Light Gage/Cold Formed Steel Wall - Continuous Bracing?

[BadgerEngineer]

Forgive me if this is a very basic question - As I am new to light gage steel design....

When you are designing a light gage exterior wall - how do you establish 'continuous lateral bracing'? Is this assumed if sheathing is on the exterior (compression) flange? If yes - is there a minimum sheathing thickness or fastening requirement?

I have always been told that bracing is effective if it prevents twist of the cross section and/OR lateral movement of the compression flange. However, I'm not sure what constitutes "preventing lateral movement" (I have always assumed that metal deck on joist or steel beams acts as bracing - so I believe this would be similar)

I'm also desiging a CFS parapet wall and trying to determine if sheathing on both sides will act as continuous lateral bracing. I woudl think the only thing that would change is that you would now have two compression flanges due to wind loading from both sides.

If possible I would like to develop full moment capacity (Sx*Fy). Obviously this is only possible if the memeber can resist LTB. Otherwise you are stuck using the nominal flexural strength charts (KL vs Mn) which greatly reduce your capacity.

Does anyone have a reference they could suggest I review for this? I have reviewed the AISI manual/spec/commentary, but haven't found anything regarding this.

I greatly appreciate your help and again I apologize if this is an elementary question.

 

[frv]

http://www.amazon.com/Bracing-Cold-Formed-Structures-Jennifer-editors/dp/0784408173

 

[ash060]

This was discussed in a previous thread about steel and wood listed below.

thread507-226616

 

[hokie66]

Sheathing or cladding does brace the flange of purlins and girts to which it is attached. But you should remember that the other flange can also go into compression due to suction. So the inside flange usually needs bracing by continuous bridging at intervals which depend on the member size and loading. You are barking up the wrong tree in trying to develop the full capacity of light gauge sections. It is very complicated, and for usual applications, you are best to use tabulated load tables by the manufacturers.

 

[Galambos]

go to the steelnetwork.com and download their software 'steelsmart' and read the manual thoroughly.

that should help you out.

 

[DaveAtkins]

If you have sheathing on both sides of the studs, and you use bridging at 4'-0" oc, you can assume the studs develop their full moment capacity.

DaveAtkins

 

[BadgerEngineer]

Dave - Do you have a reference regarding this? Your response seems to differ greatly from Hokie who suggests to never use fully braced walls.

Have others seen this regarding flexural bracing requirements:
"Bracing is effective if it prevents twist of the cross section and/OR lateral movement of the compression flange"

I don't think bridging restraints lateral movement of the compression flange - would you say it prevents twisting? Since its at the center of the section - I would think no.

I have read the post suggested above regarding braced walls. Posters seems to suggest that bridging is only effective for bracing in compression and that sheathing must be used for flexural bracing. Possibly the differnce in responses above has to do with the terminology - Compression vs Flexural bracing.

Your thoughts?

 

[ash060]

An angle with a CRC or Flat straps with blocking are the 2 common ways of bridging. They are considered to prevent the section from twisting.

The book frv mentioned does have some design examples.

So does AISI framing design guide

 

[faromic80]

dietrich.com also has a lot of good information on their website. you can try and request a catalog for a reference to design values