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Re: 1970's Panelized Roof - Question

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I'd look at the condition personally rather than relying on the
contractor.  Beam sizes were typically 5 1/8, 6 3/4,  8 3/4, & 10 3/4.
Depths were in 1 1/2" increments with depths at 24, 25.5, 27 etc.
Weyerhaeuser did list 3/4 inch increments, but they were not the
common.  Also field verifying may show cantilevers over columns by as
much as 20% of the span.  And I'm used to seeing panelized roofs with
2x4 @ 24, not 16, and purlins of 4x12 (marginal) or 4x14.  I also seem
to remember designing panelized roofs, that we referred to as
trampolines, @ 10 psf dl when there was a t-bar clg.  I don't know how
you can verify the make up of the GLB, but we worked with Bevon-Heron
extensively in the mid 70's and everything we did specified combination
24F lumber with tension lam top and bottom.

I think it's critical that you control the placement as increasing shear
capacity is usually easier than bending. I've used new platforms that
span between glb lines to not load the purlins or subpurlins.  I've also
used platforms that point load the purlins which are then reinforced by
bolting members to each side as required, below the subpurlins. I've
also used the tension cable a couple of times, but found bolting members
to existing was usually easier with regards to dealing with the contractor.

The contractor wants to place the units for minimal duct work, but when
you tell them to figure the additional engineering and structural
retrofitting costs to handle the units where he wants them, the cost of
additional duct work is easier to swallow.

Good luck

Steve P

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