] [Thread] [Date] [Author]

["Effland, Greg" <geeffland(--nospam--at)butlermfg.com>]

Bill,

Here are some solutions...

1) The first approach is to attempt using the roof panel as a diaphragm.
Here is where the building code year matters...

If using a building code which uses the 1986 AISI w/ 89 Addendum (or
without)... 
Section C3.1.3 gives purlin bending allowables for beams (purlins) having
one flange attached to a deck (yes, Tension Flange Bracing).  This
definitely gives the allowance for screw down roofs.  Some manufacturers
have done additional diaphragm and purlin bracing tests (not "Base Tests")
to verify that the same holds true for standing seam roof panels (some are
better at bracing the purlins then others).

If using a building code which uses the 1996 AISI... 
Section C3.1.3 gives purlin bending allowables for beams (purlins) having
one flange through-fastened to deck or sheating.  This ONLY gives the
allowance for screw down roofs.  The 1996 code added a section C3.1.4 for
Beams having one flange fastened to a standing seam roof system.  This
section requires that the purlin be designed using discrete point bracing
unless a "Base Test" has been performed for the purlin and roof system.  The
is currently some discussion whether the "Base Test" accurately depicts what
it is intended to...  Personally I have not witnessed any of these tests so
my current opinion is based solely on here-say.  Opinion or not though it is
written in the code!

That would be my first approach because it could avoid (additional) purlin
bracing altogether.

2) Second approach would be as James had mentioned.  If you can and it is
not cost prohibitive then you could add an inside liner panel under the
purlins (screw attachment assumed).  This would effectively fully brace your
purlin for uplift and allow the use of section C3.1.1 for determining your
allowable bending moment.

  Some manufacturers have done additional diaphragm and purlin bracing tests
to verify that the same holds true for standing seam roof panels (some are
better at bracing the purlins then others).

3) Third approach - Add purlin braces between purlins and/or Provide a load
path to get the purlin roll forces out (the or part comes from adding
anti-roll purlin clips inlue or purlin braces).

	a)Z-Purlins with a low roof slope, and upslope purlin roll forces
only!!! ... You could turn some of the higher Z-purlins downslope (ballpark
for conversation would be 1 per 5 to 10 purlins).  These downslope purlins
would conteract the upslope roll forces and internally dissipate the roll
forces.

	b) Provide a bracing method to transfer the forces out of the purlin
braces (i.e. strapping, panel diaphragm, force cancellation, etc.)

	c) follow the load path on to the footings (... fyi, after the
forces are transferred into the frames the force is typically small enough
in frame terms to not control over other loads such as wind/seismic/ or
snow)

Assuming you didn't already know, braces are most effective around the
highest moment areas... for beams this is typically near the middle (but not
always)... basically create a small enough Lb around the moment for the
purlin to work.  

--------

Some of the above answers depend on which approach is taken... some of the
items in #3 assume a discrete point brace solution although similar
procedures could be used on other schemes.  One thing you will also want to
consider is the design of the purlin brace system.  Refer to AISI D3.2.1 for
design values.  If using the 1996 AISI note that cosine(theta) terms have
been added to the equations and that a new eqn. D3.2.1-1 has been added
renumbering the old equations up by one (these are in the 1999 Supplement
#1).  Code defined brace forces are restraints at:
1) Supports only (i.e. anti-roll clips)
2) 3rd Point bracing
3) Midspan bracing

If you want forces for other conditions then you will have to test.  Just
make sure your tests can back up your design procedures.  From looking at
the computer models that Murray & Neubert did for the proposed changes to
the 1996 purlin brace equations, and having run several myself (+2000 runs),
this can be a tedious task.

Hope this helps,
Greg Effland, P.E.
KC MO, USA

-----Original Message-----
From: Bill Polhemus [mailto:bill(--nospam--at)polhemus.cc]
Sent: Wednesday, June 12, 2002 11:33 AM
To: seaint(--nospam--at)seaint.org
Subject: "Zee"-Shape Purlins Resisting Uplift Loads


Has anyone here (maybe you who work in the metal building world) had
experience with resisting uplift on zees used as purlins? I'm wondering
about the "usual and customary" method of bracing the bottom flange of the
zee against lateral torsional buckling due to wind uplift.



William L. Polhemus, Jr. P.E.
Polhemus Engineering Company
Katy, TX, USA
Phone (281) 492-2251
FAX (281) 492-8203
email bill(--nospam--at)polhemus.cc



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