Maine DOT should consider Timber Bridges for as many new bridges as possible, because steel beams, and even the rebar for reinforced concrete bridge decks, must be imported from outside Maine at high cost, but we grow plenty of wood right here in Maine.
Economic Benefits of Timber Bridges
Timber Bridges offer a significant economic development opportunity for heavily forested states like Maine. When new bridges are constructed with steel beams, all that expensive steel must be imported from far away, which means that big chunks of our money leave Maine to pay for the steel. If, instead, we can build more bridges from timber, grown and processed here in Maine, we could provide more local jobs, and prevent millions of dollars from leaving the state each year.
Exterior view of the Smith Millenium Bridge, built in 1999 - 2000 in Plymouth, NH.
Timber Bridges also have significant environmental benefits
Trees grow by themselves using solar energy, so far less embedded fossil energy is used compared to either steel or reinforced concrete (smaller carbon footprint!)
Timber Bridges typically last far longer than reinforced concrete, which is both an environmental and economic benefit
About half the weight of the timber in a timber bridge is carbon, which is stored, or sequestered, for the life of the bridge
At the end of a timber bridge’s life, the wood can be recycled, or burned for fuel.
One recent example of a timber bridge in the northeastern United States is the Smith Millenium Bridge over the Baker River in Plymouth, NH. This bridge is a single 140 foot span built with laminated timber beams in 1999 - 2000, to replace an older bridge, and has the same 100,000 pound load limit as bridges on the interstate highway system.
Interior view of the Smith Millenium Bridge
Another recent example is in Ashtabula County, Ohio, which has sixteen timber bridges of various ages. The newest bridge, completed in 2008, is called the Smolen-Gulf Bridge. This 600 foot long timber covered bridge has four 150 foot spans, and uses the Pratt Truss design with laminated arches. The framing material is Southern Yellow Pine and the bridge uses concrete piers and abutments.
The Smolen-Gulf Bridge, built in 2008 in Ashtabula County, Ohio
Long Life Expectancy
Contrary to what many people first assume, a timber bridge can last longer than typical reinforced concrete bridges. Vermont has around 80 wooden bridges still in regular use, with many of them being 80 to 100 years old, and the Smolen-Gulf Bridge in Ohio has an expected life of 100 years.
High Weight Limits
Many older timber bridges have weight limits which seem low by modern standards, but that is usually because they were designed and built 100 or more years ago, well before the advent of 50 ton trucks, and not because of any inherent limitations of timber bridges. Timber bridges can now be built to any needed weight limit, using laminated beams and Cross Laminated Timber (CLT) panels.
As previously mentioned, the Smith Millenium Bridge in Plymouth, NH, has a weight limit of 50 tons, the same as for bridges on the Interstate highway system. Other recent bridges boast much higher weight limits.
The Akkerwinde heavy traffic bridge, at Sneek, NL.
For example, the Akkerwinde Heavy Traffic Bridge, a highway overpass at Sneek, in the Netherlands, is designed to carry 60 tons, and these are probably metric tonnes, which is equivalent to about 66 American short tons.
Other timber bridges are designed for even heavier loads. The 200 meter long Kjollister Bridge over the Rena River in Norway connects two parts of a military training area and is designed to carry heavy military loads, presumably including army tanks, although the weight limit is not stated.
The Kjollister Bridge over the Rena River in Norway.
Nordic Structures, of Chibougamau, QC, has built several timber bridges with very high load limits to carry extra heavy logging trucks over rivers in Northern Quebec.
A timber arch bridge over the Temiscamie River, in Quebec.
The Maicasagi River Bridge near Chibougamau, Quebec, is a 200 foot long timber box girder bridge designed to carry extra large 180 ton logging trucks!
A timber box girder bridge with a 180 ton weight limit, over the Maicasagi River near Chibougamau, Quebec.
Another notable timber bridge in Northern Quebec, this one over the Mistasinni River.
Timber Bridges in Sweden
Over the past few decades timber bridges have become popular in other countries for both the environmental and economic benefits. In 1990 there were only 10 timber bridges in Sweden, but by 2015 there were 800, with many built by Martinson’s Timber.
The Alvesbackabron pedestrian bridge, is a cable stayed bridge with timber deck, in Skelleftea, Sweden, about 500 miles north of Stockholm. This bridge has a main span of 400 feet, the same as the well known Two Cent Bridge in Waterville, ME.
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Maine DOT
The Maine DOT work plan for the next three years includes numerous bridge projects that could be good candidates for timber bridges. Some of the best candidates would be the smaller bridges, on less traveled state roads, in the heavily wooded parts of Maine. For example there are two planned bridge replacements in the Town of Andover in Oxford ounty, over the West Branch of the Ellis River.
Birds eye view of the existing North Main Street Bridge in Andover, ME.
The Maine DOT work plan for years 2016-2017-2018 is at:
Bridge Deck Repair
Locally grown timber in the form of large Cross Laminated Timber ( CLT ) panels can also be used to replace deteriorating reinforced concrete bridge decks on existing bridges. Custom designed and prefabricated CLT bridge decks could be built in sections under cover in a factory, transported to the work site by truck, and lifted into place with a crane, saving months of on-site form work for reinforced concrete decks. Furthermore, the lighter weight of a timber bridge deck could be used to increase the load limit of a bridge being repaired, without replacing the existing steel support beams.
CLT panels being installed as the bridge deck on a new timber bridge in Quebec.
Bill Basford,
Email: wbasford@gwi.net,
Cell: 207-314-8194
Updated Dec 30, 2016
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