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Re: Rigid vs. Flexible Diaphragm
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Rigid vs. Flexible Diaphragm
- From: Mlcse(--nospam--at)aol.com
- Date: Thu, 6 May 2004 03:01:17 EDT
Question about calculation of the holdown deflection.
If the holdown is rated for 2000 pounds of uplift that results in 1/8" deflection, why not consider a linear relationship for uplift. If the uplift force is 1000 pounds than the holdown deflection is (1000)(0.125")/(2000) = 0.0625" uplift. The relationship most likely isn't perfectly linear, but its probably not a bad approach. If the shear wall is a 2:1 aspect ratio, than the horizontal deflection is (2/1)(0.0625") = 0.125" deflection.
We have written our program using this approach which self iterates to a solution where all walls and frames must have the same deflection (tolerance cutoff of 0.005" between maximum and minimum shear wall deflection). Forces to individual walls are based upon all walls having the same deflection (pure translation). Holdown sizes are checked on each iteration, and the sizes automatically increased to the appropriate size for the given uplift size. The holdown contributes greatly to the shearwall drift unless you have a very stiff holdown. But if you assume 1/16" oversized holes for a bolted holdown, and the shear wall is 2:1 aspect ratio, than you have 1/8" of shearwall horizontal deflection just due to the bolt holes slip in the wood post which is about 20% of your allowable drift for an 8 foot tall wall, and you haven't put any load on the wall yet (assumption is that any uplift on post must move 1/16" to engage the bolts to resist the uplift). I have found that for the short walls, the nail slip and holdown movement are the major contributor to shear wall deflection.
In a message dated 5/5/2004 9:11:13 PM Pacific Standard Time, newabhaju(--nospam--at)hotmail.com writes:
Let me first agree with you that whether you use FDA or RDA, good detailing
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
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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