Timber has been growing in popularity as a material for bridges in the Nordic countries largely as a result of a scheme launched in 1994. The programme, which consisted of three phases and was only completed in 2001, was aimed at increasing the competitiveness of the material compared with steel and concrete (see box).

In the case of the Flisa Bridge, it has certainly done the trick, albeit with the help of other site constraints - the choice of material was also restricted by the fact that the existing piers had to be reused, and that the clearance from the river could not be reduced. The original bridge at this location was a three-span steel truss structure that was almost 100 years old; it was only wide enough to carry a single lane of traffic, and had a weight limit of 8t imposed on it. Norwegian Public Roads Administration wanted to increase the capacity of the structure to 13t, and widen it so that it could carry two lanes of traffic and a walkway for pedestrians.

The stone piers and foundations of the bridge were still in very good condition, and the Roads Administration decided that the most cost-effective solution would be to retain these and build a new superstructure on top of them - saving about US$1million. Architect Yngve Olav Aartun explains how the concept design development was carried out:

"We got about 20 people together in a room to try and come up with the best solution possible for this structure, taking into account the constraints such as re-using the existing piers, and retaining the headroom underneath the deck. In the end, the original layout, with three truss spans, still made sense - although at this point it was still possible that we might choose a steel truss."

In the end, the deciding factor was cost - to build a steel superstructure for this design would have cost 50% more than a wooden superstructure. The low cost of the production process was cited as one of the main reasons for this differential - producing glulam beams is cheaper than fabricating steel beams, particularly in Scandinavia where labour costs are high. Maintenance costs were also taken into account - the cost of regular repainting of the steel compared with timber, which generally requires an initial coating of creosote and no further maintenance, provided the design detailing ensures proper protection from the weather.

As Aartun explains, the design development was a difficult process with the need to balance the structural requirements of the bridge with the aesthetic elements. "I was afraid that the bridge would look very heavy on the piers because of its width, and the sections were growing bigger and bigger. Some of the proportions were destroyed as the sections got larger, and we had to make alterations to certain members to balance them out." Designing the wind bracing for the structure was particularly difficult, he says. The resulting bridge is 10m wide, 180m long and has a main span of 71m. The main truss beams are up to 600mm by 700mm in cross-section, and they are up to 28m long.

The superstructure is actually a combination of steel and wood - steel cross-beams are bolted to the top of each pier to allow for the wider structure, and the wooden trusses are supported on these. The glulam beams that form the superstructure are impregnated with creosote in a pressure vacuum tank by manufacturer Moelven Limtre, and have additional weather protection installed on site.

Copper plate cladding is fixed on the top of all the main beams and the through-joints - the idea is that any moisture that finds its way underneath the cladding will react with the metal to form copper salts, which act as a preservative. Stensby says that monitoring of moisture content has been carried out on the Evenstad bridge which was built in 1996; humidity in the wood has reached a maximum of only 12% during this period, and it must be at least 20% to create the right conditions for the wood to rot.

The production of glue-laminated wood beams for structural elements is now fairly straightforward, says Moelven Limtre technical director Age Holmestad, with the only limit on size being considerations such as the size of the machinery used to produce them, and the tank in which they are impregnated with creosote. Structural elements are made up using sections of wood about 30mm thick - this can be anything up to 45mm, although there is some concern about loss of capacity when thicker sections are used.

Both sides of the wood are planed, and then glue applied and the two pieces are pressed together in a machine overnight. Curved presses are used for elements such as arch beams. The process is repeated until the element is the required thickness. The main development that has taken place over the last decade, reducing the cost of glulam as a structural material, is the mechanisation of the jointing process. Joints are connected into the timber members by the insertion of steel plates and pins into the ends of the beams; originally the drilling was carried out by hand, and because of the accuracy required, was a slow and expensive procedure.

This is now a mechanised process and as such, much cheaper to carry out. According to Holmestad, Moelven Limtre is also investigating the possibility of introducing fibre-reinforced polymers into glulam beams, an adaptation which can increase the capacity of the beam quite considerably - of course there is also a corresponding increase in cost, which means that as yet, this product will have limited applications. The Flisa Bridge also has a wooden deck - a stress-laminated construction which is commonly used in the USA and is cheap, easy to build and very stiff.

It consists of simple wooden planks positioned thin edge downwards, next to each other across the full width of the deck. Holes are drilled across the full width at about 600mm centres, and steel tie rods are inserted and stressed. An asphalt surfacing is applied to the top of the wood.

Wood makes headway

The use of timber as a structural material has come back into fashion as a result of a number of high profile projects - although the focus is now largely on promoting it for bridge projects, its large scale use in major schemes such as the new airport terminal at Gardemoen in Oslo, and the Olympic Skating Hall in Hamar, have helped it to become more widely accepted.

The Nordic timber bridge programme was a cooperation with participants from Norway, Finland and Sweden; Denmark also participated during the first two phases, and a representative from Estonia was an observer during the whole programme. The scheme was divided into 20 projects, each with a remit to examine a different aspect of the subject, such as market conditions, structural design, durability and so on.

Some of the working groups produced results that formed an important source of information for the development of Eurocodes, such as the fatigue properties of joints. Three pan-Nordic conferences were organised, as well as a number of national events, and publication of information at other conferences, and in journals and newspapers. The main result of the programme has been the construction of many new timber bridges in the region, the majority of them for pedestrians.

However, the last two or three years has seen engineers becoming more confident with timber and the number of bridges for heavy traffic is now starting to rise. Substantial structures are being built such as the 182m-long Vihantasalmi Bridge in Finland and a new five-span glulam truss bridge with a total length of 180m that was built in Norway, as well as the Flisa Bridge.

Timber has a special resonance with Scandinavians, because it is regarded as a natural, home-grown material that has a low environmental impact and fits well into its surroundings. The Norwegian Public Roads Authority recognises that this can also be a useful public-relations tool. Senior engineer and timber bridge advisor Trond Arne Stensby sums it up succinctly: "When we open a timber bridge, there is a band, a party, a big celebration; when we open a concrete bridge, there might be a couple of lines in the local paper."

Client: Norwegian Public Roads Administration

Architect: Plan Arkitekter

Consultant: Norconsult

Contractor: Moelven Limtre

Printer friendly version

Email this article to a friend