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RE: Wind loads: ASD or Strength?[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Wind loads: ASD or Strength?
- From: "Doug Mayer" <doug.mayer(--nospam--at)taylorteter.com>
- Date: Tue, 17 Feb 2009 10:27:22 -0800
It sounds like the engineer is a little bit confused. First, the design loads should be based on C&C pressures as the wood stud in OOP bending is not a MWFRS. Second, the wind force level is dependent on the load combinations used and looking at the ASD load combinations indicates that wind is a service level load. I don’t think there is any particular section in the code that specifically says this, but it should be obvious by looking at the load combos. Also, per section 2306.2.1 of the CBC (should be the same in the IBC) there is a bending stress increase for OOP wall stud design under wind loading. I don’t think your engineer was using this, but I just wanted to point it out.
Doug Mayer, SE
I agree with you. But what confuses me is that it is an out of plane calculation that is in question, and as you stated the 40% increase is only for the diaphragms and shear wall capacities resulting from in plane loads.
I would be willing to bet that the confusion is coming from IBC section 2306.4.1, which allows the tabulated values for shearwall resistance from Table 2306.4.1 to be increased 40% for wind loads when using ASD. The choice of ASD is a choice of load combos, as you said, and happens after V is calculated. Dividing the load is bad practice, though, because this increase in resistance applies only to shearwalls and diaphragms, not to everything connected above & below or drag connections.
I gave a peer
review a few weeks ago. I called the engineer for designing a wall stud for
seismic loads only, from his calculations the C&C wind loads were about
10psf higher than the seismic.
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