US 0743A Timber Bridges

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november 2006

translate august 2007

Technical Guide

Timber BridgesHow to ensure their durability


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The Technical Department for Transport, Roads and Bridges Engineering and Road Safety (Service

d'tudes techniques des routes et autoroutes - Stra) is a technical department within the Ministry of

Transport and Infrastructure. Its field of activities is the road, the transportation and the engineering

structures.

The Stra supports the public ownerThe Stra supplies State agencies and local communities (counties, large cities and urban communities)

with informations, methodologies and tools suited to the specificities of the networks in order to:

improve the projects quality;

help with the asset management;

define, apply and evaluate the public policies;

guarantee the coherence of the road network and state of the art;

put forward the public interests, in particular within the framework of European standardization;

bring an expertise on complex projects.

The Stra, producer of the state of the artWithin a very large scale, beyond the road and engineering structures, in the field oftransport, intermodality, sustainable development, the Stra:

takes into account the needs of project owners and prime contractors, managers and operators;

fosters the exchanges of experience;

evaluates technical progress and the scientific results;

develops knowledge and good practices through technical guides, softwares;

contributes to the training and information of the technical community.

The Stra, a work in partnership The Stra associates all the players of the French road community to its action: operational services;

research organizations; Scientific and Technical Network (Rseau Scientifique et Technique de

l'Equipement RST), in particular the Public Works Regional Engineering Offices (Centresd'tudes techniques de l'Equipement CETE), companies and professional organizations;motorway concessionary operators; other organizations such as French Rail Network Company

(Rseau Ferr de France RFF) and French Waterways Network (Voies Navigables de France -

VNF); Departments like the department for Ecology and Sustainable Development

The Stra regularly exchanges its experience and projects with its foreign counterparts, throughbilateral co-operations, presentations in conferences and congresses, by welcoming delegations,

through missions and expertises in other countries. It takes part in the European standardization

commissions and many authorities and international working groups. The Stra is an organization

for technical approval, as an EOTA member (European Organisation for Technical Approvals).


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With the publics environmental concerns, building owners are rediscovering the warmappearance of wood. Moreover, timber bridges are light and easy to erect, something appreciablein light of the reduction of problems during work.

But certain bridges, although built of preserved wood during the past decades, already showserious damage. When they must be demolished, the owners are responsible for the toxic wastesresulting from the treated wood.

It was necessary to reaffirm that the durability of timber structures rests mainly on the quality ofmaintenance, and above all on the initial choice of good constructive provisions, to protect asensitive material of organic origin.

There should be no opposition to well-designed, treated wooden structures. It is in fact advisableto combine the two approaches. A well-designed, sheltered wooden structure represents a long-lasting development, which may then be legitimately treated by fungicides and insecticides, whichwould have been less effective on a bad design.

Further, for coverings and guard rails in contact with the public, exotic woods from forestsmanaged with no compromise for the future, that remain durable, require no treatment and pose

no health hazard, have no reason to be excluded.

To point out these pitfalls, and to promote a material too long forgotten, Straconsidered itusefulto publish a guide book devoted to bridges, for the building owners who choose to buildwith wood.

J. BerthellemyTechnical Director at the Center for Structures of Stra

(Technical Center for Highways and Motorways).This document was written by:

Vincent BARBIER, CETE Est (Technical Engineering Center for Infrastructure, East);

Jacques BERTHELLEMY, Stra;Dominique CALVI;Stella JELDEN, CETE Est;

Jean-Louis CHAZELAS, LCPC (Central Public Works Research Laboratory);Pierre CORFDIR, CETE Est;

Jrome LAPLANE, architect representing the CNDB (National Committee for Timber);Robert LEROY, LCPC;

in a work group led byJacques BERTHELLEMY,from a first project drawn up byVincent BARBIER,with the ENSTIB,(National Teaching-Institute for the Techniques and Industries of Timber ) in pinal.

We also thank, for their comments and observations:

Hlne ABEL-MICHEL, Nathalie ODENT and Michel FRAGNET Stra;Thierry KRETZ, LCPC;Daniel POINEAU, retired engineer,Sandrine ROCARD, Emilie DERIVIRE and Frederic LERAY,

Ministry for Ecology and Sustainable Development;Bernard REY, architect SNCF;Serge LENEV, CTBA;Tarek FAR, CETE the Mediterranean;Pierre TROUILLET, M.C.S.A.C.


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Synopsis

1. - General Presentation..............................................................................................................................................8

1.1. - WOOD:A LITTLE KNOWN MATERIAL TODAY ............................................................................................... 81.2. - HISTORY OF TIMBER BRIDGES ................................................................................................................... 10

1.2.1. The first bridges.................................................................................................................................... 101.2.2. - Evolution of bridge construction and maintenance.......................................................................................... 111.2.3. - Durability, a forgotten design cri terion......................................................................................................... 121.2.4. - A n exception: timber bridges of Switzerland................................................................................................. 161.2.5. Timber bridges of N orth America............................................................................................................. 171.2.6. - The return of wood.................................................................................................................................. 18

2. Wood - The material...........................................................................................................................................21

2.1. - GENERAL.................................................................................................................................................. 212.1.1 Occurrence and availability....................................................................................................................... 21

Metropolitan Woods (or indigenous Woodsof Europe).............................................................................................. 21The particular case of French Guiana......................................................................................................................... 22The Northern woods.................................................................................................................................................. 23Tropical woods........................................................................................................................................................... 23

2.1.2. Sawn and reconstituted products............................................................................................................... 23Sawing........................................................................................................................................................................ 24Glued-laminated wood ( Glulam or glue-lam )............................................................................................................. 24Industrial products LVL, LSL and PSL.................................................................................................................... 27Panels......................................................................................................................................................................... 27

2.2. - ANATOMY OF WOOD................................................................................................................................. 282.2.1. Untreated wood : a natural, living material................................................................................................. 282.2.2. From the macroscopic to the microscopic...................................................................................................... 29

Wood is strong in both compression and bending....................................................................................................... 292.2.3. - Observation of the log.............................................................................................................................. 302.2.4. - The ligneous plan................................................................................................................................... 32

The ligneous plan of the coniferous trees.................................................................................................................... 33The ligneous plan of leafy trees................................................................................................................................... 34Ligneous plan and properties of wood......................................................................................................................... 34

2.2.5. - Cellular structure and chemical composition.................................................................................................. 362.3. - PHYSICAL PROPERTIES .............................................................................................................................. 37

2.3.1. - Wood and water..................................................................................................................................... 37Wood moisture........................................................................................................................................................... 37Drying........................................................................................................................................................................ 38An anisotropic shrinkage............................................................................................................................................. 40Influence of water on mechanical properties............................................................................................................... 44

2.3.2. - Behavior with respect to fire...................................................................................................................... 44Flammability, reaction to fire....................................................................................................................................... 44

Stability with fire......................................................................................................................................................... 452.4. - MECHANICAL PROPERTIES OF WOOD ........................................................................................................ 46

2.4.1. - Mechanical properties.............................................................................................................................. 46Density....................................................................................................................................................................... 46Orthotropism............................................................................................................................................................. 46Rheology of wood....................................................................................................................................................... 48Factors influencing performance................................................................................................................................. 49

2.4.2. - Dynamic Damping................................................................................................................................. 492.4.3. - Classification of solid wood....................................................................................................................... 49

Methods..................................................................................................................................................................... 49Singularities of wood................................................................................................................................................... 52

2.4.4. - Creep................................................................................................................................................... 532.5. - DURABILITY .............................................................................................................................................. 53

2.5.1. - A ggressors............................................................................................................................................ 53Fungi.......................................................................................................................................................................... 53Insects........................................................................................................................................................................ 55


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Marine borers............................................................................................................................................................. 57Sun and rain................................................................................................................................................................ 57

2.5.2. - Preservation........................................................................................................................................... 57General principle........................................................................................................................................................ 57Classes of employment (standard EN 335).................................................................................................................. 60Natural durability according to wood type................................................................................................................... 60Natural durability and class of employment................................................................................................................. 61Impregnability............................................................................................................................................................. 62

2.5.3. Preservation treatments........................................................................................................................... 63Products..................................................................................................................................................................... 63Implementation .......................................................................................................................................................... 64Requirements of penetration and retention.................................................................................................................. 65Guarantees.................................................................................................................................................................. 66

2.5.4. - Finishes................................................................................................................................................ 67Protective coatings...................................................................................................................................................... 67Varnishes and paint.............................................................................................................................................. 67Other 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 .............................................. 693.2. PRESERVATION TREATMENTS FOR CLASS OF EMPLOYMENT 2................................................................... 693.3. PRESERVATION TREATMENTS FOR CLASSES OF EMPLOYMENT 3, 4AND 5................................................. 69

3.4. - REGULATIONS APPLICABLE TO TREATED WOOD, CONCERNING THE USE OF TOXIC PRODUCTS................. 703.4.1. - General regulation context........................................................................................................................ 703.4.2. Regulation situation of traditional chemical treatments................................................................................... 71

Arsenic salts (CCA)..................................................................................................................................................... 71Pentachlorophenol (PCP)............................................................................................................................................ 71Creosote..................................................................................................................................................................... 72

3.4.3. - A lternative treatments............................................................................................................................. 72Woods treated at high temperature.............................................................................................................................. 72Substitutes for CCA .................................................................................................................................................... 73

3.5. - REGULATIONS CONCERNING TREATED WOOD WASTE:.............................................................................. 733.5.1. - Demolition of old structures...................................................................................................................... 733.5.2. - 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:.................................................................................................................... 74Particular case of wood waste contaminated by xylophagous insects (A rticle 10 of the decree of October 2, 1992)............... 75

3.5.4. - Obligation of the building owner, producer of waste........................................................................................ 753.6. - CONCLUSION ............................................................................................................................................ 77

4. - Design of engineering structures in wood..................................................................................78

4.1 - Types of structures adapted to wood..........................................................................................784.1.1. - Principles of use of wood in bridges....................................................................... ......................... 78

Timber bridges and heavy vehicle traffic................................................................................................................... 78General design principles............................................................................................................................................ 78Importance of association of wood and other materials.............................................................................................. 79

4.1.2. - V arious structures.................................................................................................................................. 81Arch bridges............................................................................................................................................................... 81

The composite timber-concrete bridges....................................................................................................................... 83Farm bridges............................................................................................................................................................... 86Lattice beam bridges................................................................................................................................................... 87Strut frame bridges...................................................................................................................................................... 89Suspension bridges or stayed...................................................................................................................................... 92Composite timber- steel bridges................................................................................................................................. 93

4.1.3. - Examples of footbridges........................................................................................................................... 94Vaires footbridge........................................................................................................................................................ 94Ajoux footbridge......................................................................................................................................................... 94Footbridge at Saint-Jorioz ........................................................................................................................................... 95Footbridge in Grigny.................................................................................................................................................. 95

Bridges with full side beams........................................................................................................................................ 974.2. DISEASES AND CAUSES OF DAMAGE........................................................................................................ 1004.2.1. Lack of drainage and venti lation............................................................................................................ 100


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Sealing...................................................................................................................................................................... 100Wood badly ventilated............................................................................................................................................... 100Assembly trapping water........................................................................................................................................... 101Flat surface............................................................................................................................................................... 102

4.2.2. - Solar A ggression and humidity gradient.................................................................................................... 1024.2.3. - Other causes........................................................................................................................................ 1034.2.4. - Maintenance........................................................................................................................................ 104

4.3. - CONSTRUCTIVE PROVISIONS................................................................................................................... 1054.3.1. - General Rules...................................................................................................................................... 105

4.3.2. - Covered bridges.................................................................................................................................... 1054.3.3. - Boarding............................................................................................................................................. 106Boarding arrangements............................................................................................................................................. 107Protective hoods....................................................................................................................................................... 108Protection of the end wood............................................................................................................................. 110Handrail.................................................................................................................................................................... 111

4.3.4. - Joints................................................................................................................................................. 113Some rules about joints............................................................................................................................................. 113Ventilation of wood.................................................................................................................................................. 116

Joining using supports............................................................................................................................................... 118Water traps............................................................................................................................................................... 119

4.3.5. - Flooring decks..................................................................................................................................... 119Wood flooring.......................................................................................................................................................... 120Bituminous flooring.................................................................................................................................................. 120

4.4. - CHOICE OF WOODS................................................................................................................................. 1224.4.1. - Wood in class of employment 2................................................................................................................ 122

Parts concerned........................................................................................................................................................ 122Woods usable........................................................................................................................................................... 122

Treatments................................................................................................................................................................ 1224.4.2. - Wood in class of employment 3................................................................................................................ 122

Introductory remark.................................................................................................................................................. 122Parts of structure concerned...................................................................................................................................... 122Woods usable........................................................................................................................................................... 122

4.4.3. Special case of boarding......................................................................................................................... 123Generalities............................................................................................................................................................... 123Woods with sufficient natural durability.................................................................................................................... 123Durability conferred by treatment............................................................................................................................. 123

4.4.4. - Wood in class of employment 4 and 5....................................................................................................... 124Parts concerned........................................................................................................................................................ 124

Types of wood usable............................................................................................................................................... 1244.4.5. Summary table of choice of woods............................................................................................................ 125

5. Help for writing order......................................................................................................................................126

5.1. - DEFINITION OF THE ORDER.................................................................................................................... 1265.1.1. - The program of the structure................................................................................................................... 1265.1.2. - Qualification of the company................................................................................................................... 1275.1.3. Project Management............................................................................................................................. 1275.1.4. - E xternal Control................................................................................................................................. 1275.2.1. - Documents to be supplied by the contractor................................................................................................. 128

5.2.2. - Plan of quality assurance (PAQ)............................................................................................................ 1285.2.3. Execution procedures............................................................................................................................ 1305.2.4. - Constructive Provisions.......................................................................................................................... 1305.2.5. Regulation texts and calculations............................................................................................................ 1315.2.6. - Forces, stresses, justifications................................................................................................................... 131

5.3. - SOURCE, QUALITY AND PREPARATION OF MATERIALS............................................................................. 1315.3.1. Wood material.................................................................................................................................... 131

Types of wood.......................................................................................................................................................... 131Wood humidity......................................................................................................................................................... 132Mechanical classification........................................................................................................................................... 132Section of woods and tolerances............................................................................................................................... 132Adhesive................................................................................................................................................................... 133

Tropical woods......................................................................................................................................................... 133

Chemical interaction with metal ................................................................................................................................ 133Receiving.................................................................................................................................................................. 133Requirements, implementation.................................................................................................................................. 134


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Certificates, attestations............................................................................................................................................. 134Finish....................................................................................................................................................................... 134

5.4. EXECUTION OF THE WORK..................................................................................................................... 1355.4.1. - Execution and assembly of the wooden structure.......................................................................................... 1355.4.2. - Tests of the structure............................................................................................................................. 1355.4.3. - Internal control.................................................................................................................................... 1355.4.4. - External control................................................................................................................................... 135

5.5. - THE UNIT AND CONTRACT PRICE SCHEDULE (CPS) ................................................................................. 1355.5.1. - Price of framework................................................................................................................................ 136

5.5.2. - Price of boarding.................................................................................................................................. 1365.5.3. - Price of pedestrian flooring...................................................................................................................... 1365.5.4. - Price of on-site assembly......................................................................................................................... 1365.5.5. - Steel Price for assemblies........................................................................................................................ 1375.5.6. - Price of protective coating........................................................................................................................ 1375.5.7. - Price of tests........................................................................................................................................ 137

5.6. - FOLLOW-UP OF THE STRUCTURE ............................................................................................................. 1375.7. - SUMMARY:WHO DOES WHAT?................................................................................................................. 139

6. - Appendices.........................................................................................................................................................140

6.1. - LEXICON ................................................................................................................................................. 1406.2. - CLASSIFICATION PROCEDURE OF THE AUTHORIZED SUBSTANCES ........................................................... 143

6.3. - EUROPEAN AND FRENCH REGULATIONS: HEALTH, ENVIRONMENT AND BIOCIDES DIRECTIVE ............. 1446.4. - PRINCIPAL PRODUCTS USED FOR WOOD PRESERVATION IN FRANCE,AND REGULATIONS........................ 1476.5. WASTE CLASSIFICATION PROCEDURE ..................................................................................................... 148

Structure of the classification of waste....................................................................................................................... 1486.6. - DANGEROUSWASTE ............................................................................................................................... 151

Components which make waste dangerous ................................................................. ............................... 151Properties that make waste dangerous....................................................................................................... 152

6.7. - HOW TO FILL IN AN INDUSTRIAL WASTE FOLLOW-UP FORM (BSDI)......................................................... 1546.8. - REGULATION RELATING TO WASTE ......................................................................................................... 1566.9 STANDARD SECTIONS.............................................................................................................................. 1576.10 - BIBLIOGRAPHICAL REFERENCES............................................................................................................. 158PRINCIPAL STANDARDS.................................................................................................................................... 158

V ocabulary.................................................................................................................................................... 158Safeguarding of wood........................................................................................................................................ 158Classification.................................................................................................................................................. 160Manufacture and tolerances................................................................................................................................ 161Joints............................................................................................................................................................ 161Adhesives...................................................................................................................................................... 162Tests and measurements.................................................................................................................................... 162Surface coatings............................................................................................................................................... 163Panels........................................................................................................................................................... 163DTU............................................................................................................................................................ 164Waste regulations............................................................................................................................................ 164

6.11 - GUIDES AND OTHER TECHNICAL DOCUMENTS....................................................................................... 1656.12. - TABLES OF THE FIGURES AND TABLES ................................................................................................... 167

6.13. - TABLE OF THE PHOTOGRAPHS............................................................................................................... 1696.14. - USEFUL ADDRESSES OTHER THAN STRA .............................................................................................. 172


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1. -General Presentation

1.1. - Wood: a little known material today

Wood is a material that we are rediscovering, because of technical progress in the wood trades, its warmappearance, and the publics concerns with the environment. Wood contributes to the renewal of thearchitectural quality of structures, in both urban and rural areas.

In addition timber structures are light and easy to erect, a not unappreciable fact when the hindrances toexisting roads must be reduced as much as possible. The footbridge at Vaires-sur-Marne, erected in a fewhours, at night, in one piece, is a shining example of this. Wooden engineering structures, particularlyfootbridges, thus have a large development potential.

Photograph 1: Erection of the railway-station bridge at Vaires-sur-Marne.

However the initial color of a new wooden structure changes in fact to grey after a few years exposure toweather, and only regular application of preservative will prevent this.

Further, certain woods from temperate forests that are used for outdoor construction, were treated withinsecticides and fungicides that may be harmful to health. Treated woods require precautions during usewhere the health of the workers who have to machine, bore or cut a contaminated material is concerned.Moreover, with respect to the environment, the treatments limit the possibilities of timber recycling at thestructures end-of-life.

European directive 2003/2/EC of January 6, forbids the use of some of these toxic products, in particularthose based on arsenic salts.

Exotic woods generally do not show such disadvantages but are likely on the other hand to come fromcountries where the exploitation of forests does not meet the current requirements of durablemanagement, implemented in French forests since the Rio agreements of June 1992.

It is true that timber construction makes it possible to trap carbon dioxide during the structureslifetime. Carbon dioxide is the major cause of the greenhouse effect after water vapor, and a long-lasting

wooden structure thus fixes carbon. On this subject, the application in France of the 1997 Kyotoagreements resulted in a charter aimed at increasing timbers share of the construction industry.


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The 2004 Climate Plan envisaged for example actions on this topic with the inventory and display of theamount of wood used in the construction industry, the evaluation by lEquipement des engagements dessignataires de laccord cadre Wood construction - environment of March 28, 2001, and theexemplarity of the State which committed to using wood in public projects.

However, no mechanism is in place to compensate a client who builds in wood for this carbon sink, eitheras part of an emissions permit contract or by another system. At the European level, in 2005 a two-phasequota system was set up, but it concerns only the producers of energy and the industrialists emitting largequantities of carbon dioxide.

It is only at the international level that each State will be accountable for its carbon wells and may benefitfrom them since that will give rise to international credits exchangeable between States. But timberconstructions do not involve sufficient quantities of carbon and the Kyoto protocol applies only to arablelands, pastures and forests under the term of carbon wells.

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Certain structures completed in the past twenty years show problems of premature damage. The lifespanof recent structures is too often less than twenty years, whereas certain very old structures are still in goodcondition. The bridge at Lucerne, in Switzerland, that endured more than six centuries before anaccidental fire in 1993, is a well-known example.

This important discrepancy in the lifespan of wooden structures may be explained by a loss ofcompetence in the use of the material. Since the 14th century, the use of wood has declined in France,giving way to the use of stone, a more expensive material but requiring less maintenance than wood.Maintenance was considered too constraining by the building owners. In the 20th century, with thedevelopment of concrete and metal, this phenomenon was further accentuated. In France, the importantideas about the behavior of wood and construction regulations were even forgotten with time. However,the durability of the structures rested essentially on the choice of good constructive provisions and on thequality of maintenance.

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This guide is intended for building owners and project managers who wish to have a wooden structurebuilt. Its objective is to show how to obtain a good lifespan.

Taking into account the lack of references available in France on the subject, the guide initially draws up ahistory of wooden bridges : this chapter shows to what extent the care taken in design represents the bestmethod of wood preservation and determines its lifespan. Then, the guide gives essential ideas on theanatomy and the mechanical and physical properties of this material. It also shows the aggressors and themethods of wood preservation, by underlining the disadvantages of certain chemical preservationmethods.

The guide then shows the major constructive dispositions to adopt to ensure the wood is kept dry andwell ventilated.

Finally, the last part helps the project manager to formalize good wood choices, preservation treatmentsand constructive provision requirements.

Obviously, the good lifespan of a wooden structure also implies for the client regular monitoring andmaintenance to ensure the woods good healthy condition.


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1.2. - History of timber bridges

1.2.1. The first bridges

Wood was used as far back as the Neolithic era to cross rivers. It is estimated that 17,000 years ago,covered logs laid flat made up the first wooden bridges, but with spans limited to about ten meters.Herodotus described structures with increased spans to cross the Euphrates or certain tributaries of theNile 2,000 to 3,000 years ago. Most detail is found on a bridge completed in Babylon some 2790 years ago.

Generally speaking, during antiquity, technical progress in wood structures should be credited toshipbuilding, in particular by the Egyptians, the Phoenicians, the Greeks and the Celts.

Figure 1: Egyptian ships Figure 2: bridge made of boats

The soldiers of the continental empires used floating barges as intermediate piers. One can quote asexamples the bridge of Darius over the Bosphorus in the 6th century B.C. and that of his son Xerxs overthe Dardanelles Straits, where 674 boats crossed an obstacle of about 1500 meters. The Romans started byborrowing construction techniques from the Celts, as was the case with Caesar's bridge over the Rhine

that was built to carry a Roman army into Germany. The bridge was built with simple, ready-made units,and was easy to erect and then to dismantle after the passage of the army. At 5 to 6 meters wide, it wasbuilt in only 10 days, near Neuwied, where the width of the river was 140 meters.

Figure 3: Caesars bridge over the Rhine according to the reconstruction by Andrea Palladio

Then the Romans developed new more complex structures with joints, in particular bridges with beams,strut frames and arches. Among this last type, one can quote as an example the bridge at Trajan on the

Danube, dating back to 103 and crossing 1,100 meters in 35 meter spans.


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In Asia, one found boat bridges and various types of beam, strut orportal frame or arch bridges, as well as crossings achieved with vinesuspension bridges. On the island of Java, the suspension bridge goes wayback in time. The outside cantilevering abutment was also developed inAsia to increase the spans.

Figure 4: outside cantilevering abutmentaccording to a drawing of Viollet-le-Duc

1.2.2. - Evolution of bridge construction and maintenance

During medieval times, constructors became aware that rot was woods major enemy and that it could beavoided by keeping the material dry.

In Europe, timber bridges were then very common. Charlemagne, for example, had a very large structurebuilt around 800 on the Rhine at Mainz, that was unfortunately burnt in 813. During the following

centuries, the bridges crossing the Seine in Paris were among the most renowned and contributed to thecitys historical role. The same type of structure was also found in Cologne.

The timber deck bridges were then generally built with stone piers on timber pile foundations. They weregenerally surmounted by houses, with the aim of protecting the structure from bad weather. This wasparticularly the case in Paris.

The tolls collected by the City paid for the work and ensured the subsistence of a corporation responsiblefor bridge maintenance. The techniques used were rather advanced, and both labor and raw materials wereabundant: support measures were taken at each period of low water level, parts were changed, andpreservation treatments using boiling oil were also probably implemented.

Figure 5: collapse of the bridge at Petit-Chtelet in Paris during the winter of 1407

This old engraving is in a way testimony to this expertise, but it represents the collapse of a Parisian builtbridge, that of Petit-Chtelet, in January 1407, during a flood on the Seine carrying ices : the Great Plague

of 1349 and the war, by causing terrible damage to the demography and the economy, had probably led tothe progressive abandonment of good maintenance practices on engineering structures.


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Another structure, the Notre-Dame Bridge, built out of wood from 1413 to 1420, collapsed in 1499 withits 60 houses, through lack of maintenance. The provost of the merchants and the municipal magistratespaid for this negligence with their freedom.

For precautions sake and by royal decision, no more timber bridges were built. In Italy, Andr Palladiopublished an architectural treatise recommending that if timber bridges were built they should at least becovered. In spite of the advantages of the guidelines proposed by Palladio, it seems that not much use wasmade of them in France, where timber bridges were rather badly looked upon by the bourgeoisie becauseof the rigorous maintenance requirements.

Figure 6: project for the "Pont des Arts" in Paris drawn according to the ideas of Andrea Palladio.Note the masonry piers, on timber pile foundations

The Tournelle bridge , linking Isle Saint-Louis to Pariss left bank , may be taken as characteristic of thehistory of timber bridges in France from the 14th century : built in 1369, it collapsed under a flood on theSeine. Rebuilt in 1620, it was again carried away by an exceptional ice break up in 1637. A temporarybridge built in 1640 was carried away by the Seine in 1651. It was then replaced by a stone bridge,completed in 1655, and long considered final. But the large width of its 5 piers in the river aggravated theseriousness of the Seine 1910 flood, causing its partial collapse. It was demolished after the hostilities in1919.

Figure 7: Pont-Rouge in Paris

The above engraving represents the center of Paris seen from the left bank of the Seine, around 1680. Thebridge between lIle de la Cit and lI le Saint-Louis is a timber bridge, neither covered nor built, calledPont-Rouge. The Tournelle bridge, built of stone in 1655, is located at the extreme right of the picture.

1.2.3. - Durability, a forgotten design criterion

During the 18th century, the non-temporary bridges were built in masonry. With regard to timberstructures, Perronet noted from experience, in particular with the Saint-Cloud bridge, that the lifespan of a

wooden beam left uncovered in the Paris area was limited to 25 years. This is why he recommended thecomplete covering of wooden frames by lead sheets to prevent them from rotting.


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Perronet hastened to add in his report, after an economic justification calculation: In spite of the longerlifespan we may give to timber bridges, it will always be preferable to make them entirely in masonry,when the materials are not too distant and expensive.

For financial reasons, temporary timber bridges were built in France. Thus, in 1719 in Lyon, economicconditions forced the engineer Garrin to give up the initial plan of a metallic arc, approved before 1685 byColbert, and then during construction, to not take alone the financial risks linked to the innovation: theconstruction on the Rhone was finally built out of wood.

Similarly, several construction engineers, Goiffon, Calippe and de Montpetit, proposed in vain on severaloccasions, for financing reasons, metal arches in Lyon between 1755 and 1779 to compete with stone. Afirst iron arch of 25 m span was put up in 1755 on a three-arch structure. The following arches were putup in timber for economic reasons, and the short lifespan of the structure did not allow it to become asrenowned as that of Coalbrookdale, a metal bridge completed in England in 1779 and still in service.

In the case of the bridges in Lyon, the choice of wood, as a substitute for iron, is recommended byPerronet, who has mastered the arch technique. These structures, structurally excellent, are constructed asarches, but unfortunately without considering Palladios recommendations to ensure their durability, i.e.with no covering except that of a few lead sheets. Further, the timber bridges at la Salptrire and laMulatire have only timber piling, not stone.

The Tournus bridge (figure 8), built of wood on the Saone in 1801, has masonry supports andrepresented, with spans of approximately 30 meters, Frances most successfully completed arch bridge.. I tsmechanical and aesthetic design was extremely neat, with small, radiant posts. But its designers continued,wrongly and perhaps without knowing, to respect the narrow interpretation of the Parliaments concerningold French laws prohibiting building on a wooden bridge and thus did not envisage a covering.

All these structures thus unfortunately disappeared rather quickly, because the building owners neglectedto provide the necessary maintenance resources, since wood for construction was rare and expensive atthe end of the 18th century.

Figure 8: partial elevation of the Tournus bridge .This bridge is not very different from the Roman bridge of Cologne built in 310

In Bavaria, the bridges built by Wiebeking in the 19th century were arches on the same model as thebridge at Tournus, but with much wider spans going up to 45 meters at Freyssingen in 1807 and 72 metersat Bamberg in 1809. La Planche 1 dates back to 1810, where Wiebeking, General manager of the RoadDirectorate of Bavaria, plans a bridge in Munich. But twenty years later, these bridges rotted and finally

had to be replaced and Wiebeking was subjected to the Kings mocking remarks : You are a genius of thestatic, but your bridges do not have the required durability .


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Board 1: Project of crossing of Isar in only one arch (Wiebeking, Munich 1810).


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Between 1823 and 1850, the calculations of Navier and the engineering genius of the Seguin brothers re-launched the use of wood for the decks of the first generation of suspension bridges. These bridges of thenational road network were then often granted to companies. Wood was selected for its lightness. Amongthis kind of bridge, we can mention the two bridges of the Seguin brothers on the Rhone between Tainand Tournon. The first suspension bridge, built in 1825, had two 85-meter spans. The structure was madeup of two lots of six cables of one hundred twelve strands of wire 3 mms in diameter, on which weresuspended oak beams 30 cm by 16 cm. Following the development of steam travel , this bridge was raisedand transformed into a footbridge before being destroyed, contrary to the opinion of the town council, in

1965. But another suspension bridge of the same type as the first, built in 1847, remains today atestimonial to the period.

Photographs 2: Tournon bridge of 1847

However wood in general conferred insufficient strength to the structure, and the principles of Palladiowere still often ignored. The accident at the Basse Chaine bridge built in 1834 at Angers caused 226 deathsin 1848. There were other accidents, particularly in 1852 at the Roche-Bernard on the structure of 198meter span built in 1836, then on the large bridge at Cubzac. Consequently suspension bridges withwooden decks are used only on minor roads.

Such works continued neverthelessto be built as this crossing of theMarne testifies, still in service in the1960s.(photo 3)

The beam and the first timber deckof the Grosle bridge built in 1912,were kept in service until 1973 withan 8-ton limit. They were replacedin 1977 by an aluminum girderlinked to a light-concrete slab.

Photograph 3: bridge on the Marne

The Montmerle bridge had already profited from a reinforcement of the same type, after a seriousaccident occurred because of non-respect, by a heavy truck, of the traffic signals. ( photo 4)


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Photograph 4: bridge of Montmerle (Ain)

In the 20th century we find the Tournelle bridgethat will serve as a reference point. The temporarytimber bridge (figure 9), built in 1920, did not asenvisaged allow traffic: it was prone to annoyingdynamic phenomena. In addition, the currentbridge was only completed in 1928 for lack ofcredit. During these eight years, the Parisian pressopened up and gave a last look at the prestigealready started up of the wooden structures in our

country, not missing out on the Bridges, Roads andShipping departments.

Figure 9: press article

1.2.4. - An exception: timber bridges of Switzerland

The bridge of the chapel of Lucerne (photo 5) dates from 1333. It is a striking example of longevity. Forcenturies, certain parts were rebuilt, and in 1993 most of the structure was destroyed by fire. It was rebuiltto the original in 1994.


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Photographs 5:bridge of the chapel of Lucerneand bridge in the area of Davos,probably built in the 18th century

Many other very old covered structures are still in service in Switzerland, even if they support todayreduced traffic only.

The Swiss carpenters Hans Ulrich and Jean Grubenmann were the project managers of the Schaffhousebridges (figure 10), with two spans of 60 meters in 1758, and Wettingen in 1778 whose span was 110meters. These two structures were covered. Both were unfortunately burnt in 1799 by vandalism, so thatone is unaware of how long the Schaffhouse bridge would have lasted : it had already more than 40 yearsof service at the time of its destruction.

Figure 10: Schaffhouse bridge (Switzerland)

1.2.5. Timber bridges of North America.

In the United States, it is estimated that around 10,000 covered bridges were built between 1805 and 1885.The wooden structures succeed today in still keeping an honorable share of the market, particularly onminor roads, since 7 % of bridges are still wood. This may be explained by a preserved know-how, andthe always abundant presence of forests which still cover, for example, 89 % of the surface of the State ofMaine.

Among these covered bridges are many that have lasted more than a hundred years even though at thetime no chemical preservation treatment was given.

Creosote was the subject of a US patent in 1831. To obtain this product, certain toxic wastes from the

chemical and iron and steel industries were added to carbon oils or petrol. Impregnation with arsenic,chromium and copper salts (CCA),appeared in the USA in 1933, and pentachlorophenol in 1935.


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Thanks to protection treatments against the bad weather, in particular coatings based on petroleumproducts, the cover of the covered bridges was gradually abandoned. In the United States, in Canada andin Australia, timber bridges with lattice structures were developed in the middle of the 19th century, aswere composite timber-steel structures.

The bridge known as Sioux - Narrows is located on highway 71 just north of Kenora in Ontario. Built

in 1936, it has a 64 meter isostatic span. It is a Warren bridge without cover which constitutes the largesttimber bridge of this type in North America and which was recently quoted in an OECD report as theexample of a correctly designed, built and maintained timber bridge whose lifespan might prove to becomparable to that of a steel bridge. However the structure was closed to traffic in 2003 for safety reasonsand doubled by a Bailey bridge. In spite of its great interest for local tourism, it has not yet been decidedif it will be rebuilt as original.

Further, a design of decks from pre-stressed wood was developed in Canada in the Seventies and imitatedin the United States. That consists of compressing joists using metal bars.

1.2.6. - The return of wood

For the past twenty years, there has been a renewed interest in wood in Europe. Thus in the Germaniccountries, it is used for footbridges and low-load bridges.

Glued-laminated wood was invented by the Swiss Otto Hetzer, who patented his discoveries between1891 and 1910 in Germany. Some of these discoveries were inspired by ideas published in 1561 byPhilibert de l'Orme, a contemporary of Andrea Palladio. Many technical developments were made in the20th century, mainly in North America. Glue-lam allowed construction of arch bridges where protectionagainst the rain is ensured by the roadway or long-length, large-section beam bridges

.

Photograph 6: Keystone-Wye bridge in South-Dakota.

Glue-lam arch sheltered under a flagstone (1968)

Photograph 7: Cocteau footbridge built in Nimes.

Glue-lam arch without protection against shocksand rain (1975)

Nevertheless, bad habits are hard to break: in France many bridges, like the footbridge at Montigny-les-Cormeilles had to be quickly demolished and rebuilt. In Nmes, the Cocteau footbridge, which wassubjected to the shock of an oversize vehicle in the Eighties had to be rebuilt. Moreover, the climate in thearea and water stagnations cause first localized rotting, then the arrival of Capricorn beetles. These insectsdig tunnels filled with sawdust that retain moisture inside the wood. So various opportunist fungi wererecently observed there.

At the end of the 20th century, there was finally a tendency in Germany and France to design shelteredbridges which take account of the Swiss and North-American experience of the 19th century.


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The Thalkirchen bridge built in Munich in 1991 is very original, consisting of a wooden frameworkparticularly well protected, sheltered by an orthotropic steel slab.

Photograph 8: Thalkirchen bridge in Munich (Germany)

In France, some bridges without load limitation were made from wood, like the covered bridge on theDore at Saint-Gervais-sous-Meymont, built for the Local Authority of Puy de Dme.

Photograph 9: bridge over the river Dore (Puy de Dme)

In Blagnac, the footbridge at Pinot, whose local Agency of the French Road Directorate of Haute-Garonne assured project management, constitutes with a methyl polymethacrylate plastic protectionanother original structure that combines wood with other materials to reconcile functionality,

architectural aspect and durability.


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Photograph 10: Pinot footbridge in Blagnac (Haute-Garonne)


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2. Wood - The material

2.1. - General

2.1.1 Occurrence and availability

Metropolitan Woods (or indigenous Woodsof Europe)

In France, forests cover approximately 30 % of the metropolitan area. It has been in constant progressionsince the beginning of the 19th century: in 2002, it produced 85 million m3 of wood., of which 50 millionm3 only are exploited. These 50 million m3 break down into 15 million m3 of firewood, 18 million m3 insawing, 10 million m3 in pulp and paper and 7 million m3 for panels and veneers.

Chart 1: rateof afforestation of each French region

0 - 15%

15 - 30%

30 - 45%45 - 60%

60 - 83%

On the covered area, broad-leafed trees are in the majority and represent 60 % of the wooded surface.

On the other hand, exploitation of coniferous trees is the most important: the volume of sawing ofconiferous tree represents 70 % of the total volume of sawing.

Because of prices lower than those of the broad-leafed trees, the coniferous trees are used in theconstruction industry. Moreover, the leafy trees are generally rather sensitive, making their use more

delicate because of wood shrinkage.


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Among the principal types of coniferous tree, are the fir tree, the spruce, the maritime and Scots pines, theDouglas and to a lesser extent the larch. I t will be noted that the mountain types generally have betterproperties than those of the plain thanks to a lower speed of growth, which favors a greater wooddensity.

The more common leafy types are the oak, the beech, the poplar and the chestnut. These types are readily

available.

Lastly, certain types naturally very durable , such as the locust tree (false acacia) are not very available andexist only in very small diameters.

Some of the indigenous French woods are certified by thePEFC (pan European forest council), which attests to thesource of a durably managed forest, without overexploitation.

The types most used for civil engineering structures are theDouglas, the larch and the treated pine.

The summary table, giving choices of types, gives more detail in Part 4 - chapter 4.4.5.

The particular case of French Guiana.

Since the Rio agreements in June 1992, France committed itself with the international community toensuring a durable and exemplary management of the forest belts exploited in Guiana.

A time limit was set for the preceding formula of exploitation permits. The development of the Guianaproduction forests set up by the ONF from 1993 represents an essential projection towards durable

management. As for Metropolitan France, such good management could result in certification by thePEFC.

The Guianese forest represents 96,7 % of the surface ofthe territory, or approximately eight million hectares. Thesurface area of the forests developed for exploitationrepresents 410.000 hectares, or only 5 % of the total forestarea, and production is approximately 65.000 tons of logsper annum.

Production is mostly of leafy tropical types, of average tohigh density , intended for a small part (approximately10%) for export, mainly towards the Antilles.

Chart 2: French Guiana


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The Northern woods

By Northern wood we refer to those that grow beyond 57 degrees of Northern latitude, in Finland, in theScandinavian Countries, in Russia and in Canada. The majority of Northern woods are certified by thePEFC or by the Dutch organization KEURHOUT.

These forests are essentially made up of coniferous tree (approximately 90%). Two types are abundant:the silver fir (spruce) and the red fir (Scots pine). These types grow rather slowly and have goodmechanical and durability properties.

The Northern woods are widely used in construction (low cost and good availability), particularly in theglued-laminated wood industry.

Tropical woods

The tropical forest accounts for 50% of worlds wooded area , but produces only 15% of the wood usedin France for construction.

Very many types exist, particularly leafy trees. The most common in construction are the iroko, ip, theplantation teak, the doussi, the bilinga, azob, the moabi, the movingui and the tauari. For other types,reference should be made to the atlas of tropical woods: it is particularly necessary to check themechanical properties (density, strength) and physical properties (shrinkage). Useful information(availability, special instructions) may be obtained from CIRAD (Center for International co-operation inAgronomic Research for Development, contact: [email protected]).

There are currently several certification bodies that attest to good forest management as applied to thefight against deforestation. No French regulation requires the client to demand such certification. On theother hand, a customs import document must be supplied to him by the company.

There are several forest certification systems in tropical areas: KEURHOUT, FSC (Forest Stewardship

Council) and a PAFC project (Pan African Forest Council). Only three million hectares of tropical forestare certified by the FSC and the PAFC: it is an insignificant part that corresponds to 0.2 % of the totalarea. However, many tropical operators have taken eco-certification steps involving the installation ofplans to lead little by little to the production in significant quantities of wood from certified forests. But inthe current state of the things, to require this type of certification would be excessive and would evenexclude Guiana arbitrarily for example.

Moreover, the situations of the producing countries are in constant evolution. Malaysia was thus in 1970the worlds most important exotic wood exporter, with exploitation conditions very far away from thepresent criteria of durable management. On the other hand, this country started, in 1972, towards policiesencouraging durable exploitation of forests and the production of wood of culture, particularly in the caseof teak, and even set up its own system of certification recognized by KEURHOUT.

It is primarily advisable to be sure of the source of woods used to avoid the use of wood of dubiousorigins. If the case arises, customs can indicate to the building owner the types of wood that might be atrisk in this respect, and alone are able to draw up the customs import document. CIRAD can also provideinformation on the types, their availability, the impact of their use within the framework of the process ofdurable development.

2.1.2. Sawn and reconstituted products

There is solid wood, generally sawn in standardized sections, and reconstituted products.

The reconstituted products are manufactured with wood that is ground up or unrolled, then glued. These

products are more homogeneous, because the knots, in particular, can be taken out before gluing.


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Sawing

The logs are cut into rectangular sections in thesawmills. The sections obtained are baulks, battens,rafters, slats, etc. These terms are clarified in the

appended lexicon (part 6) in chapter 6.1.

There are a certain number of standard sectionssupplied by most sawmills.

The standard sections are detailed in the appendix(part 6) in chapter 6.9.

For the different sections, the cutting is done by list: the section is specially cut for the customer, thusincreasing the cost.

The lengths start at 2 meters and increase byincrements of 0.5 meters up to ten meters.

Photograph 11: sawing on slab off-cut

Sawing is almost always done as a slab : all theboards are sawn parallel as shown in Figure 11.When the sawing is close to the core of the trunkthe boards are said to be on quarter, then furtheraway, they are said to be on false quarter andconsidered as slabs. Rarer are cuttings on quarterwhere one seeks to optimize the number of piecesobtained on quarter because they become lessdeformed with drying.

Figure 11: definition of sawing on slabs and quarters

Glued-laminated wood ( Glulam or glue-lam )

Origin

Glue-lam was invented about one century ago. The Swiss Otto HETZER had the idea to join woodboards with casein glue


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Technique

Boards (called strips) up to 5m long are dried (15 % maximum moisture because of joining), and arepurged of their peculiarities (knots, depressions, etc).The depressions are parts of the round part of thetree that appear during cutting. The strips are then abutted and glued to make continuous strips. Since the

end piece cannot be stuck due to glue absorption , gluing is completed under a minimal pressure of 2 MPaon the inclined faces that are called splice-joints.

Figure 12: detail of a butt joint( vertical finger joint )

Figure 13: effect of a diagonal grain orientationor of a shake

The plates are planed then superposed one on the other to obtain the desired section. Gluing is carriedout no more than 24 hours after planning with a thermo-hardening adhesive ( mechanically strong) that iswater-resistant The adhesive resorcinol (resorcinol phenol formalin) is most usually used outside. Thenthe whole is generally pressed between 0.4 and 1.2 MPa, for a minimum of 6 hours.

Usable types

The most common types are fir , spruce, Scots pine and Douglas .

Other types may also be used: larch, maritime pine in particular, and certain leafy types (like the iroko).

Photograph 12: beam in Glue-lam spruce


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Advantages

Glue-lam has several advantages compared to solid wood, particularly the following:

manufacture of beams of large section and long length (up to approximately 40 m);

possibility of manufacturing curved beams;

increased resistance and rigidity;

purging of peculiarities.

Strength classifications

The strength classifications of Glue-lam go from GL 20 to GL 36 (the number giving the characteristicvalue of the bending strength).

There is homogeneous Glue-lam wood(GL36h) which consists of plates having the same mechanicalcharacteristics, and the mixed Glue-lam timber (GL36c) which consists of plates having highermechanical characteristics at the ends.

GL24h for example consists of strips classified in C24 and GL24c consists of plates in C18 inside thesection and C24 outside. The mechanical classification of the woods is explained in part 2 in chapter 2.4.3Classification of solid wood.

Standard dimensions

The Glue-lam timber beams are available in standard sections and to order. Standard dimensions are:

width: from 6 to 24 cm;

height: from 10 to 60 cm;

length: up to 40 meters.

The standard strip thicknesses vary from 33 to 45 mm For exterior structures, it is recommended that thebest quality strips be used to better control wood shrinkage, and of course to design an effectiveprotection.

Examples of realization

Photograph 13: Judo Institute (Paris) Photograph 14: Pinot footbridge


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Marking and certification

Glue-lam timber is marked EC in compliance with standardEN 14080 (Construction wood - Glue-lamproducts - Requirements), that guarantees a minimal manufacturing inspection by the company and byqualified body. It is anticipated that the EC marking will be mandatory in January 2007.

There is also a quality certification ACERBOIS GLULAM, to guarantee classification of the strips, thecharacteristics of the abutments in bending, the characteristics of joining and the classification of theGlue-lam timber.

Industrial products LVL, LSL and PSL

These industrial products are obtained after taking pieces of wood then slicing (LVL and PSL) or fromlong shavings (LSL). Slices of veneer or shavings are piled up (sometimes with changes of orientation) andare stuck together.

The Lamibois or LVL (laminated veneer lumber) and the LSL (laminated strand lumber) are generally seenas thick plate, while the PSL (parallel strand lumber) makes up beams (up to 48 cm high by 28 cm wide).

These products show better mechanical characteristics than solid wood, and have a better dimensionalstability (thanks to the orientation of the veneers).

The most used type is spruce, but some applications use pine.

In the field of engineering structures, these products can be used to make struts, support for flooring forexample.

Photograph 15: LVLPhotograph 16: LSL Photograph 17: PSL

Panels

plywood is obtained from wood slabs: veneers are crossed and glued (similarities with the LVLwith cross folds);

the OSB (oriented strand board) is obtained by the joining of strips, obtained after grinding upwood, according to a favored direction;

particle boards are obtained by the joining of shavings and sawdust;

the hard fiber boards (HDF) and fairly hard (MDF) are especially used inside.


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Photograph 18: OSB Photograph 19: plywood

2.2. - Anatomy of wood

2.2.1. Untreated wood : a natural, living material

Wood is not a material manufactured for use in civil engineering. In the tree trunk it has many functions:it is the way in which nature fulfilled these functions which gives wood its properties. The tree trunkfunctions normally when it is alive: it is saturated with water and protected mainly by its bark. In civilengineering it is used dry a word to be defined more precisely - and not protected by its bark. It is fromthis point of view that the material wood will be presented here.

Wood is a living material. This must be understood in two different ways:

- wood is the material of a living organism. To know the way of life of this plant makes it possible todiscover some of these properties;

- wood adapts permanently to its environment. One of its principal components is absorbent:cellulose. In the dry state, this cellulose is permanently balanced with moisture in the air and inflatesto some extent.: wood works. It reacts too, because of the surface activity of UV on its organiccomponents. These properties must be taken into account in the use of a material, in a mannersimilar to that for other civil engineering materials: the dimensional sensitivity of wood to moisture issimilar to the dimensional sensitivity of steel or concrete to temperature, the layer of wood damagedby light on its surface is similar to the layer of burnt lime that protects limestone.

Wood, stemming from living matter, is mainly made up of carbon, oxygen, hydrogen and nitrogen, and awhole series of minerals (metal silica, calcium, potassium in particular and traces of metallic salts ).

These elements are organized in organic compounds of three principal families:

- cellulose which, in wood, is organized in elementary microfibrils, themselves agglomerated in fibers.These microfibrils comprise crystalline parts and amorphous parts. The cellulose molecule is veryabsorbent because of the hydroxyl groupings it contains. Cellulose represents 50 % of final material;

- hemicelluloses that belong to the sugar family , make up half the matrix of composite cellulose fibermaterial hemicellulose lignin matrix. It is the hemicelluloses which attract the majority of insects,

only termites being able to digest cellulose;

- lignin, another family of polymers specific to the wood, that constitutes the other half of the matrix.


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Trees belong to the plant world, reproducing by flowers and seeds (phanerogams). There are twojunctions there including:

- gymnosperms, plants with opened fruits, among which are conifers thus called because their seedsare stored in cones formed of protective scales. The wood of conifers is called resinousbecause thetree stores resins in special vesicles;

- angiosperms, plants with seed included in a fruit, among which one finds the trees called leafy.

The gymnosperms which supply coniferous timber, are further back in evolution because their moresimple structure is made up of non-specialized cells. The angiosperms that supply leafy woods are muchmore complex.

2.2.2. From the macroscopic to the microscopic

Wood is strong in both compression and bending

Mechanically, a tree trunk is a post embedded by its foot and carrying a vertical load at its head: thecrown. It thus has naturally a good compressive strength. It also ensures the resistance of the plant tohorizontal loads: the crown catching the wind is very important and the trunk thus transmits a bendingmoment and not inconsiderable shear force to the ground. Thus the trunk also has a high bendingstrength , regardless of the direction of the wind.

Figure 14: role of the trunk in the transmission of stresses applied to the tree

The symmetry of revolution of a tree trunk can be seen as a response to the fact that the wind can blow inall directions. The symmetry of revolution is no longer true for trees subjected to prevailing winds (woodat edges, isolated wood) shown at the right of figure 14: the plant reinforces its structures to offer a greaterresistance in the most stressed direction. Leafy trees generate additional wood at the side in tension(tension wood) and the coniferous trees on the compressed side (compression wood). These zones havedefects in their microstructure. Also this type of reaction wood is to be avoided in engineeringstructures.


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2.2.3. - Observation of the log

Figure 15: natural reference mark LRT identifiable on block (CNDB)

By sawing a section of the trunk one obtains a block (see figure 15). One highlights its cylindricalgeometry that leads to the fact that the tree grows by adding each year an additional layer of wood, thislayer being that located between the wood itself and the bark.

This structure induces a natural geometrical reference frame to which we will refer constantly: we willdistinguish the longitudinal direction L from the axis of the trunk, the radial direction R, corresponding tothe radii of the annual growth circles , and, locally, the direction T, tangential to the circles. In the local

plane these three directions form two by two symmetry planes that correspond to a particular mechanicalbehavior.

The observation of the transverse section below of the tree trunk, in plan TR (figure 16) (Figure 16)shows from the interior towards the exterior (from the past to the present):


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Figure 16: transverse section (plan RT) of a trunk of oak (CNDB)

- a zone of quasi-circles rings of annual increase - each one made up of a clear zone and a dark zone.It isperfect wood. The clear zone of the ring is the wood of spring, a wet period when vegetation.Awakens. Here the wood is a little more tender. The dark zone corresponds to the wood that hasgrown in the summer, a period of dryness: the wood here is less porous, harder. This perfect wood isstill called heartwood when it can be visually distinguished from sapwood;

- a zone which has the same structure of changeover but much clearer in certain cases. It issapwood.They are the recent layers of wood;

- just after the last layer of sapwood is thecambium, the layer which divides itself to manufacture the

wood of the annual layer, sapwood towards the interior, the inner bark towards outside;

- a layer of wood a little thicker, very porous, theinner bark, which constitutes the base of the bark;

- a last external layer, the outer bark, made up of inner bark cells which specialized themselves toensure peripheral protection of the trunk.

On the section of oak of figure 16 Figure 16, these various layers are characterized rather well by theircolor and their more or less porous structure.


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Figure 17: photosynthesis and circulation ofthe saps

These differences are explained by the functions ofthese various layers. I t is then necessary to reconsiderthe operation of the plant: to ensure its growth, thetree takes water and mineral salts (crude sap) in theground using its root system and transports them to

the leaves. This rise of the crude sap is ensured by thevessels (in the leafy trees) and the tracheids (in theconiferous trees) of sapwood. In the leavesphotosynthesis ensures the transformation of thiswater, mineral salts and CO2 taken in the air asphloem sap, a mixture of sugars and water which goesdown to the roots. This phloem sap is used all alongthis path to feed the plant and to manufacture tissue.

The re-descent of the phloem sap is ensured by theinner bark.

As we will see further while going down to a microscopic scale, these vertical movements are ensured bythe vertical organization of the majority of the wood cells. Horizontal and radial cells allow the horizontalmovement of the phloem sap and storage.

In the case of the oak shown in figure 16 Figure 16, perfect wood appears as dark in the center of thefigure. The movement paths of the sap were blocked by mineral and metallic salts tannins which giveit its color. I t thus plays no further role in the vertical movement of the saps. It does however retain areserve function. One speaks about heartwood only when perfect wood is differentiated by its specificcoloring compared to that of sapwood. It is the case with oak, chestnut or Scots and maritime pines. Onthe other hand fir or spruce have no differentiated sapwood, i.e. it is not possible to distinguish sapwoodfrom perfect wood.

The metallic salts that color the heartwood give it resistance to the insects called xylophagous larvaemost common in our areas: the traditional insecticide treatments are nothing more than chemical metalsalts which are forced to penetrate the wood to protect it from insects (except termites which are not justxylophagous larvae insects, but xylophagous themselves).

This natural resistance of wood to differentiated sapwood explains why a structure in oak or chestnut, if itis well protected from water stagnation, thus from rot, has nothing to fear from insects, without anytreatment. I t will be the case of well-protected parts of bridges, even if they get wet occasionally. Caremust be taken to reject parts containing too much sapwood or to demand its removal if sawing has nottotally eliminated it.

2.2.4. - The ligneous plan

On a microscopic scale there is another level of organization of wood, called the ligneous plan.Theligneous plan is the representation of the organization of the various types of wood cells. Thisligneous plan is specific to each type of wood; that