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RE: Rigid vs. Flexible Diaphragm

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Dennis:

My response is provided below

Gautam Manandhar, SE

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)?

Out-of-plane forces on the walls needs to be transferred to the lower (foundation) and the upper diaphragm. Since the hardy frame is not connected to the upper diaphragm, the top chord of the hardy frame needs to transfer the load to the king post through bending which then transfers the load to the upper diaphragm. You are right in that there may be some load distribution to the adjacent full height studs - however, the amount would depend on the bending capacity of the wall skin connecting the king post to the adjacent studs. I, however, would not depend a whole lot on the skin redistributing the out-of-plane force.

Yes, providing the king post would cost more - but is the cost of the connection of the frame top chord to the king post a big number in relation to the total cost of the project?

FYI, I try to stay away from Hardy frames because they are very stiff compared to plywood walls and strong walls. When using RDA, these frames attract too much load.


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?

This would be true if the interior partitions were in close proximicity of the hardy frame. If not, I do not see how the partitions would support the out-of-plane forces applied on the Hardy frame.


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?

IMO, both are important. If you use a shorter hardy frame that does not extend to the diaphragm, you have to reduce the shear capacity to account for the overturning forces that is transferred from the cripple wall above the frame. Also, you have to account for the additional deflection due to higher overturning forces. You also need to provide means of transferring the overturning force from the cripple wall to the hardy frame.


c) The strength of the panel (shear or not) adjacent to openings and
continuous to the double top plate.

Like I stated earlier, if the adjacent panels have plywood , there might be some redistributions; otherwise, I'd say 'forget it'.


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?

In order to transfer the load to adjacent members, the wall skin (plywood,plaster) must have adequate stiffness to be able to transfer the load. Plywood (plaster, drywall, etc) are not stiff in the out-of-plane direction.


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?

IMO, out-of-plane forces are real. Yes, if your project is surrounded by other houses, the wind load the building sees may not be the full design load. That is a choice you have to make.

My first job after graduation was to design a concrete staircase in a hotel project. I was just out of college and full of enthusiasm and wanted to produce the most economical design. I massaged my calculations until I ended up with the bare minimum required steel. After reviewing my calculations, my boss gave me a piece of advice I hold dear even now. While commending me on my attempt to come up with a very economical design, he told me that he did not expect the client to come and thank me for saving him the cost of two rebars; however, if there was a structural failure because of the two rebars I saved the client, the client would be screaming for my blood. If I were you, I would provide the connection even though the wall may not see the full design load. If the cost of providing the connection was huge, I would have second thoughts - otherwise, I would provide it and move on - especially in here in lawsuits happy America.

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?

I don't think the diaphragm has a lot to do with the out-of-plane stability of the Hardy frame. If you do not have the mechanism to transfer the out-of-plane forces to the diaphragm ( through the king post, for example) you basically have a structure that has three hinges - at the hardy frame base, at the intersection of the hardy frame and the cripple wall, and at the diaphragm level - which is unstable. In the design load condition, out-of-plane failure would have occurred before the diaphragm can contribute anything to the out-of-plane stability.


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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