Wood shear wall / diaphram capacities 2

[DRsquare]

In the Florida Code, Tables 2310.2A,B. give values of allowable shear for walls and diaphrams as a function of panel thickness, nailing etc. The footnote at the bottom of table A (but not B)states that these values are for short-term loads from wind and must be reduced 25% for normal loading. The foot note statement is repeated in 2310.3.6.

Then Sections 2313.2.4, and 2.5 state that the values in both these tables can be increased 40% for wind design.

It seems strange to give a increase twice. Is this a mistake? Any comments?

 

[lutein]

DRsquare
I have used FBC before, that clause appears strange to me as well. After discussion with several engineers and researching, we think it's a mistake. The design guide published by plywood institute did not use the 40% increase either nor did any timber design books I have seen.
However, personally, I have always treat the 40% as the saving in the back pocket; I have used the 40% in certain cases where I have overstresses.
Hope this helps. I would like to hear more opinions on this as well - Good question.

 

[FSS]

The "extra" 40% is not a new clause. It was in the SBC code from which Florida based its code. According to commentary to SBC 1999, the 40% is due to a "change in the load factor for wood structural panels from 2.8 to 2.0 (2.8/2.0=1.4). Tests conducted on walls at 1.4 times the capacities in Table 2310.2A and 2310.2B determined that the walls remained in the elastic deformation range." Don't know if that explains it clearly, but it's been around for a while.

Anyway, be more careful about reducing your values with the 0.82 factor when your studs are not Doug Fir or Pine, since they usually are SPF.

 

[DRsquare]

Thanks, I can buy that. But what bothers me is that Table 2310.2A for horizontal wood structural panel diaphrams has a foot note that states the values are already increased for short term loads from wind, and should be reduced 25% for normal loads (ie. a 33% increase for wind.) This is repeated in 2310.3.6. This is not true for wood structural panel shear walls.

Of course I've noticed that a lot of things in the world don't make sense. That's why they pay us the "big bucks", rignt?

 

[zenom]

The 25% adjustment and 40% account for different things. The shear wall and diaphragm table are based on principles of mechanics, see APA Research Reports 154 and 138. Basically a single fastener value, Z, is multiplied by load duration factor (1.6), diaphragm factor (1.1), number of nails per foot, and a few ~0.9 empirical terms sometimes apply.

The shear wall and diaphragm tables are calculated for wind or seismic (a short term load), which corresponds to a load duration factor of 1.6. If you need your shear wall to resist a long term load then a different load duration factor should apply. The 25% is from the pre-1991 NDS load duration factor which used to be 1.33. 1/1.33 = 0.75. Today if you want to adjust to a "normal" load with load duration factor of 1.0 then the adjustment would be 1/1.6 = 0.625. The codes need to be updated to reflect this. Look for a code change in the 04/05 ICC code development cycle. Also, what I describe is stated in Breyer et al. Design of Wood Structures 5th ed. I should note there is no evidence the "old" higher 0.75 adjustment is too high. The descrepency is a result of the new (in 1991) DOL factors (was 1.33, now 1.6) and diaphragm factors (was 1.3, now 1.1).

The 40% increase for wind has nothing to do with load duration. The 40% increase for wind is to lower the load factor due to historical good performance and since "we know wind loads so well".

One other thing. The framing species adjustment has changed too in the IBC. It's simpler in my opinion.

 

[DRsquare]

zenom,
Thank's for yout explaination. It's nice to know why, and not just how.