Eliminating Sag Rods - Design of Cold Formed Girt Systems - Redux 3

[Can_Est]

@Steel PE - "The AISI has a very specific set of requirements for using the reduction factors to determine the LTB capacity of the gifts. One of the big requirements is attachment of the panel to the girt every 12 inches o.c. The other is the rib spacing on the panels. IMP system have attachments ever 30” o.c. And the backside of the panel is not corrugated. So it’s a no go in accordance with the AISI."

Question - Would the inherent stiffness of the IMP (Face - insulation - face) be rigid enough to satisfy the thickness gauge and rib depth and spacing prescribed in the AISI.

Considering the Client is willing swap the cost of sag rods for the extra labour and material for 12" OC for the IMP attachment.

 

[SteelPE]

Completely random that I am actually looking at this site today as I haven't been on this site in some time.... also please forgive me if I don't really answer your question.

It's been a while since I have looked into this issue.... and I am not sure where this quote from me came from. What I am referencing in the above statement has to do with the simplified R factors used for design of lt guage girt systems. From what I recall, these R factors were not created based upon theoretical analysis but rather based upon actual full scale real world testing. These R factors allow you to calculate the capacity of the lt guage girt as long as you fall within the parameters of the test (panel type, fastener locations, girt depth etc).

The issue I have with insulated metal panel is that no one wants to use "back fasteners" that is, they only want to use the side joint fasteners to attach the insulated metal panels. Even if I could convince the client to use the back fasteners, there is no way I could get them to attach the girt to the backside of the panel @ 12" o.c.... and even if I could, I am not sure they system would fall within the parameters of the "real world tests". So I am not sure I would use the simplified method when calculating girt capacity when I am using insulated metal panel.

What I do now (and I just copied from someone else) is use torsional straps on the inside face of the panel that wraps around the inside face of the girt. I space these at regular intervals (1/3 points or 1/4 points) and then calculate the capacity of the girt system using the unbraced length charts for lt girts using the spacing between these torsional braces.

I don't typically use sag rods with these type of girt systems because the members are extremely light (members weigh about 5.5 plf) and can be adjusted by hand prior to attaching the clip to the panel.

 

[SandwichEngine]

I agree with SteelPE that the R values and other information from AISI is entirely empirical. They've only conducted testing with certain parameters so you can't use their codes to verify things outside that.

You can however, use engineering judgement. For a long time, AISI didn't have any testing or information on girts greater than 10" but you would still use 12" if you felt comfortable that the same reasoning would apply. Now they've added 12" members and confirmed that those original equations do apply.

I would be comfortable using the same logic to say that the outside fasteners of the IMPs at 30" on center would be enough to consider the outside girt flange fully braced. I'd also feel comfortable that the IMPs, which are much stiffer than a normal steel panel, are strong enough to serve the purpose. I'd think of the full depth of the panel as the depth of the corrugation. The issue is that part of that testing is using through fastened panel as a torsional brace to also brace the inside flange. I wouldn't feel comfortable assuming that the IMP connections at 30" on center are doing that as well. The pullout of the screw at 30" compared to 12" would be 2.5 times higher and well outside tested limits.

Which brings us to what I do, the same thing that SteelPE does. Use straps to brace the inside flange at intervals and design for those unbraced lengths.

 

[Can_Est]

- back fasters and 12" girts was one of the solutions we were looking at

Appreciate the time taken to respond with informative and considered posts.

 

[SteelPE]

All of the clients I deal with absolutely refuse to use back fasteners. They despise them. So we avoid them at all costs..... which really isn't much, as the attachment of the insulated metal panel to the girt system is not the fuse in the design around these parts.... it's the design of the girts themselves. So we typically end up with girts at roughly 5'-6' o.c. vertically.

That being said, the client's don't like putting the torsional straps in either... but I can get the contractor to install them.

 

[dik]

Does anyone have a good paper on the LTB of C or Z girts? I've not been able to find anything on the AISC site. I've been able to find a copy of the AISI, 2016 edition.


So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[SandwichEngine]

Dik,

You won't get anything from AISC for light gauge material. It's all AISI. LTB rarely controls cold formed members. To check them, you have to calculate a bunch of distortional buckling limit states, something like 20 or 25, and use whichever one controls. AISI has design guides that show how to do that. Worth doing once by hand to get the idea then buy a program. Worked an example years ago and it took forever. Probably more than a day to check one section. Then if that doesn't work, you have to start over. Brutal when you can buy CFS from RSG software for #120/year.

 

[dik]

I've used CUFSM program in past... What are back fasteners?

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[SteelPE]

My clients call them "back fasteners" Metl-Span calls them Blind Rivets. My clients like the FP1 attachment schedule.

I typically use the unbraced length bending charts provided by AISI. Unfortunately I have an older copy (2007). I have proven the chart capacities with the CFS program (that I use the demo version of once every few years). It's probably not the right way, but from what I can tell, it seems to work.