Moment magnification factor slender columns ACI 318-19

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1

I would like to report a possible error in RF-CONCRETE Columns in RFEM 5 when designing a slender reinforced concrete column according to ACI 318-19.
I prepared a simple verification model based on the example presented in the Dlubal Knowledge Base article:

Stability of Reinforced Concrete Structures: RFEM vs ETABS” – KB 002069

The model parameters are:

  • rectangular RC column: 1000 × 1000 mm
  • column length: 8.50 m
  • concrete: f'c = 40 MPa
  • reinforcement: approximately 1%
  • axial force: Pu = 10,000 kN
  • bending moment: Mz = 3,000 kNm
  • nonsway column
  • effective length factor in the governing direction: kz = 2.0
  • effective length: kz·L = 17.0 m
  • βdns = 0
  • Cm,z = 1.0

RF-CONCRETE Columns correctly recognizes the column as slender in the z-direction:

λz = 58.89 > λlim,z = 22.0

and calculates:

Pc,z = 46,197.1 kN

However, the program then gives:

δz = 1.0000

and therefore:

Mc,z = 3,000 kNm

i.e. no moment magnification is applied.

Additional inconsistency in the detailed output
There is also a second inconsistency in the detailed calculation output.
For the governing z-direction, RF-CONCRETE reports:
M_(2,z)=3,000 kNm
M_(2,min⁡,z)=452.4 kNm
so clearly:
M_(2,z)>M_(2,min⁡,z)

However, in the detailed output RF-CONCRETE first states:
“Was the minimum moment applied in this direction? – Yes”

while a few lines later it asks:
“Is the design moment M_(2,z)smaller than the minimum required moment according to 6.6.4.5.4?”
and answers:
“No”.

These two statements appear contradictory.
Since:
3000 kNm>452.4kNm

the minimum moment should not govern the design in this direction.

2

Hello @andre81 and welcome to the Dlubal Community :waving_hand:,

Thank you for the detailed description and the example.

The Knowledge Base article you mentioned is for RFEM 6. However, feel free to send me your RFEM 5 model, and I’ll take a closer look at the calculation and the issues you described.

4

Column_Test_ACI_v01.rf5 (1.9 MB)
Hello @hedi.boukraa
Model attached

5

Hello @andre81,

Thanks for the model. You are right on both points.

I reproduced it. With C_m,z = 1.0 and P_c,z = 53,640 kN, the magnification factor should be:
δ_z = 1.0 / (1 − 10,000 / (0.75 · 53,640)) = 1.33
so M_c,z ≈ 3,990 kNm instead of 3,000 kNm.
With C_m,z = 1.0 there is no way for δ_z to come out as 1.0.

The two contradictory lines about the minimum moment in the detailed output are connected to it, the minimum moment should not govern in the z-direction here, as you say.

The good news: this does not happen in RFEM 6. I rebuilt your column in the Concrete Design add-on with the same inputs and it gives δ_z = 1.35 and a design ratio of 1.10, i.e. the column does not pass. So if you have access to RFEM 6, I'd use it for slender column design to ACI in the meantime.

image
image1890×942 400 KB

I'm passing this to our development team. But because development focus is on RFEM 6, so I can't give you a date for a fix in RFEM 5.

Thanks again for the detailed report and the verification model :smiley:.

6

Thanks for the quick check. Unfortunately, I don't have RFEM 6 and already finalized reports with few thousands of columns...

We've noticed some strange results for columns calculated using EC. We have two columns in a nearly symmetrical segment with almost identical force values. The EC design results in significant differences between them. I haven't analyzed the details yet.