Plastic Section Modulus for Cold Formed Steel Sections
Plastic Section Modulus for Cold Formed Steel Sections
(OP)
How can we calculate the plastic section modulus for cold formed steel sections? I am the light gage speciality engineer. The EOR has asked me to design connections based on plastic section modulus for blast resistance design. I would appreciate your response.






RE: Plastic Section Modulus for Cold Formed Steel Sections
DaveAtkins
RE: Plastic Section Modulus for Cold Formed Steel Sections
RE: Plastic Section Modulus for Cold Formed Steel Sections
I very much doubt that these would even reach a plastic moment as the local plate buckling would occcur first.
Have you looked at the option of designing them as a catenary tension member. This would induce higher loads on the structure though!
There have been a lot of papers on this type of thing recently, I would suggest you google it.
RE: Plastic Section Modulus for Cold Formed Steel Sections
I agree with you all. I would not recommend to design light gage members to reach plastic moment. However, my question is if I have to do calculation plastic section modulus for light gage section, is it possible to calculate? If so, is there any reference book or section of AISI I can look into? I already googled it, and could not find any.
RE: Plastic Section Modulus for Cold Formed Steel Sections
Whether or not is means anything in reality is the question, also noted above. If you look in the NASPEC, you'll see there is no provision for CFS members reaching Mp, because most of them can't. So you couldn't really design to Mp even if you wanted to, again for most cases. Perhaps the EOR is intending for you to use the inelastic reserve capacity? I would put together some notes and give the person a call and try to clarify.
Are you using the UFC 4-010? The last edition permitted you to use the full yield stress for steel calculations, but didn't require connections to meet Mp. Basically you just remove all the safety factors or resistance factors. I don't know if this has changed in the latest version of the UFC spec.
RE: Plastic Section Modulus for Cold Formed Steel Sections
I'm curious as to where they got the idea to design connections so that the members can reach Mp. Any idea? Like many folks have typed so far, these sections can't develop anywhere near Fy*Zx.
RE: Plastic Section Modulus for Cold Formed Steel Sections
My two cents: You should be designing for combined bending AND SHEAR to elastic section properties for your connection. Although I do not know the US code, the combined Australia & New Zealand AS/NZS 4600 CFS code requires a combined check to:
(M*/phiMs)^2 + (V*/phiVv)^2 <= 1.0 (but not always; You really need to check your specific code)
Bottom line: I do not believe you are checking against a realistic condition. Your connection (more specifically the member very near or at the connection point) is more likely to fail under combined action/loading well before the plastic moment.
Good luck,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Plastic Section Modulus for Cold Formed Steel Sections
RE: Plastic Section Modulus for Cold Formed Steel Sections
"Design frame connections to surrounding walls to resist a combined ultimate loading consisting of a tension force of 35-kN/m (200-lbs/in) and a shear force of 13-kN/m (75 lbs/in). Design supporting elements and their connections based on their ultimate capacities."
RE: Plastic Section Modulus for Cold Formed Steel Sections
RE: Plastic Section Modulus for Cold Formed Steel Sections
RE: Plastic Section Modulus for Cold Formed Steel Sections
where b is the total metal width at height y above the neutral axis.
I imagine that the neutral axis may not be same neutral axis as it is for elastic bending (check for net axial force to determine that)
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Plastic Section Modulus for Cold Formed Steel Sections
A decent mechanics of materials book should give you what you need.
Link
RE: Plastic Section Modulus for Cold Formed Steel Sections
RE: Plastic Section Modulus for Cold Formed Steel Sections
First find the PNA - this is done by finding the axis about which there is an equal amount of area above and below.
Now, find the centroid for the top portion and the bottom portion seperately (from simple statics). Call the distance from the PNA to the centroid of the top portion ybar1, and the distance from the PNA to the centroid of the bottom portion ybar2. Call the Area of each portion A.
the plastic section modulus, Z, will be A*(ybar1 + ybar2) in in^3.
This is like the 5th or 6th time I've typed this out. I think I'm going to copy this into a word document for next time.
RE: Plastic Section Modulus for Cold Formed Steel Sections
RE: Plastic Section Modulus for Cold Formed Steel Sections
If anything wrong or under designed, please input.......
RE: Plastic Section Modulus for Cold Formed Steel Sections
Back to the original post, I am surprised how often this comes up, and how often StructuralEIT has to type the procedure out. This is a basic concept included in any decent mechanics of materials book.
RE: Plastic Section Modulus for Cold Formed Steel Sections
RE: Plastic Section Modulus for Cold Formed Steel Sections
You are probably right. I actually learned ASD in school, just 3 years ago. I never learned how to calc Z, but I figured I should know how so I just figured it out.
RE: Plastic Section Modulus for Cold Formed Steel Sections
RE: Plastic Section Modulus for Cold Formed Steel Sections
RE: Plastic Section Modulus for Cold Formed Steel Sections
I also enjoy going through the older specs (8th edition and earlier) to see how the equations have changed over the years. How sad is that, that I "enjoy" such things!
RE: Plastic Section Modulus for Cold Formed Steel Sections
LOL, I hope your former professor isn't on your Christmas card list, due to his short-sightedness. About 1-1.5 years before the 13th Ed. Spec. was available, the steel prof. at our school was teaching from a draft copy so that his students wouldn't have to make the trivial transition from 3rd Ed. LRFD to 12th Ed. LRFD/ASD.
You seem to have come out fine, but it seems to be from work ethic and the wilingness to study.
RE: Plastic Section Modulus for Cold Formed Steel Sections
RE: Plastic Section Modulus for Cold Formed Steel Sections
RE: Plastic Section Modulus for Cold Formed Steel Sections