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RE: Wind loads: ASD or Strength?

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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


From: Jason Christensen [mailto:jason(--nospam--at)]
Sent: Tuesday, February 17, 2009 9:43 AM
To: seaint(--nospam--at)
Subject: RE: Wind loads: ASD or Strength?


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.





From: Gordon Goodell [mailto:GordonGoodell(--nospam--at)]
Sent: Tuesday, February 17, 2009 10:36 AM
To: seaint(--nospam--at)
Subject: RE: Wind loads: ASD or Strength?




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.



Gordon Goodell


From: Jason Christensen [mailto:jason(--nospam--at)]
Sent: Tuesday, February 17, 2009 9:16 AM
To: seaint(--nospam--at)
Subject: Wind loads: ASD or Strength?


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.

The response I got was a bit perplexing. He said that the wind load (ASCE 7-05), using the MWFRS load not the C&C for OOP, was a strength level load and he then divided the MWFRS load by 1.4 and showed that that load was less than his seismic load.

I want to correct him on stating that the wind load is strength level, as well as using MWFRS in lieu of C&C for OOP. Does anyone know where, other than looking at the load combinations, that the code states that the wind loads from ASCE 7-05 are service level? For the use of the C&C loads for OOP I am going to refer him to the book "Guide to the Use of the Wind Load Provisions of ASCE 7".

Jason Christensen, S.E.
WCA Structural Engineering, Inc.
442 North Main Street
Bountiful, Utah 84010
PH. (801)298-1118
E-MAIL: jason(--nospam--at)

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