Wood and Water (part I)
Keep wood completely dry, it will last forever. Keep wood completely wet, it will last ages. We have heard these statements, yet in the real world, most wood applications don't fit into those two categories, especially timber bridges.
Periodically, we will be writing about how to get the best protection from water for your bridge structures with proper detailing.
In part I, we will be talking about timber bridge guide railings, specifically the guide rail posts.
It is important to know that glued laminated timber is manufactured with a moisture content of 12% to 14%. That is relatively dry for an exterior use application. Even with the oil borne preservatives protecting the wood, water can and does, enter the wood cells in service. Members such as rail posts with exposed tops are highly susceptible to wetting and drying causing checks, splits, water saturation and eventually deterioration.
In 1996, we set out to develop a capping system for our bridge post tops that would protect them from the elements, yet allow air circulation. We selected an untreated Douglas Fir, glu-lam beam, 6 3/4" x 10 1/2" as the post. Buried it and capped it with a custon manufactured, Polyethylene plastic cap.
After 17 years it was time to measure the results of effectiveness. (mostly, we had to remove it because the post rotted out at the ground line)
The post was taken into our shop and moisture readings were read at varying locations.
The following are the measured readings:
Post face : ground 20.4% mid 13.0 % top 11.2 %
Post top : front 13.8% mid 12% rear 13.4%
As the measurements and photos show, the top of the post was relatively dry and exhibited no signs of checking or splitting which is normally seen on uncovered tops proving that proper protection of exposed end grain on timber bridge members is paramount in providing longevity and serviceability to the structure. Whether it be polyethylene plastic, or some other means of covering, the investment will be well spent.
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