Question 1

What is KL/r and why does it matter?

Accepted Answer

KL/r is the slenderness ratio — effective length KL divided by radius of gyration r. AISC 360-22 §E1 limits KL/r to 200 for most columns. Higher slenderness → lower buckling resistance. Both in-plane (x-x) and out-of-plane (y-y) axes must be checked.

Question 2

How does AISC 360-22 Chapter E handle flexural buckling?

Accepted Answer

§E3 computes the critical stress Fe = π²E / (KL/r)². The compressive stress ratio Fcr/Fy is then determined from the appropriate column curve (Table E3.1 for W shapes): Fe/Fy ≤ 0.44 → Fcr = 0.877Fe; Fe/Fy > 0.44 → Fcr = 0.658Fy (elastic-plateau transition). Pn = Fcr × A.

Question 3

What are the LRFD and ASD load factors?

Accepted Answer

LRFD (Load and Resistance Factor Design): γD = 1.2, γL = 1.6. So Pu = 1.2PD + 1.6PL. LRFD resistance factors: φc = 0.9 for compression. ASD (Allowable Strength Design): ΩD = 1.0, ΩL = 1.6. So Pa = PD + 1.6PL (for dead + live only). ASD safety factors: Ωc = 1.67 for compression.

Question 4

How is the moment magnifier δ computed?

Accepted Answer

§C2.2: δ = Cm / (1 − P/Pcr) where Pcr = Fe·A. Cm = 0.85 for braced columns with end moments within ±⅓ of uniform moment diagram. Unbraced frames use Cm = 0.85 (no sidesway) or 1.0 (sidesway). This calculator uses Cm = 0.85 for braced, 1.0 for unbraced.

Question 5

What is the combined N+M interaction check (Ch. H)?

Accepted Answer

AISC 360-22 Eq. H1-1a and H1-1b: when Pu/φPn ≥ 0.2 → Pu/φPn + (8/9)(Mu/φMn) ≤ 1.0; when Pu/φPn < 0.2 → (Pu/2φPn) + (Mu/φMn) ≤ 1.0. Both axes are checked; the governing axis determines the utilization ratio.

AISC Column Calculator