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RE: Hold downs with "Perforated Shear Wall Method" and "Force Transfer Method"

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That 10 psf minimum pressure for MWFRS would NOT apply to uplift as that
section is written.  As that section is written the 10 psf is a pure
horizontal force put on the vertical projection/outline of the structure.

If you want to talk the specific wind pressures, then we can open up that
can of worms again.  I would argue that in the case that I described (check
of overtuning and net uplift of a shearwall) the MWFRS pressures would be
used to get the overturning force, but C&C pressures would be used for the
net uplift.  This is because the overturning portion is being used to
overall lateral support the entire structure and the entire structure gets
MWFRS, but the net uplift is purely due to a tributary uplift area of the
shearwall, which is NOT wind on the whole structure.

As to number of holddowns, you SHOULD in general get more holddowns in a
traditional segmented or discrete shearwall system than if you do a
perferated shearwall.  The idea behind a perferated shear wall is that the
WHOLE wall length is the shearwall and thus only needs holddowns at the
ends...and there is enough "stuff" above and below any opennings to
sufficiently engage the whole wall as one large shearwall, even if it does
have holes.  OTOH, a segmented/discrete will have several shearwalls along
the length of the overall wall section...and each shearwall will have to
have holddowns to resist overturning.  And if those discrete/segmented
shearwalls are tall, skinny walls, then they will likely have beefier
holddowns than if you have some nice short, squat walls (moment
arms...moment arms).

Regards,

Scott
Adrian, MI

-----Original Message-----
From: Joseph R. Grill [mailto:jrgrill(--nospam--at)cableone.net] 
Sent: Friday, May 23, 2008 10:55 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Hold downs with "Perforated Shear Wall Method" and "Force
Transfer Method"


I'm going to confuse the issue some more.  O.K. in using the net uplift in
overturning calculations that means the roof uplift forces are loading the
MWFRS.  ASCE 7-05 (6.1.4.1) uses a 10psf minimum on the vertical plane. What
about the horizontal plane for uplift?  I don't see a minimum required for
that. For the uplift can you just use the vertical component of the outward
forces on the roof from 7-05.  Or is there a minimum that I am not seeing?

I guess the reasons for these questions is I have a small box addition. I am
getting a lot of hold downs using the traditional segmented design. I am
going to get some real comments from the number of hold downs for this small
structure.  Therefore, I started looking at the other two methods to cut
down on the number of hold downs.  But as things go sometimes, there are
more questions that come up.

Joe

-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)umich.edu] 
Sent: Friday, May 23, 2008 7:01 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Hold downs with "Perforated Shear Wall Method" and "Force
Transfer Method"

I think I understand the intent of what you are saying, but wording does
bother me.

With wind or seismic in one direction so that the shearwall is not loaded,
but there is overall uplift on that wall, there would be no possible
"double-dipping" as the wall would not be subjected to overturning (assuming
a simple diaphragm, thus no torsional effects).

With the wind or seismic in the other direction, the wall (if a shearwall)
should be exposed to both overturning and potentially net uplift (due to
wind uplift or the vertical component of a seismic force...overall, more
logical to think about it in terms of net wind uplift) AT THE SAME TIME.
Thus, the same dead load should be resisting both the uplift AND the
overturning.  In other words, your FBD of the shearwall will have the wind
overturning force, the net uplift, and the 60% of the dead load that would
work to resist both...all as loads applied to the shearwall.  I don't
consider that "double-dipping".  I agree that if, for some reason, you check
the wall seperately for uplift and for overturning, then the same dead load
should not be used in both check...but I would think you should be checking
both uplift and overturning in the same check.

Am I missing something?

Regards,

Scott
Adrian, MI

-----Original Message-----
From: Showalter, Buddy [mailto:Buddy_Showalter(--nospam--at)afandpa.org] On Behalf Of AWC
Info
Sent: Friday, May 23, 2008 9:47 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Hold downs with "Perforated Shear Wall Method" and "Force
Transfer Method"


One caveat however, the WFCM does not use dead load to offset uplift and use
the same dead load again to offset overturning (double-dipping). So in the
WFCM tables, we only show dead load offsetting uplift loads.



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