Timber Bridges and Foundations - Forestry Commission · PDF fileTimber Bridges and Foundations...

INNOVATIVE TIMBER ENGINEERINGFOR THE COUNTRYSIDE - InTeC

Timber Bridges andFoundations

Timber Bridges and Foundations

A report produced for the Forestry Commission

PREPARED BY:

G Freedman - FCE (InTeC chairman)C Mettem, P Larsen, S Edwards - TRADA Technology

T Reynolds, V Enjily - BRE

November 2002

BRE Ltd, Bucknalls Lane, Garston, Watford, WD2 7JR01923 664000

TRADA Technology Ltd, Stocking Lane, Hughenden Valley, High Wycombe, HP14 4ND01494 563091

Forestry Civil Engineering, Greenside, Peebles, EH45 8JA01721 720 448

© Building Research Establishment Ltd 2002© TRADA Technology Ltd 2002

© Forestry Civil Engineering 2002

Front Cover Picture - Footbridge at Garpenburg, Sweden, with timber caisson foundations (photo BRE)

InTeC Timber Bridges and Foundations

Nov 2002 TTL, BRE, FCE

EXECUTIVE SUMMARY

Bridges are one of the highest forms of civil engineering - few other structurescommand the same combination of functionality and visual impact. In the UnitedKingdom bridge building in timber has been very limited. This is in marked contrast tothe initiatives which have taken place in North America (USDA Forest Service TimberBridge Initiative), Canada, and Northern Europe (Nordic Timber Bridge Programme).World-wide, the use of timber for bridges is experiencing a major revival. In mostindustrialised countries other than the UK, timber is widely and increasingly beingused, for vehicular, as well as for pedestrian bridges. The strength, lightness inweight, energy absorption and environmental features of timber make it highlydesirable for bridge construction.

Although there is an established history, and a continued use, of timber for bridges inthe United Kingdom applications tend to be limited both in span and capacity, than ismerited by the virtues of this aesthetic, sustainable material. Experience elsewhere inthe world is showing that with correct design, timber is also a durable material forvehicle carrying bridge structures and, additionally, piled foundations. Nevertheless,this aspect remains a significant query in the minds of many mainstream designers,both engineers and architects, who advise UK clients. Revitalised timber bridgeactivities elsewhere are impressing UK specialists. Nevertheless, there is a greatneed to disseminate awareness and knowledge to mainstream designers,commissioners of projects and the public. At present, timber bridge producers in theUK are a small, niche sector of the UK timber industry, and some firms are really onlyrepresentatives of producers that are adding the main value elsewhere in Europe.

Timber engineers have the expertise to provide aesthetically exciting, well-protected,and durable bridge structures. To achieve impact, economic drivers must beharnessed, to unlock consumer and specifier indifference. Key motivators include:

• National cycle routes• City regeneration, calling for aesthetically exciting, well-performing links.• Canal and rail regeneration• Marina and dockside development• Housing developments, with associated bridging needs.• Forest roads and infrastructure maintenance in remote regions.• Linking to value-added forest products.

The use of sustainably grown and locally produced timber for bridge, foundation andsea defence engineering will increasingly be seen as favourable. In addition there areconcerns and moves in Europe away from the use of timber treatments such ascreosote and Copper Chrome Arsenic. Applied research and development,demonstration projects, and benchmarking involving the use of domestic growntimber are seen as vital. Above all, however, well-informed promotion is recognisedas of paramount importance in unlocking demand for timber bridges as flagshipprojects in sustainable development, environmental protection, and improvements tothe quality of life.



Contents

EXECUTIVE SUMMARY

1.0 INTRODUCTION AND PROJECT BACKGROUND 1

2.0 THE HISTORY OF TIMBER BRIDGES 5

2.1 Bridges in ancient history 52.2 Mediaeval bridges 62.3 The Renaissance and the growth of trade 62.4 Long spans - the triumphs of bridge carpentry 72.5 The dawning of industrialisation 82.6 Laminated timber - from mechanical to reliable adhesive technology 82.7 The railway era 92.8 Protective design lessons from history 112.9 Maintenance of historic timber bridges in Britain 112.10 New materials 12

3.0 THE OVERSEAS DEVELOPMENT OF TIMBER BRIDGES 13

3.1 Relevant history 143.2 The way forward to make use of international research 18

4.0 CURRENT UK POSITION 20

5.0 CATEGORIES OF TIMBER BRIDGES 24

5.1 Categories of use 245.2 Locations 24

6.0 STRUCTURAL FORMS 26

6.1 General 266.2 Beams, including bowed types, no arch action 296.3 Arches 296.4 Girder beams & trusses 296.5 Lift & swing bridges 296.6 Further design fundamentals 29

7.0 MATERIALS 31

7.1 Principal elements 317.2 Decks & decking – UK current practice 367.3 Parapets & handrails 377.4 Connections 38

8 DURABILITY 40

8.1 Detailing 408.2 Natural durability 428.3 Preservative treatments 44



9.0 TIMBER FOUNDATIONS 47

9.1 History and overseas Use 479.2 Durability of timber piles 489.3 Traditional timber species and treatments 499.4 Marine structures 499.5 Pile driving and design 509.6 Other geotechnical uses for timber 52

10.0 BRIDGE DESIGN PRACTICE 55

10.1 General practice for design of bridges in the UK 5510.2 Deflection limits 6010.3 Eurocode 5 6010.4 Overseas practice - Decks: 61

11.0 FUTURE CHALLENGES 65

11.1 High efficiency composite materials 6511.2 New adhesive bonding technologies 6511.3 Steel reinforced timber 6611.4 Timber concrete composites 6611.5 Deck protection systems 66

12.0 PRIORITY WORK AREAS 67

12.1 Innovative Timber Engineering for the Countryside 6712.2 prEN Eurocode 5, Part 2 6713.0 CONCLUSIONS 68

REFERENCES AND BIBLIOGRAPHY 69



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1.0 INTRODUCTION

Bridges are one of the highest forms of civil engineering - few other structurescommand the same combination of functionality and visual impact. In the UnitedKingdom bridge building in timber has been very limited. This is in marked contrast tothe initiatives which have taken place in North America (USDA Forest Service TimberBridge Initiative), Canada, and Northern Europe (Nordic Timber Bridge Programme).World-wide, the use of timber for bridges is experiencing a major revival. In mostindustrialised countries other than the UK, timber is widely and increasingly beingused, for vehicular, as well as for pedestrian bridges. The strength, lightness inweight, energy absorption and environmental features of timber make it highlydesirable for bridge construction.

Although there is an established history, and a continued use, of timber for bridges inthe United Kingdom applications tend to be quite limited - although some very fineshort span timber footbridges are constructed. Experience elsewhere in the world isshowing that with correct design, timber is also a capable material for vehicle carryingbridge structures and, additionally, piled foundations. Nevertheless, this aspectremains a significant query in the minds of many mainstream designers, bothengineers and architects, who advise UK clients. Revitalised timber bridge activitieselsewhere are impressing UK specialists. Nevertheless, there is a great need todisseminate awareness and knowledge to mainstream designers, commissioners ofprojects and the public. At present, timber bridge producers in the UK are a small,niche sector of the UK timber industry, and some firms are really only representativesof producers that are adding the main value elsewhere in Europe.

To illustrate the extent of use elsewhere, the United States Department of Agriculturereports that approximately 41,700 road bridges of over 6 m span are made of timber,and improvements are continually being introduced, through the federal HighwayAdministration Timber Bridge Programme. Also in North America, a number ofsignificant modern timber bridge innovations were first introduced in Canada, in the1970’s. These included the stressed laminated deck, details of which were added tothe Ontario Bridge Code at that time. Since then, use of the material has continued,and the technologies have further improved, with several additional innovations suchas new types of structural deck, and prefabrication systems. North Americanexperience has been that in situations where salts and other de-icing chemicals areextensively applied, modern timber bridges are more durable than concretestructures.

In Finland, about 700 timber bridges are owned by the Finnish Road Administration,and along with other Nordic countries, (Denmark, Finland, Norway and Sweden), adevelopment programme has been in progress since 1994, to extend relevanttechniques and experience. Due to the investments into research activities applied totimber bridges in the Nordic Region, and the increase of the general interest in theuse of natural materials, timber bridges have become again a real alternative in bridgeengineering (Figure 1). In continental Europe, particularly but not exclusively the alpineregions, impressive modern bridge structures are also to be seen, and majorcontributions have been made to developing harmonised codes and guidancedocuments, spearheaded by the new bridges Eurocode itself.



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Figure 1: Modern timber road bridge - Evenstad, Norway; 5 spans of bowstringtrusses; 180m total length; creosote treated pine glulam; internally flitched steel

gusset plates, attached with stainless steel dowels. (photo CM/Trada)

The advantages of timber for bridges is also recognised by quite a large number ofemerging countries, such as the West African territories, notably Ghana; countries inCentral and South America, as well as a number of Asian regions. In developingcountries, the revival of interest in timber bridges in the fully industrialised zones of theworld encourages a futuristic view, rather than a “poor material” attitude. For thosewith rapidly growing populations, this is eminently appropriate, not only from anenvironmental viewpoint, but also in order to be able to avoid expensive importedtechnologies and materials.

Bridge clients, engineers and architects are beginning to become aware once morethat bridges using this traditional material can be designed, fabricated and constructedin exciting new ways, as well as being created in forms sensitive to past traditions.Developments such as new, efficient connection techniques, and the introduction ofmodern wood-based composites which can be preservatively treated inenvironmentally acceptable ways, are further encouraging innovations.To re-establish timber bridges in the UK, a great deal needs to be done, especially interms of “Knowledge Re-Packaging” and technical dissemination. Architecturally



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pleasing solutions need to be backed up by the ability and confidence to provide goodprotective design measures and to overcome prejudices about lack of longevity.Modern timber bridges need to be seen as more than just a routine, and possiblypoorer alternative to concrete or steel bridges. Timber is a renewable constructionmaterial with impeccable "green" credentials. Trees, while they grow, absorb carbondioxide and release oxygen. 1 cubic metre of dry softwood represents around 611kgof carbon dioxide that has been removed from the atmosphere. In addition, forestsalso provide areas for wildlife and recreation. Timber is light to transport, easy tohandle and work with on site, and has a natural empathy with the landscape.

To ensure a viable future timber supply chain for engineered, exterior structures,including bridges, the industry needs to grow both the high-profile, spectacularprojects, and also the bread-and-butter access structures and smaller bridges that areof great amenity and community value. Producers and advocates of timber bridgesalso need to establish, sustain and grow their abilities to meet exacting performancerequirements, in terms of safety, serviceability, and design life, as well as providingclient satisfaction through elegance, tactility, warmth and craftsmanship. Contractors,looking for rapid delivery, and even faster erection, seek standard solutions. Theimportance of minimising road or track closures is paramount, and competinganswers, especially steel footbridges, are fully geared up to these demands.

Softwood timber production in the UK has doubled in the last 10 years and is about todouble again in the next 10 years but pulp, paper and board markets are so saturatedthat new markets need to be developed. The structural market is poorly penetrated byhome grown products and timber is the UK’s second biggest import. The focus of thisresearch is to utilise poorer quality home grown timber for the high quality structuralmarket. The timber-housing sector is growing steadily but needs a boost and anystructural developments will be welcome. Rural structures e.g. bridges, towers andcrash barriers are high profile uses will help move timber into the public eye and actas a catalyst for other developments. Research is essential to support these uses inthe UK.

To compound an already serious situation the value of the raw material has droppeddramatically over the last 5 years and this has led to a drop in harvesting and ageneral weakness in the industry at a time, ironically, when Forestry has been granted‘Industry Cluster’ status in Scotland. This means that it is one of the country’s 5 ‘coreindustries’ employing a large number of people and as such, requires to flourish forthe sake of the economy. These factors point to the desperate need for the creation ofnew initiatives, in the knowledge that they will be well supported by Governmentagencies.

Innovative Timber Engineering for the Countryside:

Against this background Forestry Civil Engineering (FCE) of Forest Enterprise (FE)and the two major players in timber research, Building Research Establishment(BRE) and Timber Research and Development Association (TRADA), came togetherto gather ideas. It was during initial meetings that agreement was quickly reached onthe focus being “Countryside and Highway” and that to utilise lower quality homegrown timber in high value added products “Engineering” would be required.Innovative ideas for research projects were put forward and immediately somecommon factors surfaced. Timber has some very desirable properties but it isrelatively low in stiffness compared with steel. In the interest of sustainability andoptimal utilisation of existing forest and woodland resources, there is a desire toinclude the use of lower grade material. This led us to accept that composites with



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steel, high quality timber or fibre reinforced polymer composites (FRPs) would berequired to develop a product in which timber could display its best value.

The objective of the InTeC project is to stimulate by research and demonstration theuse of timber for road bridges as well as pedestrian traffic, and the use of timber forother related civil engineering including abutments, retaining walls and foundations.



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2.0 THE HISTORY OF TIMBER BRIDGES

2.1 Bridges in ancient history

Timber is a traditional bridge building material, with examples in authenticated recordsdating to as long ago as 600 years BC. It is suspected that even before this, ancientcultures, including those in China, Persia, the Asian subcontinent and around theMediterranean rim, had quite sophisticated timber bridge structures. Roman bridgesare recorded in works quite accessible today. Julius Caesar himself, for example,records a large timber bridge in Italy, whilst Padillio (1518 – 1580) discusses anotherbig bridge which was used by the Romans to cross the Rhine into Germany. There isalso some evidence that the Roman bridge in London was by no means a crude orsimple structure (O'Connor, 1993).

One of the largest and best documented of the Roman timber bridges was built overthe Danube, in what is now Bulgaria, in 104 AD. This is often known as “Trajan’sBridge” (Figure 2), because its images are recorded on Trajan’s Column, nowstanding in Rome. This bridge consisted of 20 piers up to 45m high, each joined by asemi-circular timber arch of about 52m span. The thrusts in the triangulatedtimberwork, correctly transmitted into the masonry piers according to modernengineering concepts, seemed to be fully understood by the Roman engineers, whoconstructed and rapidly erected this prodigious feat. Methods of timber conversionand treatment for durability were also recorded in contemporary Latin texts.

Figure 2: An arch of Trajan's bridge, modelled by architectural historians,Florence University. (photo CM/Trada)



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2.2 Mediaeval bridges

The oldest timber bridges that still exist in Europe date from the late mediaeval period,that is from the 14th to the early 16th century. Many of these are covered bridges,owing their longevity to this simple structural protective device. Several examples ofthese ancient bridges are in Lucerne, for example the Kapell bridge and the Spreuerbridge.

Other such timber bridges, which are very important from both the historical and alsothe technical point of view, are those built by the State of Berne during the 16thcentury. These include bridges at Neubrugg 1532, Gummenen 1555, Wangen 1559and Aarberg, 1568. These are still in good condition, most of them having theiroriginal main elements, and some still carrying heavy traffic. It is to be emphasisedyet again that all of these bridges follow the same structural principle; that is protectionof the timber against direct wetting from rain, sleet and snow, by means of a duo-pitched roof with a large overhang.

2.3 The renaissance & the growth of trade

A large number of timber bridges which are still on record, and sometimes still in use,were built from the 16th through to the 18th century, when increasing trade andtransport needs resulted in the construction of new and better roads. As a result ofthe beginnings of an understanding of engineering principles, during the spread oflearning after the Renaissance, more technically advanced designs began to appear,and new construction techniques were introduced. These included arches, trussesand suspension bridges.

Palladio, mentioned earlier as the recorder of Roman bridges, also documented andillustrated a series of his own “Inventions”. The sites of some, such as the often-illustrated Cismone Bridge, have been rediscovered and archeologically investigated.Less well known are some ably-conceived timber trussed arch bridges, also by thissame influential architect.

Leonardo da Vinci, (1452-1519) Italian painter, sculptor, architect, engineer andscientist, was one of the greatest figures of the Italian Renaissance. He was active inFlorence, Milan, and, from 1516, in France. Amongst his design sketches and notesare a series of ingenious timber bridges, several of which have been modelled inrecent exhibitions of his life and works (Figure 3).



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Figure 3: Bridge designs by Leonardo da Vinci (photo CM/Trada)

2.4 Long spans – the triumphs of bridge carpentry

During the 18th century, very long timber bridge spans were achieved through the useof arched trusses. Typical European examples include a Rhine bridge, constructedat Schaffhausen in 1758 by Hans Ulrich Grubenmann. This had an overall span of119m, with the construction including a redundant pier at mid-span, which thisfamous bridge builder was obliged to include at the behest of the dubious“Bergermeisters” of the commissioning town. The structure had laminated archedribs, each with a depth of about 2 metres and comprising seven courses of timber,notched and banded together. This same pioneer of timber bridging constructed anumber of other impressive structures, all of which had complex end-jointing details,and many other advanced features.

Expansion of trade and business in North America also gave rise to some very largetimber arched spans, one of the most noteworthy being the “Colossus Bridge” overthe Schuylkill river at Philadelphia, USA. This was constructed in 1812 by LewisWernwag, and had an amazing free span of 340 feet (102 m). The laminated archelements each comprised six 6 x 14 inch (150 x 350 mm) heart-sawn baulks ofsoftwood, separated, but strongly linked together with iron bands and threaded rods.



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2.5 The dawning of industrialisation

The next stages in the evolution of timber construction saw a gradual transition fromcarpentry to engineering principles. This entailed the greater use of metallic fastenersin the form of bolts, rods, spikes, straps and other devices, such as special keys.These developments also involved the greater use of side-lapped members, ratherthan members intersecting in a single plain through mortises, tenons and other suchcarpentry joints. There was also an increasing reliance upon triangulation, and insome instances standard designs accompanied by published calculations.

Truss systems started to be introduced for timber bridging, particularly in NorthAmerica, where the European custom of roofing timber bridges had been adopted.Entrepreneurs such as Palmer, Town, Long and Howe introduced “Patented” TrussSystems. Town and Howe trusses in particular were very successful, owing theirpopularity to their simplicity and ease of construction from a relatively standardisedrange of member sizes. Many covered bridges of these types have remained in use inNorth America for over a hundred years. They are now regarded as part of thehistorical industrial heritage, and even have “Preservation Societies” dedicated to theirupkeep. A few bridges of the Howe type were also built in Europe, and some of thesetoo remain in use.

Although the records are generally difficult to obtain, it seems likely that earlysuspension bridges used timber walkways and support gantries, along with othernatural materials as the cables and suspenders. Such bridges must have pre-datedarches and trusses, but by their nature they would have been regarded as lesspermanent affairs. However there do exist 19th century photographs by Forrest, theScottish plant collector, of suspension bridges in China, using timber and othermaterials, which are probably directly similar to centuries old designs. It is alsoevident that the suspension bridge goes back long into history, from some of theforms of such bridges that are still built in remote regions of Asia and the SouthPacific, without the benefit of any metal parts or cables.

In the 19th Century, impressive suspension bridges created very long spans usingsteel cabling along with stiffening trusses and decking in timber. A good example isthe footbridge in Ojuela, Mexico, which was built in 1892. This has a span of 278m,and is still in use today.

Through European development aid, particularly from Switzerland, impressivemodern steel and timber suspension bridges, for which a series of design manuals isavailable, have been constructed in Nepal. The mountainous terrain, use of packanimals, and extreme inaccessibility of some regions, makes these structures acontinued necessity of life.

2.6 Laminated timber – from mechanical to reliable adhesive technology

Very early applications of mechanical laminating are discussed by Newlands (1857).For example, he cites the knowledge, on the part of Col. M. Emy, in France,commencing in 1819, of the much earlier mechanical laminating system of PhilibertDe Lorme. Newlands also discusses a report for the Society for the Encouragementof National Industry in 1831, by Emy, publishing his laminating inventions andtechniques. He illustrates a roof for a shed at Marac, near Bayonne, and a “riding-house” (cavalry training structure) at Libourne. Newlands then shows a Gothic churchroof at Grassendale, near Liverpool, which he states followed the Emy system. It is



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not at present known whether this still stands. He also gives quite elaborate details ofvarious forms of bending apparatus, manufacturing for curved roof laminations (“ofthe bending of timber”).

The Timber Development Association (forerunner of TRADA) historic photographicarchives contain several examples of mechanically laminated worsted mills, in theBradford region of England. Booth (1964) discusses mechanically laminated railwaystation roofs, such as GWR, Bath (by Brunel), as well as dealing extensively withrailway bridges, as indicated below. James (1982) provides densely annotated lists ofpotential primary sources for those able to pursue early American and otherinternational (e.g. Russian) mechanically laminated bridges.

Developments in the use of glued, as opposed to mechanically laminated timber,began surprisingly early, and in Europe it was established by the start of the 19th

century. During 1807 – 1809 a Bavarian engineer named Wiebeking developedhorizontally laminated timber arch bridges with spans of up to 60 m. Most of hisbridges used very thick iron bolted or rod-connected laminations of oak.However in 1809, the first major glued laminated timber bridge structure was built byWiebeking, at Altenmarkt. This had ribbed laminations fabricated in situ, working(presumably with great difficulty) from scaffolding and temporary piling. Thinnerspruce boards were used for this bridge than with the mechanically laminated oaktypes that he had built previously, and there was an appreciation of the benefits ofstaggering end-joints, relative to adjacent laminations.

Further evidence of the well-established nature of glulam is the mid-nineteenthcentury Congregational Sunday School roof in Manchester, 1864, documented byBooth (1971) and surviving until demolition in 1963. A former schoolroom, now usedas the Southampton Register Office, in Southampton, 1860 is documented by theGLTA, and is also corroborated by Booth, as the earliest known use of glulam archesin a building. Yeomans cites the “German Gymnasium” in London as another still-standing structure with more than one hundred years service. Privatecorrespondence and photographs, courtesy of P. J. Steer, show a nineteenth centurymusic hall in Nottingham during recent restorations, that is glulam roofed, and still inuse.

By the start of the twentieth century, patents were being taken out for glulam inGermany. In Switzerland, certain structures, laminated with casein adhesive, wereconstructed that still stand today (Chugg, 1962). In 1939, in the USA, a landmarktechnical publication appeared that strongly influenced subsequent North Americancodes. This was entitled “The glued laminated wooden arch”, by T. R. C. Wilson(1939) of the USDA. Evidently, glued laminated softwood bridges were wellestablished by then, since a footbridge of such construction is included in thisreference.

2.7 The railway era

The civil engineering construction associated with the rapid 19th century developmentof the railways made extensive use of timber bridging. Some of the finest examplesincluded Brunel’s designs, although there were also other successful British railwaypioneers using timber for bridges and for other structures, including the Greens (John,and his son Benjamin) in the North East of England. For the Newcastle and NorthShields Railway, these engineers continued the use of mechanically laminated timberfor structures such as Ouseburn Viaduct.



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Returning briefly to the famous Isambard Kingdom Brunel, there is only space to saythat he made extensive use of timber for many railway viaducts, which were builtacross the valleys of south-west England and South Wales. Surprisinglysophisticated concepts were involved, including the use of timbers that werepreservatively-treated using chemicals applied under pressure. An early process ofthis type was Kyanising (1832, Kyan’s patent, using chloride of mercury, Newlands p.106).

Brunel planned his designs to allow maintenance to be carried out on thesestructures without interrupting the passage of trains. He built forty-three viaducts inCornwall alone, spanning a total of eight kilometres. The last of Brunel’s timberrailway viaducts were only dismantled in South Wales the 1930’s, and generally thesestructures were replaced only to construct bridges able to carry much heavier traffic,rather than because of deterioration through decay. At Barmouth estuary, in NorthWales, a timber railway viaduct designed and constructed according to similarprinciples remains in use today (Figure 4), with pitch pine piles having been replacedby the extremely durable greenheart timber.

Figure 4: Barmouth Bridge - one half mile long timber trestle pile viaduct completedin 1867, the only large timber viaduct in Britain still in use. It spans the Mawddach

estuary on 113 short spans. There are two steel girders at the north end, one of whichused to swing to allow ships up river.

For a wealth of further information on timber railway bridges in England in thenineteenth century, the copious and scholarly work of Booth (1996) is an essentialstarting point for the serious historian as it contains many secondary referencesources, including Booth’s own. These would lead to many prime source references,many of which are available in UK libraries such as the Science Museum (ImperialCollege) and archives such as those of the Great Western Railway.

As mentioned above, the “Kyanising Process”, and similar methods, were known toBrunel’s contemporaries. Other chemical treatment processes of that era aredescribed by Newlands. These include, for example, Sir William Burnett, 1838,chloride of zinc; Payne’s patent. 1841, sulphate of iron and muriate of lime; the earlyuse of tars and essential oils (Newlands cites as an example a 1737 patent by oneAlexander Emerson); Bethell, who gave the basis for modern creosote treatments, in1838.



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Early patents for American bridges, during the “Palmer, Town, Long and Howe” era,often involved the co-incident publication of patents concerning timber treatments.Later, in North America, in the 1930s, the widespread industrial introduction ofpressure preservation processes, using substances such as creosote, is said tohave led to an expansion of the use of timber for large truss and girder bridge forms.An impressive example was constructed at Sioux Narrows, in Kenora, Ontario, wherelarge, preservative treated Douglas fir members were arranged in a box-Howe Trusspattern, to create what was for many years the world’s longest single-span highwaytraffic bridge. At 64m main span, this bridge still remains in service.

2.8 Protective design lessons from history

Although decay has always been one of the factors affecting the service life of timberbridges, they have more often been destroyed by war, natural disasters and fire. It isknown from the durability records of ancient timber churches, cathedrals and houses,as well as roofed bridges, that preserving timber structures with adequate protectivedesign measures considerably reduces decay risks. The importance of goodprotective design detailing is a lesson from history that cannot be emphasised toostrongly, in the context of modern timber bridges.

Figure 5: Good protective design features - in 1,000 year old Norwegian stavechurch. Left; Stone sill elevates timber ground sill, sacrificial boarding protectsexterior of corner posts and cladding. Right; Elevated post base, drip moulding atcladding bottoms. Note also extensive use of pitch preservative in both cases. (photoCM/Trada)

2.9 Maintenance of historic timber bridges in Britain

Many historic timber bridges may still be found throughout Europe, including Britain,and fortunately, nowadays, restoration work is undertaken to preserve them. The



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continuous maintenance and replacement of timber elements, using like-for-likecarpentry, in the Lucerne covered bridges has already been mentioned, as has therestoration work on Barmouth viaduct.

Until quite recently, the Scottish East Coast main line railway crossed one of the fewsurviving timber viaducts, over a peat bog, near Inverness. The live track no longerpasses over this structure, which has, however, been conserved.

In 1915, John Saner, engineer to the Weaver Navigation System in North WestEngland, designed a structure known as Dutton Horse Bridge. This has twin ellipticalspans skewing across the River Weaver’s sluice channel, which leads eventually tothe Mersey. The bridge is constructed from mechanically laminated greenheart, andis believed to be the oldest such structure remaining in service in the world.Greenheart planked caissons and many other impressive engineering features are tobe found in and around this outstanding structure. The durability of this structure isunsurprising however, since greenheart is renowned for its longevity. In Guyana itself,its country of origin, marine jetties of over one hundred years in age stand, even insuch adverse, warm sea-water and termite ridden conditions, whilst the cathedral ofGeorgetown is claimed to be the tallest (greenheart) nineteenth century ecclesiasticalbuilding in the world. Two examples of King post truss vehicular bridges in the riverSpey region of Scotland were used as lecture examples by TRADA until quiterecently, and were said to be still in service. There has not yet been opportunity toverify their current status.

2.10 New materials

Following the epoch making construction of Ironbridge in Shropshire, England, (with aframework arrangement based on contemporary timber designs!) the 19th and early20th centuries saw the rapid spread of the industrialised use of the ‘new’ materials,iron, steel and later on, reinforced and pre-stressed concrete. This completely alteredthe concepts of bridge construction, making the increased requirements regardinglonger spans, larger roadways, and higher loads achievable with ease. However withhigher frequency, heavier traffic, and the need to guarantee an all-year-round use ofthe roads, problems have arisen with these ‘modern’ materials. Besides faults due toinadequate design and execution, which may happen with all materials, highmaintenance costs have been incurred as a result of the use of salt as a de-icingagent on roads. This has caused corrosion problems with reinforcing bars and pre-stressing steels in concrete bridges, as well as deterioration of paints and membersurfaces in steel structures. The lesson has gradually been learnt that adequateprotective measures against direct and indirect hazards of the climate are alsonecessary for these so-called “durable” materials, and that such measures invokeconsiderable penalties in terms of whole-life costs.

The complete replacement of quite new bridges necessitated by poor durability hasalso demonstrated the high costs of dismantling concrete structures. In the face ofthis situation, timber engineers have recognised new opportunities, proposing theirmaterial to solve some of the problems that bridge engineers have beenencountering. Timber engineering itself also has become armed with “new”materials, several of which have very high performance, low variability and excellentreliability, thus offering additional advantages over the “traditional” version of solidsawn timber.



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3.0 OVERSEAS DEVELOPMENT OF TIMBER BRIDGES

The USA, Canada, Australasia and the rest of Europe are well ahead of the UK in thedesign and production of “modern” timber bridges:

• Glulam timber and transverse decks• Longitudinal glulam decks• Dowel-laminated, longitudinal panel decks• Stress laminated decks A common factor between modern bridge designs is load sharing through compositeaction which distinguishes them from the old “stick” designs. Timber for bridges hasadvantages over other structural materials which have been recognised overseas butignored in the UK. Timber is:- • Durable and long-lasting - with modern treatments bridges are expected to last at

least 50 years.• Simple construction ~ Construction usually demands low skills and simple

equipment available locally. Maintenance is also within the scope of local labour• Prefabricated Components ~ Modern timber bridges are either entirely factory

made or factory component manufactured thus assuring good quality• Wood has high strength to weight ratio ~ This saves in foundations and gives

confidence to reuse old foundations. Crane loadings are reduced and money issaved.

• Competitive ~ Small span rural bridges can be built in timber at a significantlylower cost than from steel or concrete.

• Aesthetics ~ Timber is natural and is appreciated by everyone and looks as goodin the countryside as in an urban location.

• Chemically stable ~ Timber is not affected by de-icing salts as is steel andconcrete

• Expansion ~ Timber does not expand and contract much with heat so roadsurfaces can be continuous over them without the need for troublesome joints.

• Renewable and Sustainable ~ This is important to the economy• Removes Carbon from Atmosphere and locks it on the Ground ~ This is of

ultimate important in today’s environmentally conscience world. There is a lot of catching up to do and much development is needed to enablemodern bridge ideas to be imported to the UK and then assimilate them to UKpractice, codes and materials.



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3.1 Relevant History Scandinavia: Around 1990 the Norwegian/Nordic Timber Council took steps to plan the introductionof more timber bridges to the public road network (notable examples are shown inFigures 6 to 8). Their reasons were not in the first instance economic but more to useindigenous materials and later to assess the whole value when practice had producedthe best solutions. Otto Kleppe, Chief Bridge Engineer for the Government in Oslo,travelled the world to study old timber bridges in order to gain insight into efficientdesign and durability. He learned lessons from 100 year old covered timber bridgesas well as the latest forms of modern stress laminated decks. He returned to Norwayand has engineered the development of some remarkable new timber structures, notonly on the public road network, but also over motorways. Norway is fortunate inhaving vast reserves of very high quality timber available which makes the task ofproducing elegant long spans much easier. The Norwegians have worked on many fronts with a view to providing a full range oftimber bridge solutions and included experimental work with preservatives. A verysuccessful design is a combination of longitudinal glulam beams with CCA treatmentsubsequently stress laminated and treated with creosote. They have developed veryhigh quality jointing systems which permit large king post and truss structures andhave innovative ideas allowing timber crash barriers. The high quality structures areprotected using copper sheeting on the structure and bitumen compounds on thedeck. In both Norway and Sweden simple stress laminated decks are factory madefor minor road and forestry road bridges. These low cost options are treated withpreservative but not protected in any other way. Even with a shorter life thesestructures will have a very competitive whole life cost.

Figure 6: Vihantasalmi bridge, Finland - Glulam king-post trusses each spanning42m; composite concrete-steel-glulam deck. (photo Nordic Timber Council)



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Figure 7: Evenstad bridge, Norway - glulam truss beams each spanning 36m with

stress laminated timber deck. (photo Nordic Timber Council)

Figure 8: Sinettäjoki footbridge, Finland - glulam king post trusses spanning

18.8m with lumber deck. (photo Nordic Timber Council)



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USA and Canada: Timber, which was readily available in enormous quantities, played a major role as aconstruction material in the development of North America since the early pioneerdays. Indeed, it is estimated that by 1900 half of the total forest area of the continentwas felled. Timber trestles were used extensively to span gorges and rivers for thetranscontinental railways. Many century old covered road bridges are still in serviceand are considered to be heritage items. With a main span of 64m, the SiouxNarrows bridge, built in 1936 in Kenoria, Ontario, is one of the worlds longest singlespan wood highway bridges (Figure 9)

Figure 9: Sioux Narrows Bridge - Howe trusses formed from solid sawn Douglas

fir (photo Canadian Wood Council)

Currently in the USA there are nearly 600,000 bridges, 7% of which are timber and afurther 7.3% have timber decks. Recent studies have shown that 240,000 of thesebridges are classified structurally deficient or functionally obsolete. This critical stateof affairs prompted Congress into introducing the Timber Bridge Initiative (TBI) in 1989and another similar programme which promotes demonstration bridges, researchand information transfer. Under the programme a 50% grant in available to build abridge which demonstrates modern technology. Timber structures declined in number from 50 years ago when large trees becamescarce and concrete technology became reliable. However the modern materials,concrete and steel, have not been without their problems and since the middle 70’smuch research effort has been undertaken to utilise smaller wood sections to buildlarge structures. It was in Canada in the 1970’s the real pioneering work was carriedout on the stress-laminated decks which has become important to the new bridgeinitiative in the USA and could become a very useful concept in the UK. Timber engineering and technology has benefited greatly through these programmesand many hundreds of bridges have been built although much remains to be done.



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There are so many avenues of help in the USA especially as these initiatives weretaken by government agencies but personal contacts will be crucial to ensure theaccelerated programme necessary in the UK, in order to catch up. Australasia: In Australia and New Zealand the increased production of plantation timber hasgenerated a modern timber engineering industry. The UK should have beenshadowing these recent impressive developments. Many are the same as those inAmerica and are necessary in the UK to catch up and create some high valuemarkets for our new increasing production. Although there are many initiatives intransportation structures, there are also significant ideas in building, from which UKpractice could benefit. Developing World: There has been substantial experience involving the UK timber researchorganisations, and TRL (see website http://www.trl.co.uk/bridges.htm), in theoverseas development uses of timber for bridges. These have been carried outthrough assistance provided via organisations such as United Nations IndustrialDevelopment Organization (reported in Anon 1985) and the UK Department forInternational Development (DFID, formerly ODA). For example, prefabricated modular timber road bridges have been successfullyintroduced into a significant number of developing countries on four continents. Thefirst of a series of standard designs for modular timber road bridges was prototyped inKenya, some thirty five years ago. Further development work continued in CentralAmerica the Far East and elsewhere. This development included contributions oflocal expertise, and associated professional training. Similar road bridges are stillbeing produced, in accordance with well-tried design manuals and drawings, usinglocal timbers and labour, to the great advantage of rural communities in more thantwo dozen countries, in all of the tropical continents. Extensions to the originaldesigns, and substantially new types of standard design, have subsequently beenadded.

Figure 10: UNIDO prefabricated bridge (photo UNIDO)



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It has been found that by giving ministry departments, and their associatedprofessionals, renewed confidence in timber in communally important, heavy-dutyapplications such as these, there is spin-off resulting in an enormously improvedusage for smaller-scale applications. These are initiated and executed entirely bycommunities themselves, on their own initiative. This is another important lesson thatthe timber industry needs to take on board, here in our own country. 3.2 The Way Forward to Make use of International Research When specific areas of research are identified and funding is in place for individualprojects, the initial literature search will be extended using references and contacts.Research partnerships will be explored and when the missing knowledge is identifiedwork will begin. Much will be assimilation of past international work to accord with UKtimber species. There are a number of codes of practice in existence which will be ofuse but climatic conditions, safety regimes, species differences etc will create manytransfer problems. The mission of InTeC is not just to carry out the research, solvethe problems and show that things can work but also to produce the codes andguidance so that the ideas are taken up. This part of InTeC’s work will be timeconsuming, but if the past successes of concrete and steel as construction materialsare to be emulated then the information circle must be closed. Young designers willnot adopt timber unless it is made easy, logical and sensible. Some Specific International Ideas likely to be Transferred to the UK: Stress laminated bridge decks are certain to become a useful solution for minor ruralbridges and they are a timely product of recent ability and need. Accurate sawing isessential. Safe bacteriological treatment is demanded and high tensile stressing ofsteel tendons is the key to the structure. All of these are now available at low cost.The technology level is not high and their production could become a cottage industry.This idea has arrived at a time when UK softwood timber production is about todouble again for the second successive decade and funds for rural bridging are low. Abutments for bridges and retaining walls have traditionally been constructed from‘permanent materials’ like concrete and masonry but the question of ‘life cycle’ needsto be addressed. A public road bridge in the UK is designed for a 120 year life butforestry bridges are designed for 50 years as that is the economic cycle of theindustry, being the growth time from plant to mature tree. We have, in the past,expected buildings to last forever given enough maintenance, but supermarketbuildings are now financially appraised over 7 years, that being the predictable tradingprojection limit. Perhaps it is time to look at structures with a shorter life, provided thatshort life gives a unit cost per year less than the more permanent structure. A timberbridge deck and abutments could be constructed for a 20 or 30 year life, require nomaintenance and be replaced within the unit annual cost of the ‘permanent structure’. Appraisal of Timber Structures: Overseas countries have realised that timber structures enhance the environment,local economies, society, aesthetics etc. factors which must be introduced into anyappraisal. Energy values are used in appraisals, where the inputs to refine thecomponent materials and the energy to carry out the construction and demolition areevaluated to calculate a whole life cost. Timber structures excel in all of thecomponents of appraisal. A local timber industry creates stable rural employmentwhich tourism can latch onto. Timber structures are popular with rural people and



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their visitors. The western society accepts that timber structures in the countrysidemake sense. An international research check has proved that we are behind the rest of thedeveloped world in this area of applied engineering. This does, however, mean thatwith careful planning and study of current research we can catch up quickly and thenconcentrate our efforts on the most relevant areas. It means that much of ourresearch will be to assimilate ideas developed for other species. The most valuablegain however will be in finding the ways to extend already good ideas. This is mucheasier when a fresh mind takes up a partly developed piece of work. InTeCresearchers bring that quality and with the correct funding many exciting extensions ofexisting work that could bring benefits to the UK. A programme similar the TimberBridge Initiative in the USA would be welcomed in the UK and could become thecornerstone for future development while bringing forward the many bridgereplacements necessary in the UK.



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4.0 CURRENT UK POSITION Unfortunately, the official bridge design scene, viewed from the position of the averagecivil and structural consulting engineer in the United Kingdom, includes nothing toencourage the use of timber. There is no British Standard dealing specifically withdesign. The BS 5400 series only covers steel, concrete, and steel-concretecomposite bridges. This absence of a British Standard is thought to have inhibited thespecification of timber as the structural medium for many footbridges, as well ashaving resulted in a number of designs whose performance has not been entirelysatisfactory. Although authorities such as Highways Agency (HA) have their ownstandards, recognising timber in footbridges to a small degree, the absence of a maincode of practice and accompanying support standards, is a serious deterrent.Awakening awareness by some influential specialists within authorities of thedevelopment of new Eurocodes relating to bridges, is likely to provide better hope forthe future, provided that this is seized as an opportunity by the timber industry itself. Timber interests in the UK were extremely impressed by the manner in which the USNational Timber Bridge Initiative was launched (USDA 1983), and by its subsequentsuccess. Their programme involves many demonstration timber bridges, togetherwith research and technology transfer. Starting from a relatively small financial basis,it was difficult to see how anything comparable could possibly be started in the UnitedKingdom. However, there are now some positive signs. Work was carried out about sevenyears ago, with support by DETR. This led to two preliminary study reports (Mettem1993) and (Mettem 1994). A pilot project, termed “Innovative Timber Engineering forthe Countryside”, has been initiated involving BRE, TRADA Technology, and ForestryCivil Engineering, with support from the Forestry Commission, and this report relatesto this particular project. The second positive step is that active work has now beenstarted on an EN version of Eurocode 5: Design of Timber Structures Part 2: Bridges.This is scheduled for issue for public comment in 2003, with the target of a final draftfor printing in 2004. The principal Eurocode 5 for timber structures, to which thebridges part refers for all of its main technology, is ahead of this, and has alreadybeen strongly promulgated and supported by design guidance, involving TRADATechnology and its various industrial and research partners. BS DD ENV 1995-1-1was issued in 1994 and is already used in practice mainly by more experiencedtimber engineering designers. A BS EN version is expected to be published early in2004, before which, training will be given to all practising designers and new students. Current Requirements: Current requirements for bridges are generally formulated independently of thematerials to be used. In general terms, bridge design has to fulfil certain mainrequirements which can be related to timber and wood-based materials as follows: Load Capacity & Vehicle Clearances: Modern timber bridge designs for vehicular traffic are perfectly possible, and are infact already being designed and constructed in a number of countries and regions.Appropriately designed timber and composite deck systems can provide forincreasing traffic loads. Clearances, given in regulations, are taken into accountwherever necessary. For road bridges, vehicle size obviously affects the design of



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both the carriageway widths, and also, in the case of covered and arched bridges, theoverhead clearance. Long Spans: Earlier limitations of timber brought about by its availability only in the sawn form nolonger apply. Glued laminated timber, structural timber composites - STCs (Mettem1996) improved strength grading procedures, reliable connection techniques and theuse of other materials acting compositely in conjunction with timber, all help to makelong spans possible. Roadway Surface Conditions: Normally there is a requirement that there shall be no difference in the surfaceconditions and levels between the bridge and the connecting road pavements. Withappropriate deck systems and sealed wearing surfaces, such requirements can befulfilled using timber structures. Routing of the Bridge: Modern bridges have to be integrated into the general route-planning scheme.Consequently skew, cambered and curved deck bridges are often required. Suchforms are attainable with timber bridges. Recent Developments in Timber Bridge Decks: Timber & Concrete Composite Decks Timber and concrete composite decks have existed in regions such as New Zealandand North America for decades. Early systems comprised nailed laminated deckingwith un-reinforced concrete and a thin asphalt surface. More recently, thickerreinforced-concrete layers and shear connectors have been added, giving greatercomposite action. The effective width of the concrete flange is determined as for aconcrete T-Section. All of the shear force transmission between the two materialstakes place via special, strength-calculated connectors, and not by natural bonds. Notensile strength is recognised within the concrete layer. Some design rules for thisform of construction are given in Eurocode 5 Part 1-1, the general design document,whilst supplementary rules are contained within prEN Eurocode 5 Part 2, Bridges. Developments in the composite timber/concrete deck continue, for example: • More stable laminated timber decking, using post-tensioning systems.• More efficient shear connecting systems between concrete and timber, to achieve

more reliable composite action during the service life of the deck.• Better systems to seal the concrete surface, and to provide protective and hard

wearing road surfaces.

Laminated timber deck plates are made of individual laminations which are heldtogether by nailing or adhesive bonding. In the case of pre-stressed plates,discussed below, there is in addition a permanent lateral pressure, which guaranteescontinued friction between the faces of the laminations, and according to the type ofconstruction, between any un-bonded adjacent faces which may exist between theindividual slabs.



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Pre-Stressed Timber Decks

Pre-stressing in a timber bridge deck is defined as a permanent effect due tocontrolled forces and/or deformations imposed upon the structure. The plates arenormally pre-stressed by means of steel bars or tendons. Pre-stressed decks intimber bridges first appeared in Canada, where they were introduced as a repairmethod for nailed timber-laminated decks. The correct choice of materials andspecification of moisture contents overcame early problems with loss in pre-stressingforce due to timber shrinkage.

These forms of deck are now very common in North America, and their use has beenspreading to other regions where modern timber bridge developments are occurring.These include Australia, and Finland, Norway and Sweden. The additional step ofusing glued laminated timber rather than solid sawn timber for the decks wasprobably taken first in Switzerland. Recently in Australia and Scandinavia, progresshas been made in utilising other modern STCs such as Laminated Veneer Lumber(LVL), for which reliable pressure preservative treatment processes have now beendeveloped. Glued laminated timber and STCs are always supplied at low, factory-conditioned moisture contents, and with these, early problems in loss of pre-stressinghave been completely overcome. Decks with no re-stressing requirements are beingachieved by following design recommendations such as those given in prENEurocode 5 Part 2, Bridges. Pre-stressing bars are also now sometimes bonded in,resulting in a high degree of corrosion resistance and good load carrying capacity.

Dowel-Type Fasteners & Mechanically Laminated Bridge Structures:

The term “dowel-type fastener” is used throughout the structural timber Eurocodes,and in the latest edition of BS 5268 (BSI 2002), to refer to fasteners whose cross-section is essentially of a cylindrically prismatic form, and whose function is totransmit forces in lateral shear between adjacent layers of the timber. In the contextof current codes, such fasteners essentially consist of steels of adequate and definedstrength, although research is now in progress on the use of non-metallic, and inparticular Fibre Reinforced Plastics (FRP) dowels. Bolts, lag screws and plain steelrods acting in transverse shear are all examples of dowel-type fasteners that arecommonly employed in bridge design.

Over the past twenty-five years, timber engineering researchers have extensivelyexplored the design theories associated with these types of device, and the theoriesare now well adapted to reflect real fastener behaviour in actual structures.Essentially, the theories depend upon a knowledge of the behaviour of the fastener asa rod-like device, which tends to embed itself elasto-plastically into the surroundingtimber. The response of the latter is modelled as a yielding elastic foundation. At thesame time, allowance is made for the tendency for the steel of the fastener itself toyield plastically, and to form plastic hinges at various points, whose locations dependupon the exact interface arrangements.

The development and use of such theories for the design of dowel-type fasteners,along with methods enabling designers to predict changes in the effective sectionmodulus, due to slip between adjacent layers, has enabled the accurate design oflarge and impressive modern mechanically laminated timber bridge designs.Mechanically laminated timber bridges are designed and constructed throughoutEurope, but are particularly prevalent in the UK, Netherlands and Germany. Verydense, durable tropical hardwood timbers are normally used for these designs, with



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one particular timber , being found to be very successful. This is the hardwood named“Ekki” in the UK, and “Azobé” in Continental Europe (the same species of timber ineither case, namely Lophira Alata).

5.0 CATEGORIES OF TIMBER BRIDGES



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5.1 Categories of use

Three broad categories of use can be considered, as follows:

a) Highway and adopted road bridges.b) Footbridges.c) Footbridges with occasional vehicular access (e.g. farm, golf-course and parkland

bridges).

Category a) in timber is extremely rare in the UK, and in the main restricted to specialand historic structures. The present volumes of use in categories b) and c) aremodest, but growing, with opportunities for bridge and associated timber suppliersand engineers, especially in the light of the positive factors mentioned in theintroduction. Even in category b) however, there are significant obstacles to theprocurement of timber, where influential authorities are involved. The HighwaysAgency inventory for England, for example, contains only one timber pedestriancrossing bridge over the roads for which they are responsible. It is the intention of thepresent project to conduct more thorough market research, costing studies andbusiness potential investigations.

5.2 Locations

Generically, locations for footbridges and light vehicular bridges can be divided intothe following four types of crossing:

• Over roads – general access.• Over rivers, canals and other water features.• Associated with the leisure industry, various crossings, including the three types

above.• Over railways– general access. Bridges associated with alternative modes of transport, such as cycling, mightarguably be regarded as a separate category. However, provision of routes andfacilities for serious and mundane access to work, education, and other aspects ofdaily life by such means can hardly be argued to have reached the stage to warrantseparation from the leisure category. a) Road crossings Many footbridges are used to provide safe pedestrian crossings. Timber is permitted,as well as steel and concrete. However, the Highways Agency (formerly, through aDepartment of Transport document, currently undergoing revision) points out thefollowing, in its Standard BD 29/87 (DoT/HMSO 1987): "A footbridge is the least suitable form of crossing for disabled people and should onlybe provided when other forms of crossing – e.g. a crossing at grade or a subway aredeemed to be unsuitable." Timber has only a small share of this market. Furthermore, its share is probably evensmaller, as a result of some unfortunate instances of glulam bridges de-laminatingduring the 1980’s. These were manufactured by firms that were not members of theGlued Laminated Timber Association (GLTA). Lack of independent third partymanufacturing control and certification is recognised to have been part of the



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problem. However, the failures achieved notoriety for the timber industry as a whole,causing a major setback to information dissemination and promotional exercises. b) Crossing rivers, canals and other water features This is an important market, with timber footbridges, boardwalks and piers having asizeable portion of the total. Often the surroundings and environment are such as tosuggest the choice of timber as the most sympathetic material. Timber weathersparticularly well in marine environments compared with steel or reinforced concrete. c) Associated with the leisure industry. This is generally an expanding and promising market for timber bridges and otherlandscape features. Example applications include golf courses, theme parks, visitorcentres, wildlife and animal sanctuaries and nature reserves. d) Crossing railways Timber has only a very small share of this market. Historically, the extensive facilitiesfor iron and steelwork available to railway builders tended to facilitate the choice ofmetal in the first instance. There are some modern examples of timber stationstructures, and a few of timber footbridge crossings. Not all of the engineersconcerned are opposed to this material, but railway structural engineers have acautious approach, and need assistance to specify in performance terms, rather thanby prescription. Reorganisation of the administration of the national rail network hasalso made it difficult, in recent years, to decide where best to focus impact.



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6.0 STRUCTURAL FORMS 6.1 General To describe the majority of footbridge and light vehicular access types, the followingfive categories of structural form have been devised. The forms refer to the principalstructural elements of the bridge: • Beams, including bowed types, no arch action.• Arches.• Girder beams and trusses.• Lift and swing bridges.• Cable stayed and suspension types.

These five categories of bridge based upon the form of the principal members led tothe summary shown in Figure 11 and Table 1 (below). This classification also relatesto the usual static system for the principal members, connected in turn to thestructural analysis that will be required in the design. Also shown in Table 1 is theform, or shape alternatives for the principal members, and an indication of thematerials which are commonly chosen for each of the types.



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A wide range of structural forms is available for bridge solutions in timber. Within thisreport, the most common and appropriate are broadly categorised into five main types,as follows:

a) Beams, including bowed types, no arch action.

b) Arches.

c) Girder beams and trusses.

d) Lift & Swing Bridges.

e) Cable Stayed & Suspension Types.

Figure 11 Five principal types of timber bridge.(Not to scale).



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Table 1: Structural forms of timber bridge

Structural form

a) Beams b) Arches c) Girder trusses d) Lift &swing

e) Cable Stayed& Suspension

Figure 1

Usualstatic

system

Single simplysupportedspan; flat orbowed(positive pre-camber), but noarch action.

3-pinned, round& parabolic.

Single simplysupported span;Triangulated, e.g.King post trusses.Bow-string trusses.

Two-leafcantilever.

Timber masts; steelcables/links.

Additional forms

Multiple simplysupportedspans.Cantilever sidespanssupportingsuspendedcentral span.

Two-pinned,multiple spans.

Multiple simplysupported spans.Multiple bow-strings.

Single-leafcantilever

Single or twintowers withside spans.

Form ofprincipalmember

s

Straight, lightlycurved or pre-cambered.

Circular orparabolic (widerange of radii).

Parallel or near-parallel chorded(often Warren orPratt trusses).

Main (lower)beams straightor single-tapered

Balance(higher) beamsstraight ordouble-tapered.

Deck beamsstraight or tapered

Towers, parallelmasts or A-frames.

Commonmaterials

Sawn timber –softwood orhardwood.

Timber poles –natural,debarked orturned/profiled.

Glulam.

Mechlam.

Glulam.

Mechlam.

Sawn timber–softwood orhardwood.

Glulam.

Mechlam.

Beams –sawn timber –softwood orhardwood.

Glulam.

Portals –glulam,mechlam.

Towers andmasts – Sawntimber, glulam,timber poles –natural,debarked orturned/profiled.

Decks as in 1.



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6.2 Beams, including bowed types, no arch action

Beam bridges range from a single, simply supported span, to multiple spans andcantilever arrangements. In laminated construction, pre-camber, and slightly bowedforms (without expressly designed arch action) are quite common. Span ranges forbeam bridges may be from as little as 3m for a very small solid timber footbridge, toabout 24m in bowed laminated construction.

6.3 Arches

Site, terrain and clearance considerations may lead to choice of the arched form,which is architecturally very striking. Much larger spans are possible than withbeams, in the order of 12m to 70m being feasible. Various deck arrangements andpositioning levels can also be provided.

6.4 Girder beams & trusses

Trussed girders provide greater load carrying capacity and stiffness than simplebeams. Various trussing arrangements are possible. Girders are often formed fromseveral lines of trusses. These require to be cross-linked with bracing, and the designmay involve other lateral members, such as transoms. Deck levels may also bevaried. Camber and light curvature are often applied. Well-designed timber girderbridges are architecturally pleasing. Viewers “read” the structural forms, andappropriate designs can be conceived for both urban and rural situations. Individualspans for bridges formed from girders of this type are likely to range from about 9 m to45 m.

Modern timber engineering versions of several traditional timber bridge forms havealso appeared recently in the Nordic regions, for example. Both “bow string” and “Kingpost truss” types have been given an updated treatment through the use of newconnections technologies, innovative deck types, and environmentally sensitive timbertreatment processes. Use of these forms has been extended into multiple spanscreating some of the longest timber bridges constructed in modern times.

6.5 Lift & swing bridges

There are several practical and available moveable bridge forms in timber. Theseinclude bascule bridges, which can be lifted by tilting, and swing bridges. It is ofpractical importance in dockland, harbour, and inland waterway situations to be able toobtain clearance for waterway traffic. Recently, in such areas, many regeneration andrefurbishment schemes have been undertaken. These continue to be needed withexpansion of walking and cycle routes, and linking-up of riverside districts. Moderntimber design methods, materials and fabrication concepts can provide similarsolutions to those used in the past for industrial duties. Either traditional orcontemporary architectural styles are possible. Spans for this type of bridge tend tobe fairly modest, with those in excess of about 24 m being uncommon.



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6.6 Further design fundamentals

Once the principal structural form has been selected, further fundamental structuralconsiderations are necessary. The elevation of the deck in relation to the remainder ofthe structure is another very important design issue. The option of roofing the bridgemust also be taken into account at the early design and cost estimating stages.According to the authorities concerned, regulations exist which may affect severalother key elements of the bridge structure, both at ground level and above. Theseinclude for example abutments, piers or other supports, deck width, roof clearance (ifprovided), stair or ramp accesses, and parapet height and design.

a)Deck levelsThere are usually several options in choosing the elevation of the deck. For mostforms of bridge shown in Table 1, these are fundamentally low-level, mid-level or high-level decks. The choice of deck level has considerable influence on the architecturalform and engineering design of the structure. It also relates to planning considerationsand functional aspects. The former includes for example headroom for vehicles orvessels beneath the bridge. The latter include the measure of protection provided bythe deck to the remainder of the structure.

Possible elevations of the deck are interpreted in relation to the principal structuralforms in Table 2. This second table also incorporates some notes on variations onthe basic forms. It mentions for example roofed bridges. Although these areuncommon in the UK, they are not unknown.

Structuralform

a) Beams a) Arches a) Girder trusses a) Lift &swing

a) Cable Stayed& Suspension

Figure 1

Deckelevations

Over beams.

Betweenbeams.

Over thecrown ofarch(es).

Over girders.

Between girders.

Deck itself, asfor 6.

Deck itself, as for 6.

Additions &variations

Roof.

Beams as part ofparapet.

Mansard archwith stairs.

Tied arch,tangent at deckor other level.

Skewed plan form.

Roof. Roof supports aspart of main girders.

Trusses as part ofparapet

Parallel or pitched topchords

Generallykeeps toclassical formof Dutchdrawbridge.

Vessel passagealso achievable byswing-bridge(another cantileverform).

Single cable, centralin plan, from A-framed tower(s), orsimilar principle usingstays.

Table 2: Forms of timber bridge, deck elevations and variations



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b) Materials and durabilityCorrect choice of materials, also understanding and applying best practice options toachieve durability, are such vital aspects of timber bridge design that these topics arediscussed under major sub-headings below.

7.0 MATERIALS

7.1 Principal elements

In relation to the categories of principal structural forms shown in Table 1, thematerials that are commonly used as elements are shown in the table. In essence,these are as follows:

a) Sawn timber.b) Timber in round and pole forms.c) Glued laminated timber (glulam).d) Other structural timber compositese) Mechanically laminated timber (“mechlam”).

a) Sawn Timber:This may be used for all of the forms in Table 1 which do not involve significantcurvature of the members and for which adequate lengths can be obtained to meetthe main spans and to provide the other structural spanning requirements. Sawntimbers can range from small sections of softwood or hardwood, suitable for thesimplest of short-span beam bridges, through larger sections, more usuallyhardwood, to very long lengths of specialist hardwoods that can be used for thebiggest members, such as masts for cable stayed bridges and for pilings. The latterare still in some cases hewn, rather than sawn sections, although technically this haslittle effect to the designer.

The types of hardwood used for the intermediate applications include temperatespecies such as oak, often British grown, and established tropical hardwoods that areavailable for structures in the UK. BS 5268: Part 2 lists data for oak, and also fortwelve tropical hardwoods. Typical examples of the latter are Iroko (West Africa, e.g.Ghana) Strength class D40; Keruing (South East Asia, e.g. Malaysia) D50 and Ekki(West Africa e.g. Cameroon, Ghana) D60. For the largest lengths and cross-sections, including big beams and masts, Greenheart (Guyana) D70 and Basralocus(Dicorynia guianensis- not listed in BS 5256) are used.

Sustainability: Mention of tropical timbers clearly immediately raises the issue ofsustainability. The key organisation for producers is the ITTO (International TropicalTimber Organisation) which facilitates discussion, consultation and international co-operation on issues relating to the international trade and utilisation of tropical timberand the sustainable management of its resource base. Regarding certification, it hasthis to say, in one of its most recent (2000) reports:

“Certification is a process which has, to date, not found favour with many producermembers, largely because it is seen to be discriminatory. But as countries makeprogress towards sustainable management, certification may become increasingly



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attractive. Indeed, Malaysia has already established its own national certificationscheme, and in Indonesia the project ‘Training development on the assessment ofsustainable forest management in Indonesia’ (PD01/95) is being operated jointly withthe Indonesian Eco-labelling Working Group, one of the aims of which is to developtraining materials for the inspection of sustainable forest management.”

Principal ITTO exporter countries are listed in Table 3. Ghana is working towards asimilar position to Malaysia, in having its own, nationally-based scheme. Goodprogress towards Certified sustainability is also being made independently by somesmall tropical producing countries such as Fiji, Honduras, and several African andCaribbean countries that have plantations of non-indigenous species, such as teak.This material is particularly suited to bridges and other landscaping applications.

ITTO exporters of tropical timberMalaysiaIndonesiaBrazilCameroonPapua New GuineaGabonCôte d’IvoireGhana

Table 3: ITTO tropical hardwood producers and exporters

Softwoods: Where smaller sawn cross-sections and lengths are required, there arebetter opportunities in bridges than in the building market generally for specially valuedBritish grown softwoods. The great majority of British softwood production is of Sitkaand Norway spruce, non-durable species that are hard to treat with preservatives, andunsuited to prolonged external exposure.

Those British softwoods worthy of consideration include Scots Pine (SS grade =C22), which has good preservative retention, and Larch (three British grown species,SS grade = C24), which has a good degree of natural durability. Douglas fir grown inthe UK also has a degree of natural durability, and sufficient availability to beconsidered in some regions. In comparison with imported Douglas fir (SS grade =C18), it seems to have suffered from an unduly cautious down-rating of its strengthproperties. However, this has recently been rectified, for larger cross-sections atleast, by a Code amendment approved but awaiting printing.

b) Timber in round and pole forms.All three of the British softwood species or groups mentioned above have a longrecord of accomplishment of good service in exposed conditions as power-line andtelegraph poles. Suitable treatment regimes for these are well established, and thesehave been supplemented by further quite recent research into the topic of durability.

Poles themselves are also used, as the main beams for small or medium spanfootbridges, and occasionally for small masts. They are usually of softwood, with apreference for species with a degree of natural durability such as Larch and Douglas



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fir. Debarked, straight poles are a simple version of this class of element. A limit ontaper, such as 10mm per metre, is recommended. Strength grading rules for roundtimber in uses other than power lines and telegraph poles are not yet fullystandardised. In the past, similar ad hoc grading rules and stresses have beenderived for other projects. These have included applications in developing countries,where the use of plantation softwood poles is of great economical importance.

Advantages of using round timbers include the retention of the natural strength of thetree form itself, and, in appropriate situations, a good appearance. Disadvantagesinclude response of the round timber to drying, and difficulties in connecting suchelements neatly and efficiently. Both of these have, to some extent, been overcome,as a result of applied research and developments of the technologies concerned.Treatment techniques for poles are well understood and documented.

In countries such as Switzerland and Austria, where a great deal of use is made oftimber for bridges, round timbers are also partially shaped. Profiles that are usedinclude circular with one slot, to relieve radial drying stresses and eliminate splitting;circular with one or two flats; and circular with one V-shaped segment removed.Connection systems have also been developed for these more sophisticated forms ofround timber.

c) Glued laminated timber (glulam).Glued laminated timber (glulam) bridge elements are manufactured to BS EN 386,and other supporting standards, to which this principal document refers. Others coverstrength grading of laminations; adhesives; and end-joint testing. Strength classes forglulam are contained in prEN 1194. Both softwood and hardwood laminations areused for bridges, the latter to a far greater extent than in glulam beams for buildings.The British timber code and its related standards used a system of gradinglaminations and performing design calculations which was peculiar to the UK, butwhich stood the test of time (8). Some of the procedures and requirements describedin the former British Standard for glulam, BS4978 are still followed for hardwoodglulam, since the European documents have been developed principally withsoftwoods in mind. An introduction to glulam production and strength classes isavailable in STEP/EUROFORTECH Volume 1, Lecture A8.

Glulam bridge beams are possibly more common in one particular laminatedhardwood, namely Iroko, than in any of the softwoods. This timber has found favourfor its combination of good durability, the ability to be bent and glued, and its goodjoinery properties. Substitutes are now being considered, because Iroko is underpressure through perceived sustainability issues, and may even be coming intogenuine shortage from some forest regions. Alternatives might include Dahoma(Piptadeniastrum africanum). This has been used successfully in several vehicularbridges in its country of origin, to demonstrate the concept of sustainability throughchoice of alternative (“Secondary”) species. Since these bridges were built in a highlytermite-susceptible region, pressure preservative treatment was used in addition toselecting a durable species.

Where laminated softwoods are specified, European redwood rather than whitewoodis preferred for external structures, by Nordic glulam producers, due to its greateramenability to pressure preservative treatment. Douglas fir was hitherto more widelyused in these situations in the UK. Also, there are no technical reasons why larchshould not be chosen. Indeed, one particular specialist timber engineeringmanufacturer prefers this timber, finding that it bonds very well. It was the preferred



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choice for Scottish fishing boats, including laminated keels, but is now uncommon inthe timber engineering industry.

Unlike the case in Germany, laminations for external structures such as bridges arenot restricted, in Britain, to a maximum of 33mm. In that country, this is understood tobe a precaution to ensure that full clamping pressure is applied throughout the depthof the elements. However, a prescriptive standard of this nature would no longer belikely, within the harmonised European system. The matter would be left to thediscretion of individual manufacturers, under third party quality assurance controls.

The normally permitted maximum of 45mm for straight laminations is quite commonfor glulam in both softwoods and hardwoods. Permitted adhesive types are of courseselected from the most rigorous exterior/high hazard exposure category, and thisnormally indicates a phenol/resorcinol formaldehyde type. Provided that the adhesivespecification and manufacturing procedures are correct, including quality control testsin relation to the finger joints, there seems to be no reason to believe that 45mm thicklaminations, including those from selected hardwoods, are unsuitable.

d) Other structural timber composites.The 'family’ of structural timber composites (STCs) is growing. Glulam is really thebest known, and longest established, structural timber composite, but it has beenjoined by other products that are manufactured from veneers, strands and flakes.These are dried, graded and reconstituted, using modern synthetic adhesives, appliedunder heat and pressure. The exact processes vary, but long, prismatic structuralsections always ensue, as opposed to wide, flat boards.

The newer STCs are still not as widely known as glulam, by generalist engineers andarchitects. These tend to take a long time to become aware of such changes, butpublicity by major European producers, and support work on codes and standards, isbeginning to take effect.

STCs are manufactured using well established techniques and materials that havebeen developed over many years for the production of structural wood-based boardmaterials. Indeed, each member of the ‘family’ of STCs has its ‘relatives’ in thestructural board materials range, most of which have long-standing references bycodes such as BS 5268. Several types of STC are suitable for bridges, and instancesof their application for this purpose can already be cited. The following outlines thethree main types of STC, commenting on their potential for this purpose :

Laminated veneer lumber (LVL): Bonding together dried, graded, spliced andtrimmed veneers that are peeled from a log, in much the same way as makingplywood, produces laminated veneer lumber (LVL). Once pressed and trimmed, theresulting long panels are sliced into prismatic structural-sized sections. Unlikeplywood, successive veneers are generally orientated in a common grain direction,although a hybrid product, that has every fifth veneer laid orthogonally, has also beenfound useful for diaphragm applications, including bridge decks.

European LVL is manufactured in Finland, from Norway spruce, by Finnforest Oyunder the name Kerto LVL. The standard (all veneers longitudinal) type is namedKerto S, and the special (cross-veneered) type is Kerto Q.



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LVL is also manufactured elsewhere in the world, the great majority of themanufacturing machinery emanating from Finland. In Australia, for example, LVLmade from Radiata pine is applied in bridges.

LVL is found to be amenable to full penetration by pressure-applied preservatives, andis thus one of the best established of the new STCs for exterior structures.

Parallel strand lumber (PSL): Parallel strand lumber is manufactured from peeledveneers that are cut into long strands. These are then coated with adhesive andcombined under heat and pressure, in a quasi-extrusion process, to form structural-sized sections in long lengths.

PSL is manufactured in the USA, by TrusJoist MacMillan, under the name Parallam. Itis made mainly from Southern yellow pine and Douglas fir.

PSL is also understood to be amenable to pressure-applied preservatives, but thereis less experience in its use for exterior structures in Europe, possibly due in part topricing differentials, compared with LVL.

Laminated strand lumber (LSL): Laminated strand lumber is produced by bondingtogether flakes of wood, again under heat and pressure, to produce structuralsections.

LSL is also manufactured in the USA, by Trus Joist MacMillan, under the nameIntrallam. It is produced mainly from Aspen strands, a timber that is perishable(extremely non-durable), rendering this an unlikely choice for bridge structures, even ifpreservatives were to be introduced.

Advantages of STCs: The major advantages of STCs are that large dimensions areavailable, with higher characteristic strength values than those of the raw materialitself. This is brought about by defect dispersal within the manufacturing processes.These products are manufactured at a low timber moisture content, their dimensionsare accurate, and when installed, moisture-related movements, such as shrinkage,twisting and warping, are virtually eliminated.

The strength of solid timber sections depends largely on the influence of defects,such as knots and irregular or sloping grain, rather than on the inherent strength of theclear straight grained species. Clear, kiln dried timber is normally at least two and ahalf times stronger than average quality commercial sawn timber, at air driedmoisture content. When making composites, the veneers, strands or flakes arerecombined. This dispersal of defects produces a material, which has significantlymore consistent structural properties than solid timber. The longer spans that canachieved often mean that fewer intermediate supports are required, and simplerstructural systems are possible. Construction times can be significantly reduced, bytaking advantage of these features, in combination with a number of innovative,partially prefabricated techniques for element and connection formation.

Guidance and standards for STCs: The design of elements and components usingSTCs may, in general terms, be undertaken in accordance with the rules given in BS5268, or with those in DD ENV 1995-1-1 Eurocode 5 Design of timber structures Part1.1. There are at present no British or European Standards for STCs, and as a matterof principle, no materials have their design data included as part of the Eurocodes. Infact, it is considered an advantage of the newer Eurocodes that they are more “open”to the introduction of such innovative products.



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Suitable information on generically classified STCs is shortly to be provided, inintended European Standards (ENs). Examples of existing Technical Approvals thataddress STCs such as Kerto are the British Board of Agreement Certificates.Meanwhile, procedures are also being developed to establish European HarmonisedTechnical Approvals that will address materials such as these.

e) Mechanically laminated timber (“mechlam”).Mechanically laminated members are termed "mechlam" as a convenientabbreviation in this review, although it should be clarified that this is not a universallyrecognised term. Recently encountered has been an interesting example of amechanically laminated Greenheart bridge, which was built in Cheshire in 1915 andwhich, having remained in good condition, has just been refurbished.

The modern manufacturing process, which was developed in Germany, and usedquite extensively there and in the Netherlands, has become quite familiar in the UK.Numerous examples of bridges containing members of this type are to be found,ranging from simple short-span beam bridges, to the more ambitious types such asarches and cable stayed structures.

Formerly, the timber used was almost exclusively Ekki, or Azobé, as it is known inContinental Europe. Recently, experiments and a few actual applications haveoccurred using oak. The design of mechlam structures involves some specialconsiderations involving slip between the layers that leads to incomplete compositebehaviour. This affects ultimate limit states, as well as serviceability design. Some ofthe fundamental principles are provided by Eurocode 5 Part 1-1, and STEP LectureB11 also explains the basis of the computations, with elementary examples, based onthis code. However, mechlam timber bridges are now offered in Western Europe,including UK, by some half a dozen firms, on a design, supply and erect basis. All ofthese types of supplier tend to guard precise details of the full basis of design frommainstream practitioners, as well as from organisations such as BRE and TRADATechnology.

7.2 Decks & decking – UK current practice

Structural diaphragm decks do not at present form part of the British timber bridgedesigner’s vocabulary. They are an extremely significant item that needs to bebrought forward for their attention, in order to improve efficiency, as outlined in theIntroduction. For convenience however, the following only discusses decks that spanas secondary or tertiary items between transoms or stringers, and which do not actas composite diaphragms. The latter are briefly introduced in Section 10.5 of thisreport, dealing with Overseas Practice – Decks.

The commonest form of simple one-way spanning, non-diaphragm deck usesspaced sawn planks. These are usually laid transverse, but are sometimes placedlongitudinally. The deck planks can be softwood or hardwood, with certain hardwoodspreferred for maximum wear and durability. In connection with wear, designers are ineither case advised to discount a proportion of the section, as newly-placed, whencalculating for strength and stiffness. This point is addressed specifically by prENEurocode 5 Part 2.

Softwood decking planks can be specified as either GS or SS grade to BS 4978.Suitable preservative treatment may be considered. This would tend to lead specifierstowards timbers such as Scots pine/European redwood, Douglas fir and larch.



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Hardwood decking planks are usually from a naturally durable species, such as Iroko,Jarrah or Ekki, and are specified as HS grade to BS 5756.

Slipperiness of timber decks in general is recognised as something to avoid. It is anissue also related to maintenance and upkeep. It is an aspect of timber footbridgesthat would merit attention that is more specific to these structures, where the questionof incline of ramps and arches also enters into the equation.

Where foot grip is especially important, profiled decking planks provide a goodsolution. Hardwood profiled planks, in timbers such as ekki, are marketed extensivelyas a separate purchasable item, by timber bridge suppliers, for customers’ use inlandscaping and crossing structures beyond the realm of bridges. In footbridgesspecifically, at higher gradients, profiled planks are sometimes used in conjunctionwith kick-plates, which are nailed down to the deck. Unless these are well executedhowever, they are a notorious source of early wear and hence maintenance cost.Recently a proprietary form of profiled, treated softwood decking board has alsobecome available. This has embedded inserts of non-slip material, which are groovedinto each castellation of the profile.

It is generally felt that the gaps between simple decking in rural footbridges should notbe less than 5mm, in order that dirt and debris can pass through the deck. This alsoallows air to circulate around the planks, thereby avoiding damp pockets where fungaldecay can start. Larger gaps are sometimes used, and in remote country areas,deliberate gaps of up to 25mm have been specified. For certain bridges over roadsand railways, particularly in more urban environments, gaps in the walkway are notpermitted, due to concern over vandals dropping objects onto vehicles or personsbelow. This has led some designers to use glulam beams, which can provide thespanning medium for the bridge, as well as the deck. Such laminated decks areabutted together, to provide the walkway. An alternative solution has been to useplywood decking with additional non-slip surfaces, but this does not seem to have hada good record. Wear has been rapid and it has become evident that plywood deckingrequires special attention to drainage details.

LVL decks, with appropriate pressure preservative treatment and added wearprotection, have started to occur in a few instances, in the UK. This type ofprogressive solution is moving towards the concept of treating the deck both as awalking surface, and as a structural diaphragm. As mentioned above, this isdiscussed further in section 8.5.

7.3 Parapets & handrails

The primary function of the handrails and parapets is of course the protection ofbridge users. Occasionally, in very remote areas such as forests and moorland trails,bridges are built with no parapet, or with only one handrail. A pair are however thenorm. Various configurations are used, with the choice primarily depending on thefollowing:

a) The type of footbridge user (for example – pedestrians only, or cyclists andpedestrians).

b) The nature of the site and locality, for example whether it is a rural or urban

location, and whether it passes over a main road, railway or a stream.



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The first item dictates the height and strength requirements for the parapet, asdiscussed in more detail below. The second affects the degree of openness that ispermitted for the handrail, intermediate rails (if any), spindles and posts.

Common solutions for bridges in rural locations involve cantilevered handrails. Abetter, and very traditional arrangement in these types of location may be for thevertical post to be triangulated, by projecting out the decking members in the vicinity ofthe post, and adding a diagonal raking member. This type of design is often adoptedfor short-span bridges, where the main beam is of limited depth.

For suburban bridges, it is not uncommon for mesh infill, or even solid, surface-profiled metal sheet, to be fixed to the handrail, to prevent children from falling throughthe gaps. The cheap, but somewhat inelegant solution is mesh infill. A betteralternative is light section, close centred spindles with adequate intermediatelongitudinal rails for stiffness.

In fully urban areas, an altogether lesser degree of openness is usually required,whilst for bridges over trunk roads, motorways and railways, there is serious concernover objects being accidentally kicked, or deliberately dropped, onto the highway, rails,or traffic. In applications such as these, authorities will invariably stipulate the requireddimensions of enclosure that will normally prevent an open solution. This has resultedin a number of bridges where the deck is located near the centre, or towards the baseof the main beams. These then provide the lower half or two-thirds of the parapet.This type of arrangement is common in glulam footbridges with through decks.

Larger, girder truss bridges ( for example the type illustrated in Figure 11 c), also oftenincorporate part of the structural girder depth into the parapet. This of course hasfurther detailing implications, but such structures tend to be offered by specialists whohave evolved practical and acceptable solutions that comply with the rules andcustoms of the various European countries in which they operate.

Both softwoods and hardwoods are used for handrails, with the latter, in a suitablespecies, preferred for durability and smoothness to touch. Most, if not all, of thesmoothest-to-hand timbers used in joinery are of tropical origin. External weatheringtends to aggravate splinter pick-up in open grained species. Hence, this is a particularissue that needs to be clarified, in relation to user-inhibitions through preference foravoiding tropical timbers, because of perceived sustainability questions. Besidessustainability, however, is the matter of toxicity to skin of the splinters of somespecies. Good detailing of parapets and handrails is thus undoubtedly an aspect ofthe furtherance of timber ridges that requires co-operation between timber engineersand wood technologists.

Regarding grade and strength class specifications, softwood handrail members willagain be GS or SS grade to BS4978, assigned to the appropriate strength class, andwith suitable preservative treatment, if required. Hardwoods are usually from anaturally durable species, such as Iroko or Opepe, and are HS grade to BS 5756 andthus to the appropriate hardwood strength class (D Classes).

Parapet and handrail detailing to achieve protective design against long-termdeterioration, through weathering and decay, is addressed by several Nordic andGerman-language publications. Some indication of the type of guidance available canbe seen in STEP Lecture E17 (Fisher 1995). This is another aspect that will be wellworth further attention for UK design guidance.



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7.4 Connections

Connections within modern timber bridge structures is an enormous subject, thatcould quite easily occupy a two year applied research and “knowledge repackaging”project, in its own right. TRADA Technology is currently engaged in a separate projectto this, under DETR Partners in Innovation funding, entitled “Connections IT Toolbox”.Good introductory texts for engineers on mechanically fastened timber joints ingeneral are also contained in STEP Volume 1, Lectures C1 to C19. The TRADATechnology Eurocode Design Guidance Documents already published also addressthe topic extensively.

Mechanical fasteners and connectors for bridge structures are at present normally ofsteel, and quite often of stainless steel specifications, rather than from plain carbonsteel. In virtually all instances, some form of corrosion protection is required on otherthat stainless items. Where flitched or spliced joints involve the use of steel plates,these are usually specified with a thickness of not less than 6mm, following steelbridge design practice. Again corrosion protection is essential and the Eurocodes 5(both Part 1-1 and Part 2), supported by the documents described above, provide anentry point.

Signposts to research on bonded-in connections, a lot of which is highly relevant tobridges, are given in the state of art reference cited above. Work has also beenstarted on bonded-in non-metallic (Fibre Reinforced Polymer, or FRP) connectionsthat may in future be relevant, especially in view of their potential corrosion resistance,as well as their high tensile strength (Bainbridge et al, 2000).Fatigue within timber structural connections for bridges is another aspect thatresearchers have started to address. Some success has already been achieved,showing this to be not a hyper-critical issue, but one that can be handled usingestablished timber research and code formatting techniques.



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8.0 DURABILITY

Timber, when suitably protected, can be remarkably durable and can outlast in certainconditions other materials such as metals, brick, stone and concrete. Timber isinvulnerable to salt water, either from sea or de-icing salts, and freeze-thaw action.In a timber bridge, elements which are not covered will frequently attain moisturecontents above 20% - the threshold for fungal decay. The threshold for insect attackis even lower, at 12%, although only sapwood and decayed heartwood is vulnerable.Preservative treatment will be necessary only if the natural durability of a timber isinsufficient to meet the required service life.

8.1 Detailing

Bridges are a particularly exacting application, and ensuring that the timber membershave adequate durability is a vital consideration. Before considering this item from theperspective of material selection, it is important to note that much can be achieved interms of increased durability by means of improved detailing. Indeed the converse isalso unfortunately true, in that if poor detailing is provided then premature failure oftimber components can occur.

Table 4 identifies seven susceptible parts of a timber bridge in general. Most of thesepoints apply to all types of bridge, irrespective of the precise form of the structure. Thetable then exemplifies poor detailing aspects and gives better alternatives. At thisstage, the items in the table are regarded as pointers for guidance, and assuggestions for closer attention, rather than definitive solutions. It is anticipated that itwill be necessary to pay considerable attention to detailing, and that these aspects willrequire discussion by timber experts and bridge manufacturers, in conjunction withthe analysis of the survey results.

The benefits of effective and well maintained finishes have been very apparent in thesurvey work which has already been performed and which continues. Modern waterrepellent finishes offer a considerable measure of protection to exterior timberstructures such as bridges. The prevention of weathering of the timber surface itselfhas an important role in this respect. A high specification of finish and a goodmaintenance programme for the same would always be advocated in addition to thecorrect detailing and choice of durable species mentioned above.



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Part of the structure Examples of poordetailing

Examples of betterdetailing

End grain of members ingenerale.g. beams

Exposed end grain, leadingto fissures, unattractive andultimately a seat of decay

Protected end grain eg: byattaching other timbermembers having side grain,or by ventilatedcapping/sealing

Upper edges of exposedmemberse.g. beams and handrails

Flat upper edges wherewater lies and which trapdirt, especially whenweathered/ fissured

Chamfered and slopedupper edges which freelydrain

Edges protected byventilated capping

Joinery details e.g.:handrails, parapet to beamconnections

Details which trap moisturein mortises, fixing holes,recesses etc.

Freely draining, ventilated,flush details

Raise parapet above splashlevel with separate drainedkerb

Decking and itsattachments

Deck which is tight jointedor with a sealed surface butwhich merely trapsmoistureAttachments to beamswhich form traps

DPC between deck andbeams

Deck which freely drains,laterally and longitudinally,even when worn

Drip mouldings beneathdeck boards

DPC between deck andbeams

Intersection points caneasily form moisture/dirtentrapment regions.

Not easily avoided, butdetail for maximumventilation and drainage eg:by drilling/arranging gaps

Member intersection points,column bases, especiallywith steelwork

Intersection points can trapmoisture and remain damp.

Design steelwork to allowdrainage and ventilation.Avoid details which allowthe collection of water .

Bearing points, supports,bank seats etc. Poorly ventilated,

susceptible to silting up, dirtand debris entrapment

As well raised fromsurroundings, eg: bymasonry and supportingsteel, as possible

Table 4: Susceptible parts of timber bridge structures, with examples ofdetailing weaknesses and improvements



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8.2 Natural durability

The biological natural durability of timber is due to the anatomy of the timber speciesand in some cases the presence of naturally occurring extractives within theheartwood. Each timber species has its own characteristic set of these chemicals,some of which are toxic to wood-destroying organisms. Even when the detailing is asgood as possible, for an exacting, fully exposed application of timber such as abridge, it is advisable to consider the use of a timber which falls into a naturaldurability category which is at least as good as "moderately durable", Table 5. It isimportant to note that the biological natural durability of a timber refers only to itsheartwood.

Such classifications are well-established in Britain for all of the better-knownconstruction timbers, both softwoods and hardwoods, including all of those listed inBS 5268: Part 2. Table 6 shows the natural durability classifications of the twelvetropical hardwoods listed in the code, together with European redwood and Douglasfir, for comparison. These classifications are based principally but not exclusivelyupon traditional ground contact stake tests. It should be noted that the ratings relate toUK conditions, which do not include a termite hazard, but which represent a high riskfrom fungal attack.

Exposure trials have been conducted using EN 330 "L-joint" type specimens, both toassess natural (untreated) durability, and to evaluate various forms of preservativetreatment beneath a coating. In due course, the information from this project will be ofvalue to bridge designers, especially when they consider the joinery items such asparapets and handrails. Certified, sustainable and naturally durable timbers, including,for example, plantation teak from various sources, may be admitted for the mostexposed parts of superstructures, such as handrails and parapets, on the basis ofevidence from such tests.

Durability Category Approximate life in ground contact, 50mm x50mm section (years)

Very durable More than 25

Durable 15 – 25

Moderately durable 10 – 15

Non-durable 5 – 10

Perishable Less than 5

Table 5: Natural durability categories



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Timber, Standard Name; Species Region of Origin Natural durability

BalauDense Shorea spp

SE Asia Durable

Ekki (Azobe)Lophira alata

W Africa Very durable

GreenheartOcotea rodiaei

Guyana Very durable

IrokoMilicia excelsa


JarrahEucalyptus marginata

W. Australia Very durable

KapurDryobalanops spp

SE Asia Very durable

KarriEucalyptus disersicolor

W. Australia Durable

KempasKoompasia malaccensis

SE Asia Durable

KeruingDipterocarpus spp

SE Asia Moderately durable

MerbauIntsia spp

SE Asia Durable

OpepeNauclea diderrichii


TeakTectona grandis

SE Asia Very durable

Douglas firPseudotsuga menziesii

N America Moderately durable

European Larch Larix decidua (L. uropaea)

Europe, incl. UK Moderately durable

Scots pine/European redwoodPinus sylvestris

Europe, incl. UK Non-durable

Table 6: Natural durability classifications of the twelve tropical hardwoodslisted in BS 5268: Part 2, and of Douglas fir and European redwood



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8.3 Preservative treatment

In the modern philosophy of designing for durability, the use of chemicals to treat thetimber, normally through pressure application, is regarded as the third line of defence,following good detailing and species selection. Increasingly, though, the mostsustainable use of our principal commercial timbers (i.e. non-durable softwoods) is toextend their service lives through preservative treatment. This allows more thansufficient time for forest growth to compensate for the consumption of timber.However, evidence from around Europe suggests that traditional preservative activeingredients are going to come under increasing environmental scrutiny and legislation.In Denmark and the Netherlands, legislation has already instigated restrictions on theuse of copper/chromium/arsenic (CCA) - the most widely used wood preservative.Amendment to the EC Marketing and Use Directive for arsenic will limit CCA to a fewderogated uses including bridges. Creosote is due to be withdrawn forpublic/domestic use in the EC in 2003, but will still be available for industrialapplications such as utility poles and bridges. Of the 150 bridges constructedrecently under the Nordic Timber Bridge Project (Nordic Timber Council, 1999), themajority were either creosote or CCA treated. Clearly up to date guidance is neededfor timber bridge designers and certain clients may demand "environmentally friendly"solutions.

In the United Kingdom only formulations approved under the Control of PesticidesRegulations 1986 by the HSE Pesticides Safety Directive are used for timbertreatment, and formal authorisation procedures are in place to ensure that operationscomply with legislation relating to aspects such as employee health, safety, materialcontrol and waste disposal. Treated timber is, of course, far more widely used inapplications such as fencing, decking and utility poles than bridges. The latter may,however, come under greater scrutiny because of their siting over sensitivewatercourses.

For timber bridge members the principal chemical preservative treatments applicableare either Copper Chrome Arsenic (CCA) or creosote, applied under pressure,although there are some alternatives on the market. In general, hardwoods either donot require treatment or are difficult to treat due to poor penetration, whilst mostsoftwoods are treated. Spruce and hemlock are difficult to treat, although penetrationcan be improved by incising. BRE Digest 429 (1998) gives guidance on both naturaldurability and resistance to preservative treatment.

Copper Chrome Arsenic:

CCA (marketed under the trademarks of "Celcure" or "Tanalith") is a water-bornepreservative with a particular application for softwood glulam bridge beams, usually inconjunction with European redwood as the timber. Both pre-treatment of individuallaminations and treatment of the entire member after complete manufacture andmachining are known. Pressure cylinders of up to 25m length are available. Timber isdried before application of CCA under high pressure/vacuum process and allowed todry again for between 7 to 14 days during which fixation occurs. The advantages ofCCA treatment are that the timber is free from odour and that paints and varnishescan be applied. CCA treated timbers (such as off-cuts) should not be burnt in an openfire, since this releases the preservative elements in the ash and smoke. Waste CCAtreated wood should either be burnt with flue gas recovery or put into landfill.



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Creosote:

Creosote is a complex mixture of over 300 substances derived from the distillation ofcoal tar and tends to have a poor image due to an association with odorous gardenfences and weeping telegraph poles. In fact, creosote is a very long serving andeffective wood preservative which has low water solubility and is biodegradable whendispersed in soil. There are many instances of creosoted timber structures and woodpiles still giving good service after 100 years of ground contact, and timber currentlytreated by the pressure process can offer a service life of at least 40 years. Althoughfresh creosote will burn the skin, requiring gloves to be worn during handling of treatedlumber, it is not a systemic poison. Creosote also protects wood from thedevelopment of splits. Creosoted timber should not be used for parts of the bridgewhich come into contact with unprotected skin, such as handrails. Freshly creosotedtimbers may cause the formation of on oil sheen if in contact with water.

There are two forms of creosote treatment, full cell and empty cell. In the full cellprocess all the available voids in the wood structure are filled as far as possible withcreosote by first applying a vacuum to the timber, then flooding the pressure cylinderwith preservative. After the vacuum is released atmospheric pressure forces thecreosote deep into the structure. Further application of pressure after this stageachieves even greater penetration. At the end of the cycle, a second short periodunder vacuum is applied to withdraw a small amount of preservative from the surfaceof the timber leaving it dry and in a reasonable state for handling. In the empty cellprocess a longer period under vacuum is applied to remove a greater amount ofpreservative, leaving the voids in the wood only partly filled but with the internal wallsof the wood cells coated. Although creosote is used undiluted by solvents, freshlytreated timber is normally allowed to dry for up to 7 days to allow the more volatilecomponents to evaporate.

Alternatives:

Many preservative products have been developed over the last 10 years that haveaimed to provide alternatives to CCA whilst providing equivalent performance in thefield. A number of these have focused on removing the arsenic compound from thepreservative formulations, such as CCB systems that use boron. There are alsosystems available that are both chromium and arsenic free where tebunconazole andboron based preservative have replaced those ingredients. These preservatives stillprovide the timber with its characteristic green colouration, and are increasing used inmarkets such as garden decking. All these products are approved for use inPesticides 2001.

Selection system for timber bridges:

Selection of an appropriate system for timber bridge component should follow theEuropean Standards below:

1. Design bridge (timber components in contact with the water, soil and out ofground contact)

2. Assess the hazard class of the end use of the timber (EN 335-1). For example atimber in freshwater contact is Hazard Class 4. Out of ground contact timber isHazard Class 3 and is a less challenging environment for the timber component.

3. Select wood species (EN 350-1 Natural durability)



46

4. Is wood species natural durability sufficient for the performance required for theHazard Class? (EN 460 Links durability to Hazard Class)

5. If species is insufficiently naturally durable then select and specify the preservative(EN 599-1 and required treatment result EN 351-1 and DD 239)

Alternatively a similar philosophy is passed through in BS 5589 and BS 5268 withspecifying based on preservative treatment schedules applied to particular timberspecies being fit for purpose.



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9.0 TIMBER FOUNDATIONS

Timber piles are a highly suitable choice of foundation, given appropriate groundconditions, for many structures including bridges. Timber piles are economical, easyto transport, handle, cut to length and work with on site. They are particularly suitedfor locations with access difficulties or where minimal disturbance is a priority suchas structures in the countryside and canal-side and shoreline sites where excavationsand the delivery of concrete would pose problems. Short driven timber piles can bethe solution for foundations in ground with a high water table or where firm strataexists below surface material of loose sand, soft clays, highly organic soils or fill.Timber piles are also resistant to acidic and alkaline soils, and soils with high sulphateor free carbon dioxide content. Treated or durable timber can also be used for theconstruction of wingwalls and bank seats, as well as for foundation pads and footings.

Timber is a sustainable construction material with obvious environmental advantagesover both steel and concrete. Trees, while they grow, adsorb carbon dioxide andrelease oxygen. Forests provide areas for wildlife and recreation. One of thesuggested methods of reducing global warming has been to create carbon sinks - tolock up carbon for long periods of time. Using timber for foundations would effectivelyachieve this.

9.1 History and overseas use

Timber has been used for piled foundations for centuries. Before 1900 nearly all pileswere either untreated wood or stone. Old London Bridge was founded in 1176 onstone filled starlings constructed from elm piles, and lasted 600 years (Nash, 1981).The City of Louisiana is founded on timber piles, so too is Pont Notredame bridge inParis, The Royal Place in Amsterdam, The National Theatre of Finland, The Dome ofUtrecht and The Reichstag in Berlin. In 1902 the Campanile Tower in Venice wasrebuilt on the 1000 year old piles, still in excellent condition, which supported theoriginal structure (Haldeman, 1982).

More recently, Graham (2000) reports on the use of 30 tonne capacity timber piles forthe foundations of the Cargo Terminal at John F. Kennedy Airport. Timber piles werealso used for the 210m diameter Louisiana Superdome supporting 130,000 m3 ofconcrete and 18,000 tonnes of steel. Timber piles with 70 tonne design loads are inuse on a 300m long viaduct near Winnemuca, Nevada. In Canada alone over30,000m3 of treated wood piles are used annually. Most of the deep foundationsupport for highway bridges in North America comprise of treated timber piles. TheUS Army Corps of Engineers used over six million timber piles to construct the locksand dams for the Inland Waterway System.



48

Figure 12: Timber trestle piles supporting a concrete deck bridge (Bellingen, NSW) (photo Kardon Piling)

9.2 Durability of timber piles

Timber piles, when driven in below the ground water level, are virtually immune tobiological degradation and can have an almost indefinite life. Timber piles have beenrecovered from the remains of Roman and medieval constructions in a state ofperfect preservation. The section of a pile above ground water level is, however,vulnerable to decay and one option is to terminate the pile below the water table andcontinue the foundations in a different material such as concrete. In the past this wasaccomplished with stone or masonry. The timber piles of historic buildings may decayif the local water table is lowered below the tops of the piles for long periods, either byabstraction, drainage or de-watering for nearby excavations. Both York Minster andthe Mansion House in London were built in marshy ground on timber foundationswhich subsequently degraded due to drainage. Bouteje and Bravery(1968) report onsimilar problems with buildings in Stockholm.

Above the water table in soil, fungi attack will lead to severe deterioration of untreatednon-durable timber in a matter of months only. Under fresh water such as in riversand lakes but also in soil below the water table the outer layer of sapwood ofuntreated timber will become infected by anaerobic bacteria which can degrade thestrength properties over a time period of decades. In central Europe untreatedspecies of non-durable softwoods such as Scots pine and Norway spruce wereformerly used extensively.

Timber piles are highly resistant to both acid and alkaline soil conditions. In Austrailia,at the Ulan coal mine, treated timber piles were chosen for a bridge carrying oretrucks because high free carbon dioxide levels and extreme acidity in the soil wouldhave destroyed both steel and concrete piles. Timber piles were also used for thefoundations of the Brambles Container Terminal in Burnie Tasmania (soil pH 11.5)and the Auburn, N.S.W., Waste Transfer Station (soil pH 2.5).



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9.3 Traditional timber species and treatments

BS 8004 (1986) gives guidance on the use of timber for piles, and states that in theUnited Kingdom Douglas fir imported in sections up to 400mm square and 15mlength is the most common softwood used for piles. Pitch pine is also available insections up to 500mm square. Greenheart was formerly the most commonly usedhardwood, imported rough-hewn in sections up to 475mm square and up to 24mlong. Other suitable tropical hardwoods given by the Code include ekki, jarrah, andopepe. In the past, domestic grown hardwoods such as oak, beech, ash and sweetchestnut have been used for piles. Elm is also durable below ground, so much so thatit was used for water pipes and also coffins. In Scandinavia and Central EuropeNorway spruce, Scots pine and to a lesser degree fir and larch have historically beenused (Peek and Willeitner, 1981). In the US and Canada southern pine is usedextensively as well as larch and western red cedar.

The Romans are known to have treated timber for pilings by smearing the wood withcedar oil, pitch and then charring. Pressure injection of coal-tar creosote began inEngland in 1838. Following the use of pressure impregnated railway sleepers (orrailroad ties) in the United States the process was first applied to foundation pilings inthe early 1880's. Today, pressure impregnation of creosote or copper-chrome-arsenic (CCA) are the two main types of chemical wood preservation applied totimber used for piles. Only softwoods are suitable for chemical impregnation, withspruce and hemlock being difficult to impregnate. Both types of preservative areapplied during a high pressure/vacuum process. In the case of CCA preservative,which is water-borne, the chromium acts as an oxidising agent and the metalsbecome highly fixed into the wood structure. Creosote, which is derived from thedistillation of coal tar, may be applied as a "full cell" or "empty cell" process. In thelatter case a vacuum is applied post application to remove surplus creosote whichmight otherwise bleed under the influence of sunlight. For softwood timber pilestimber selected with a thick sapwood layer which adsorbs preservative treatmentbetter than heartwood is beneficial since this provides a thick protective layer of wellimpregnated material. Spikes or hooks should not be used to handle treated timberpiles since this may expose less well protected wood in the inside of the log. All cut-offs and drill holes should be liberally applied with preservative. For hardwoods thevulnerable sapwood is removed and the timber is normally supplied squared off.

9.4 Marine structures

Timber is favoured for marine works because of its ability to absorb impacts, its easeof handling over water, and the poor performance, historically, of cast iron, steel andreinforced concrete. Timber is used for groynes and sea defence works as well asjetties and dolphins.

In seawater and brackish estuary waters untreated timbers are liable to attack bymarine borers, around the British Isles principally the mollusc Teredo (the shipworm)and crustacean Limnoria (the gribble). Teredo bores circular tunnels 15mm indiameter and up to 150mm long horizontally and vertically in timbers leading,ultimately, to severe weakening. Occasionally Teredo damage is observed in timberswhich have been floated in marine waters prior to sawing, the damage beingcharacterised by lack of bore dust and the chalky white calcareous tunnel linings(Desch and Dinwoodie, 1996). Limnoria creates shallow tunnels approximately2.5mm in diameter and penetrating less than 15mm in depth, the extensive nature of



50

which leads to erosion. Another crustacean Chelura, is associated with attack byLimnoria, but cannot by itself burrow very far into timber.

The Sea Action Committee of the Institution of Civil Engineers (ICE 1947) foundLimnoria and Chelura to be active in British waters, with Terodo active south of theMersey and Humber. Greenheart, kauri and jarrah were found very resistant to marineborers, while oak and untreated softwoods were not resistant. Borer attack was foundto be limited in polluted water such as in docks, although this observation may not berelevant nowadays. Greenheart was found in excellent condition after 60 yearsservice in Liverpool and similarly Danzig fir after 52 years service in the Thames atNorthfleet. Creosoted Baltic pine (i.e. slower grown Scots pine) was recommendedfor British ports on the grounds of its useful economic life. Other suitable softwoodsinclude Douglas fir, Western hemlock and European larch. The Handbook ofHardwoods (HMSO 1972) lists a number of tropical hardwoods recognised as beingresistant to marine borers such as basralocus, belian, okan as well as the Australianhardwoods jarrah, ironbark, southern blue gum and turpentine, the latter beingparticularly long favoured. Currently there are no FSC approved sources ofgreenheart or ekki which were traditionally used for marine piles, although possiblealternatives include Acariquara and Purpleheart.

In tropical waters untreated timber piles of non-durable species can have a useful lifeof only a few months. For softwoods, combined treatment of CCA and creosote hasbeen found very effective. Hardwoods, such as terpentine, will also benefit greatlyfrom the provision of an outer barrier layer of treated timber. Timber piles may also beprotected by providing copper or aluminium sheaths, and there has been somedevelopment of the use of PVC (Heinz, 1975). Methods of repair of marine borerattacked piles include jacketing with concrete. The principal inspection method formarine timber piles is by diver.

Abrasion resistance is an important design consideration for marine structures suchas sea defences and groynes, particularly on shingle beaches. Timber can withstandwear in the marine situation better than either steel or reinforced concrete, withtropical hardwoods being particularly durable. Dense softwoods such as Douglas firand pitch pine also perform well. Timber structures can be protected from scoursimply by providing a sacrificial layer of planking.

9.5 Pile driving and design

All timber piles are displacement piles, therefore suitable ground conditions must existfor their use. Conventional pile drivers are used to insert timber piles with the normalweight of a drop hammer being 1.5 times the weight of the pile. A long narrow drophammer increases the chance that the pile is hit axially, avoiding damage to the pileand maximising the downward impulse. Diesel hammers are also suitable, includingthose that run on bio-diesel. Care must be taken with all hammers not to over-stressthe pile or to cause splitting of the pile toe. A helmet or cap protects the pile head fromfracturing or brooming, and in addition the pile may be banded to prevent splitting.Hard driving should not be continued in an attempt to meet a prescribed set, sincethis may result in the pile head disintegrating. Where there is a surface layer of hardfill a pre-bore may be performed. Timber piles can also be inserted using vibratorymethods. Groups of timber piles inserted into soft clays may need to be loadedtemporally to prevent the effect on soil pore water pressures causing buoyancy.Timber piles can be spliced and extended in length using short sections of steel tube,angle or plates to reach loadbearing strata.



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Figure 13: Timber pile driving (photo Kardon Piling)

Timber piles support loads by end bearing, shaft friction or combined end bearing andfriction depending on the nature of the strata into which they are inserted. Driven thinend down, trees make natural tapered piles enhancing shaft friction. Timber has ahigh strength to weight ratio, and is particularly strong in compression parallel to grain.The timber selected for piles should be straight grained and free from defects and, ingeneral, suitable material is obtained from SS grades and better. The centreline of asawn pile should not deviate by more than 25mm throughout its length, and for roundpiles a deviation of up to 25mm on a 6m chord may be permitted.



52

Piles are designed as columns, but consideration should be given to bracing forunsupported lengths above ground level. BS 5268 (2002) may be used to calculatethe allowable compression parallel to grain. Stresses on the timber during insertionare usually far higher than those encountered in service. Similar equations exist forthe loadbearing capacity of timber piles to those of steel and concrete, including thosebased on dynamic pile driving formulae. Appropriate factors of safety for foundationdesign are given in BS 8004 (1986) and BS 6349 (1988). To reduce the need forexcessively hard driving the working loads are often limited to 300kN (30 tonnes) on a300mm x 300mm pile. The American Wood Preservers Institute (2000) and CanadianWood Council (1991) also give guidance and design examples on the use of timberpiles.

9.6 Other geotechnical uses for timber

Timber is attractive and its use for earth retaining structures such as bridgeabutments is suitable in sensitive countryside, in particular for forest tracks. Inaddition, there is the obvious benefit of utilising an inexpensive, locally producedbuilding material in situations were the delivery of heavier materials would poseproblems. Timber can easily be combined with soil anchors and geotextiles in thesame way as concrete or steel. Round timber and sheet piles can provide aneconomical wall for moderate heights of retained material. Examples of interlockingtimber sheet piles are given in BS 6349-2 (1988).

Further demonstration of the suitability of treated timber as foundation material isprovided by the permanent wood foundation or PWF (CWC, 1997). PWF is a loadbearing wood-frame system designed as a foundation for light-frame construction forresidential houses, commercial premises such as hotels and factories, andagricultural buildings. Its use dates back to 1967 in Alberta, Canada. Water-basedpressure treated timber is laid directly onto a granular drainage layer 300mm deep.This drainage layer prevents hydrostatic pressure building up against the foundations,and allows timber framed basements to be constructed in suitable locations. Allconnectors are corrosion resistant. A polythene moisture barrier extends over theoutside of the walls below ground level terminating at the top of the drainage layer(Figure 14). A separate moisture barrier exists under the floor of the basement, eitherover-site concrete or a suspended timber floor. There is no need for a damp proofcourse between the timber footing plate and the bottom rails of the wall panels which,like timber frame, comprise the main support of the structure.



53

Figure 14: Permanent Wood Foundation (illustration Canadian Wood Council)

Stanchions and columns can be founded on footings comprising two layers of nailedtreated timber running at 90 degrees to each other (Figure 15). The timber footing islaid on a thin layer of sand over undisturbed soil. A steel plate placed over the top ofthe footing helps to transfer the load from the column over the timbers. The principaladvantages of these treated timber foundations are that they are economical, fast,require less plant, and do not need measures to protect them from freezing - this lastaspect being particularly important in Canada. Timber foundations also makeexcellent usage of an abundant, renewable and local material. Timber foundationscan also take the form of embedded poles with concrete pads and collars. Examplesof these types of foundation for countryside structures, together with half round timberfootings and an example of a simple wood foundation for a footbridge are given byJayanetti (1990).



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Figure 15: Timber column base (illustration Canadian Wood Council)



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10.0 BRIDGE DESIGN PRACTICE

10.1 General practice for design of bridges in the UK

Bridges constructed from steel or concrete, whether highway or footbridges, aregenerally designed using the BS 5400 series of Standards. This series comprises tenparts, as follows:

Part 1 General statement giving design objectives and definitions.

Part 2 Specification for loads.

Part 3-5 Codes of practice for design of steel, concrete and compositebridges.

Part 6-8 Specifications for materials and workmanship for steel, concrete andcomposite bridges.

Part 9 Specification for bridge bearings.

Part 10 Code of practice for fatigue.

The partial factor design process for a bridge will primarily involve only two of theabove parts. Firstly the loads, and the partial safety factors for the loads, are obtainedfrom Part 2. Secondly the design (including partial safety factors for materials) isundertaken in accordance with Part 3,4 or 5 depending on which constructionmaterial is being used.

In the case of road bridges or bridges over roads, the Highways Agency (formerlyDepartment of Transport) has produced a number of Department Standards whichoccasionally override the requirements of the BS 5400 Series. One such DepartmentStandard BD29/87 gives directions to engineers on how to design footbridges,including timber types.



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An Outline Procedure For The Design Of Timber Footbridges Over Roads OrIn Urban Areas

In outline, the procedure may be considered in three stages, as follows:

1. Establish the general arrangements for the bridge, taking note of the requirements for layout and minimum dimensions given in DoT Departmental Standard BD29/87.

2. Evaluate the loads acting on the bridge, using the unfactored loads of BS 5400: Part 2, unless these are made more onerous by an HA Standard.

3. Design the members of the bridge in accordance with BS 5268: Part 2 which is a permissible stress code, used principally for timber members in buildings.BS 5268 does however contain sufficient basic materials properties, fastener design information and member design procedures for simpler types of timber bridge, as explained above in the section dealing with materials.

An Outline Procedure For Design Of Timber Footbridges In Suburban Or RuralAreas

Many engineers specialising in timber bridges consider the BS 5400/HA provisions forminimum dimensions and loading too severe for lightly trafficked footbridges insuburban and rural areas. For such footbridges, typical alternative procedures areexemplified as follows:

1. Establish the general arrangements for the bridge taking note of the minimumdimensional recommendations given in publications such as "Footbridges in theCountryside, Design and Construction" (Countryside Commission for Scotland,1989).

2. Evaluate the loads acting on the bridge using the unfactored loads of BS 5400:Part 2, or consider making them less onerous on the basic of recommendationsgiven in publications such as the above.

3. Design the bridge members in accordance with BS 5268: Part 2.

Examples of how the "Countryside Commission for Scotland" publicationrecommends less onerous minimum dimensions and loadings are given in Tables 7and 8 below:

Source of data DoT StandardBD29/87

"Countryside Commission forScotland" publication

Location of bridge Urban area Accessiblerural area(two-way

traffic)

Inaccessiblerural area (one-

way traffic)

Pedestrians only 1800 1200 900People with disabilities 2000 1700 1200

Table 7: Recommended deck widths (mm) for alternative bridge locations



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Source of data BS 5400: Part 2 "Countryside Commission forScotland" publication

Location of bridge Urban Area Rural Area

Vertical imposeduniformly distributedload on bridge deck(kN/m‘)

5.0 2.3 – 3.2

Horizontal load permetre (kN/m)perpendicular tohandrail

1.4 0.74 – 1.4

Table 7: Recommended imposed loadings for alternative bridge locations

Highways Agency criteria for layout and dimensions of footbridges

Layout of Footbridge

The HA (former DoT) Standard BD29/87 stipulates several criteria relating to thelayout of footbridges, some of which are quite fundamental to the bridge design.These may be summarised as follows:

Access: Access to footbridges located adjacent to carriageways should be sited sothat pedestrians walking down the access face on-coming traffic. Plain rampedaccess is preferred to stairs as it is more satisfactory for people in wheelchairs andpedestrians pushing prams. However wherever possible both forms of accessshould be provided.

Layout: The main span of a footbridge should, wherever possible, be at right angles tothe road carriageway.

Supports: Where a footbridge crosses a dual carriageway, preference should begiven to spanning both carriageways in a single span, to avoid the need for a supportin the central reserve. Supports which may be subject to collisions by errant vehiclesshall be designed to resist collision loading.



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Dimensions for footbridges

Width of bridges for pedestrians only:

The dimensions of the clear widths of the main span, ramps and stairs of a footbridgewould be derived on the basis of the information in Table 9 (below) meet the peakpedestrian traffic.

Gradient Clear width (mm)

< 1/20 300mm per 20 persons per minute

Steps or > 1/20 300mm per 14 persons per minute

Table 9: Recommended clear widths (mm) for alternative bridge gradients

Minimum widths of 1800mm for general purposes, or 2000mm stipulated for bridgesenhanced for use by disabled persons. Where bridges are to be designed for thecombined use of pedestrians and cyclists, further width requirements apply. Theserange from a 1200mm wide footpath separated by a white line from a 500mm widecycle track, to a 1950mm wide lane for each, separated by railings. The DoT criteriafor heights of parapets vary according to the types and combinations of bridge user.They are summarised in Table 10, below.

Type of footbridge Parapet height (m)

Pedestrians only, where bridge is in area ofhigh prevailing winds or with headroomunder bridge greater than 10m

1.15 – 1.30

Pedestrians and cyclists 1.4

Table 10: Height requirements for bridge parapets

Stairway requirements may be summarised as follows:

Maximum number of stairs in a flight is 20Riser dimension < 150mmTread dimension > 300mm.

It is a preference that ramps should not be steeper than 1 in 20. However if limitationsof space dictate then steeper ramps may be used, up to a maximum gradient of 1 in12. For ramps with gradients greater than 1 in 20, landings must be provided in orderthat the rise of any ramp section does not exceed 3.5m

Evaluation of loads using BS 5400: Part 2



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As mentioned above, timber footbridges are designed using the unfactored nominalloads of BS 5400: Part 2. Interestingly, the use of unfactored nominal loads instructural design is not only limited to timber, with the design of foundations and thatfor aluminium structures also being based on unfactored loads. Where adequatestatistical distributions are available, the nominal loads in BS 5400: Part 2 are thoseappertaining to a return period of 120 years.

The following types of nominal load relating to footbridges are considered by BS 5400:Part 2

1. Permanent loads

• Dead-weight of structural elements• Superimposed dead – road surfacing, etc. 2. Live loads from pedestrian traffic • Nominal vertical live load• Nominal load on pedestrian parapets 3. Wind loads • Transverse• Longitudinal• Vertical

4. Loads from temperature effects

5. Erection loads

BS 5400: Part 2 suggests that in most cases snow loads can be ignored. This islogical since the full pedestrian design load is improbable under heavy snow falls ofthe duration likely to be experienced in the UK.

The maximum wind gust speed is evaluated by applying gust factors to mean hourlywind speeds extracted from a map of isotachs. The magnitude of the gust factordepends on the height of the bridge, and the horizontal wind loaded length. Forfootbridges only, BS 5400: Part 2 allows the following reductions in wind load:

1. The mean hourly wind speed is reduced by 0.94, which is a conversion factor toobtain 50-year return period values from 120-year return period values.

2. A reduction in the gust factor when the bridge is located in urban areas or a ruralenvironment with many windbreaks.

To use BS 5268: Part 2 for the design of bridge members, the designer has to decideupon the service conditions for the bridge. This mainly involves deciding the exposureand duration clauses that are appropriate for the member concerned. Experience hasshown that designers usually are on the conservative side by choosing to designmembers using wet exposure stresses. With BS 5268, the threshold moisturecontent between wet and dry exposure conditions is 18%. Hence this is oftenunnecessarily conservative. For a vertical imposed uniformly distributed loading which



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represents a crowded bridge, experience has shown that designers usually selectmedium-term duration. This is the duration class used with BS 5268 for snow loadingin the UK. Horizontal loads on handrails or parapets are usually designated as short-term. This is the same load duration class as that arising from the case of a manstanding on a roof member.

10.2 Deflection limits

The limited guidance given in BS 5268: Part 2 regarding deflection limit is of littlerelevance to the design of bridge members. Enquiries indicate that a static deflectionlimit of Span/200 under imposed load only is often used for beam members. The"Countryside Commission for Scotland" publication recommends a tighter limit ofSpan/240 under total loading. Lightly precambered glulam bridge beams are oftendesigned using a deflection limit for live load only, which is permitted in principle byBS 5268.

10.3 Eurocode 5

As explained above, it has proven possible to design acceptable timber footbridgesusing BS 5268: Part 2 recommendations, supplemented by additional guidance fromelsewhere. The eventual publication of EC5, Part 2 will be a great step forward andwill be welcomed by everybody involved. Meanwhile, EC5, Part I is shortly to beavailable. Thus even at this stage, the Eurocodes will bring advantages to the moresophisticated aspects of timber engineering such as bridges.

EC5 introduces limit states design to timber for the first time in the UK. It is thereforea more radical change for timber than the introduction of Eurocodes for other majorstructural materials. EC5 contains the essential rules for design, but unlike the BritishCode, BS 5268, it does not include material properties, tables of fastener loads andother such other design information. An immediately obvious change is that whereverpossible, EC5 uses equations rather than tables. Also much of the nomenclature andterminology is considerable different.

As in all instances of limit states design codes, EC5 will require clear thinking aboutthe distinction between ultimate and serviceability limit states. The latter are, ofcourse associated with deflections, deformations and vibration. EC5 treats thesematters in a considerably more sophisticated manner than their coverage in BS 5268.As has already been pointed out, BS 5268 gives no adequate guidance related todeflection limits for bridges. Furthermore it is likely that the Working Group dealingwith EC5, Part 2 will require to give considerable thought to the serviceability topics.

EC5 offers a number of advantages over BS 5268. It provides the opportunity todesign with a wide selection of materials and components. The use of characteristicvalues for materials, based directly on test results, means that new materials andcomponents, which have achieved suitable technical approval can more easily beassimilated, thus facilitating development and innovation. More guidance is given onthe design of built-up components than in BS 5268, and EC5 provides a unifieddesign and safety basis for laterally loaded dowel-type joints (nails, staples, screws,bolts and steel dowels). The ENV contains no information on the design of joints usingconnectors such as toothed plates, shear plates and split rings, but a procedure isbeing developed through other sponsored research programmes. Interim guidance onthe design of such joints is contained in the UK NAD.



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10.4 Overseas practice - Decks

The principal function of a deck is to distribute the loads produced by the traffic to thesupporting elements of the bridge. If the deck is designed as a structural diaphragm, itcan be used as a major spanning element in its own right. It can also brace othermain components, and transfer horizontal wind or brake loads through to other partsof the structure, and ultimately to the foundations.

Dependent on its location, a secondary function of the deck may be to protect themain structure from moisture and mechanical damage from traffic. An effective anddurable protection of substantial parts of the timber structure may be achieved withclosed, high-level decks.

Structural timber decks have been used in North America since at least the 1930s.Initially, nailed laminated construction was used. Timber-concrete composites werealso introduced at quite an early stage. Later, glued laminated beams connected withshear devices started to be used extensively. Nowadays, structural timber decksexist in many designs, using half-round timbers, sawn timber, glued laminated timber,and structural timber composites of various types.

In parts of Europe, particularly the German-speaking countries, structural decks of allthe types mentioned above are found. A more traditional type, used on lighter bridges,including footbridges, is the two-layer herringbone boarded pattern. This does notmake a major structural diaphragm contribution, in comparison with stressedlaminated decks, for example, but it does contribute to the lateral bracing of thestructure. A good example is in the large Main-Donau Canal 190 m long tensionribbon bridge, described in STEP Lecture E17. There is significant coverage ofstructural deck design in prEN Eurocode 5 Part 2.

Research and practise in structural decks of various forms has also occurred in theNordic countries and in Australia and New Zealand.



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Common Timber Deck Types In Canada:

Longitudinal and transverse nail-laminated (LNL/TNL) decks:

In longitudinal laminated decks, the timber laminations are orientated parallel to thedirection of the traffic, whereas in transverse laminated decks the laminates areorientated perpendicular to the direction of the traffic flow.

Figure 16: Longitudinal laminated deck ( left) and transverse laminated deck(right)

Nailed laminated decks consist of planks of timber laid on edge side by side. Thelaminations are nailed together to form a slab. Nails are driven through the faces ofthe planks to fix them together laterally.

1. Timber-concrete composite (TCC) decks: A concrete topping is applied to a timber deck, traditionally a longitudinal naillaminated type, giving the slab a concrete compression zone and wearing course.Shear keys are required between the timber and the concrete layers. 2. Longitudinal or transverse stress laminated (LSL/TSL) decks: In addition to nails, post tensioning is applied using high strength steel to improve theload transfer of the deck. Although the stiffness of the timber is low perpendicular tothe grain, pre-stressing allows a plate action in the deck. Edge members are oftenmade of hardwood or steel, in order to avoid damage due to high bearing stressesperpendicular to the grain at the pre-stressing points.



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Figure 17: Tension rods 3. Floor beam ( or timber tie) decks: This type is a beam and plank deck. the planks overlay heavy transverse beams orties. 4. Two layer plank decks:

As above, but with two layers of heavy planks.



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Primaryhighways

subjected tohigh volume

of traffic

New bridgeson

secondaryroads with

mediumvolume of

traffic

New bridgeson

secondaryroads withlow volume

of traffic

Lightly usedpark and

forestaccess

roads withoccasional

heavyvehicles

Very lowvolume roadswith little or

nocommercial

traffic

Deckreplacement on

steel trussbridges

Longitudinalnail laminated

Economicalwhensupported ontimber pilebents

Economicalalternative toreplacingconcrete

Can give higherlive load capacity

Speedyreplacement

Transversenail

laminated

On steel ortimbergirders

Timber-concrete

composite

Spans up to7m

Longitudinalstress

laminated

Viable option Will span upto 8m for286mm deepdecks andlonger forlarger timbersizes

Transversestress

laminated

Not yet used,but showspromise

On steel ortimbergirders

Floor beam Economical

Two layerplank decks

Economical

Table 11: Summary of the principal applications for various combinations oftimber decks, beams and other structures used for highway bridges in Canada



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11.0 FUTURE CHALLENGES

Fresh challenges constantly arise, both for timber engineering in general, andspecifically for bridges. Salient aspects of timber engineering materials and designrelating to timber bridges which are still under development are outlined as follows:

11.1 High efficiency composite materials

Although timber provides high specific strength and stiffness, until recently, its naturalvariability remained a challenge. However, through efficient strength gradingprocedures and associated quality assurance measures, timber and wood-basedcomposites offer variability levels as low as those attained in metallic engineeringmaterials, and certainly much lower than in many commonly used grades ofconcrete.

High-strength softwood glued laminated beams, using laminations of adequatecontrolled quality and regulated end jointing techniques, can be produced withcharacteristic bending strengths up to typically 36 N/mm2. By substituting STCs forthe outer laminations, composite elements of 15~25% higher bending strength maybe guaranteed, without increase in weight. Such composite materials are comparablein strength to high-performance reinforced concrete, but have a mass of only one-quarter of the latter. European research is also in progress in which, by utilisingstructural composites based on temperate hardwood veneers, bridge beams withcharacteristic bending strengths greater than 60 N/mm2 are feasible, with weightincreases of only 15%.

11.2 New adhesive bonding technologies

The classical timber engineering adhesives have been proven adequate for small andmedium spans in buildings. However new technical and economical requirementshave been driving towards improved adhesives and new bonding techniques.Adhesive bonding is only now becoming accepted in the field of bridge engineering ingeneral, with strides having been made in resin bonding technologies for therefurbishment and upgrading of steel and concrete structures. Until recently, thesetechniques have depended upon the use of steel as the bonded-on reinforcingmedium. Recently however, field trials and in situ monitoring have commenced onbridges reinforced with Fibre Reinforced Plastics (FRPs), involving such advancedmaterials as Carbon Fibre.

In the light of such developments, the climate of acceptance has grown for thepossibility of the greater use of bonding technologies in timber bridge engineering.prEN Eurocode 5 Part 2 Bridges contains an Informative Annex on the use of “Glued-in Steel Rods”. These rules have recently been supported by a major EuropeanCollaborative research Programme, known as “GIROD”. This will permit bonded-inrods to form safety critical connections in timber bridge structures, and also providethe basis for calculations concerning bonded-in reinforcements for applications suchas large structural deck plates.

At present, the Eurocode Informative Annex design recommendations apply only tosteel rod-type reinforcements and connections, these having already been subjected



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to earlier research programmes including outdoor exposure assessments. However,this research is now being extended to embrace bonded-in materials of otherclasses, including FRP Pultrusions joined to timber and timber composites (Mettem,1996).

Bridge parts depending upon adhesive bonding for their manufacture and assemblyentail substantial manufacturing planning and production arrangements, to ensurethat the conditions for correct control are met. In large decks and other bridgestructural members, sections of up to about 1m2 of cross-sectional area have so farbeen produced, and this upper limit is constantly being expanded.

11.3 Steel reinforced timber

As mentioned above in connection with new bonding technologies, steelreinforcements are already being used within timber elements contained in bridgestructures. In prEN Eurocode 5 Part 2 Bridges, design rules can be found forreinforced members, transversely reinforced timber, and for deck plates containingsuch reinforcements.

Most frequently, these steel reinforced timber elements serve as high duty deckplates, although local reinforcing also takes place in stress-critical zones in the mainstructural members. Reinforcement is especially worth considering where elementaldesign is restricted through the relatively low tensile strength of timber transverse tothe grain direction. It is also possible to use certain techniques to improve shearresistance, in situations where the action effects tend to cause sliding of adjacentfibres (known as shear parallel to the grain). Another instance where this type ofreinforcement may be considered is at notches and at other abrupt changes insection, where stress concentrations can cause Mode I or Mixed Mode I/II crackgrowth and fracture. Finally, it is considered in some instances where highlyconcentrated action effects are to be transferred between parts of the structure, andwhere greater strength can be attained if such forces are transferred to zones deepwithin the receiving members.

Where timber road bridge decks are steel reinforced, in general, the bars or tendonsare located in directions inclined or parallel to the major fibre directions of the parentwood. In some instances, bonded-in steel-reinforcements are pre-stressed, tocompensate for shrinkage effects which may occur with the components in service

11.4 Timber-concrete composites

Current timber-concrete composites are described above, and as stated, designprinciples and application rules are already given in the structural timber Eurocodes.Challenges which remain for such systems, include searching for ways of achievingstill further efficiency in the composite interaction benefits that can be achieved withbetter, more durable, shear key systems. Effort is also concentrated on securingcontinuous improvements in the sealing and moisture protection of these types ofdeck.

11.5 Deck protection systems

Better deck protection systems are being given a high priority in several of theresearch programmes outside the UK. Higher traffic speeds and intensities of wheelpassage have drawn attention to the need for greater deck protection against wetting



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that is spread from the vehicles themselves. Deck systems and structural elementsboth need carefully designed protection to avoid cumulative damage from theseeffects. To some extent, the timber deck and the main structure may be treatedindependently in this regard.

12.0 PRIORITY WORK AREAS

This Report has set out the state of the art of timber bridge design in the UK, withwider references to Europe in general. From this, it is possible to identify the prioritywork areas in which it is felt that effort should be concentrated, from a UK point ofview.

12.1 Innovative Timber Engineering for the Countryside

The above is the title of the Pilot Project in which BRE and TRADA Technology areengaged as Technology Providers.

1. Review serviceability criteria for footbridges in all materials.

2. Continue bridge survey work, paying special attention to serviceability performance and durability aspects.

3. Carry out design studies leading to the construction of prototypes which are to be used for serviceability assessments and durability monitoring.

4. Produce nationally applicable design guidance.

5. Maximise the use of UK grown species in sizes (round and sawn) likely to be available locally for bridge construction

Within item 3 above, the initial design studies have already led to the conclusion thatthere are four areas where supplementary design guidance is required. These are asfollows:

1. The design and detailing of bracing systems to ensure member stability and to resist horizontal wind loads

2. Methods to ensure satisfactory vibration performance of bridges under human footfalls

3. Simple methods to ensure that the excitation of bridges by wind is avoided.

4. In the case of mechanically laminated beams, a simplified method of evaluating deflections is required.

12.2 prEN Eurocode 5, Part 2

The Project Team has identified several priority areas coinciding with those felt to benecessary in the UK. The following have been suggested:

1. The dynamic behaviour of bridges under pedestrian and wind loadings.

2. The fatigue behaviour of connections.



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3. Timber-concrete composite behaviour.

4. Bonded-in steel rods.

5. Durability requirements for timber bridges, including those for the connectors.



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13.0 CONCLUSIONS

Timber engineers have the expertise to provide aesthetically exciting, well-protected,and durable bridge structures. To achieve impact, economic drivers must beharnessed, to unlock consumer and specifier indifference. Key motivators include:

• National cycle routes• City regeneration, calling for aesthetically exciting, well-performing links.• Canal and rail regeneration• Marina and dockside development• Housing developments, with associated bridging needs.• Forest roads and infrastructure maintenance in remote regions.• Linking to value-added forest products.

The use of sustainably grown and locally produced timber for bridge, foundation andsea defence engineering will increasingly be seen as favourable. In addition there areconcerns and moves in Europe away from the use of timber treatments such ascreosote and Copper Chrome Arsenic. Applied research and development,demonstration projects, and benchmarking involving the use of domestic growntimber are seen as vital. Above all, however, well-informed promotion is recognisedas of paramount importance in unlocking demand for timber bridges as flagshipprojects in sustainable development, environmental protection, and improvements tothe quality of life.



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