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Re: Timber Unit Weight

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----- Original Message -----
From: Fredericks, Douglas/SAC <DFreder1(--nospam--at)>
To: <seaint(--nospam--at)>
Sent: Thursday, June 17, 1999 2:21 PM
Subject: Timber Unit Weight

> For the unit weight of timber, AASHTO requires 50 pcf.
> The AITC Glulam tables for Douglas Fir-Larch lists 35 pcf.  I believe the
> difference in these two unit weights is due to expected in service
> content and the different specific gravities of various wood species.
[Bruce Pooley comment] The difference is also due to the pressure treatment
of the wood. See the AITC 109 for required retentions. For creosote, the
retention is 8 pcf for above ground contact and 10 pcf for ground contact.
Earlier standards required up to 12 pcf.

Species could also contribute to the larger unit weight. For example,
Southern Pine specific gravity is normally considered to be higher than
Douglas Fir-Larch (AITC lists SP at 36 pcf for beam weights).

> Just for kicks, I checked into what the unit weight of Douglas Fir would
> at the fiber saturation point, about 30% moisture content.  Using the
> equation at the bottom of Table M2, Appendix M, of the NDS I get a unit
> weight at 30% mc of about 37 pcf.
[Bruce Pooley comment] What version of NDS did you use? I looked in my 1997
edition and did not find the equation that you referred to. The weight you
calculated seems to be quite low for going from 12% to 30% moisture content.
I will continue to research this and post my findings later. I think their
is an adjustment to the specific gravity that is missing.

The wood will probably only increase to a slightly higher moisture content.
It is very doubtful that the wood will go to the saturation point. Wood will
probably have highest moisture content at bearing areas and connections.
Field surveys by the Forest Products Laboratories indicate that moisture
content (MC) of wood under deck and paving will remain near the initial
moisture content, and will increase based on climatic conditions. Generally,
MC ranges from 16% to 25%.

> Am I doing something wrong?
> --------------------------------------------
> Specifically,
> Unit Weight = 62.4*[G/(1+0.009*G*mc)]*(1+mc/100), G is .51, mc is 30%, so
> Unit Weight = 62.4*[0.51/(1+0.009*0.51*30)]*(1+30/100) = 36.4pcf
> Doug Fredericks/CH2M HILL
> 2485 Natomas Park Drive, Suite 600
> Sacramento, CA  95833
> 916.920.0212 x341
> 916.614.3541 (fax)