A few months have passed since the Second International Timber Bridge Conference at Las Vegas. It was three days of sharing experiences and knowledge regarding advancements and past histories of timber bridge construction and uses worldwide.
The organizers have placed all of the presentations and presentations (PDF format) from the conference on the National Center for Wood Transportation Structures' website. In addition, the summary conference slide show with pictures is available.
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%
Moisture reading locations on post top. F = front R = rear
Moisture reading locations along post face
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.
For those who are familiar with designing wood members, you may also be aware that the allowable design values for dimension and sawn Southern Pine timber have been reduced. Tests have indicated that the juvenile wood, used in todays markets, have lower allowable strength values than those that have been published before.
As a designer, it is important to know that these reductions are not applicable to the design of Glued Laminated Timber members. Our laminator, ALAMCO Wood Products have provided us with the following from the APA-AITC white paper on Glue Lam lumber grading (select link below)
photos courtesy of H.E. Bergeron Engineers, North Conway, NH
For many years, glued laminated decking has been a preferred choice for existing steel truss bridges for both vehicular and pedestrian applications. And for good reasons...Take a look at dead loads. Timber weighs in at 50 PCF while concrete tips the scales at 150 PCF. That's a factor of 3 whereas the thickness required doesn't come near that factor increase.
Glued laminated timber also benefits from the quick assembly time it provides thanks to extensive prefabrication of the material at the plant. Panels can be manufactured continuous over multiple floor beams increasing the structural design while saving on installation times. In most cases, a deck for a truss can be set within two days depending upon site characteristics. And of course, treated decking is resistant to road salts, which probably was a main contributor to the need for a bridge repair.
Feel free to contact us at 607.562.8110 to learn more about decking for trussed bridge structures.
Glulam decking on pedestrian truss rehab
photos courtesy of Larson Design Group, Williamsport, PA.
