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Timber Deck Rating

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CoreyR

CoreyR

Structural
Feb 7, 2003
3
I am in the process of doing an AASHTO live load rating on a small timber bridge (timber stringers and timber decking) and have a question regarding the deck rating portion. The deck is comprised of two layers of 2 1/2" thick timbers. The bottom layer runs perpendicular to the stringers and the top layer runs at 45 degrees to the bottom layer. It's my assumption that only the bottom layer of timbers was intended to be the "deck" and the top layer was intended to be a wearing surface of sorts and thus I was not intending to count on that as addition structural capacity but can't find anything in AASHTO or elsewhere to confirm or deny my assumptions. Anyone else ever run into something similar?

Thanks
Corey
 
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Corey,
Your assumption that only the bottom layer is structural is reasonable, according to my very old book on the subject. The reason for 45 degree skew for the top planks is to minimize wear, as such configuration is more durable than the one with planks parallel or perpendicular to the axis of the bridge.
For the top layer, assume some reasonable wear (say .5"), and use this layer to distribute the wheel load to the bottom planks. Typically, 1:1 ratio is being used for the distribution of the wheel load, but that's very conservative, and 1:1.5 could be more appropriate.
The distribution could be even higher, if the deck's planks were nailed together.
 
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The U.S.D.A Forest Service publication "Timber Bridges Design, Construction, Inspection, and Maintenance" seems to verify your assumption. There is a design example for a transverse timber plank deck (4x12) with a 2" timber wearing surface. It gives no structural contribution from the wearing surface. (Example 7-13, page 7-138)
 
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