Stiffener Design as per AISC vs CSA

[EngDM]

Here is my situation. I am designing stiffener plates for a column bearing on a beam. CSA gives one equation, and AISC gives another. They take different approaches for determining what the bearing length N (lb in AISC) is for web crippling calculations. CSA takes presumably takes N as much larger (they only provide one example using this equation, and it is connecting to concrete), allowing you to use the entire bearing width of either the cap plate or the concrete wall you are bearing on (or use the baseplate from the column if you are loading from above). AISC takes N as t+5*tp in accordance with DG-24.

For many calcualtions where CSA does not strictly have instructions, they themselves defer to AISC 360.

My question, if you could call it that, is if CSA typically defers to AISC, why do these equations/approaches not follow suit? The CSA value for web crippling seems MUCH more unconservative in comparison to AISC. Without the forethought to consult AISC, a designer trying to comply with CSA might not think twice on this calculation.

I'm curious what the other Canadians on here do in cases like these. A lot of the times, if calculations are provided in AISC, the CSA version is quite close (for example welds, we have different phi factors but it works out to the roughly same once you actually perform the calculation). Do you use the more conservative method, justified by the use of alternate codes sentences in the NBCC?

 

[skeletron]

Question 1:

...if CSA typically defers to AISC, why do these equations/approaches not follow suit?

Yep. This is a common question for fabricators and connection designers. If I understand correctly CISC's research aims to satisfy Limit States Design and also Canadian fabrication practices. They also do their own testing or at the very least their own review of the existing material within a Canadian design context. AISC is generally a more established and funded organization, with a heavier flow of input from bigger budget research schools. The CISC committee works closely with AISC but doesn't necessarily adopt all provisions without their own review. My understanding of committees is that they agree on the "best solution" based on their evidence, but that committee agreement isn't necessarily the de facto solution...it's just the one they agreed upon at that time, based on their information, and someone else could come along in 5 years and do 31 full-scale tests that prove otherwise.

Question 2:

I'm curious what the other Canadians on here do in cases like these...Do you use the more conservative method, justified by the use of alternate codes sentences in the NBCC?

The compliant way to design these cases would be to follow the Canadian steel (CSA, CISC, NBCC) approach first. If there is ambiguity or gaps in the approach, then borrowing from another established system (AISC) would be appropriate. However, you need to be really cautious about "mixing and matching" codes to cherry pick the most advantageous answer or if the value (...say a capacity) was used as an input later on. I think the best practice would be to note it clearly in your design notes and add a descriptive rationale to the calculation.

This comes up a lot in connection design where AISC has a buffet of examples and solutions and CISC has less material available to guide a designer through the design. I think it makes sense to fill in the gaps with AISC in connection design, but you need to somehow resolve it to a Canadian context.

For example, up until recently CISC didn't have a guide to design a "Canadian Moment Connection" and they still don't really have a "Vertical Brace Connection" guide. The designer was/is left to piecing together clauses from S16 together with the general framework provided by DG-4 and DG-16 (R.I.P. now DG-39). Although the temptation would have been to turn to software (VAConnect, RAM Connections, etc.), my experience was that the calculation submittals would get kicked back from the EOR (not a steel connection designer) because it didn't meet the Canadian design specifications (i.e. NBCC requirements). So, while the connection probably physically works regardless of which side of the 49th they were on, the person wetting the seal surely does not want to risk being in such obvious non-compliance with the law (i.e. the building code). In many cases we would develop descriptive spreadsheets that presented a "Canadian" translation of the methodology and a rationale for some of the decisions where gaps existed. File under: engineering judgement.

I think you can get away with this in connection design because "...connection design is the last bastion of rationale design..." (William Thornton). But I believe it is much harder for macro-scope design or concepts that are explicitly called out in S16. By the way, your question referenced DG-24. So I'm confused whether you are running into an HSS connection design issue or whether Clause 14.3.2 (web crippling and bearing) applies?

 

[human909]

As an aside...

In the case of stiffener plates, in most cases I wouldn't have thought the extra effort in calculation time is worth the worry about minor code differences. Just design it XX thick and move on.

(I suppose if theses are 1m deep beams and the connection occurs several dozen times then it is worth the extra calculation time. But if it is more regular beams of less section depth I wouldn't worry.)

 

[EngDM]

By the way, your question referenced DG-24. So I'm confused whether you are running into an HSS connection design issue or whether Clause 14.3.2 (web crippling and bearing) applies?

Thank you for the detailed response above. To address your last question, DG24 has an example for a steel beam bearing on an HSS, and it goes through all of the limit states for both the HSS and the steel beam. This example is referenced by RISA Connection.

I do notice that the CSA and AISC web crippling equations are quite different, so where CSA allows for a higher bearing length than AISC, their rationale may be more conservative in other assumptions in the equation.

I guess at the end of the day I want to make sure not only that we are code compliant, but that I have a rational understanding of how the equation is meant to be interpretted and the limitations of the equation. With how vague this particular section is, and the lack of examples, it made sense to defer to AISC since they actually check it for bearing on another steel member, and not just concrete. For instance, there doesn't appear to be any limitations on the length of N as per CSA, but I'd have to imagine after a certain length of N, especially when bearing on a column cap plate connection, that the plate just begins to yield and not provide any additional bearing capacity.