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Re: Pitched Tapered Glu-Lam

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Bdpooley(--nospam--at) wrote:
> Indeed the 15 psi is a low value for radial tension (tension perpendicular to
> grain) for Western Species (includes Douglas Fir -Larch, Hemfir, etc.) glulam.
> However, I disagree with the statement that it is a catastrophic failure.
> Double pitched and tapered (DPT) glulam beams will sag, but I have yet to hear
> of a beam that has fallen to the floor/ground. I investigated one situation
> where the DPT beams sagged two feet under a snow load of 60 or more psf that
> were designed for 30 psf. One of the beams even fractured to the extent that
> the bottom tension lamination was broken, but it did not fall. If you know of
> a failure, I would appreciate hearing from you.
>  AITC now has a recommendation for designing DPT glulam beams for Western
> species. It shows how to design reinforcement (steel rod or rebar in epoxy)
> to prevent the "smile" that can occur due to the radial tension. Southern Pine
> has a much better value for radial tension and many of those designs do not
> require reinforcement.
> Contact AITC at 303-792-9559 for more information.
> Bruce Pooley, P. E.

I observed several glulams which failed during the 1996-1997 winter snow
in the Yakima Washington area.  Overall I was amazed that the beams were
still "in place" after the snow.  Several truss chords had broken
completely in to but the truss did not fall.

Specifically regarding radial tension.  There was one beam in a CA
storage room that did fail and collapse onto the bins of fruit.  Several
of the others in this structure had severe cracks but did not fall. 
This was a situation where the combined bending moment and the begining
of the  curved section in radial tension combined to cause the failure
of the beam under record snow loads.


Jill T. Shuttleworth, P.E., S.E.
Sunnyside, WA