2nd order analysis-AISC 341 vs SEAOC Vol 3

[McEngr]

Shouldn't the AISC manual be checking all members for second order magnification effects? Seaoc is doing this and I believe this is an oversight by the authors of the manual. Can someone confirm?

 

[Amry69]

Only necesry if Ms/Mp is greater than 0.1.

Is than the difference?

 

[McEngr]

Interesting you say that Amry. The only time it is considered for SCBF's is in the moment magnification of the beams. It almost seems like they don't include it in the analysis because the moments are small in all the other members (ie concentric). In the SEAOC Vol 3, they calculate the column moments with amplified moments due to vertical and lateral load induced magnification beta factors. The SEAOC manual is more frustrating than the AISC Seismic Manual examples because they don't clearly state the problem. I believe they just force feed you to accept that the column has a moment and you need to deal with the P-Delta effects. "Don't question where, just understand the concept!"

 

[McEngr]

BTW, only chevron braced beams are considered. I believe the beams (that are in the middle of the interior braces) don't get heavy consideration either.

 

[McEngr]

I'm hoping that if I have to detail an SCBF connection, that they are ok with the uniform force method with concentric workpoints. I can't find an example problem that illustrates the offset (gusset to beam only or gusset to column only). There's an example of the 4 types of uniform force method calcs in the AISC example problems, but that's a ton of work to explore possibilities for an exam!

In addition, I think that if the offset were to occur, one would have to include moment magnification if the eccentricity of the welds becomes too great. Where that cutoff is, no one knows...

 

[HMitchell]

Shouldn't the AISC manual be checking all members for second order magnification effects? Seaoc is doing this and I believe this is an oversight by the authors of the manual. Can someone confirm?

McEngr- I'd like to help track down the reason for the difference you've found between design examples. Can you provide a reference to the SEAOC design example that is checking second order effects and the similar AISC example that is not?

Heath Mitchell

AISC Steel Solutions Center

 

[McEngr]

Heath,

Thank you for the response! I do not have my AISC 341 at work with me today (at home). So, I will respond this evening. Thanks!

 

[McEngr]

SEAOC assumes that because the SCBF beams have at least partial fixity to the columns that the second order effects of the columns need to be considered. In my opinion, second order effects will always need to be considered on any of the column lateral force resisting systems. It seems that it's an omission on the manuals part. See example 3.8 of AISC 341-05 Design Manual.

Another issue that SEAOC presents is the discussion of column strength and the maximum force delivered by the system on SCBF's. it assumes 1/2 brace force of one floor and the full brace force of another and compares it with the overstrength calculation. This is discussed in the early pages (I think page 25?). I digress for now until I hear from you, Heath. Thanks again!