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RE: Sg rods at purlins

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Although not that common anymore, sag rags were used in purlins too. Some of the old designs had channel sections for the purlins. On a sloping roof. the sag rods would transfer the component of the roof gravity load, in the direction parallel to the slope, to the ridge beam. With a shorter spacing of the sag rods between the purlins, the purlin will see relatively a smaller magnitude of any lateral load.

Without additional information, it is difficult to comment on Morris's project. It depends on how the angle members are connected near the ridge of the roof. If these members on either slopes are not connected together at the ridge, they may not function as either sag rods or as purlin braces.


--- On Fri, 11/14/08, Harold Sprague <spraguehope(--nospam--at)> wrote:
From: Harold Sprague <spraguehope(--nospam--at)>
Subject: RE: Sg rods at purlins
To: seaint(--nospam--at)
Date: Friday, November 14, 2008, 1:56 AM

Sag rods are used for girts as opposed to purlins.  I would guess from you discription that they are secondary bracing elements for wind uplift when the bottom of the zee purlins go into compression.  Unfortunately, these are generally deterimined directly from testing as opposed to deriving formulas based on testing. That is how the PEMB industry justifies the use of standing seam clips to provide bracing for the compression flanges in the zee purlins for gravity loads. 
Regards, Harold Sprague

> Date: Thu, 13 Nov 2008 15:01:20 -0900
> From: dmorris(--nospam--at)
> To: seaint(--nospam--at)
> Subject: Sg rods at purlins
> 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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