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Re: Rigid vs. Flexible Diaphragm

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Dennis,

>From a high wind low seismic point of view your qualitative assessments
don't necessarily hold water because out of plane loads are very real and
significant. In high wind you have a lateral load and wall suction applied
simultaneously failure at the window area can eliminate the effectiveness of
the shear wall on either side and cause a collapse. So here  we establish a
discrete load path for a framed opening, apply a value to the load, and
design the members for that stress. The trib. areas include the window also
not just the area above the window. To me if the load is discernable then
you have to put a value to it and come up with quantitative results. But
again your prospective comes from a whole different type of load than mine
does so things you have to worry about I don't and things I have to worry
about you don't.

Does the "beam" of the Hardie frame require full lateral bracing for the
values they give you?

Just my thoughts,
Rand



----- Original Message ----- 
From: "Dennis Wish" <dennis.wish(--nospam--at)verizon.net>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, April 30, 2004 12:12 AM
Subject: RE: Rigid vs. Flexible Diaphragm


> Joe, Gautam and Oshin,
>
> I understand your point of view on the king post, it may be a good
> deterrent as it mitigates potential out-of-plane damage. As Joe
> suggested, the use of a Tube Steel column may be a wise choice, but it
> can also impede installation of electrical or diagonal plumbing stacks.
>
> Playing Devil's Advocate, lets assume you may be missing some realities;
> that while the king post is continuous so are the studs between the
> header and the next header. It would be difficult to try and balance
> out-of-plane forces based on rigidity unless you are willing to design
> the entire strength of the out-of-plane reaction at the end of the
> headers and transfer it to a continuous column. This is costly and not
> often practical. How many of you actually calculate the reaction at the
> hinge and do you apply it to the king post as a concentrated load. If
> you do, what is the reaction based on? Is it the wind load normal to the
> wall and tributary to each end of the header (50% of normal force by
> nature of simple beam)?
>
> OR do you consider:
>
> a) Interior partitions that are perpendicular to the wall and act to
> brace the wall by the nature of the lap of the top plate and connection
> of sheathing at inside corners?
>
> b) The strength of the king-posts; with the exception of tall walls Mike
> Cochran pointed out quite correctly that most hinges will occur above
> the center of the wall and close to the double plate. When placing
> frames we always should try to place them as close if not flush to the
> top plate, but there are times when we can't extend them high enough
> without losing important capacity and it is at this time that we need to
> compensate for the in-plane shears at the expense of out-of-plane
> potential damage due to the hinge. Which is more important?
>
> c) The strength of the panel (shear or not) adjacent to openings and
> continuous to the double top plate.
>
> d) Imposing the initial load into the stiffest member in the walls
> adjacent to the header (and this might be a double or triple post
> supporting a girder truss or drag truss?
>
> Finally, is it our professional responsibility to try and design the
> perfect performing model OR do we consider the potential out-of-plane
> force to be a low risk for failure of the wall at the hinge point?
>
> What we do know is that IF (and I am sticking my foot in my mouth,
> Gautam, on this one) the diaphragm is rigid then the failure at the
> hinge is not due to diaphragm deflection as it might be in a URM or high
> wall mass building, but due to a once in 75 year wind event. Considering
> actual wind load design to be less than 90 mph Exposure C in Southern
> California, will the wall fail due to this type of load?
>
> Some good stuff to toss around if your game :>)
>
> Regards to all
> Dennis
>
>
> Dennis S. Wish, PE
>
>
> California Professional Engineer
>
> Structural Engineering Consultant
>
> dennis.wish(--nospam--at)verizon.net
>
> http://www.structuralist.net
>
>
>
>
> -----Original Message-----
> From: G M [mailto:newabhaju(--nospam--at)hotmail.com]
> Sent: Thursday, April 29, 2004 4:34 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Rigid vs. Flexible Diaphragm
>
> Dennis:
>
> To add to Joe's comment, one has to make sure the king post is
> adequately
> connected to the frame so as to transfer out-of-plane wind load.
>
> Gautam
>
>
> >From: Jnapd(--nospam--at)aol.com
> >Reply-To: <seaint(--nospam--at)seaint.org>
> >To: seaint(--nospam--at)seaint.org
> >Subject: Re: Rigid vs. Flexible Diaphragm
> >Date: Thu, 29 Apr 2004 14:35:22 EDT
> >
> >Dennis:
> >
> >The king post (full height) should be designed for the wind load over
> the
> >window/doors and the trib area of the wall.  You may have 6x12's for 20
>
> >plus foot
> >openings.
> >
> >
> >Joe Venuti
> >Johnson & Nielsen Associates
> >Palm Springs,  CA
>
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