sag-rod supported girts (girt design) 3

[jetboat]

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?

 

[DaveAtkins]

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

 

[JAE]

Read this one: thread507-115545

 

[JAE]

...and this one

thread507-184458

 

[hokie66]

Don't you have purlin bridging in the States? In Australia, our girt and purlin manufacturers make clip in bridging sections.

b

http://www.bluescopesteel.com.au/go/product/lysaght-zed-and-cee-purlins-and-girts

 

[GalileoG]

If memory serves me right, it braces along one direction only.

 

[civeng80]

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 !

 

[hokie66]

And a good one to you as well, civeng. But I've got to get you to spell Hokie correctly.

 

[BAretired]

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

 

[JAE]

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.

 

[BAretired]

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

 

[JAE]

BAretired,

When they hang the girt onto the columns, there is a natural, self-weight sag that occurs. The contractor with either ignore it (if it is not noticable) or push it up to a more level condition. We usually will note on our plans to ensure a minimum amount of downward sag, perhaps L/480 or something of the sort.

It isn't much and it isn't really visible and doesn't affect any other aspects of the building.

 

[jetboat]

DaveAtkins,

Would you have a copy of lecture notes for a reference?

 

[hokie66]

I agree with BA about the sag in the girts. Installing "sag rods" and then insisting that the girts sag goes against the grain with me. I like everything to be straight and level. But then I am used to using a purpose designed system to ensure this occurs.

 

[JAE]

hokie66,
I guess I don't disagree wth your desire for everything to be straight. I just know that most girt conditions occur in more industrial facilities where that sort of straightness isn't all that necessary.

If you think about it - ALL of your horizontal beams in roofs and floors are definitely not straight at all. Why does the girt have to be perfectly straight when its natural sag helps the stability.

 

[BAretired]

Let's say that the girts are C sections. The load on the wall will be suction pressure in order to cause compression in the interior flange.

If a C section is installed with web upward, then the shear center is above the web. Load is applied by exterior paneling at about the middle of the exterior flange. This is well below the shear center, so the channel rotation causes the interior flange to buckle downward. This is what we want.

If the channel is installed with web down, the opposite is true and the inside flange tends to buckle upward. So perhaps the correct way to install a C channel girt is web up (flanges down).



Best regards,

BA

 

[JAE]

You are correct - something to consider

 

[hokie66]

But don't you use Z girts rather than C's in most cases? We do, as you can lap the Z's.

Your theory is still good for a Z installed with the inside flange up, which is the predominant way, I think. But if the inside flange is down, as is common in some industrial buildings to allow easier cleaning of debris, then the load is above the shear centre.

 

[JAE]

Then arch it upwards if that is the case?

Most of the heavy warehouses we have done use wide flanges for the girders. This is due to the long spacing between columns which in turn are located to work with the racking inside the buildings.

 

[hokie66]

Do you mean rolled steel sections for the girts? That sounds like a lot of work in screwing the sheeting.