Are We Doing P-Delta Effects for Concrete Members Twice?

Before we interpret this topic, you may be interested to check out our article explaining: 
Step by Step – How to Compute P-Delta effect using ETABS according to ASCE 7-10

Both ASCE 7-10 and ACI 318-11 require amplification of member forces to account for P-delta effects, as part of seismic analysis or as part of member design. Does that mean we are doing it twice? If not, which document to follow for this purpose?

Certainly we are not doing it twice. Slenderness and stability are related. You do not deal with them separately.

ASCE 7-10 requires us to consider P-Δ effects in seismic design – either in structural analysis itself or by multiplying the results of analysis by a factor 1.0/(1- θ).

ACI 318-11 Section 10.10.2 provides us with three options for dealing with slenderness effects: (1) Nonlinear second-order analysis, in accordance with Section 10.10.3, (2) Elastic second-order analysis in accordance with Section 10.10.4, (3) Moment magnification procedure of Section 10.10.5.

In seismic design, since the member forces already include the effects of P-Δ by the ASCE 7 requirement, the requirements in ACI 318 Section 10.10.4 are automatically met, and as a result, no further moment magnification by Section 10.10.5 is necessary.

Please note that, in my opinion, because ASCE 7 is adopted by the IBC for the purpose of defining the seismic design forces, it governs over a material standard like ACI 318 when it comes to matters concerning such forces. So, when designing columns for seismic forces, the P-Δ effects are required to be included by the provisions of ASCE 7, rather than the provisions of ACI 318. This is important because ASCE 7 imposes an upper limit on the stability coefficient θ, which ACI 318 Section 10.10.5 does not include.

In wind design, where the IBC and ASCE 7 have not addressed stability, we are free to use ACI 318 requirements as they are presented in that standard, unmodified.

Via S.K.Ghosh Associates

Post Author: Zahi Baroudi

One thought on “Are We Doing P-Delta Effects for Concrete Members Twice?

  • Scott B

    (December 23, 2016 - 3:49 PM)

    Actually ACI does have an upper limit on second order effects which is related to the theta factor in ASCE 7. See ACI 318-11 section 10.10.2.1 and commentary section R10.10.2.1. This upper limit would apply to both wind and seismic.

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