Timber Recreation Bridges – Materials
For main structural components such as stringers, truss and arch members, solid sawn and glued laminated materials are compared to balance strength properties, camber requirements and material economy. We recommend these materials be treated with an oil borne preservative.
Oil borne preservatives are waterproof. This is important for larger members. The oil borne preservatives prevent water moving in and out of the wood and limit dimensional changes due to swelling and shrinking. These members are pre-framed prior to treatment for easy assembly and added durability. Oil borne preservatives have consistently demonstrated a service life in excess of 50 years for well designed and detailed structures.
Waterborne preservatives are used for components that will be in intimate skin contact. There are a variety of preservatives available depending on the wood species.
Preservatives using light solvent carriers offer performance similar to oil borne with a dry to the touch surface, but at a premium price.
The materials and treatment used for the decking and railing can vary based on the intended use of the structure. Often solid sawn treated lumber is sufficient. Glued laminated or dowel laminated materials are only used when loadings are excessive or there is exceptional sensitivity to the dimensional stability of the members (i.e. reduce cupping and warping).
When treated lumber is used the treatment depends on the level of contact expected. Oil borne preservatives are the most durable and can be conditioned to reduce the surface oil, but if it is anticipated people will congregate on the bridge and lean over the railing oil residue may stain a white shirt. In this case we recommend a waterborne preservative for the railing and possibly the decking. Waterborne preservatives can be painted or stained, but this will require on-going maintenance. We recommend leaving the natural weathered appearance.
Composite lumbers have been used for rail components. Their service life is harder to predict based the lack of history and variability of the various products available. Composite decking is not recommended for bridges requiring a wheel load. Composite decking manufacturers do not provide design data and most products are not available in suitable thickness. Naturally durable wood species can also be considered when treated wood is not desired. The service life varies by species. Composite lumbers and naturally durable species are more expensive than treated wood.
Service life for decking and railing can vary by the materials used. Often the components are replaced for aesthetic reasons before they wear out. Most options provide 15 to 25 years with minimal maintenance.
All hardware used for Wheeler bridges is hot-dipped galvanized and should last the life of the structure. Periodic tightening may be required.