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# Re: More questions about rigid plywood Diaphragms

• To: seaint(--nospam--at)seaint.org
• Subject: Re: More questions about rigid plywood Diaphragms
• From: jbotch(--nospam--at)isp1.blissnet.com
• Date: Fri, 02 Jul 1999 19:57:57 -0600

```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
time.  IS THERE A RATIONAL METHOD FOR CALCULATING THE DEFLECTION OF A
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)aol.com 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
>

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