Clause 8.7 specifies how to determine the effective length (KL) of compression members based on their end restraint conditions. Table 11 provides idealized effective length factors for six standard boundary conditions. For columns in frames, Annex D provides a more rigorous approach using relative stiffness parameters.
Key Requirements
•Effective length KL = K × L, where K is the effective length factor and L is the actual unsupported length
•K depends on end conditions — degree of rotational and translational restraint
•For non-sway frames (braced): K ranges from 0.65 to 1.0
•For sway frames (unbraced): K ranges from 1.0 to 2.0 (or more)
•For columns in multi-storey frames, use Annex D alignment charts or simplified formulae
Reference Tables
Table 11 — Effective Length Factors for Compression Members (Clause 8.7)
Boundary Condition
Schematic
K (Recommended)
K (Code Value)
Both ends fixed (rotation and translation)
Fixed–Fixed
0.65
0.65
One end fixed, other pinned
Fixed–Pinned
0.8
0.8
Both ends pinned (translation restrained)
Pinned–Pinned
1
1
One end fixed, other end free to rotate but not translate
Fixed–Roller
1.2
1.2
One end fixed, other end completely free (cantilever)
Fixed–Free
2
2
Both ends fixed in rotation, one end free to translate (sway)
Fixed–Fixed (sway)
1.2
1.2
These are idealized values. Actual end conditions in practice are between these extremes. For partial fixity in frames, use Annex D. In doubt, use the more conservative (higher) K value.
Formulas
KL = K × L
Effective length of a compression member
KL = Effective length (mm)K = Effective length factor from Table 11 or Annex DL = Actual unsupported length between restraints (mm)
Practical Notes
✓In typical building frames with rigid beam-column connections and bracing, K = 0.65 to 0.85 is appropriate. Pure K = 1.0 (both ends pinned) is rarely the actual condition for building columns.
✓For a column in a braced frame (non-sway), K is always ≤ 1.0. For unbraced (sway) frames, K ≥ 1.0. This distinction is critical — a sway column with K = 2.0 has four times lower buckling load than K = 1.0.
✓In truss compression members (like top chord or struts), the effective length depends on the restraint from gusset plates. Typically K = 0.85 to 1.0 in plane, and the full length out-of-plane.
Common Mistakes
⚠Assuming K = 1.0 for all columns — real building columns with beam connections typically have K < 1.0 (braced) or K > 1.0 (sway).
⚠Not distinguishing between sway and non-sway frames — the effective length factor doubles from ~0.65 (braced fixed-fixed) to ~1.2 (sway fixed-fixed).
⚠Using in-plane effective length for out-of-plane buckling — columns must be checked about both axes with their respective KL values.