modifications to G, apparent stiffness?

[kipfoot]

I'm having trouble reconciling wood shear wall deflection calculations done according to equation (4.3-1) of the AFPA SDPWS-2008. My software result doesn't match my hand calcs and spreadsheet so i wonder if I'm missing something.

Specifically, are there modifications that must be done to G, apparent shear stiffness, from table 4.3A that would make my numbers screwy? There are footnotes to the table, but any other adjustments I should be considering in my calcs?

 

[JoshPlumSE]

You have to look in the commentary to 4.3.1. Specifically example C4.3.2-1 "Calculate the Apparent Shear Stiffness, Ga in SDPWS Table 4.3A".

That walks you through the nitty-gritty details about how they came up with the Ga values. Note: you can assume different levels of fastener slip (if you like) and develop your own alternative Ga values.

 

[kipfoot]

Thanks for the reply. I'm still confused because I can only replicate the values for the sheathing that's used in the example. When I try with input for different thicknesses and nails my spreadsheet falls apart.

It's interesting to note that one can use an alternative fastener slip value, Vn. Have you done this by using the actual fastener load per nail rather than 1.4Vs(ASD) in the equation?

 

[JoshPlumSE]

In concept you can use the actual fastener load per nail to come up with a stiffer panel. Essentially considering the actual stiffness of the wall at the given load level rather than assuming a stiffness based on the fastener failure load level. I have not done that personally.

Also, the concept should, of course, be valid for other panels and fastener spacing and such. It's been a while since I did these calcs, so I can't easily point you towards any stumbling blocks. Just that it should work and that I remember it working when I did it back when.

 

[kipfoot]

I like the idea of using actual load per nail when determining e(sub)n. The mistake I was making was that I used the same formula to determine e(sub)n when looking at other sheathing thicknesses.

For the benefit of anybody else who stumbles on this thread: The divisor and exponent in the equation for e(sub)n changes with sheathing thickness according to Table C4.2.2D.