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RE: LGSS - Light Gauge Steel Stud question

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Bao,
I appreciate you spending the time with me to discuss this issue. Here is the solution that I came up with:
1)  I have modified the design by tieing the webs together in order to prevent them from buckeling independently inward or outward. Each end of the bolt is tightly secured with nuts on each face of webs. The bolts (braces) are placed within 12" from each end and at the middle of the header width up to 6'-0" wide. Wider headers follow the same configuration with the brace bolts added every three feet.
2) At the Garage header (which takes the least load of 5'-0" tributary roof loading) I will be installing the bolt braces at 3'-0" on center and filling the void of the box beam with expandable polyurethane foam which will create the rigidity that we need. 
 
I thank you again. I would also be interested in your spreadsheet for web crippling. I remember the last spreadsheet you sent - I believe it was to determine the section properties of various cross sections.
 
Regards 
 

Dennis S. Wish PE

 
-----Original Message-----
From: B D Pham [mailto:bpham(--nospam--at)airmail.net]
Sent: Wednesday, March 18, 1998 11:40 AM
To: seaoc(--nospam--at)seaoc.org
Subject: Re: LGSS - Light Gauge Steel Stud question


>4) the foam idea would have been ideal since it would have taken >most of the gravity load off the webs and if any buckeling occurs, >would probably be very small and enough for the gypsum to control.
 
The company I used to work for used sprayed foam (polyurethane) for insulation and it seem to be relatively rigid after it harden.  I think it is a good idea to use the foam in this situation.

>Question: How is buckeling calculated when the beam is a built-up >box beam  with a top and bottom track to distribut load? I assume the >H/t ratio of the member for buckeling is determined by loading the >individual section, not the built-up member. Is this true? 
 
This is true with built-up box beam when calculating the shear and web crippling strength. Individual member will be calculated separately and carry its share of the load.

>Furthermore, a pony wall (cripple wall) is constructed above the >header with a continuous track supporting the trusses at each cripple >stud. Is the complete assembly considered in reducing the potential >for web crippling by creating a deeper composite type beam?
 
 
I doubt that the cripple wall will reduce the load that might cause web crippling.  The cripple wall is there as a mean to transfer the gravity roof loads to the beam.  And ultimately all the roof loads will be load on to the beam and then to the support.

>I think a lot of this has to do with engineering judgement than whether >or not the web will cripple in reality. Fortunatly, I was not the engineer >who designed these members or I would have followed the >manufactures design guides. The engineer who did the design also >released a book recently on Steel Stud Construction however, I have >reviewed other work he designed and found them to be less than >adequate. 
 
In reality the beam might never experience its full design load and web might not cripple.  But the possibility of the web will cripple is definitely there,  otherwise the AISI code would not require a member to be check for  web crippling failure and also I have seen the web cripple during a construction due to excessive load and the web was not reinforce.  I think how to prevent the web from crippling is per engineering judgement.
 
I also have a spreadsheet that would calculate the shear and web crippling strength.  Let me know if you are interested in it.

Bao D. Pham