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RE: Sg rods at purlins

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My understanding is that sag rods are only used on wall girts. They help
reduce deflection of the girts under their own self-weight: noting they are
bending about their weak axis vertically.

Of more importance to light weight sections in general is the bridging or
bracing members to reduce lateral torsional buckling. Proprietary bridging
systems tend to be a rolled formed channel, with special fastenings. For
larger sections c-section bridging is used with angle brackets to fasten to
the web of the girt/purlin: need at least one fastener either side of
midpoint of the web. Refer, Lysaght, Stramit, Fielders, Stratco girt and
purlin manuals for the way we do it here in Australia.

However, I noticed the ASCE guide to bracing cold-formed structures uses
simpler angle sections for bracing, the angle is continuous and passed
through slotted holes in the sections being braced.

The offset you mention suggests the angles may be some form of
bracing/bridging. If putting extra purlins in, then load widths reduced, and
bracing requirements also reduced. Though suggesting upgrade for higher
loads so net effect maybe requirement for more bridging. But then the
purlin/girt manufacturers load capacity tables should identify such need.

Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
South Australia

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