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RE: AISI Cold Formed Steel Design

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Why not use light gage tension straping in an x-configuration from girt to girt? This should provide positive bracing of the inboard flange.
 
 
Jim K.
-----Original Message-----
From: David Maynard [mailto:davemaynard(--nospam--at)vcn.com]
Sent: Thursday, December 08, 2005 3:43 PM
To: seaint(--nospam--at)seaint.org
Subject: AISI Cold Formed Steel Design

I’m looking for a little assistance on this one.  To preface, I’ve worked with the AISI about, oh, 3 times over the past 8 years.  I have recently, due to new job, moved up to the 2001/2002 design manuals for AISI CFS Design.  That being said, here’s what I got.

 

I’m looking at a wall girt design.  The span is 32’-0” between supports with a 2’-0” overhang off of each support (total length of 36’-0”).  I would like to try and utilize the 10C3.5X105 for this application, expecting to have girts spaced at 4’-0” o.c.  The wind velocity for the area is 90 mph, in which I am using a 20 psf leeward wind pressure for design.  The walls are uninsulated.

 

I think I am having trouble applying this design code.  I see in Section C3.1.3 of the AISI Standard for North America, there is a section that discusses “Beams Having One Flange Through-Fastened to Deck or Sheathing.”  When I apply this, my section fails.  Which is fine, because I don’t have any lateral bracing (as is part of the notes in applying this section). 

 

So, now I have to supply some sort of lateral bracing, right????

 

Once I do so, am I now governed by Lateral Torsional Buckling (section C3.1.2)???

 

The “lateral bracing” that I am considering is similar to floor joist blocking.  Between two girts, at say the 1/3 points, I would install a vertical CEE of the same size as the girts.  I’d weld this “brace” to both members.  Girts would be installed in pairs that are connected to one another.  So, with respect to lateral bracing, is having two girts braced to one another adequate, or do I have to connect all of the girts together???  In other words, would this type of construction constitute a quality lateral brace for this application???

 

Once I get this squared away, I have to and look at the combination of Bending and Shear, as well as web crippling.

 

And this brings me to another situation.  I see that there are tables in the AISI Manual Cold Formed Steel Design, Table II-7b (for LRFD).  Is there a basic assumption of full lateral bracing to be considered when using these tables.

 

Finally, does it sound as though I am applying things correctly?

 

ANY help would be greatly appreciated.

 

Sincerely,

David Maynard, PE

Gillette, Wyoming