P-delta Effect

[SPR Baker]

Dear All
I am not clear on upper limit of1.4 on the second to primary moment ratio.
M_2nd-order=< 1.4M_1st-order. the question is which moment ( M_2nd-order (Pdelta(big))(Sideway only) or M_2nd-order (P delta(big)+delta(small))(Sideway+slenderness effect) )to compare with M_1st-order?
Thank you

 

[271828]

This is similar to the AISC limit on second-order amplification (B2 factor) in the Effective Length Method, Appendix 7, and recommended limit on the Direct Analysis Method in the commentary to Chapter C.

If the second-order effect is large, the system might be close to buckling. If it is close to buckling then a more sophisticated analysis might be warranted.

 

[DaveAtkins]

This section of ACI is all about big delta, not little delta. Is that your question?

DaveAtkins

 

[271828]

Dave, the commentary shown above, last paragraph, seems to agree with you.

I could see this applying to P-little delta in a few odd cases. For example, we once had a tall concrete column at the back of a theater stage. P-little delta played a big role. I could see limiting the amplification (B1 factor in AISC Appendix 8) for that also. I am not sure that's the intention of the section in question, though.

 

[EZBuilding]

I believe this applies to both Big P-Delta in consideration of your global lateral system and Little P-Delta in consideration of local column effects.

 

[NRam]

Both P-δ and P-Δ.
If you assume your frame is nonsway, then there will be no P-Δ effect (6.6.4.5 accounts for P-δ effect. Member deflection as it approaches buckling), yet you're still required to perform the upper limit check.
Therefore P-δ must be included in addition to P-Δ for frames that are not nonsway.

 

[sticksandtriangles]

I agree with others, include P little and P big if you are in the sway area.

S&T -

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