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Re: Cutting a Glulam Beam

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----- Original Message -----
From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Wednesday, June 30, 1999 11:54 AM
Subject: Cutting a Glulam Beam


> Shafat,
>
> As noted by many other responses to this query, the lamination grades for
the
> outer laminations are different than the core laminations.  So, cutting
off 3
> laminations at the bottom of the beam would remove *all* the higher
quality
> laminations.  A question that remains is, if you do this, will the beam
still
> be structurally adequate?

Bruce Pooley comment: An analysis would be needed based on the allowable
stresses (see earlier post that provides the tabular design values)
>
> Since the core laminations may be a different species as well as grade
from
> the outer laminations, the E-value would also be different.  Therefore, if
> you remove the outer laminations, and transform the remaining section, you
> would end up with a T-section, which moves the neutral axis up, which
> increases the tension stress in the lower lamination.

Bruce Pooley comment: It is possible that the beam may be manufactured using
different species and the E value as well as all the other design stresses
would be different. ASTM D3737 uses the transformed section. In fact, it is
used even when all the laminations are of the same species, but different
grades that are assigned different E values. For example, laminating grades
(see Standard Grading rules #17, West Coast Lumber, 1996) L1 has an E value
of 2,100,000 psi, L2D =1,980,000 psi, L2 = 1,710,000 psi and L3 = 1,600,000
psi.
>
> Putting a steel plate on the bottom and making it composite, moves the
> transformed section neutral axis down, increasing compression stresses in
the
> top.  Damned if you do; damned if you don't!
>
> As has been pointed out, putting a post in would create tension in the
top,
> and unless the beam was fabricated as a "balanced" lamination combination,
> the quality of the top laminations for tension stresses is unknown.  Also,
> IIRC, top laminations subjected to compressive stresses are also permitted
to
> have butt joints instead of finger joints --- and glued butt joints have
no
> reliable tension value.

Bruce Pooley comment:The quality of the top laminations (now in tension) are
known and values can be assigned. Recently APA has tested beams and has
developed allowable design values for extreme fiber in bending based on
these top laminations in tension. These values are shown in NER 486. AITC is
also developing similar design values.

 Butt joints are not allowed in glued laminated timber in accordance with
ANSI/AITC A190.1 (the manufacturing standard). All end joints (finger joints
are mostly used) are qualified by strength and are tested daily for QC
purposes.
>
> Good luck.  (Why are so many responders wishing you this?)
>
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona
>
> Shafat wrote:
>
> >>One of my clients wishes to install a garage door to his two car garage.
> The header beam is too low and needs to be reduced in depth by about 4.5
> inches. One of the solutions proposed by the client's contractor is to
> remove the bottom three laminations and add a new post + pad  about
> mid-span. The current Glulam beam is 21 inches deep. It works as a 16.5
> inch beam for the reduced span.
>
> Question: Is there a problem if he cuts 4.5 inches from the bottom of the
> Glulam?<<
>