A RENAISSANCE IN TIMBER BRIDGES
An article by Christopher J Mettem and Stuart Edwards, Timber Engineering Group, TRADA Technology, published in TRADA Timber Industry Yearbook 2001.
Timber bridges are experiencing a renaissance and a number of factors are combining to encourage further development – not only footbridges but also large impressive structures. The key drivers are sustainability, aesthetic appeal, new materials and jointing techniques, advanced preservative treatments, prefabrication and the availability of good practice guidance to ensure durability and value for money.
With sustainability now a priority in construction, timber bridges are increasingly common, not only in the countryside but also in urban areas. There is plenty more scope – it is estimated that 1,000 new footbridges are needed to bring all UK footpaths and bridleways into good order. As a naturally renewable resource requiring little energy in production, timber has excellent ‘green’ credentials as well as compatibility with most surroundings.
‘Timber’ includes not only solid material but also many wood-based construction materials such as glued laminated timber (Glulam), mechanically laminated timber and plywood. These have now been joined by structural timber composites (STCs) such as laminated veneer lumber (LVL), parallel strand lumber (PSL) and laminated strand lumber (LSL). Modern wood-based composites, allowing for much longer spans, open up new opportunities for timber bridge design.
Despite the many benefits, designers have often been deterred from using timber and its derivatives by a lack of information – there is currently no specific British Standard for the design of timber bridges with the BS 5400 series only covering steel, concrete and steel-concrete composite structures. Timber designs have had to be based on BS 5268 Part 2, the permissible stress timber code for buildings, cross-referenced with advice from BS 5400 “Steel, concrete and composite bridges”. This deficiency can now be rectified by using the structural Eurocodes which have the advantage of harmonising design between different materials and types of structure. Examples in bridging include timber and concrete composite construction and stressed laminated timber deck plates where the design takes the best of each material for the benefit of the structure as a whole.
Eurocode 5 Part 1-1 “Design of timber structures – General rules and rules for buildings” (EC5-1.1), published in 1994 as a Draft for Development, DDENV, contains the general design principles for timber structures. However, Eurocode 5 Part 2 “Design of timber structures – Bridges” (EC5-2), published in 1997 as an ENV, deals exclusively with timber bridges including larger, more complex structures and those capable of carrying vehicles. It embodies new principles and advice, not previously available, which are useful for the design of even simple bridges, notably avoiding excessive vibration which can worry pedestrians.
EC5-2 also gives guidance on new methods of fastening. Connection methods for timber components normally consist of metallic fasteners such as nails, screws, bolts, dowels, shear plates and split rings or timber engineering hardware ranging from simple plates to prefabricated devices such as hangers, brackets and straps. EC5-2, however, also covers adhesive-bonded rods, a growing area of research which introduces possibilities such as high capacity stiff load-transfer, neat durable detailing and materials like FRP (fibre reinforced polymer) composites in place of metallic components. TRADA Technology has recently embarked on a new research programme on the subject involving partners in Germany and Sweden as well as UK suppliers of FRPs.
There are certain categories of bridge that will require further reference to national documents, for example: bridges over roads, railway crossing footbridges and bridges in rural areas.
Durability is largely a matter of design, the specification of an appropriate timber species and a sensible maintenance schedule. Lack of understanding in detailing and reliance on preservative treatments as a first line of defence against high risk environments have been the principal causes of failures. An information sheet published by TRADA Technology, Durability in Design, contains valuable information.
A structural timber protection plan to ensure longevity divides into four key elements:
Good conceptual design to prevent or minimise direct exposure of major structural elements to the weather.
Optimum choice of materials, installed at appropriate moisture contents, with the moisture controlled during the structure’s life time. Inclusion, where necessary, of exceptionally durable timbers.
Detailing which covers and protects the main structure, protection for the end grain of timber and specific shelter for particularly susceptible parts.
Application, where necessary, of in-depth preservative treatment under pressure, applied at appropriate stages in manufacture. Protection of the completed structure with properly maintained surface finishes.
When such protection measures are carried out, timber bridges are particularly cost competitive – there are many examples of bridges which have lasted for decades and even centuries.
Photos courtesy Svenska Träboar AB
