Timber Bridges

Christopher J. Mettem

• Verlag: Routledge
• Jahr: 2013
• ISBN:
  9781135720636
• Auflage: 1
• Thema: Technologie et ingénierie
• Thema: Ingénierie de la construction et de l'architecture

Bridges built in timber are enjoying a significant revival, both for pedestrian and light traffic and increasingly for heavier loadings and longer spans. Timber's high strength-to-weight ratio, combined with the ease and speed of construction inherent in the off-site prefabrication methods used, make a timber bridge a suitable option in many different scenarios.

This handbook gives technical guidance on forms, materials, structural design and construction techniques suitable for both small and large timber bridges. Eurocode 5 Part Two (BS EN 1995-2) for the first time provides an international standard for the construction of timber bridges, removing a potential obstacle for engineers where timber construction for bridges has not – in recent centuries at least – been usual.

Clearly illustrated throughout, this guide explains how to make use of this oldest construction material in a modern context to create sustainable, aesthetically pleasing, practical and durable bridges. Worldwide examples include Tourand Creek Bridge, Canada; Toijala, Finland; Punt la Resgia, Switzerland; Pont de Crest, France; Almorere Pylon Bridge, the Netherlands.

1 Benefits of timber bridges

1.1 Essentials

Bridges in timber for pedestrians, cyclists and equestrians are enjoying a significant revival. Recently, they have also come to be regarded as suitable for longer spans and heavier loadings, including regular vehicular traffic (Figure 1.1). The high strength-to-weight ratio has long been recognised, and for this reason loads upon new or existing foundations are kept to a minimum, a motive for several of the projects illustrated herein. With timber engineering, off-site fabrication has always been completely normal, ensuring considerable versatility. Prefabrication in sections takes place in factory environments with strict levels of quality control fully addressed by European Standards and codes. Large components or even entire bridges are often rapidly lifted into place and assembled, as in Figure 1.2, and again as exemplified in later chapters. Ease of erection is especially significant in situations where minimal disruption of traffic is essential.

Timber is ideal for applications where aesthetics are important, and the awakened public and professional awareness of its sustainability has undoubtedly increased for these reasons. There are suitable forms of bridge for both rural and urban environments, and a range of surfaces, colours and types of finish are provided.

Figure 1.1 Timber provides excellent bridges for pedestrians, cyclists and equestrians. It is also a suitable material for longer spans and heavier loadings, including regular vehicular traffic.
A: Crossing the river Neckar, Remseck, Germany.
B: Vihantasalmi Bridge, Mäntyharju, Finland
Photo A: © STEP
Photo B: © CJM

Figure 1.2 By nature lightweight and suitable for prefabrication, timber bridges for footpaths and cycleways minimise time on site. Here a construction in 2008 at Traunreut, in Germany, has round-turned glulam and protected rectangular-section glulam stringers with round hollow section steel. A roof fabricated from innovative structural laminated panel material completes the project
Photos© F. Miebach/Schaffitzel

This publication contains comprehensive information on timber bridges of moderate span, suitable for relatively light types of traffic, since these represent the majority of present applications in the United Kingdom. However, it also exhibits large modern engineering achievements beyond these shores. In interviews with construction professionals, and during presentations at training events, astonishment is often expressed at the scope and scale of timber bridge developments elsewhere. The advent of the unified suite of structural Eurocodes, including one specifically addressing timber bridges, has removed a major obstacle in the eyes of British engineers and potential approvers. There is a perception here that timber bridges lack any prospect of longevity, yet the evidence of still-standing medieval structures disproves this notion. Within this book there are examples of bridge accomplishments that embrace the very latest timber technology and engineering principles, and that are confidently expected to fulfil the demands of formally defined design lives of up to 120 years.

In the last 25 years, there has been considerable international cooperation on modern timber bridges amongst experts and specialist fabricators. The Nordic Timber Bridge Project and other similar initiatives have resulted in hundreds of new bridges in the northern latitudes, where forest products are naturally abundant. While the majority of these are pedestrian types, new heavy-traffic bridges have also been installed. The advent of efficient supply chains linked with electronic access to technical information has led to opportunities for timely deliveries to the United Kingdom of prefabricated components similar to those used in Denmark, Finland, Norway and Sweden. This can involve energy-efficient methods of supply, including rail and water transportation of large, sectionalised parts for bridges. The Nordic project has included scientific life cycle assessments that support environmental declarations, so these are not just vague ‘wood is good’ statements. Elsewhere in Europe, timber engineering activity has also grown, and a number of illustrations shown in this publication relate to bridge projects in France, as well as in the German-speaking countries of Europe. In North America and in the Antipodes, renewed timber bridge programmes are also well established, with published design documents available, to which roads and traffic authorities routinely make reference.

United in their conviction as to the efficiency, durability and other benefits of these bridges, timber engineers around the world nevertheless need to respond to differing national and regional priorities and motives. To those having some familiarity with the subject, these may be recognised in many of the illustrations used throughout, but to illustrate this variety further, selected case studies are presented in Chapter 9.

1.2 Durability

By far the most effective way to ensure durability and ease of maintenance of any timber structure is to incorporate protective design measures at the initial design stage. In Chapter 2, examples are given of timber bridges that have lasted for centuries, due to protective design features – in particular, covers over part of the structure, or indeed a complete ‘overcoat’ in the form of a roof. Designing for durability is examined in detail in Chapter 3, while materials choices and their factors are discussed in Chapter 4. At this stage, it is emphasised that the species of timber selected for bridging should always possess a degree of natural durability, even when the options of including preservative treatment or adding full or partial coverings are chosen.

At an early stage in the concept design, it is essential to adopt a decisive protection strategy. The three-pronged approach entails careful timber selection and specification; surface treatment and possibly in-depth treatment with preservatives; and physical protection of key parts of the bridge using measures detailed herein. Standard durability classifications are provided for all of the commonly available bridge timbers. These ratings are also linked to hazard definition classifications, which are discussed in Chapter 3. The primary choice of timber species and material format is crucial. It interrelates not only with technical design matters such as structural calculations, for which it is necessary to select a strength class or a nominated timber species, but also with issues such as available cross-sectional sizes, lengths, and amenability to pressure preservative treatment if stipulated.

1.3 Benefits

Using timber in construction in general, including bridges, has important environmental benefits, including:

very low embodied energy during manufacture;

low mass, lessening transport and foundation construction energy;

positive carbon balance;

the opportunity to use a perpetually renewable material – hence encouraging the sustainable management of forests and woodlands.

The Pont de Crest (Figure 1.3) is a recent example of a bridge for which timber was specifically chosen on environmental grounds. Because of faith in the quality of the local forest products and the skills of their carpenters, timber was the preference of the citizens of Crest in Drôme, France for this attractive and successful structure.

Figure 1.3 The Pont de Crest constructed in 2001 to cross the river Drôme, in France. With a length of 94 m, this bridge is distinguished by its tree-like architecture, conceived to serve the community whose livelihood is heavily dependent upon forestry and its products. The beams, struts and a composite under-deck diaphragm are all produced from local Douglas fir. Several innovative features are included in the connections and in the protective design details. The bridge carries a local secondary road and is designed for a 10-tonne capacity, although normally vehicles of only 3.5 tonnes are permitted
Photo © CJM

1.3.1 Sustainability

Uniquely amongst structural materials, timber is completely renewable by nature, bringing clear environmental benefits. These include a very low embodied energy content in converted products, especially if they are derived from relatively local sources. Applications such as bridging provide skilled design and manufacturing opportunities for rural and regional communities, leading to the expansion of forest and woodland protection and viable cyclical harvesting. While they are still growing vigorously, trees absorb carbon dioxide, but this benign effect is only experienced if the natural resource is correctly protected, utilised and renewed when the time is right.

Well-managed timber production has a tradition spanning back for centuries in Britain, and in most other parts of Europe, as well as the whole of Scandinavia, and this can be sustained through innovative and efficient practices. For several centuries, softwoods have provided the ‘traditional’ structural timber in most parts of Europe, but over the past few decades there has been a strong revival of interest in the use of broad-leaved timbers such as oak, ash, beech and sweet chestnut. Specifiers can have a major influence on such choices. Careful design and detailing reduces reliance upon chemical preservatives. The choice of durable timbers for exposed applications such as timber bridges can also keep treatments and finishes to a minimum.

Recently, all of the Forestry Commission woodlands in England, Wales and Scotland were assessed against FSC- and ISO-recognised schemes. This showed that the best possible practices of management and replanting have been taking place. This excellent record is reflected also in the private sector of arboriculture in Britain. In the Nordic and North American regions, FSC and PEFC schemes apply comprehensively, and their abundant supplies of forest proffer considerably greater opportunities, both in timber bridging and also in building design, than are currently realised.

1.3.2 Forestry benefits

Engaging sustainable forestry involves making conscious decisions on the future of the land and biosphere, paying careful attention to four fundamental aspects:

1 Physical, biological and ecological considerations.

2 Social, political and cultural aspirations.

3 Economic and financial opportunities and constraints.

4 Growing, managing and fulfilling technological aspects – at all stages in the forestry–timber chain.

With very few exceptions (Figure 1.4), the most durable species of timber are obtained from tropical sources. Those readily available in Europe for bridge construction are described in Chapter 4. Reliance on the natural durability of the heartwood is only one of the three main avenues leading towards the lifetime performance now expected of permanent bridges. Nevertheless, the larger cross-sections and greater available lengths of tropical timbers mean that these types are particularly suited to civil engineering applications. There may still be some confusion about...