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Re: Sg rods at purlins

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Drew,
<rant on>
Sorry, but I have to cringe every time that I hear the term "sag rods". A
"sag rod" is strictly an alignment component. I know of very few times that
I have encountered a design where the "sag rod" was not also required as a
stability brace for axial or moment effects. This applies to both
manufactured and conventional construction.

The problem with referring to these as sag rods is that the field personnel
may not install them because they have taken measures to ensure the
supported member is aligned during construction so the sag rod was an
unnecessary waste of time - for the past 30 years. Please use the term
flange brace or stabilizer - especially on construction drawings. I think
that Newman uses an adequate term in his book - but I've forgotten it at the
moment.
<rant off>

Okay, I feel better.

If you do a quick check you will be able to determine a couple things about
your situation:
1) Does the Z purlin require a brace-point to achieve the original design
load capacity (probably uplift)? If so, the angle is a stability brace.
2) Is the roof cladding through-fastened vs. sliding clip standing seam?
AISI standard permits the use of a reduced Z section strength limit without
discrete bracing under certain conditions. Some stability bracing is still a
good idea on 24' span, anyway.

Purlin stabilizer bracing has been poorly done by many manufacturers over
the years. The most neglected consideration being accumulated forces in the
stabilizer line.

I would strongly recommend that, at the very least, you replace any such
angles that you remove. Better yet, check the design requirements with the
standards. Since you mention upgraded gravity loads and added purlins you
may need to include improved stabilizers and anchorage.

Regards
Paul

> From: Drew Morris <dmorris(--nospam--at)bbfm.com>

> I have a question on sag rods.  The project that I am working on is a
> pre-manufactured building (unknown manufacturer) with a roof slope of
> 1:12.  The 8" deep roof zee purlins span 24 feet and there is an
> L3/4x3/4x 12 gage angle at the midspan that intersects the purlins about
> 5" down from the top.  Based on the photos, the angles are not
> continuous from purlin to purlin and seem to be offset vertically, i.e.
> come in different heights on either side of the purlins.  We are
> upgrading the roof system for higher snow and seismic loads and need to
> infill with new matching purlins to get the spacing down.  Do you have
> any idea whether these angles are sag rods?  Since the roof panels are
> already inplace, our thinking was that the sag rods could be cut away
> since the new infill purlins could be installed and screwed to the roof
> panels.


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