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Re: Three Hinge Glulam Arch - More Information

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Chris

I looked farther into this subject and found that the deflection due to
change in moisture content  that you alluded to for the haunched or tudor
arches is also applicable to any curved wood (glulam) member. Reference in
AITC Timber Construction Manual, 4th Edition, on page 5-275, is to Technical
Bulletin No. 1069, Fabrication and Design of Glued Laminated Wood Structural
Members (U.S. Department of Agriculture, 1954), and AITC Technical Note 2.

TB 1069 discusses when this deflection may be of considerable importance.

"In the case of three-hinged arches of such shape that they are horizontal,
or nearly so, at the crest of a roof, such effects may be of considerable
importance. In such arches there may be shrinkage enough to form a
depression, or trough, at the crest of the roof that will create serious
drainage problems.  For an arch of this type, consideration should be given
to the moisture content of the member at the time of fabrication, the
moisture content to be expected in service, and the change in angle between
the two ends of the member that will result from changes in moisture content
and the consequent shrinkage across the grain."

You may also want to look at AITC 104, Typical Construction Details, that
show typical heel and crown connections of arches.

Contact AITC at 303-792-9559 or download/order on the AITC website
www.aitc-glulam.org

Copies of TB 1069 are not available. If you would like more information from
TB 1069, let me know and I can possibly fax it to you.

Bruce Pooley
----- Original Message -----
From: "Bruce Pooley" <bdpooley(--nospam--at)home.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Tuesday, May 01, 2001 2:11 PM
Subject: Re: Three Hinge Glulam Arch


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Chris

Since your arch is being designed for wet-use conditions (EMC of the wood
16% and greater) both the Modulus of Elasticity and radial stress design
values  would be affected by the wet-use factor. Csub D (wet-use factor) for
MOE is 0.833 and 0.875 for radial stress.

Methods and equations in the AITC Timber Construction Manual can be used
(for the 4th Edition, see pages 5-294 through 5-306).

Bruce Pooley
Timber Design
3448 South Newland Court
Lakewood, CO 80227
----- Original Message -----
From: "CDaniels" <ced(--nospam--at)larsondesigngroup.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Tuesday, May 01, 2001 12:52 PM
Subject: Three Hinge Glulam Arch


I am currently designing a 3-hinge glulam arch roof (r=31.33', span=59') for
a roadway sand and salt shed.  I am using STAAD to model the arches (I
divided the arch into 20 equal straight line segments).  I did this for more
or less the convenience of manipulating the different load combinations
required.  My question is there any criteria I should be looking at for
deflection due to seasoning (i.e. shrinkage or swelling).  I know AITC has a
method for computing the seasoning effects on deflection and radial stress
for haunched or tudor arches but I have a true circular, constant section
arch and I expect the equilibrium moisture content inside the building to be
18 to 25%.  I don't any experience with glulam arch design.  Any input would
be appreciated.

Thanks,

Chris Daniels
Larson Design Group
ced(--nospam--at)larsondesigngroup.com

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