Trough bridges are constructed from two main girders made from laminated timber which are arranged at the railing level. Reinforcing frames made from steel are fitted between the main girders at a distance of approx. 2.0 m. Both open and sealed road surfaces can be added to the steel cross beams. As a rule, the horizontal reinforcement is completed in the form of a round steel lattice. To ensure a long service life, all of the supporting timber elements have casing on their top and their side.
Trough bridges offer a good cost-benefit balance with a visually appealing appearance. They are frequently used as single span bridges.
Structural timber protection is the key to the longevity of a timber bridge. Structural timber protection intends for all structural timber components to be completely protected. In this respect, the following steps are taken into consideration: Installation of technically dried timber with a timber humidity of 12% in order to prevent insect infestation, protection against weathering, ingress of moisture, UV rays due to the installation of projections and skirting in the design as well as good air circulation to encourage drying out. Constructions designed in this way satisfy the requirements of protected timber bridges pursuant to DIN EN 1995-2 NA. This means that loss events are prevented and maintenance costs are minimised, and the use of timber preserving chemicals can be reduced and/or avoided. Structures protected according to DIN EN 1995-2 have a service life of 60 - 80 years*. On a trough bridge, the main supporting structure - consisting of two girders made from laminated timber - is protected by side cladding in the form of solid timber and a top cover made from sheet metal.
* Source: Redemption amounts calculation regulations - ABBV, BMVBS, status 2010 and study of DGfH 2006
Here is a selection of laminated timber trough bridges.