Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: AISI Cold Formed Steel Design

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Probably too late for the project but ...

> From: "David Maynard" <davemaynard(--nospam--at)vcn.com>

> I'm looking at a wall girt design.  The span is 32'-0" between supports with

> girts spaced at 4'-0" o.c.  The wind velocity for the area is 90 mph, in

> uninsulated.

> 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

Be aware that this clause applies to the "free" (interior) flange when
that flange is in compression (e.g. net pressure away from the exterior
face of the exterior cladding). Typically, the cladding fully braces the
exterior compression flange (e.g. net pressure toward the exterior face
of the exterior cladding). Lots of caveats to consider.

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

Yes. Or use heavier sections or closer spacing or built-up sections
(e.g. channels back to back stitched), in that order. Extra rows of
girts may be less expensive than adding L-T bracing (material and labour
considered).

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

Yes.

> 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???

Yes, in this case. The "blocking" channel prevents the torsional
response and the cladding provides the lateral resistance. Block
alternate pairs vertically. You can't do much better than this.

Others have suggested strapping and threaded rods. Be careful. I have
seen both of these incorrectly specified and incorrectly installed. Both
are tension-only braces but the rod does have some limited compression
capacity (typical rod kL/r = 250 to 500).

You are trying to brace the interior flange. For a channel, the
compression flange will tend to rotate toward the open side (e.g. in the
direction that the unbraced compression flange is pointing). So, if the
channels are installed opening up, you need to anchor toward the base
and vice versa.

Discontinuous braces (rods, rule of thumb) must be angled from inner 1/3
(compression side) to outer 1/3 (tension side) of adjacent member with
cladding fastened at tension side. Otherwise, they are like straps and
need to be anchored to a rafter or foundation or other substantial
member for the purposes of torsional restraint.

Continuous brace (e.g. straps) must fasten inner flange (compression
side) of all members to anchor point (foundation or beam). Members must
be facing in the same direction. Straps make my skin crawl for a lot of
really good reasons.

> I was under the impressing that the purpose for sag rods was to help with
> weak axis bending.  I wasn't aware of any help through lateral or torsional
> restraint.  Am I wrong on this???

Weak axis bending for the purposes of construction alignment, ergo, "sag
rods" or "sag straps". For alignment purposes, it doesn't matter much
how they are installed. However, the name stuck and is applied to all
similar bracing including lateral-torsional. Regardless of the
structural purpose, they are occasionally removed after construction.

Double nutting rods on opposite sides of web is probably a good idea for
a couple reasons:
1) locks the nuts,
2) holds the rod for tension and compression.
Even with cold formed girts (especially 32' span capable) I wouldn't be
concerned about requiring washers.

I would avoid welding gage metal (unless the welder REALLY is capable)
and use bolts or self-drillers with a clip angle.

> 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.

I don't have a copy of the manual. The basis should be indicated at the
reference point or in footnotes.

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

Yes.

> David Maynard, PE

-- 
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********