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Re: Wind loads: ASD or Strength?

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As to wind being a "strenght level" load (like seismic is in the code) or service "level load" - that is easy...wind is NOT strength level like seismic.  There should be zero confusion (although that does not mean that some still get confused).

Now, the issue of C&C vs MWFRS pressures is a wee bit more messy and is not suprising that many get confused (although if everyone just listened to me and did it "my way" we would be WAY better off...this is for all things not just structurally things <grin>).  I will spare you my take on take on C&C vs MWFRS as 1) I have offered it before and 2) it would long winded (pun intended) as I tend to be long wind at times (OK, all of can stop laughing and nodding your head up and down) but would likely be especially long wind this time as my figures are well rested after my tranplant...but long winded is tough to on my iPod Touch (which is where this being typed).  But if you want long winded, then I can always fire up the laptop...just say the word and I will "make your day".

HTH,

Scott
Adrian, MI

On Feb 17, 2009, at 2:14 PM, Andy Heigley <aheigley(--nospam--at)jgaeng.com> wrote:

Jason:

I don't know where he's finding the wind pressure to be a strength level load, especially if you are using the Allowable Load Combinations of the building code...  I would tell him that he's not following the Allowable Load combinations of the Building Code, and what he's using is not listed as an alternative load combination either...

However, I have found information regarding the use of MWFRS pressures for the design of studs themselves...

In the Wood Frame Construction Manual, in their Exterior Wall stud Bending calculations, it describes the design of the stud to check the stud for 'bending only' using C & C design loads.  In addition to that, it uses MWFRS pressures in combination with Axial loading to check combined bending and axial.

Also, in the AISI standard for CFSF wall studs, the procedure I listed above is also described.

I know in the past, I've always designed for C&C loading with axial only...  But it seems as though you'll get a little benefit in MWFRS pressures with axial loads.

Obviously the connection for OoP forces would be governed by C&C loads.

One more thing, maybe he got confused on the stipulation that for members designed for C&C forces, they can be multiplied by 0.7 for deflection calculations...

Andy Heigley, PE


JASON CHRISTENSEN wrote:

Sorry if this has posted multiple times,  our email seems to be messed up.  So I sent this again using a different email.  



 
 
From: jtt_christensen(--nospam--at)msn.com
To: seaint(--nospam--at)seaint.org
Subject: Wind loads: ASD or Strength?
Date: Tue, 17 Feb 2009 09:20:23 -0700


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)wcaeng.com

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