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?

 

[haynewp]

I have reviwed severl WF girt metal buildings due to the long spans and/or deflection critical finish.

I think the Z girst are usually installed with the inside flange pointing upward.

 

[kslee1000]

jetboat:

I am not an expert on paneling, the following are merely personal thinking and observation.

In between end supports, the sag-rods provide stability to the paneling system in the direction of gravity. In another words, the rods "brace", and thus minimize the movement in that direction. I think the tendency of the channel to rotate around the rod is prevented by the jamming force from the fastening nuts, therefore, buckling is prevented as well. So, it is justfied to say the girt is braced at sag-rod.

At where I work, there are hundreds of steel building with steel paneling. The buildings are ranged from 30-90 years old, the height are ranged from 45' to 200'. Due to lake effect, the wind is a building killer. However, the damages are limited to wall/roof panels (completely torn from wall), so far haven't encounter problem with girt, except some were weakened by rust/corrosion. I guess this phenomenon can be explained by comparing the relative rigidity of the steel pannel, panel-girt connections, and the girt. Obviously, the weakest one has to break first.

Consult with a wall panel manufacture on usual wall span and member sizes, I think size of rod and spacing play an important role in the whole business.

 

[LOKSTR]

jetboat,

Here is an Canadian experimental study of girts under negative pressure which will clarify your doubts.

http://cisc.ca/files/technical/techdocs/faq/girts.PDF

LOKSTR

 

[kslee1000]

Very good experimental study.

 

[civeng80]

Apologies hokie I guess its time for me to think about reading glasses!

As a interest what diameter rods do they use in the States?

Here they use to be 16mm but now have gone down to 12mm (1/2 inch rods) and they look very flimsy some purlin Companies actually use bridging throughout which is much better.

 

[DaveAtkins]

5/8" diameter on a project I engineered last year.

Which brings up another point--a 5/8" diameter rod which is 5' long (a typical girt spacing) can take 1000# of compression (Allowable Stress Design). So the sag rods can also resist upward girt buckling.

DaveAtkins

 

[SteelPE]

I have begun to see a lot of contractors installing flange bracing on the backside of the girts. The bracing is typically a light guage strap that is attached to the girt and to the interior face of the panel. This detail only works if you are working with a foam panel siding.