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Re: Moisture Content in Timber

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As a point of interest, the Canadian codes specify s-dry at 19% regardless
of whether the timber is kiln dried or air dried.  It is difficult to
guarantee that every piece of lumber be it long span trusses or stud walls
wall have this specification at the time of installation.  I've never seen
contractors on site with moisture meters making sure the lumber is in good
condition.

One obvious problem resides in the fact that lumber may be shipped to site
as per spec, but during storage on site or during construction a good rain
can really effect the moisture content as in most cases the building
envelope isn't up yet.

I've worked on some long span timber trusses that were built in the forties.
At this point the checks and splits in the timber are at the point where
clamps and stiffening elements no longer suffice.  There will have to be a
significant intervention taken.

I've never designed long span wood trusses and so am not familiar with the
design parameters that would dictate a big difference in strength capacity
from 16-19% moisture content.  In my mind the issue has less to do with
strength capacity and more to do with serviceability such as warping and
drywall screws popping out as wood dries out.

Warping can be a structural issue if very serious, I've not seen an example
of this yet in the field though.

regards,

Hans E. Boge, P. Eng.
--
Boge & Boge (1980) Ltd.       Phone: (204) 942-7276
Consulting Engineers               Fax:  (204) 942-7288
268 Ellen St.                        e-mail:  hboge(--nospam--at)boge-boge.com
Winnipeg, MB                Web page:  www.boge-boge.com
R3A 1A7                                         Canada
-----Original Message-----
From: Richard Flower <RLFlower(--nospam--at)worldnet.att.net>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Tuesday, July 18, 2000 10:03 AM
Subject: Re: Moisture Content in Timber


>This one is for all you wood experts:
>
>The 1997 edition of the National Design Specifications, Section 4.1.4
>appears to suggest a 19% moisture content as the expected service condition
>for structural timber. However, the 1997 UBC Table 16-E footnote 1 says
>"Seasoned lumber is lumber having a moisture content of less than 16% at
the
>time of installation...". If both these statements are accepted as
>guidelines for design, then, does that mean that timber cannot be
considered
>seasoned when used for structural purposes?
>
>This makes a considerable difference in the design of long spaned members.
>
>Regardless of how structural lumber is specified for a project, much of the
>structural lumber that is delivered to a construction site is delivered
>"wet". Yet, these members can be expected to dry out over time - in fact,
>much of the moisture content is lost even before the members are placed in
>service. Also, members of relatively smaller cross-section would "air-dry"
>more quickly than members of larger cross-section. Yet, I have not found
>provisions that consider this fact.
>
>Any comments on this issue would be appreciated.
>
>-Richard L. Flower, P. E.
>
>
>
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