Effective Length of Column

The effective length (KL) of a compression member is the length of an equivalent pin-ended column that would buckle at the same load — it converts the real column, with its particular end + sway restraint, into the standard Euler case. K (effective length factor) depends on rotational fixity at the ends and whether the frame can sway: K = 0.5 (both ends fixed, no sway), 0.7 (one fixed-one pinned), 1.0 (both pinned, no sway), and ≥1.0 up to 2.0+ for sway/cantilever columns.

It is fundamental to column design: the slenderness ratio uses KL/r, and both IS 456 Annex E (RCC, by frame braced/unbraced + end conditions) and IS 800 Table 11 (steel) tabulate effective lengths. Using the wrong K — especially treating a sway frame as braced — is a classic and dangerous design error that under-estimates buckling effects.