Forming Channels from Thick Plates. Ducitility concerns?

[Jerehmy]

Hello,

We do a lot of work for this manufacturer. They have in house engineers but they aren't PE's in some states, which is where our firm comes in. We will stamp drawings/calcs for states they aren't PE's in.

Problem: They are taking 3/8" and 5/16" plate and bending them into channels. They word it like this "(C10) Designation represents a SS 3/8" formed plate to match C10x15.3 Channel."

The steel is SS type 304L.

Two questions:
1) Does this constitute a cold formed section now? Should I design with AISI?
2) More importantly, does this type of steel even allow cold forming? Are you allowed to form sections this thick? It seems to me you will get significant yielding at the outside corners since the sections are much much thicker than cold formed sections. Aren't there ductility requirements for cold formed sections? Are my concerns valid?

They've maybe done this once before or twice before but for some reason I didn't take issue with it. I'm going to talk to my boss about it in a bit but I was also like to hear what the community has to say about this.

Thanks in advance!

 

[Jerehmy]

I'm sorry, the material is ASTM A240 316L, not 304L

 

[EngineeringEric]

I'd check the bending guidelines from AISC but i think you are not going to do it. And the shape of a C10x15.3 is not from a single width (ie the Tw =/= tf; .24"=/=.43") So i guess they want the outside shape dimensions, with rolled corners? I bet one can get C10x15.3 in SS or a comparable shape.

AISC V13, table 10-12 states that the minimum interior radi for up to 3/4" is 1.5T... or 1.75 for A514... no 316 or 304.

 

[Jerehmy]

http://www.aisc.org/store/p-1145-de...on-guidelines-for-cold-bending-of-plates.aspx

going to download this.

Do you consider it a hot rolled section or a cold formed section?

 

[dhengr]

Jerehmy:
I haven’t worked much with forming SS, but done plenty with carbon stl., so just my 2 cents worth. How are they forming these channels, roll forming, more likely break forming, break press, how long? Can they apply any heat before/during forming? Forming to what inside radius vs. thickness? How do they treat the edge of the plates before forming, round them a little? Have you seem samples of this work and found any cracking? Make them do some testing on tighter radius bends and to more than 90̊; see when you start causing cracking. I’ll have to take a look at the ASTM Spec. and so should you. I assume the rolling grain in the original strip or pl. is in the same direction as the length of the channel, problematic? Small lengths would be better formed with the original rolling grain running across the bend lines. How are these formed sections being used? If the major stresses (bending, normal stresses) are parallel to the length of the member usually forming is not much of a problem. If you have significant stresses which might cross a forming crack that’s another matter.

 

[Jerehmy]

They're being used as beams and the bend radius is ~ 1/2". I have the ASTM book next to me, and from my research the steel type doesn't seem to be apart of the approved materials for cold work.

That being said it is extremely ductile with a Fy = 25ksi , Fu = 70ksi and 40% elongation (10% is minimum according to AISI and Fu/Fy >= 1.08). So it doesn't seem like ductility is an issue.

I think I am going to have to call them tomorrow and see how their engineers handle this because it is ballooning into a larger issue and taking up hours on a small job. But, I'd like to know how to handle this in the future.

All those forming questions I could not answer at this time but their shop is literally 3 miles from where I work so I could go look at it if I had to .

Thanks for the response! From what I gathered the beam isn't going to crack or fail but I want to do this the right way regardless.

 

[JStephen]

We've run into situations in the past where it was cheaper to buy flat bar and have it bent into angles than to buy the stainless steel angles. I doubt the channels that big are readily available in stainless. One alternative would be built-up sections, which presents its own issues.

Note that the bending and torsional properties would be different from a rolled channel, so they're not automatically interchangeable. In particular, some of the torsional properties are related to the largest-inscribed circle, which will be considerably smaller with the formed section since it doesn't have the tapered flange and fillet radius.

 

[Jerehmy]

Yeah I have a spreadsheet that calculates every geometric property you would ever need for cold form steel sections so I use it for these sections. I forget why I even made it because you can get all the properties from a table but I'm glad I have it now haha.

 

[SAIL3]

my understanding is that certain grades of SS square and rectangular Tubing is made out of bent-up pl and then seam welded...the OP's concern about the residual stress/ductility of the corner area is something I encountered in a project I designed a few years ago. After erection of the structure a crack was discovered in one of the SS colums(304SS). It was about 16" long and right along the corner radius and relatively straight as if following a peak stress. I never did find out what was the real cause but, if I remember correctly there may have been a certain loading that produced a stress that in combination with any residual stress in the corner would result in a max local stress resulting in a straight-line crack as shown in the photo attached. To me the the most important feature was how staight the crack was which indicated to me that a certain type of condition existed for the length of that area.

 

[TLHS]

What was the repair on that? Drill out the tips of the cracks at each end and weld the seam back together?

 

[JAE]

I may be mistaken but AISC 360 doesn't deal with stainless steel. The AISI spec refers to "other" steels via their Appendix A but don't specifically list ASTM A240...so not sure that code would apply either.

 

[SAIL3]

yes, drilled holes @ each end of crack....then welded a bent-up pl to the corner of the col....

 

[Jerehmy]

JAE

Yeah one of the requirements for appendix A is for it be part of ASTM A6, A500, A847, A924, or A568. I checked the ASTM book and it isn't. But the ASTM to be honest is kind of confusing because a steel can conform to more than one, especially when they have "referenced sections". But even after going through about half of them it doesn't appear as though its part of any of those sections.

I'm going to design it with AISI since it seems like the more applicable code. The fact that its very ductile quells some of my concerns of the residual stress. The AISC article I bought for 10$ didn't help at all haha. Oh well. I'm going to have to phone the fabricator and maybe I can go up and look at a couple pieces and maybe do some testing!

Thoughts?

 

[JAE]

Makes sense to use AISI as you have to use something.
I think the visual observation would be good on some fabricated pieces to verify no cracking.

I wonder if you could also perform a dye test on a small percent (10%?) to check for micro cracking at the bends.

 

[topsisu]

You might want to look at AISC Design Guide 27 "Structural Stainless Steel" that was published in 2013

 

[dhengr]

Jerehmy:
I would tend toward using the basics of the AISC Manual, probably using ASD for simplicity. The difference btwn. those and AISI, are that AISI pays particular attention to the thin plate buckling problem which exists on almost every element of all of their cross sections in every form of usage. That design spec. is driven by element buckling and cross section buckling which then leads to allowable stresses. Your material thickness and cross section fits the normal ranges covered by AISC. The fact that your particular ASTM Spec. is not explicitly listed does not change the basic Engineering Mechanics and Strength of Materials (Theory of Elasticity) which are the foundation of all our work. And now, it seems, even that has been rectified with yet another profitable Design Guide, without which we can’t be Structural Engineers any longer. The fact that all of the newer codes are so garbaged up with so many multipliers, divisors and other modifiers (load factors, resistance factors, reliability indexes, standard deviations dependant upon the day of the week and the moon phase, etc.) makes it almost impossible to find the basic governing principles of Engineering Mechanicals for all the other stuff, but they do still govern whatever we do, whether resistance factor is .91 or .89.

I would change the rolling grain orientation when I could. I would like to inspect shapes that they have already made for any cracking and for final material thickness in the bend area. I would have them do some sample bend testing to try to hone in on material thickness vs. bend radius and angle of bend vs. onset of cracking. JAE’s idea of some dye pen. testing is a good one, that kinda went without saying as part of my testing comments in my first post. I certainly would get a copy of that new Design Guide 27 on Stainless Steel. Without a doubt it will have much useful info. in it.

 

[SAIL3]

I agree with dhengr entirely....I use AISC ASD for design of SS....with the material thickeness described in the OP's case it is nowhere near the problems associated with AISI....his mention of the rolling grain orientation is important for corner radiuses, though. The stress/strain curve for SS is not as well defined as for carbon steel near the yield point but I have ignored that in the past. If one wants to go crazy, I believe that Europe has a SS Design Manual.

 

[SAIL3]

in all fairness, please strike that last comment on the European SS Design Manual as I have not read thru it.

 

[chicopee]

Jerehmy, one thing that can be done to strengthen the sides of the channel is to weld stiffners inside the channels where connections are to be made. We had to do this on many of our structural steel beams and lifting beams.

 

[Jerehmy]

The reason I said to use the AISI instead of the AISC was for two reasons:

1) When I modeled it in RAM, I had to make a custom section. The custom section takes the form of a C section without lips. Ram recognizes this and applies the AISI code to it through design and I couldn't find ANYWHERE to let me change which code it applies to the section. This is what got me thinking about all this in the first place. I was going to check it with my AISC beam Mathcad sheet just to be sure. RAM was mostly for analysis not design.

2) The AISI code is for materials up to 1" thickness (according to Appendix A). The fact that they're bending the plats when they're cold obviously makes them cold formed. The AISI code seems to be a more in depth analysis but for the larger thickness I obviously don't have to worry about things like distortional buckling.

I checked out that AISC Design Guide, $60 for non-member. I'll have to see if my boss will let me get it. He's been doing this for years and years without a hiccup but I'm just curious what the design guide even says. I have a feeling it might not say anything that we haven't already discussed here, but who knows.


If I do get the design guide, I'll be sure to say whether it had anything useful in it or not!