances................................................................................................................................161
Joints............................................................................................................................................................ 161
Adhesives...................................................................................................................................................... 162
Tests and measurements.................................................................................................................................... 162
Surface coatings............................................................................................................................................... 163
Panels........................................................................................................................................................... 163
DTU............................................................................................................................................................ 164
Waste regulations............................................................................................................................................ 164
6.11 - GUIDES AND OTHER TECHNICAL DOCUMENTS....................................................................................... 165
6.12. - TABLES OF THE FIGURES AND TABLES................................................................................................... 167
6.13. - TABLE OF THE PHOTOGRAPHS............................................................................................................... 169
6.14. - USEFUL ADDRESSES OTHER THAN SÉTRA.............................................................................................. 172
