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Re: Rigid vs. Flexible Diaphragm

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On a recent project I found just the opposite to be. The 10 ft. Hardy
frame deflected 0.29" while the 10 ft. high shear walls deflected about
0.15" each. This from my program using the 4 part equation for deflection
of shear walls and the City of LA test data for Hardy frames.

Stan Scholl, P.E.
Laguna Beach, CA


On Wed, 05 May 2004 17:17:12 +0000 "G M" <newabhaju(--nospam--at)hotmail.com> writes:
> 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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> 
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