End Moment Magnification in SP Column

[mcc202]

It is my understanding of ACI that when analyzing concrete columns subject to sway that the end moments obtained from a second order analysis may be utilized in lieu of magnifying the end moments, but we would still be obligated to check moment magnification along the member length. Is there a way to turn off end moment magnification in SP Column, but leave on the member length moment magnification? We are analyzing frames in RAM using a P-Delta analysis, but completing the column design in SP Column.

Thanks!

 

[GC_Hopi]

I am not an expert user of SP column and am unable to open it at the moment. But does the code point to you back to the nonsway method for member length moment magnification so you could run SP Column using non sway to the get that information. Then there is probably some hand calcs you would need to wrap everything up ie combine your results to get the final design loads and final column design.

 

[mcc202]

Yes...in a round about way. ACI references 6.6.4.5 for the calculation of member length slenderness effects, which is the Non-Sway Moment Magnification Method. I suppose I could then run SP Column as Non-Sway with the 2nd order moments input from our Ram 2nd Order Analysis. That's what I was leaning towards doing, I just needed a sanity check.

Well, as soon as you check Non-Sway in SP Column, you loose your K factor. So I may be doing this by hand.

Thanks for the reply.

 

[Celt83]

Well, as soon as you check Non-Sway in SP Column, you loose your K factor. So I may be doing this by hand.

K maxes out as 1 for the non-sway magnification procedure.

If your not applying any load to the face of the column then your RAM 2nd order end moments are likely going to be your M1, M2 for the non-sway magnification procedure which will then capture P-delta (little D) and the along the length increases. Since your amplifying the 2nd order moments you reduce the max K value to 1 instead of the infinite cap on the sway K.

ACI really needs to work out a better way to handle all of this because from various testing there isn't a single piece of software out there that produces fully code compliant column designs per ACI, everything requires manually intervention.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[Celt83]

Bentley's community page on the topic

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[Settingsun]

KootK runs through the ACI process chapter and verse in this discussion:

https://www.eng-tips.com/viewthread.cfm?qid=457880

Further down, I quote Richard Furlong in 1981 saying that ACI procedures probably aren't being applied correctly so don't hold your breath waiting for code simplification/clarification. Although not sure why software doesn't handle code design properly when P-DELTA analysis is done as it seems fairly straightforward.

 

[mcc202]

Thanks everyone. Does anyone have a copy of Richard Furlong's paper? I have now seen that document referenced in several threads...seems worth a read.

 

[IDS]

The paper abstract is available here:

https://www.concrete.org/publications/internationalconcreteabstractsportal.aspx?m=details&ID=9593

The full paper is a "Member Benefit", so freely available to ACI members.


Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Settingsun]

If you can't get the Furlong paper, you could read/watch the AISC's lecture on the direct analysis method. It goes over much the same ground as the Furlong paper but in the context of steel design. The main difference is in the stiffness reductions (material specific), otherwise the P-Delta analysis requirements to use K=1.0 (and the rationale for K=1.0) are common. The AISC lecture also covers the effective length method and its shortcomings, which Furlong didn't cover because concrete design didn't use it.

Video:

https://youtu.be/iOH6kaXX3Mk

PowerPoint:

https://www.aisc.org/globalassets/c...-approach-to-design-for-stability-handout.pdf