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 §E3 handle flexural buckling?

Accepted Answer

§E3 computes the elastic critical stress Fe = π²E / (KL/r)². The compressive stress ratio Fe/Fy then selects the appropriate column curve: Fe/Fy ≤ 0.44 → Fcr = 0.877Fe (elastic); Fe/Fy > 0.44 → Fcr = 0.658^(Fy/Fe)·Fy (inelastic plateau). Pn = Fcr × A.

Question 3

What is §E4 torsional and flexural-torsional buckling?

Accepted Answer

§E4 covers torsional buckling and flexural-torsional buckling for columns where the torsional mode may govern. For doubly symmetric W-shapes, torsional buckling is rarely governing because the elastic critical stress Fe_t is well above Fe from major-axis flexural buckling. However, asymmetric or certain mono-symmetric sections may require this check. AISC Design Guide 25 provides the full methodology.

Question 4

What is §E7 slender-element reduction?

Accepted Answer

§E7 applies when a section element (flange or web) is classified as Slender. The slender-element reduction factor Q = Qs·Qa reduces the effective area and critical stress. Qs = 1 for non-slender flanges; for slender flanges Qs = (1−0.22/√λ_f)/√λ_f. Qa uses the same form for webs. The reduced capacity φPn_e = 0.9·Fcr_e·Ae must be compared against the non-slender capacity.

Question 5

LRFD vs ASD — what are the load factors and resistance factors?

Accepted Answer

LRFD (Load and Resistance Factor Design): γD = 1.2, γL = 1.6 → Pu = 1.2PD + 1.6PL. φc = 0.9 for compression. ASD (Allowable Strength Design): ΩD = 1.0, ΩL = 1.6 → Pa = PD + 1.6PL. Ωc = 1.67 for compression. The utilization ratio must stay ≤ 1.0 in both methods.

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