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Re: Conventional Lumber Diaphragms

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Nels, I think the problem with 97 UBC is that it left out a para. in a section
that was in the 94 UBC, which gave allow. shear values for lumber-decked
diaphragms. I think it was an oversight, not intentional. It wasn't in the 1st or
2nd printing of 97 UBC.

As regards wood structural panel sheathing over spaced boards for re-roofing, I
believe it has become common practice to generate some diaphragm capacity and
strengthen roof. It also allows other types of roofing such as asphalt comp.
shingles or tiles to be installed as roofing, instead of wood shakes which were
common in Southern CA.  Spaced board roofs would have no diaphragm capacity.

Diagonal sheathing for wall bracing was accepted as prescriptive "conventional"
construction. I never saw a published shear value for it, but there is some old
reports circa 1930s or 40s from HHFA (precursor to HUD) describing wall racking
tests and comparing shear stiffness and strength performance compared to other
forms of construction, I believe including plywood sheathing. Wood structural
panel sheathing is better, of course.

John Rose/APA, Tacoma

NRoselund(--nospam--at) wrote:

> I have a project for an addition to an old house and garage.  New
> construction is to match the old, or old style to the extent possible.
> 1.  Sections 2315.3.1 and 2315.2 of the '97 UBC give instructions for
> construction of diaphragms of diagonal 1x lumber.  Table 23-II-G specifies
> span-width ratios for diagonal-board-sheathed horizontal and vertical
> diaphragms.  I can't find allowable values.  Am I overlooking a Table or
> Section of the Code?
> Appendix Chapter 1 on the UCBC has allowable values for horizontal diaphragms
> of wood diagonal sheathing with either roofing or hardwood flooring
> coverings, but this is a code for existing construction, not new, and does
> not include values for shear walls.
> 2.  Section 2320.12.9, Roof Sheathing, allows spaced roof sheathing for
> conventional construction.  Chapter 15 allows spaced board sheathing under
> tile and wood shingle roofs.  Why does re-roofing universally include plywood
> sheathing over existing spaced sheathing?  (At least in S.CA it does.)
> Nels Roselund
> Structural Engineer