Member stability check as per Canadian Steel code design CSA S16:2019

[TTTKAO]

Hello All,

I am designing a building with Canadian steel design code CSA S16:2019. This building's analysis is following Simplified stability analysis method as per clause 8.4.3 by use of RISA 3D. For all the members designed by the software, I am wondering if i still need do member stability check as per chapter 9, my feeling is i should do stability check as well. Let's take building vertical brace for example, i will get a design force from software as per external force, and as per chapter 9, there is load requirement and displacement requirement due to building column stability which is supported by vertical brace. so i should consider the sum of these two forces together to design the vertical brace? May I ask who is familiar with CSA S16 code can help to verify my understanding ?

Thank you!

 

[TLHS]

They're not written all that clearly, but section 8 is mostly about stability of the overall structure and section 9 is mostly about stability of individual members and elements.

I'd heavily suggest reading the commentary in the handbook for both of those sections.

 

[TTTKAO]

They're not written all that clearly, but section 8 is mostly about stability of the overall structure and section 9 is mostly about stability of individual members and elements.

I'd heavily suggest reading the commentary in the handbook for both of those sections.

Thank you for the information. i did check the commentary, i should include this member stability load as per my understanding of the code, but i asked some engineers, the answer is they didn't consider this load when using Canadian code before....

 

[jayrod12]

I believe the notional loads indicated for stability are only required to be considered if there is no other lateral load on the structure. Essentially they're ensuring that the structure has some nominal lateral capacity, even the the imposed design loading doesn't actually directly apply lateral loads.

Therefore, if you have designed the structure for other lateral load considerations, i.e. seismic or wind, you can often omit including notional loading.

 

[canwesteng]

No, notional loads are included in wind and seismic cases as well. They account for the frame being out of true and also a fudge factor for residual stresses in the members.

 

[jayrod12]

I can tell you that I don't believe this is considered by most designers. At least not in my locale. In fact, when I do consider notional loading on structures, I'm looked at like I'm crazy.

But in all fairness, we are generally fairly conservative in our lateral load magnitudes and therefore the 0.005 times the gravity loads is eaten up in the conservative lateral load determination. If we sharpened our pencils on the lateral loads, we can likely reduce it by the same amount as the notional loading.

Now, with the OP's question being about clause 9, if they're using RISA to do the design, it takes into account second-order analysis directly. At least that's what I understand.

 

[TTTKAO]

Hello All, thank you for all the discussion above, the notional load is required as per structure stability analysis not required by member stability. for Canadian code, you have to consider notional load if you want to avoid direct analysis. For US code, if your structure is rigid enough, notional load is only required for gravity load combinations. under this situation, if your wind load or seismic load is big enough, you can argue notional load won't control, and ignore notional load. Second order analysis is another factor need to be considered for structure stability.

Back to my original question. member stability is caused by member misalignment and this factor is not covered under structure stability factors. if a member is contributing system stability and member stability at the same time. wondering if we need sum both of the force for design ?

 

[canwesteng]

I'm pretty sure you're incorrect on all counts. There is no direct analysis in S16, and notional loads are required in all cases. AISC 360 requires notional loads in all load cases, and act in the same direction as the lateral loads.

 

[canwesteng]

To answer the original question though, you should be adding the brace force from chapter 9 to the forces that come from analysis, if the brace is restraining the column from buckling.

 

[jayrod12]

I'm pretty sure you're incorrect on all counts. There is no direct analysis in S16, and notional loads are required in all cases. AISC 360 requires notional loads in all load cases, and act in the same direction as the lateral loads.

Then what is 9.6.2 indicating if there's no direct method?

 

[TTTKAO]

I'm pretty sure you're incorrect on all counts. There is no direct analysis in S16, and notional loads are required in all cases. AISC 360 requires notional loads in all load cases, and act in the same direction as the lateral loads.

Check AISC360-18 C1.1 for Direct analysis method for design. check CSA S16:19 annex o for direct analysis method. you may argue that this doesn't exact word, for my understanding, this advanced analysis method is the same thing or idea as the direct analysis method in AISC360

 

[TTTKAO]

To answer the original question though, you should be adding the brace force from chapter 9 to the forces that come from analysis, if the brace is restraining the column from buckling.

thank you, i have the same understanding or feeling. but i can't find direct clause to support this. wondering if you notice any this kinds of word in any design guide or commentary?

Thanks

 

[canwesteng]

Then what is 9.6.2 indicating if there's no direct method?

Direct method not direct analysis, it's kind of going the opposite direction to direct analysis. Direct method is just a method to calculate the brace force, direct analysis is meant to analyse the entire structure. Annex O is like direct analysis, but I don't think it's meant to be used as the default analysis, only in special situations.

I don't have any backup for considering both global forces and local stability forces in the bracing. I'm not sure how to reason that they won't happen together though.