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RE: Rigid vs. Flexible Diaphragm

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Dennis:

You indicated in another post that the manufacturer has checked hardy frame for out-of-plane forces. So I guess that issue is settled for now. I wanted to comment on response about the rigidiity of the hardy frame.

In response to :

"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."

You wrote:

"You hit the nail on the head here - they do attract load. This is the reason
for using them as they fit into spaces where conventional plywood walls do
not work. I would not arbitrarily use Hardy Frame, ShearMax or Strongwalls
where I could construct a traditional plywood shearwall - it is not
economical.  Furthermore, you are not likely to use only RDA for
light-framing alone. If you do, you leave yourself liable for potential
open-front or soft-story failures. The code (at least the 97 UBC) does not
say you can't use an open-front design - it's just that historically we know
in Southern California that it does not work - especially where there is a
living structure above. If you attempt to balance based on the worst case
results from Flexible (wind and seismic) and Rigid then you need to reverse
engineer the RDA to push the shear back into the diaphragm for it to be
distributed to other walls. This can cause a domino effect - especially if,
as you pointed out, you introduce a stiffer wall such as a Hardy frame into
the model.)"


I agree they attract load - the only problem I have is that they attract too much load if once does a RDA. For argurments sake, I calculated the stiffess of 24 inch wide hardy frame, simpson strong wall, and a 4 ft plywood wall (1/2" ply with 10d nails @ 4" o.c. with a PHD6 each end). I used PHD6 in order to match the size of the anchor bolts (7/8") with HF and SW.

Cap Delfection Stiffness (Cap/Deflection)
For HF8x 24 7/8,        2728            .203              13438
For SW24x8               1610           .389                4139
For 4' PLy 1840 .446 4122 (based on UBC equations)

The validity of comparing the stiffness with a 4 ft ply may be questionable; it is provided for comparisions only.

If one did the FDA method, the demand in the SW and HF would be the same. But if one did the RDA, the demand on the HF could be as high as 1610*13438/4139 = 5227 in which case HF would be under designed. Of course, this comparision is crude as the stiffness of other walls would also have an effect on the net distribution (RDA assumption); but I hope you see the point I am making.

I agree with you I should not just use RDA - I don't. I used the envelope method. In my spreadsheet, my wall stiffness is based on the capacity rather than demand. This helps establish the wall rigidity rather than reiterating for every small change in force redistribution; also it is in line with the fact that the rigidity of the strong wall and hardy frame is based on the capacity rather than demand. So, in terms of analysis, it takes one iteration only. Of course, the validity of this assumption could be questioned - but I had to have a starting point.

Gautam

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