Australia has a rich heritage of timber bridges. In the early expansion of our road system, the abundance of strong and durable hardwoods made timber bridges a very cost-effective means of crossing creeks and rivers. Many of the bridges on minor roads are still the original timber bridges made from round timber members.
Timber is still a viable material for the construction of new road and pedestrian bridges. In most cases, manufactured wood products are used to provide the main structural elements. There is plenty of scope for both technical and aesthetic innovation in the design and construction of new timber bridges.
Modern bridges are designed to comply with AusRoads standards for bridges. The AusRoad standard specifies loads and geometry of bridges including geometry of horizontal and vertical curves and kerbs and handrails. Depending on the location of the bridge, there may be constraints on the span underneath – clearances for trains (if the bridge is over a railway line) or navigation clearance for waterways.
• The performance of the timber materials can be modelled with the Timber Structures code.
• The form of the bridge can make use of arches, trusses, beams. The system chosen will address the functional requirements of the bridge and the aesthetics of the structure.
• Detailing of the bridge will be in accordance with requirements given in the AusRoads standard.
Strategies for Limiting Deterioration
Most bridges are exposed to the weather. Sun and rain can cause deterioration of exposed timber elements. There are some strategies that can be used to limit the extent of the deterioration.
Bridge decks form a plane for the traffic. The near-flat surface catches water and may not shed it quickly. Protection for the deck can be provided by a surface seal. This generally gives a flatter, more even running surface and sheds the water prior to it reaching the deck. This is quite appropriate for road and pedestrian bridges.
Beams that support the deck are often protected from exposure to the weather by the deck itself. However, truss bridges, bridges supported by deep beams or arches can have the major structural elements exposed. Protection can be enhanced by treatments, surface coatings or both. Maintenance schedules for these elements will include regular inspection, rejuvenation of treatments, and replacement of protective coatings.
Hand Rails and Guard Rails
Hand rails and guard rails are by nature exposed, so cannot be protected by a bitumen seal. A cover plate can be installed on the outside of the handrail post. In the event of deterioration of the exposed face of the post, it will be possible to remove the cover plate and replace it, leaving the post itself protected and not interfering with the service of the bridge. It is good to have rails placed so that the upper surface is not level, and water drainage is facilitated.
A number of bridges in use in Australia today are over 100 years old and have provided service for much longer than a reasonable design life. It is a worthwhile challenge to keep these older bridges safe and serviceable at a reasonable cost.
Maintenance includes the following basic steps:
- Inspection – this aims to detect deterioration associated with rot and insect attack early before the strength is compromised. The zones with the most potential for degradation are the ground-line and the deck where water can soak into it. Stringers and half caps, particularly on the outside of the bridge tend to get wet and can show signs of deterioration. Regular inspection and testing the soundness of these is essential.
- Treatment – it is possible to halt the advancement of decay by replenishing treatments. Boron and similar treatments diffuse slowly into the timber from pressed chemical rods inserted into holes. Where the wood is wet, the diffusion is accelerated, and hence the treatment matches the level of risk of decay.
- Replacement – where the deterioration is too advanced, then replacement of the damaged elements is an option. This is to be avoided as it may impact on the function of the bridge.
Unfortunately, it is often not possible to obtain timber in the species and member sizes used in the original bridges.
- Manufactured Wood Products such as LVL or glulam are frequently used to replace members with large cross sections.
- Pile replacements are possible, though awkward.
- New deck replacement techniques have proved both possible and economical. By protecting the new deck with a bitumen seal, a much longer life can be expected. Stress laminated decks
Stress Laminated Decks
Stress laminated decks make use of commercial sized timber, but by acting together as a composite, share their strength so that the properties of the composite timber deck are close to the average over all of the pieces. The composite action is achieved by stressing them together so that friction forces them to act as a single large block of timber:
- A stress lam deck consists of pieces of treated timber laid on their edge side by side for the entire width and length of the deck. The boards are positioned so that the butt joints between the boards are evenly distributed longitudinally throughout the deck.
- The entire deck is prestressed by tensioning steel bars that run across the deck at regular intervals. The prestress keeps all of the pieces squashed up together, and distributes the load under individual vehicle tyres to all of the timber in the deck.Rails and other bridge hardware are fixed to the side of the deck.
- A bitumen surface is bonded to the top of the deck to keep water away from the timber and to prevent mechanical damage to the upper edge of the boards.
- Stress laminated bridge decks have made a significant contribution to the refurbishment of many older timber road bridges.
- The principle is relatively simple, and there are a number of different configurations available to achieve different spans and loadings:
- A stress-lam deck replaces the entire superstructure of a timber bridge – stringers, cross members, decking. The deck is designed to span between the piles.
Traditional Australian timber road bridges had different forms depending on their spans:
- For smaller spans, road bridges made use of round stringers as simply-supported beams, made from essentially whole hardwood trees. Decking was placed across the stringers to give a running surface. Many bridges did not have a seal on top of the decking, but more recently, bituminous concrete is placed over the decking to protect the decking and stringers from water.
- For larger spans timber trusses were used.
- The trusses could be above the deck level or below the deck level. Bolted connections are used extensively.
Designers of modern timber traffic bridges can use a variety of construction methods to satisfy demanding structural requirements, yet provide an aesthetically pleasing and innovative structure that can reflect contemporary architecture and become a feature of their environment. The following are some examples:
- Round timbers in a truss-type structure, with connections between the major members a significant aspect of the design.
- Round timber piles, with a truss structure used to span between the piles constructed of sawn hardwood timber. Bolted connections are used extensively.
Using a large number of very closely spaced nail-plated trusses. These trusses were trusses of the type used in domestic construction, so required smaller sized members and commonly available technology to fabricate.
- Some impressive spans can be achieved by using manufactured wood products. They can carry full traffic loads and be detailed to ensure that the main structural members have a measure of protection from the elements.
Arches can be created by using curved glulam as the main structural elements
The cable-stayed bridge is a structural form that we more often associate with steel bridges, but is cost effective and attractive when used with timber as well.
Although foot bridges experience lighter loads than traffic bridges, they still have to be designed for reasonably high live loads. (If it is possible to fit a vehicle on the bridge, then it must be designed to take its weight, even if driving on the bridge by the public is prohibited!). Many of the Scandanavian bridges shown here are designed to carry the weight of snow clearing machines!)
There can be a large range in the architecture used for pedestrian bridges and walkways. By careful attention to detailing of joints, handrails and the upper surface of structural members, footbridges can enjoy a high level of durability. A roof over the foot bridge not only offers protection from the weather for people using the bridge, but also protects the deck and other structural members of the bridge.
The structural forms of pedestrian bridges are the same as those used for traffic bridges. Smaller spans commonly use glulam girders or small trusses.