LAND DEVELOPMENT BRIDGES
COMPARING CONCRETE and TIMBER MATERIALS
This installment of A Bridge Moment provides some factors land developers should consider before installing a timber bridge on your next land development project. If you are a land development civil engineer or a project manager for a land development company and a bridge is required on a current or upcoming project, think twice before you consider building a timber bridge. Concrete bridges cost about the same and will last 2 to 3 times longer with minimal maintenance compared to timber bridge that will likely require maintenance before the community is turned over to the CDD or HOA. Consider the following facts comparing a concrete bridge to a timber bridge. If you are interested in a design-build cost proposal for your land development bridge project, contact Ralph Verrastro, PE. Go to www.rv-pe.com for more information.
1. DESIGN LIFE: Concrete 75 = years and Timber = 30 years.
2. MAINTENANCE COSTS: Timber has very high annual maintenance especially if a timber deck is used and Concrete has minimal to no maintenance costs.
3. Timber and concrete bridges designed to AASHTO design standards have similar initial construction costs but the life cycle cost of a concrete bridge is much less.
4. Timber materials continue to lose strength over time due to a natural process called fungal decay and concrete continues to gain strength over time due a natural process called hydration.
5. Timber bridges utilize short 10’ to 15’ spans requiring numerous piers that impede water flow and concrete bridges utilize 30’ to 40’ spans requiring a reduced number of piers in the flow way.
6. Timber materials must be treated with caustic preservatives to prevent deterioration that can leach out into sensitive environments and concrete is naturally durable and preservatives are not required.
7. Wood products are limited to shapes available from sawmills or glue-lam fabricators and concrete can be formed into any shape imaginable on-site or from a precast concrete manufacturer.
8. Timber bridges can easily catch fire and concrete is non-combustible.
9. Timber materials absorb water especially after forming cracks (checks) due to shrinkage and concrete is very dense and water tight (think swimming pools).
10.Timber piles are relatively short and need plastic wraps for protection when installed in water and concrete piles are available in lengths of 100 feet or longer and no wraps are required in water.
11. Timber bridges are not considered on government roadway projects and concrete bridges are preferred by FDOT and many other state DOTs.
