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# RE: Rigid vs. Flexible Diaphragm

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Rigid vs. Flexible Diaphragm
• From: "Dennis Wish" <dennis.wish(--nospam--at)verizon.net>
• Date: Thu, 29 Apr 2004 22:12:10 -0700

```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
connected to the frame so as to transfer out-of-plane wind load.

Gautam

>From: Jnapd(--nospam--at)aol.com
>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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