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Re: More questions about rigid plywood Diaphragms

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The depth of analysis required to draw meaningful conclusions about the
displacement and deflection of single/two story light-frame construction should be
moot.  Research report published by the APA (#138, Sept 1993) conducted numerous
full scale trials of plywood shearwalls per ATC-7 guidelines:  displacements,
deflections, failure modes, shear flow about discontinuities...more.  The report
states that "the deflection is virtually the same as that of the identical
diaphragm without openings".  Yes, I had a hard time with that one, especially
since no additional detailing was present for shear flow about the discontinuities
(diaphram failed at 1314 plf).  The mode of failure warrants additional attention
to discontinuities in the plane of the diaphram.  Its scary to think how far this
diaphram may have gone with a few CS16 straps.  Given the body of empirical tests
available for scrutiny, I am not inclined to delve deeper into global models for
light-frame construction, yet genuinely strive to incorporate the analysis and
detailing of  local models and systems (diaphrams) to satisfy my intent to provide
a nominal level of capacity to broken/stepped plates, CA framing, vaulted/trussed,
cantilever, suspended and other bizarro configurations I'm asked to consider.  Your
not alone Joe.

jbotch(--nospam--at) wrote:

> I hope you all have a nice independance day holidy.
> I'm going to try to ask the same question to this topic that I did last
> week, different wording maybe.  We're still all into the discussion of
> rigid plywood diaphragms vs flexible.  A little more to the point this
> PLYWOOD SHEARWALL WITH OPENINGS?  Is the reason there was no reply to my
> last message, a stupid question, maybey ignorance on my part, or are the
> rest of you in the same boat as I?  Calculating loads to shearwalls
> assuming a rigid diaphragm, is not an overwhelming task.  Most of us
> have either been given a spreadsheet to do this or we've produced our
> own.  Calculating loads to walls for flexible diaphragms is also no big
> deal.  My problem (once again) is how to determine the rigidity of a
> plywood shearwall with openings.  Maybe the rest of you design buildings
> with nice neet walls of discreet height and length.  Frankly in 15 years
> I've yet to see it.  Once again, my opinion is if you can't determine
> the wall deflection, hence its rigidity, THE ARGUMENT BETWEEN FLEXIBLE
> VS RIGID IS BOGUS.  You can't make the "rigid"  assumption of a rigid
> diaphragm and then distribute the loads by an arm-waving assumption at
> the walls.
> Come on people, tell me I'm crazy, tell me why I'm crazy, give me
> something more than unsubstantiated emotion.
> Joe Grill
> Seaintonln(--nospam--at) wrote:
> >
> > It's no secret that I have strong opinions as to the consideration of rigid
> > diaphragms in wood framed construction. However, I need some advise to see if
> > my specific opinions regarding roof diaphragms are valid.
> >
> > 1. I would argue that a Gable roof or any roof exceeding a slope of 3:12
> > would be considered flexible by nature of the mechanical connections at the
> > peak. These can not be considered rigid and therefore the diaphragm will tend
> > to bend at the ridge rather than force deflection normal to the walls. The
> > hinge at the peak of the roof becomes the weak link.
> >
> > 2. I would argue that any scissor truss or vaulted ceiling which exceeds a
> > 3:12 slope could not be designed as rigid by nature of it's performance.
> >
> > 3. I might be more inclined to accept a minimally sloped roof (1/4:12) as
> > having greater ridgidity - however, I am still reluctant to treat it as a
> > rigid diaphragm by nature of the quality of construction unknowns.
> >
> > I don't want to waste time working through the analysis for rigidity if my
> > professional judgment tells me that the calculations are not adequate to
> > determine deflection of a pitched roof.
> > What are the opinions of others? Also, what would be my risk of liability if
> > I choose to argue this point and have the building official accept my
> > argument?
> >
> > Thanks
> > Dennis Wish PE
> >