Steel Design
Core concepts
Understand tension member design (gross yielding, net section fracture, block shear), compression members (Euler buckling, effective length factor K, slenderness limits), and flexural members (yielding, lateral-torsional buckling, local buckling of flanges and webs). Know bolt and weld design: shear, bearing, tear-out, and combined actions. Be comfortable with compact, noncompact, and slender section classifications.
Common traps
- Effective length K: a fixed-fixed column is not K=1; a pinned-pinned column is. Support conditions change capacity dramatically.
- Net area: deduct bolt holes plus a stagger allowance for zig-zag failure paths.
- Shear lag: the effective net area for a member connected by only some of its elements is reduced.
- Block shear: the failure combines tension rupture on one plane and shear yield/rupture on another - check both.
Worked rule of thumb
For a column, compute KL/r. As a rough guide, KL/r near 200 is the practical upper limit for compression members, and capacity drops sharply as slenderness rises. If KL/r is below about 50, yielding tends to govern; above that, buckling increasingly controls. Always compare the factored demand to φPn from the governing limit state - the lowest φPn among all applicable limit states governs.
Remember that lateral-torsional buckling depends on the unbraced length Lb relative to Lp and Lr; between them, capacity interpolates linearly.