Modern Timber Bridges: An Alternative for Mississippi
Recent studies have highlighted the deteriorating conditions of U.S. bridges, with some 147,000 labeled obsolete or structurally deficient. On a nationwide basis, “rural” bridges accounted for 80 percent of bridges on the 2010 National Bridge Inventory. Of these rural bridges, about half are classified structurally deficient or functionally obsolete.
Structurally deficient means the bridge is restricted to light traffic only because it has deteriorated structural components, is closed, or needs immediate rehabilitation to remain open.
A functionally obsolete bridge is one that has inadequate deck geometry, is improperly aligned with the approaching roadway, has inadequate load-carrying capacity, or has insufficient under-clearance.
Rural bridges typically serve low volumes of traffic but are essential for transport of agricultural crops from field to market. Bridges on rural roads tend to be older and have a higher percentage of structural problems than bridges in urban areas.
The maintenance responsibility for these rural bridges falls primarily on state and county governments. Nationwide, county governments have maintenance responsibility for over 50 percent of these rural bridges, while state governments maintain 42 percent. County governments usually have more bridge problems than funds available to correct them. This is also true here in Mississippi.
According to the 2010 National Bridge Inventory, there are 17,038 bridges in the state, including bridges on and off the Federal Aid Secondary System. Of these, 4,120 bridges (24 percent) need to be replaced or rehabilitated.
These bridges are crucial to retain rural access for emergency vehicles (fire and ambulance), service vehicles (telephone and electricity), and school buses. Trucks carrying farm and timber crops also rely on rural bridges for transport from the field to the mills. In addition, rural economies depend on the availability of reliable truck transportation to develop more diversified economies.
Whose problem is it? The responsibility rests largely with local governments because in Mississippi, county governments have maintenance responsibilities for 81.4 percent of the deficient or obsolete bridges. The solution to the rural bridge problem will require a combination of efforts by legislators, engineers, local officials, researchers, and others. County officials and engineers need options to help solve these bridge problems.
A Positive Development
During the late 1970s and throughout the 1980s, treated timber received increasing attention by structural engineers and county decision-makers. The modern timber bridge is the focus of a national initiative funded by Congress through the USDA Forest Service.
The National Timber Bridge Initiative provides funding for research, information about modern timber bridges, and construction of demonstration timber bridges throughout the United States. One such demonstration is in Tallahatchie County. The Mississippi State University Extension Service, Tallahatchie County, and the USDA Forest Service cooperated to fund and install a modern timber bridge demonstration in Mississippi. The demonstration provides engineers, government officials, and others a tangible example of modern timber bridge technology.
What Is a Modern Timber Bridge?
Bridges made of treated timber are not new, but the use of treated timber in engineered, prefabricated bridge components is new. Researchers have developed innovative, cost-effective applications for treated timber in small-bridge construction (20- to 60-foot spans) over the last 15 years. These designs include solid sawn lumber and new wood products such as glulam, laminated veneer lumber, and others in the construction of factory-made timber bridge components manufactured and shipped to the bridge site for installation.
Modern timber bridges incorporate new technology into designs like these:
• glulam girder and transverse decks
• longitudinal glulam decks
• dowel-laminated, longitudinal panel decks
• stress-laminated decks
Advantages of Modern Timber Bridges
With modern treatment, timber bridges are expected to last 50 years or longer. Many timber railroad bridges built in Mississippi during the 1920s are still structurally sound and in service.
Local crews can construct modern timber bridges using equipment generally available locally. Timber bridges are easily repaired, upgraded, or modified.
Because modern timber bridges are factory-made, quality control is enhanced and the need for field fabrication is minimized.
High Strength-to-Weight Ratio
Wood is lighter than most other bridge materials, which means lighter construction equipment can be used for installation. Lighter bridge weights can also allow use of existing foundations. These factors can lead to construction savings.
Modern timber bridges can be an economical alternative. Bridge costs vary and tend to be site-specific, but the construction characteristics of timber, coupled with modern timber bridge performance, can make these bridges a competitive option in small-bridge construction.
Timber bridges are simple and feature the warmth and beauty of wood. They are well suited to rural or town settings.
Timber is a renewable resource important to Mississippi’s economy.
Individual timbers in these designs are fastened together by glue, steel dowels, or stressing rods (depending on design). The timbers are fastened so securely that deck components can share the load. This load-sharing is a significant improvement over the old stick-built timber bridges.
Bridge components are cut, milled, and treated before assembly to avoid damage to the preservative treatment that protects the wood. Once assembled, the prefabricated bridge sections are shipped by truck to the bridge site.
Most modern timber bridge deck systems can be installed on timber, steel, or concrete foundations, whichever is most suitable.
A Mississippi First
In the summer of 1990, Tallahatchie County (northwest Mississippi) was selected for a modern timber bridge demonstration. The bridge is on William Adams Road off Highway 32 about 6 miles from Charleston, the county seat. The bridge crosses Tillatoba Creek and lies on a paved road. The previous bridge was an old nailed-timber deck on a timber pile foundation.
The dowel-laminated deck has 15 prefabricated deck panels about 5 feet wide, 10 or 18 feet long, and 10 or 12 inches thick, depending on the span. Deck panels are joined by shiplap joints secured with galvanized spikes.
Wheeler Consolidated Inc. of St. Louis Park, Minnesota, furnished the bridge deck, and Tallahatchie County crews and equipment completed the foundation and installation. A local contractor did the pile driving for the foundation.
The old bridge was removed and the new, modern timber bridge was installed in 5 days.
Tallahatchie County Demonstration Bridge
Location: William Adams Road off Highway 32 about 6 miles east of Charleston, Mississippi.
Design: Wheeler Consolidated Inc. dowel-laminated, longitudinal deck of Southern pine. Timber pile and cap foundation. HS-20 loading. Three spans (two 10-foot spans and one 18-foot span).
Length: 38 feet
Width: 26 feet clear roadway
Dowel-laminated construction isn’t the only type of modern timber bridge design. Also attracting attention is the stress-laminated timber bridge. This design uses treated structural lumber held together by the force on transverse stress rods that pass through the deck at 2- to 4-foot intervals.
Stress lamination holds the individual timbers together, distributes the load efficiently among timbers, and permits use of shorter timbers.
Glulam is another product used in modern timber bridges. Glulam is stress-rated, structural lumber glued together by machine to form beams or deck panels. Glulam components are factory-made to precisely engineered specifications and treated before being shipped to the job site.
Glulam production is certified by the American Institute of Timber Construction, which has a glulam bridge manual available (see Additional Reading).
The potential of the modern timber bridge has created interest among engineers and local officials throughout the United States. Modern timber bridges are simple, practical, cost-effective alternatives for highway bridge construction. State highway departments in Pennsylvania, West Virginia, Minnesota, Wisconsin, and Colorado have programs to further study and use modern timber bridge construction technology.
Researchers at the University of Wisconsin, West Virginia University, Colorado State University, University of Nebraska, Pennsylvania State University, and elsewhere are continuing to refine and expand the modern timber bridge concept.
The manufacture of modern timber bridges in Mississippi can stimulate jobs, use local resources, and help solve a major rural transportation problem. Obviously, every new bridge shouldn’t be a modern timber bridge, but, in many cases where a functional, long-lasting bridge is needed, timber should be considered.
Modern timber bridge construction offers opportunities in rural development, employment, improved transportation, and timber utilization. With today’s technology in the use of wood’s structural strength and treatment techniques, modern timber bridges may provide a major solution to rural transportation problems.
Timber Bridges. 1986. Transportation Research Record 1053. Transportation Research Board, National Research Council, Washington, DC. Julia Withers, editor. p. 79.
Timber Bridges: Design, Construction, Inspection, and Maintenance. 1990. Ritter, M.A. USDA Forest Service, Engineering Staff EM7700-8 Washington, DC. p. 944. (Available from the National Timber Bridge Information Center.)
Glulam Bridge Systems. 1989. American Institute of Timber Construction, Vancouver, WA. p. 32.
Timber Bridges: An Alternative to Concrete and Steel. 1989. Bill Grove. Northern Logger and Timber Processor. October 1989. pp. 22–25.
Stress-Laminated Wood Bridge Decks: Experimental and Analytical Evaluations. 1990. M.G. Olivia et al. USDA Forest Service, Forest Products Laboratory. Research Paper FPL-495. p. 24.
Timber Bridges: Part of the Solution for Rural America. 1989. Brungraber, R. et al. Transportation Research Record 1106. Transportation Research Board, National Research Council, Washington, DC. pp. 131–139.
Cost Comparison of Timber, Steel, and Prestressed Concrete Bridges. 1990. Behr, R.A. et al. ASCE Journal of Structural Engineering, Vol. 116, No. 12. p. 3448–3457.
Wood Design Focus. 1990. Vol. 1, No. 3, Wood Products Information Center. Portland, OR. p. 23.
National Timber Bridge Information Center
USDA Forest Service
10 Canfield Street
Morgantown, WV 26505
Publication 1783 (POD-12-15)Revised by Dr. Jason S. Gordon, Associate Extension Professor, Forestry, and Dr. P. David Jones, former Assistant Extension Professor, Forest Products, from an earlier version by Dr. Bob Daniels, retired Extension Professor.