sagrod configuration

[delagina]

I'm looking at this monoslope roof sagrod configuration.
This is one of the goby drawings I have.
Does this type of sagrod configuration work?

 

[hokie66]

I had to look up "sagrod" before I realized you meant "sag rod". The sag rods would tend to keep the channels straight. It is generally better to have the toes of the channels uphill, as vertical load is then applied closer to the shear centre of the purlins.

 

[delagina]

is it ok to have the sag rod in the middle of the purlin than near the top and bottom flange?

 

[hokie66]

That is normal, and mostly just helps in erection. I assume the roof sheeting braces the top flange. If your bottom flange needs bracing for uplift, the sag rods don't do a lot of good. In that case, better to use a bridging system which gets closer to the bottom.

 

[KootK]

As hokie as intimated, much here comes down to whether or not you have roofing or in plane bracing that will serve as a competent diaphragm.

In many situations (standing seam), the roofing for a structure like this will not be a competent diaphragm and it is really the sag rods that keep a portion of the the gravity load of the roof from becoming weak axis bending the in the purlins. On a dual sloped roof, there will be connection between the sag rods on either side of the ridge so that none of the channels have to absorb the accumulated sag rod force as weak axis bending. With a mono slope roof, it's a different animal.

In the absence of a diaphragm, the down slope component of your gravity load will be:

1) loading your top most channel in weak axis bending if it is stiff enough to attract the load.
2) loading all of your channels in roughly equal week axis bending in which case the sag rod has little purpose unless it is intended for lateral torsional restraint.

I don't believe that it's normal practice to count on sag rods to restrain lateral torsional buckling in roof purlins. That said, it is a pretty normal practice to use them to restrain lateral torsional buckling in girts.

I don't love the angles. As shown, they would put the fillet welds to the channel webs in direct bending which is generally a no-no for welds. Perhaps it's acceptable in this instance with the loads being pretty minor.

This position statement by Canam may be of interested. Kind of sounds as though the official party line on this has been in flux. We've got a couple of PEMB industry experts on this forum. Hopefully one of them chimes in.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[delagina]

Kootk, if you were to design this, how would you go about it. Does it even need a sag rod? It seems it's not doing much for monoslope roof.

 

[KootK]

I need to know the diaphragm story here in order to comment. Is the roof sheathing diaphragm capable? Is there horizontal-ish bracing in the plane of the roof?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[thaidavid40]

Regarding the orientation of purlin cees and zees: in a known dirty environment it is often typical to orient the legs/flanges "downhill". This is not structurally optimal, but it does minimize the opportunity for process material to be trapped in the corners formed by members placed in the normal "uphill" orientation. Depending on the process material composition, and the moisture level under the roof, that could just prolong the structural life of the roof framing.
Dave

Thaidavid

 

[delagina]

Roof is just an R panel Ga 26. No bracing.

 

[SteelPE]

I don't know exactly what you are trying to achieve with the sag rods. In instances where I have constructed similar roofs using 8" purlins I would typically use bridging at the 1/3 points. The bridging would be a 6" 18ga metal stud screwed with a clip to the purlin. This detail was given to me by a engineer who used to work for a metal building company.

 

[delagina]

I'm not putting sag rods if not necessary. The picture was from a goby. I'm getting advice here, aside from researching of course, what's the proper (sag rod, bridging or nothing) to use for monoslope roof.

 

[KootK]

I was going to suggest the same thing as SteelPE but with hot rolled C6. Cold formed may well be cheaper if there's already other CFM on the project.

Unless you're willing to utilize each channel in weak axis bending, I feel that you still need some kind of in plane roof diaphragm. Probably discrete bracing by the sound of it. I need to know more about the building in order to recommend an appropriate scheme. Plan dimensions, supporting framing scheme... stuff of that sort.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.