huangyhg

contractor-fabricated gl arch
have a proposed job to look at involving a built-up wood arch spanning 48 feet with a 24 foot high ridge, spaced at 12' centers forming a kind of quonset structure.
the contractor/owner wants to fabricate it on site using df#2 material with special jigs, glues, bolts, and construction equipment. considering the low stress grade of df#2 for 1" members, has anyone found any allowance in the codes/standards, other than testing, to increase the bending stress above the standard 1000 to 1200 psi range, into the range of a laminated gl beam - 1600 to 2400 psi? i highly suspect not without special testing, inspection, and certification, but i thought i'd ask anyway.
mike mccann
mmc engineering
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mike,
i believe that the only stress grades that go above fb=1500 psi is machine stress rated (msr) lumber. the design values are found in mechanically graded dimension lumber (table 4c in the 2001 nds supplement). but, i don't know if they run msr for 1x wood.
you might give the apa in tacoma a call at (253) 620-7400
thanks - i'll give it a shot.
mike mccann
mmc engineering
i would not be too comfortable with an owner/contractor fabricating these...too many variables beginning with glues, temperature, humidty, etc - much less having a good design.
douglas fir is easily upgraded to higher stress allowance with scarfs and finger joints to eliminate checks, knots and voids. a laminated sample tested to ultimate and then used at 1/3 the test strength would be appropriate,(flexural of extreme fiber).
i agree with miketheengineer. most people under estimate the amount of clamping force required to develop the proper glue bond in a laminated beam. i would be skeptical that a contractor field fabricating laminated beams would be able to develop the proper bond.
for 40' arches spaced 12' o.c. i am surprised that anyone would try to field fabricate the arches. glulam plants are set up to do this sort of thing on a daily basis. it would be hard to imagen that the contractor could same money by gluing up the arches in the field. particularly now with our slow economy, plants should be hungry for work.

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