I am running a model in the Ram SS Concrete module and getting a few errors with the moment magnification factor exceeding 1.4. When I attempt to back-calculate the results the program is getting, I am not getting the same numbers. So my question here is two-fold:
1. Why aren't the results more transparent with its calculations of Cm, delta, beta, and Pc?
2. Why do my hand-calculated results not match the program results? Cm seems slightly off, and even if I use the program-calculated Cm to calculate delta, I don't get the same delta value. Is there a different equation being used than what ACI lists?
Can you confirm the following, or send your file?
I'm in Ram Manager CONNECT Edition 17.03.00.285 SES. Connect does not show any updates are available. The rest of the settings listed are already implemented. I uploaded a model too.
I also noticed that 0.65Pn is always listed as equal to Pu, which seems like a bug. (in my screenshot, 0.65 Pn = 820.36 and Pu =820.36. This is consistent even on lightly loaded columns with the same reinforcing and geometry)
And swinging back to my first comment - I really think more transparency on the calculation output would be a significant improvement to RAM. Other software like RISA has become much more engineer-friendly since making their calc output more clear.
What column was shown in your screen shot?
At grid 10/F
My results in 17.03.01.50 are a little different, but I think I can still answer.
First note that you can get a warning for "Magnification factor, delta, exceeds 1.4", but not see that in the report because the delta value we report is for the governing data point, i.e. the specific load combination and pattern that yields the highest Ld/Cap ratio. There is an enhancement request to update the report to include the slenderness calculations for the critical delta factor in addition to the critical data point, we just have to find a way to do this concisely. (Enhancement 877094: Slenderness section in concrete column report). See also: https://communities.bentley.com/products/ram-staad/f/ram-staad-forum/224441/column-magnification-factor-warning-with-non-slender-column
Second there is the issue of the minimum eccentric moment controlling in this case since the moments from the analysis are small. See: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/25803/concrete-column-minimum-eccentric-moment
As for the reported value of Pn, yes it always equals Pu (when Pu is below the plateau of the PMM curve), because we are calculating a Ld/Cap ratio of the (demand moment/capacity moment) at the given axial value and angle.
As for the delta calculation, I'm still working on my hand verification of this, my initial results are slightly off (like yours).
I see the problem with the Cm factor in this case. The program is incorrectly using the ratio of the major axis moments not the minor axis moments in the calculation of Cm associated with M1 and M2 minor. It's a reported defect already fixed for our next release (Re: Defect 813994: Cm factor and end moments calc for minor axis columns)
Answer Verified By: Brad Geyer
Thanks Seth. The Cm factor was my initial thought when I calculated the two moment ratios in my screenshot.
Regarding the 0.65Pn=Pu: That makes sense... but this all comes full circle about being more clear on the results output. I don't think you should have to read the manual to understand results output - plans examiners also need to decipher these pages without the manual. It seems "Dem/Cap" or even "demand/capacity" would be more clear than Ld/Cap because Ld is commonly used as a distance in column design. It would also be clearer if 0.65 used here was clarified as Φc, after a calculation showing that the maximum axial load is within compression-controlled strain limits.
Just my two cents