The equation I used was in the 1986 edition of the NDS. It appears at the
bottom of Table M2 "Section Props of Standard Dressed S4S Sizes" of Appendix
M. Granted, I am applying this to Glulams, not sawn lumber, but the results
should be similar.
I used the 30% mc as an upper bound, even though it should never get near
that moist. I just want to see how realistic the 50 pcf is for DF.
> -----Original Message-----
> From: Bruce D Pooley [SMTP:bdpooley(--nospam--at)home.com]
> Sent: Friday, June 18, 1999 10:24 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Timber Unit Weight
> ----- Original Message -----
> From: Fredericks, Douglas/SAC <DFreder1(--nospam--at)CH2M.com>
> To: <seaint(--nospam--at)seaint.org>
> 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
> > 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
> 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
> 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
> 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
> It is very doubtful that the wood will go to the saturation point. Wood
> 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.
> 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%,
> > 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)