sag-rod supported girts 9girt design0

huangyhg

sag-rod supported girts (girt design)
i have been told when designing girts with sag-rods that the girts can be considered laterally supported at the location of each sag-rod. i am unable to locate any technical data or examples to reference. if i were to design the girts without assuming lateral support from each sag-rod the girt sizes would become unreasonable. does anyone have suggestions or references on this topic?
from "lecture notes for structural design of industrial buildings"
prepared by james m. fisher, ph.d., p.e.
"the sag rods are often assumed to provide lateral restraint against buckling for suction loads."
daveatkins
read this one:  
...and this one
don't you have purlin bridging in the states?  in australia, our girt and purlin manufacturers make clip in bridging sections.
if memory serves me right, it braces along one direction only.
personally i wouldn't consider sag rods as providing lateral support for girts (or purlins for that matter) no matter how much literature is out there proving otherwise.  they need bridging like hokkie66 says and thats what we use in australia.  
by the way happy new year hokkie !
cheers !

and a good one to you as well, civeng.  but i've got to get you to spell hokie correctly.
well, i have never been a fan of sag rods as bracing for girts but the practice seems to be firmly entrenched in my locality and i have to admit there has never been a problem of girt buckling so far as i am aware.  of course, that does not necessarily make it right.
best regards,
ba
just to point out that in those two links that i posted above, the concept of sag rods acting as braces, only applies if the horizontal girt is installed with a slight, downward sag to begin with.  
with that initial sweep in the girt, any outward wind suction would compel the girt to buckle away from the sag rod, thus allowing the rod to work in tension, which it can do.
and in conjunction with the exterior siding, the rod-plus-siding combination creates a couple that can resist torsional twisting in the girt, thus dis-allowing lateral torsional buckling.

i do not believe that standard practice is to install girts with a slight sag.  the erectors try to get everything as plumb and level as possible.
perhaps it would be a good idea to tie the lowest girt to the foundation with another sag rod.  it would not really be a sag rod...more of a hold-down device.
best regards,
ba