Vehicle Bridges

Steel Beam Timber Deck

Vehicle bridges on the countryside network are designed for all vehicles permitted on the public road without special permission. The design loading used is the Eurocode 1: Actions on Structures. This employs a loading about 30% greater than the British Standards used for 30 years before but results in stiffer decks which is good for the modern forestry and estate traffic. These bridges are designed for low cost and are generally steel beams and timber decks. For many years concrete beams and decks were the first choice and some durability arguments are supporting their comeback. However galvanised steel beams will last 70 years in the countryside and most owners are happy to replace timber decks as part of maintenance. It means that the construction can be carried out in any weather and there is no risk of pollution of the river which is a very big consideration today. Recently precast concrete deck planks have been used in place of timber planks.

Stress Laminated Timber Road

Stress Laminated flat road bridges are usually made from full length timbers and drilled and then pressure treated before assembly. Laminates are generally 5-6m long and depths up to 250mm and always 50mm wide. The laminates are threaded onto the stressing bars which are tension jacked to create what is effectively a single solid mass of timber. The exact analysis and design involves a number of factors nut is covered by Eurocode 5. This is a very cost effective way of building forestry road bridges up to 6m span.

Concrete Deck

Concrete road bridges use prestressed beams and in situ concrete. The beams are now difficult to get as most precast yards in Scotland and England have stopped making them. The system is good and the bridges are durable but the construction must be well controlled to avoid the risk of pollution. To minimise this I developed a system where the kerbs were precast in the factory.

Bailey Bridge

Bailey Bridges were developed for the D Day landing in the 2nd Work War. Many units are still in service and are capable of taking today’s traffic. Re-use of old sections is OK as long as specialist inspection is employed to rule out fatigue cracks in the metal which can occur. New BB sections are now available and are useful in certain circumstances. A direct comparison with a steel and timber standard bridge shows the BB as more expensive but if the use is temporary and there are 2 subsequent used planned for different site they are viable but you only ever have one bridge in service. Existing BB refurbishment can be economic.

Temporary Bridges

Temporary bridges are becoming a common request in forestry especially. The choice very much depends on the span. Of course Bailey Bridges are a consideration but the cost of abutments are a major consideration and they are only viable for long spans. Short spans can be proprietary steel fabricated sections which can be easily transported about the forest. Timber stress almanited decks are excellent for short spans up to 6m.