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RE: Rigid vs. Flexible Diaphragm

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Let me first agree with you that whether you use FDA or RDA, good detailing is king.

A major contribution of plywood wall deflection is from nail slip followed by the contribution of the holddown deflection; the contribution of shear deflection and flexural deflection is generally small and is dependent on the aspect ratio. It is the nail slip that causes a whole lot of problem in calculating the wall rigidity.

Let us say a shear demand is 400 plf. 1/2" ply w/ 10d nails at 4" o.c. with a capacity of 460 plf will probably be specified. The actual rigidity calc should be based on the nail slip w/ 400 plf; the rigidity based on capacity will be lower as the nail slip will now be based on a load of 460 plf. When I first developed my program, I used the actual stiffness ( - this is also not 100% correct because the holddown contribution is based on the Simpson's catalog which is based on capacity ). This procedure required several iteration because the change of rigidity of one wall caused the redistribution of forces which then further changed rigidity (due to nail slip). I must say that I did have doubts about the wisdom of using RDA because of the number of iterations required.

The use of capacity based rigidity made iteration manageable - generally only one iteration is required. But then I am not using the actual rigidity - and that is why I said the validity may be questioned.

The more I tinkered with my program (more automation), the more I realised that capacity based rigidity is a better way to go inspite of the flaw I described above. Some of the reasons I feel capacity based rigidity is the way to go are:

1. The holddown deflection contribution is based on capacity. When I talked to Simpson, they did not have deflection values for load other than the capacity.

2. Proprietary wall deflection values are based on capacity.

3. Moment frame/cantiliver columns can now easily be incorported - once you know the section, just plug in the rigidity into the program.

4. If the building consists of plywood shear walls only, most of the walls will have a reduced rigidity and the error will tend to balance out.

5. If the building consists of a mix of plywood walls and proprietary walls, it doe not make sense to use the demand rigidity of the plywood walls and capacity rigidty of proprietary walls.

Gautam Manandhar, SE

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