----------------------------------- | :--------------: | :-------------: | | Both ends pinned | 1.0 | N/A (mechanism) | | One end fixed, one pinned | 0.85 | 2.0 | | Both ends fixed (no rotation) | 0.70 | 1.0 | | One end fixed, one free (cantilever) | 2.2 | 2.2 | | Typical floor column in braced building | 0.85 | — | | Ground floor column, pinned base | 0.90 | — |

For sway frames, K must be determined by rational buckling analysis per Clause 4.7.2, not from this table.

AS 4100 Column Curve — alpha_c Table (Selected Values)

The full alpha_c factor per AS 4100 Table 6.3.3(3) is a function of the modified slenderness lambda_n and the section constant alpha_b:

For lambda_n <= 13.5 (the plateau), alpha_c = 1.0. For Australian UC sections in braced frames, typical lambda_n values range from 25-80 for storey-height columns (3.0-5.0 m), giving alpha_c values of 0.978 to 0.706. Columns above lambda_n = 120 are uncommon in multi-storey construction — they indicate insufficient bracing or very slender sections.

Interaction Check — Biaxial Bending (Clause 8.3.4)

For columns with axial load plus moments about both axes, the interaction check per AS 4100 Clause 8.3.4 has two limits:

In-plane (strong-axis bending + axial): N*/(phi Nc) + Mx*/(phi Msx) <= 1.0

Biaxial (both axes + axial): N*/(phi Nc) + Mx*/(phi Msx) + My*/(phi Msiy) <= 1.4 ... AND (N*/(phi Nc))^1.4 + (Mx*/(phi Msx))^1.4 + (My*/(phi Msiy))^1.4 <= 1.0

The second form (exponent 1.4) is typically more conservative for intermediate utilisation levels and should always be checked when My* is significant. The calculator applies both forms and reports the governing utilisation.

Worked Example — Biaxial Loading

Problem: A 200UC52.2 column, Grade 300, effective length 4.0 m, N* = 600 kN, Mx* = 30 kN.m, My* = 15 kN.m (minor axis moment from eccentric connection).

Check 1 — Section capacities:

• phi Nc = 1305 kN (from earlier example)
• phi Msx = 0.9 x 568 x 10^3 x 300 x 10^-6 = 153 kN.m (major axis)
• phi Msiy = 0.9 x 195 x 10^3 x 300 x 10^-6 = 53 kN.m (minor axis)

Check 2 — Linear interaction (Clause 8.3.4 limit 1.4): 600/1305 + 30/153 + 15/53 = 0.460 + 0.196 + 0.283 = 0.939 <= 1.4. OK.

Check 3 — Exponent 1.4 interaction (Clause 8.3.4 governing): (0.460)^1.4 + (0.196)^1.4 + (0.283)^1.4 = 0.335 + 0.112 + 0.184 = 0.631 <= 1.0. OK.

Result: Column is adequate at 63% utilisation. The minor-axis moment from connection eccentricity accounts for 29% of the interaction capacity — always include connection eccentricity in the analysis.

Column Splice Design — AS 4100 Clause 9.4

Column splices in Australian multi-storey construction are typically located 600-1200 mm above the floor level. Per AS 4100 Clause 9.4:

• Bearing splices: Machined ends bearing directly. The splice plates carry 50% of the column capacity in the connected elements. Preferred for shop-welded columns delivered in two-storey lengths.
• Non-bearing splices: Splice plates carry 100% of the axial load plus any moment at the splice location. Used for site-bolted connections where machining is impractical.
• Minimum bolt count: 4 bolts per side per flange for W-shapes; 4 bolts per side for HSS columns.
• Splice plate thickness: Minimum 10 mm or half the column flange thickness, whichever is greater.

For a 200UC52.2 column splice (bearing type), the splice plates must carry 50% x 800 kN = 400 kN. With 8-M20 Grade 8.8/S bolts in double shear: Vf per bolt = 2 x 79.0 = 158 kN. 8 x 158 = 1264 kN > 400 kN. Bolt shear is adequate. Splice plate (200x200x10 mm, S300) bearing check controls the final design.

Related Australian Resources

• Australian Beam Sizes (UB/UC/PFC)
• Australian Steel Grades
• AS 4100 Column Buckling Guide
• Australian Beam Capacity Calculator
• All Australian References

FAQ

What is the form factor kf? The form factor kf = Ae/Ag accounts for local buckling in slender sections per AS 4100 Clause 6.2. For compact sections with λ ≤ λey, kf = 1.0. For slender elements, the effective area Ae is reduced based on the effective width per Clause 6.2.4.

Calculation Tips

• Always use the minor axis (ry) for slenderness — it governs unless bracing restrains the weak axis.
• Grade 350 provides 17% more capacity than Grade 300 for the same section size.
• Adding intermediate bracing at mid-height halves the effective length, increasing φNc significantly.
• For crane columns and columns supporting vibrating equipment, check fatigue per AS 4100 Clause 13.

What is αb for Australian UC sections? Hot-rolled UC, UB, and welded sections with fy ≤ 450 MPa use αb = 0.5 per AS 4100 Table 6.3.3(1). This corresponds to the AISC column curve. Cold-formed sections and very thick sections use different αb values.

How does the effective length factor K affect column capacity? K directly multiplies the effective length in the slenderness calculation: le = K × L. Doubling K doubles λn, which reduces αc significantly. For example, a column with K = 1.0 might have φNc = 1305 kN, while the same column with K = 2.0 (cantilever) would have φNc ≈ 480 kN — a 63% reduction. Proper end restraint characterisation per Clause 4.6.3 is critical for economic design.

What is the full AS 4100 column capacity equation? The Perry-Robertson formulation in AS 4100 Clause 6.3.3 gives αc = ξ × [1 - √(1 - (90/(ξ × λ))²)] where ξ = (λ/90)² + 1 + η and η = αb × (λ - 13.5)/(λ - 15.3 + 2050/αb). For λ ≤ 13.5, αc = 1.0. This formulation accounts for geometric imperfections (αb = 0.5 for hot-rolled sections) and residual stresses.

Can I use UB sections (Universal Beams) as columns? Yes, UB sections can be used as columns. However, UC sections are generally preferred because their more balanced flange width-to-depth ratio provides better minor-axis buckling resistance. When using a UB as a column, always check the minor axis slenderness and consider that UB sections have lower ry/rx ratios than UC sections, making minor axis buckling more critical.

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.