## Question about the interaction ratio for combined compression axial and moment formula

## Question about the interaction ratio for combined compression axial and moment formula

(OP)

Hi there

I'm using the following formula from the AISC for the combined axial compression + moment

(a) when Pr/Pc ≥ 0.2

Pr/Pc + 8/9 ( Mrx/ Mcx + Mry/ Mcy ) ≤ 1.0

(b) when Pr/Pc < 0.2

Pr/2Pc + ( Mrx/ Mcx + Mry/Mcy ) ≤ 1.0

I have all the values ready to just plug and compute. The only one I am confused about is " Mcx " which is the available flexural strength in the strong axis "x" according to the AISC. I was able to find Mcy (the available flexural strength in the weak axis) no problem by using table 3-4, as it clearly states that the selection is by Zy. I looked up my value under ΦMn of course since I am using LRFD, when I was using table 3-4. SO then I went on to look for my Mcx value and I thought I could just go to table 3-2 since it says "selection by Zx" but I see 2 LRFD values for flexural strength, one marked Mrx and one with Mpx and so I assume that I have to be looking for Mn to apply to my interaction ratio formula . Should I just use table 3-10? since table 3-10 has the Mn . Any comments ? thank you

I'm using the following formula from the AISC for the combined axial compression + moment

(a) when Pr/Pc ≥ 0.2

Pr/Pc + 8/9 ( Mrx/ Mcx + Mry/ Mcy ) ≤ 1.0

(b) when Pr/Pc < 0.2

Pr/2Pc + ( Mrx/ Mcx + Mry/Mcy ) ≤ 1.0

I have all the values ready to just plug and compute. The only one I am confused about is " Mcx " which is the available flexural strength in the strong axis "x" according to the AISC. I was able to find Mcy (the available flexural strength in the weak axis) no problem by using table 3-4, as it clearly states that the selection is by Zy. I looked up my value under ΦMn of course since I am using LRFD, when I was using table 3-4. SO then I went on to look for my Mcx value and I thought I could just go to table 3-2 since it says "selection by Zx" but I see 2 LRFD values for flexural strength, one marked Mrx and one with Mpx and so I assume that I have to be looking for Mn to apply to my interaction ratio formula . Should I just use table 3-10? since table 3-10 has the Mn . Any comments ? thank you

## RE: Question about the interaction ratio for combined compression axial and moment formula

The unbraced length determines whether your member will fail in yielding or via lateral torsional buckling (assuming the shape is compact).

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## RE: Question about the interaction ratio for combined compression axial and moment formula

-Mac

## RE: Question about the interaction ratio for combined compression axial and moment formula

## RE: Question about the interaction ratio for combined compression axial and moment formula

You can use Table 4-1 for Pc (based on the unbraced length in both axes).

You can use Table 3-10 for Mcx (based on the unbraced length in the x-axis).

[EDITED]Find Mcy from Table 3-4 (see Hokie's post below)Often with beam-columns you only have compression and strong axis bending so determining Mcy is typically unnecessary because you don't usually have Mry.

## RE: Question about the interaction ratio for combined compression axial and moment formula

## RE: Question about the interaction ratio for combined compression axial and moment formula

## RE: Question about the interaction ratio for combined compression axial and moment formula

## RE: Question about the interaction ratio for combined compression axial and moment formula