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Reduced Flexural Stiffness for Direct Analysis Method Affecting Elastic Critical Bucking Stress

Reduced Flexural Stiffness for Direct Analysis Method Affecting Elastic Critical Bucking Stress

Reduced Flexural Stiffness for Direct Analysis Method Affecting Elastic Critical Bucking Stress

(OP)
Say you are designing a column using Direct Analysis Method (DAM). You are required to reduce the flexural stiffness, EI, of your column member by 0.8, which will result in a horizontal larger displacement. When you calculate the elastic critical buckling stress, Fe, for that member using Aisc Eq. E3-4, would you factor Fe by 0.8 since it contains the variable E?

RE: Reduced Flexural Stiffness for Direct Analysis Method Affecting Elastic Critical Bucking Stress

No.

RE: Reduced Flexural Stiffness for Direct Analysis Method Affecting Elastic Critical Bucking Stress

I caught that as well. Look in the commentary for the DAM (16.1-437 in the black book).

It states that the reduced stiffness and notional load requirements only pertain to analyses for strength limit states. They do not apply to analyses of serviceability conditions of excessive deflections, vibration, etc.

They then continue stating that the reduced E is only to be applied for the second-order analysis. The elastic modulus should not be reduced in nominal strength equations that include E. The net effect is to amplify the second-order forces such that they are closer to the actual internal forces in the structure.

Maine EIT, Civil/Structural.

RE: Reduced Flexural Stiffness for Direct Analysis Method Affecting Elastic Critical Bucking Stress

(OP)
Thanks. I'm surprised that question is so clearly answered in the commentary.

RE: Reduced Flexural Stiffness for Direct Analysis Method Affecting Elastic Critical Bucking Stress

Yeah, I personally think that the way the DAM section of the spec is written doesn't make this clear at all (actually, to me, it seems to imply the opposite of the commentary). This should be more clearly spelled out that 0.8E is for the 2nd order effects alone.

Maine EIT, Civil/Structural.

RE: Reduced Flexural Stiffness for Direct Analysis Method Affecting Elastic Critical Bucking Stress

I just checked the 14th edition and it does at least note it, unlike the 13th edition. It says something like the reduced stiffness shall be used in determining the "required strengths" (not the factored resistance) but that was pretty much the only note I could find.

Maine EIT, Civil/Structural.

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