Wood Bridges and Highway Noise Barriers
Wood was probably the first material used by humans to construct a bridge and is still widely used for short and medium spans. Timber’s strength, light weight, and energy-absorbing properties are key features desirable for bridge construction. Timber is capable of supporting short-term overloads without adverse effects. Contrary to popular belief, large wood members provide good fire resistance qualities that meet or exceed those of other materials in severe fire exposures. From an economic standpoint, wood is competitive with other materials on a first-cost basis and shows advantages when life cycle costs are compared. Timber bridge construction can occur in most weather conditions without detriment to the material. Wood is not damaged by continuous freezing and thawing and resists harmful effects of de-icing agents, which cause deterioration in other bridge materials. Timber bridges do not require special equipment for installation and can normally be constructed without highly skilled labor. They also present a natural and aesthetically pleasing appearance, particularly in natural surroundings.
Glued-Laminated Timber Bridges
Another misconception about wood as a bridge material is that its use is limited to minor structures of no appreciable size. Not true with glulam, which is the most widely used modern timber bridge material. Glulam is manufactured by bonding sawn lumber laminations together with waterproof structural adhesives. Thus, glulam members are virtually unlimited in depth, width, and length and can be manufactured in a wide range of shapes. Glulam provides higher design strengths than sawn lumber and provides enhanced utilization of the available timber resource by permitting the manufacture of large wood structural elements from smaller lumber sizes. Technological advances in laminating over the years have further increased the suitability and performance of wood for modern highway bridge applications.
The USDA Forest Service has published two sets of standardized timber bridge plans (download in Publications).
Standard Plans for Southern Pine Bridges and
Standard Plans for Timber Bridge Superstructures.
Highway Noise Barriers
Noise wall barriers are typically designed to reduce the combination of diffracted and transmitted highway noise by at least 10 decibels in neighborhoods adjacent to high-traffic roads.
Most Southern Pine noise wall barriers range from 14 to 28 feet high and are constructed of horizontal 2 x 8 tongue and groove Southern Pine planks, in sections from 10 to 14 feet wide. Others are built of glued laminated Southern Pine timbers, often assembled into vertical sections. The cost of wood walls ranges from 40% to 50% less than competitive materials. The treated solid-sawn Southern Pine lumber is kiln-dried after treatment. To find sources of supply, go to SFPA’s Product Locator.
For the construction of noise abatement systems, pressure-treated Southern Pine is an excellent choice for many reasons:
Public meetings on highway noise problems have shown that many homeowners prefer wood walls because of their natural appearance and their compatibility with the surrounding area.
The flexibility of wood systems permits contractors to adjust walls and poles an inch or two in difficult terrains to ensure a snug fit, in contrast to the precision engineering required with other materials.
Wood noise walls are not only more aesthetically pleasing than other materials, but they are also less subject to road salt damage than metal and concrete.
Wood walls can be repaired faster and more economically than metal or concrete if damaged by vehicle impact.