Dr. Dan Tingley Ph.D., P.Eng. (Canada) MIE Aust, CPEng, RPEQ, Senior Engineer, Wood Research and Development
Dr. Tingley has worked in the wood products industry for over 40 years. He received his Bachelor of Science in Forest Engineering and Master of Science in Civil Engineering from the University of New Brunswick. He completed his Ph.D. at Oregon State University in Wood Science, Technology and Civil Engineering.
Dr. Tingley currently holds more than 40 published patents in the reinforced wood field in the US and other countries. He has authored over 125 conference proceedings, publications, and articles in the area of reinforcement of wood and wood composites.
Dr. Tingley is the Senior Engineer for Wood Research and Development (WRD). Dr. Tingley is currently the Chairman of the subcommittee on the Development of The Handbook of Conventional Maintenance Practices for Railway Bridges for Committee 10 – Bridge Maintenance & Construction and he also serves Chairman of the subcommittee on specifications for Committee 7- Timber Structures.
Timber Bridges – The Old Becomes New
There are over 250 thousand bridges in North America comprising over a one quarter of the 929 thousand bridges in service. During the last four decades timber bridge construction for highway/railway bridges has dropped to under 1% of the new bridge construction, while 7% of the maintenance costs for highway bridges is allocated to timber bridges. However, during the last five years the highway bridge industry has again been looking at the option of utilizing timber for construction of highway bridges. A number of reasons are driving this reconsideration.
At the head of the list is cost. Timber bridges tend to cost 40% less to construct for spans over 20m than concrete or steel. Longevity is another major consideration. Steel bridges, particularly in coastal environments, aren’t lasting past 50 years. Rust resistant steel isn’t helping extend life in these environments. Concrete bridges are frequently going out of service at 35 to 40 years due to spalling, reactive aggregate and chemical degradation, particularly in Canada with salt being applied to winter roads. Maintenance costs are tied directly to these forgoing considerations and the true maintenance costs are being calculated after these steel and concrete bridges go out of service at an early age. The maintenance costs for steel and concrete bridges aren’t favorable in comparison to a properly designed and constructed timber bridge.
Carbon friendly considerations are also playing a role with timber bridges which are 21 times more carbon friendly than steel and 16 times more carbon friendly than reinforced concrete. Heritage considerations are also playing a role currently with the public being interested in seeing less concrete and steel and more wood.
There are many factors that still stand in the way for timber to move back into an equal footing with concrete and steel. The top reasons are; lack of knowledge and understanding in the design community. Rarely do undergraduate engineering programs include timber design to say nothing of timber bridge design. Engineers design with materials they are comfortable with and timber isn’t one of those materials today. A misconception about longevity is another reason. Concrete bridge engineers will see a concrete bridge fall out of service due to spalling or reactive aggregate issues and immediately say it is a quality control issue. The cover needs to be increased, they say. When a timber bridge has extensive rot in a girder due to vertical fasteners originating from above, they say timber doesn’t last! They should recognize instead that it is a quality control issue – don’t put vertical fasteners in from the top face! They say build with concrete, it lasts and timber doesn’t.
Maintenance costs for timber bridges are perceived to be higher than concrete bridges. This isn’t true. A recent Swiss publication has shown that the maintenance cost for a properly design timber bridge is less than that of a concrete bridge.
The misconception that timber bridges cannot span great distances and carry heavy loads is another factor. This simply isn’t correct. Timber bridges can be engineered to span longer clear spans than concrete. At spans over 30 m, clear span, the dead weight of concrete becomes its biggest design load.
This presentation discusses these factors and presents large load capacity long span timber bridges as examples of what is happening today around the world with timber bridges.