Section class for bi-axial bending in Canada 2

[CDLD]

Good day!

Not sure if this is a question or a complaint... but the web section classes in S16-14 seem to be inconsistent and jumpy in regards to bi-axial bending.

If you have strong-axis bending with no axial force, you need to meet h/w <= 1700 / Fy^0.5 for class 2.
If you have weak-axis bending with no axial force, you need to meet h/w <= 1700 / Fy^0.5 for class 2.
If you have bi-axial bending (majority weak-axis stress) with no axial force, you need to meet h/w <= 525 / Fy^0.5 for class 2??

Does this seem reasonable? I am getting a lot of class 3 sections for bi-axial bending.

To me it seems that a class 2 web in bi-axial bending should have to meet 1700 / Fy^0.5, but there is no mention of this in the CISC code or commentary.

 

[CDLD]

Any insight on this?

 

[canwesteng]

Are you referring to the right code? This is in table 2, not 8.1, and makes no reference to biaxial bending.

 

[CDLD]

This table is from a guide published by CISC but it is the same table as Table 2 from the code
One of the last rows in the table refers to bi-axial bending

 

[canwesteng]

Oh yeah, gotcha. For weak axis bending and no axial force, you should be meeting 525 sqrt(Fy), not 1700. Likewise for bi axial bending. It is fairly punishing

 

[TLHS]

You should get a copy of the actual code table. The above looks like an interpretation of the code table to cover more scenarios, but it doesn't describe the base scenarios as clearly. Some of the things in the example table are discontinuous because they're taking the worst case from different scenarios in the actual code table. The Axial + Biaxial Bending scenario is just the worst case of the web values for compression+strong axis and compression+weak axis. So the first two terms are from one and the last term from the other.

 

[CDLD]

I have a copy of S16-14 and it is the same as the table above. I posted this table to avoid the trouble of scanning the hard copy.

The issue I have with the code is that for weak axis bending and no axial (ie. Cf < 0.4*0.9Cy), the class 2 limit for the web is 1700/Fy^0.5 which is the same limit as strong axis bending with no axial.

So when you have bi axial bending and no axial and Mfy/Sy > 0.9Mfx/Sx it seems unduly punishing that the class 2 web limit is 525/Fy^0.5.

 

[TLHS]

I stand corrected. There was an update I didn't incorporate because I thought it was just the addition of the storage rack parts of the code (Update 2422756_EN1 on the CSA system). It's better than where they'd left it prior to the update, though, which with a certain reading meant that all webs in weak axis bending needed to meet 525/fy^0.5 to be class 2 or better.

I agree with your read of what the code is saying. However, it may be an intentional choice rather than an oversight. Unfortunately they don't update the commentary with this sort of update. They may have used that limit because with biaxial bending the interaction becomes less clear and to some degree approximates weak axis + compression, and they may not have data for what the effects of a gradient rather than constant compression stress added to the weak axis effects may be.

They shouldn't have pushed an update like this out as a mid-code cycle change. Unfortunately they forced their own hands with the way they added webs in weak axis in the original 2014 table.

I'm going to do some research when I have time. There have been a few studies coming out of this issue it looks like.

 

[CDLD]

Fair points.
The AISC code does not seem to be as punishing. The majority of sections can attain plastic moment capacity in bi axial bending by their code.

Also, an interesting side note that I’ll follow up with when I have time is that STAAD pro does not seem to follow table 2 to the letter in their S16-14 code check. I compared one of the outputs with a spreadsheet I made and they had a class 2 section while I had class 3 based on the limits of the table above.

 

[KootK]

1) For what it's worth, back in 2008 I queried CISC as to whether or not I could use AISC provisions for Canadian designs. They said "have at 'er". Just be internally consistent and use AISC all the way rather than picking and choosing from the two codes.

2) To an extent, I do feel as though I can offer up a plausible explanation for the current form of the CISC class limits. It's all speculation though so don't treat anything that follows as any way definitive.

3) Why NOT punish the web for pure weak axis bending? Because you don't really need the web for anything in this situation. The web does not transmit primary shear between parts of the cross section that are working hard in flexure. At most, it's just kind of stabilizing the flanges rotationally.

4) Why punish the web for significant weak axis flexure in a biaxial situation? Two reasons:

a) Because of the presence of the strong axis flexure, you now do need the web for primary shear transfer between the flanges.

b) With significant weak axis flexure nearing the plastic limit, your strain profile is such that your web is very close to being heavily compressed uniformly as a result of flexure rather than the more beneficial stress gradient business. So close that it's probably prudent to assume that it is uniformly compressed.

 

[CDLD]

Kootk, you're explanation seems reasonable to me, although I still believe either CISC is being overly conservative or AISC is being under-conservative.

You might find this explanation useful:

Link:

https://www.cisc-icca.ca/wp-content/uploads/2018/08/Rev_LSDSS10E2P.pdf

 

[dik]

I never thought to ask... good point. I often use AISC stuff and occasionally Eurocode stuff, if I cannot find S16 material.

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

-Dik

 

[KootK]

You might find this explanation useful:

Indeed I do, thanks for sharing that. I took a quick look at that just now. We're in agreement that kind of supports my previous hypothesis, correct?

..although I still believe either CISC is being overly conservative or AISC is being under-conservative.

Yeah, it's hard to know what to do with the discrepancy. So I just go with the more liberal version when it suits me.

This will sound unpatriotic of me given that I'm a Canadian practicing in Canada but, in many respects I prefer the US codes (my first decade practicing was in the US).

You know how our radio stations have 30% Canadian content rules? So instead of getting the best music you get some of the best music and a lot of Gordon Lightfoot?

I feel that there's an unofficial version of this that happens in our code development process. Where there's alternate, Canadian research we use that. So our codes wind up being 90% the same as the US codes but with some, occasionally poorly coordinated, Gordon Lightfoot thrown in where where Gordon has things to say about the buckling of uniformly compressed plates.

 

[canwesteng]

Often that canadian research ends up getting copied over to AISC, like weld directionality, but generally we are playing catch up. I think CSA is generally doing a very good job - but the problem is AISC is next door and they are unquestionably the most advanced codes for steel design there is, so it's tempting to just use their spec.