Steel Column (NZS 3404:1997)

How it calculates

The Steel Column (NZS 3404:1997) calculator designs hot-rolled steel columns and posts using limit state design per NZS 3404.1:1997 (Amendment 2). It runs a first-order elastic analysis per Clause 4.4.2 to determine demands, then applies NZS 3404 capacity equations for all relevant limit states.

Structural analysis

The calculator performs FEA on the column as a beam-column, resolving axial forces, bending moments, and deflections under applied loads. End conditions (pinned, fixed, roller) are specified at each support. Concentrated axial loads and distributed lateral loads can be applied at any height. The member is assumed to be of uniform cross-section along its full length. Interaction limit states are conservatively evaluated at the maximum individual demand in each span regardless of load case.

Axial section capacity

Section capacity in compression is checked per NZS 3404.1:1997 Section 6:

utilization = N / (phi * Ns) ≤ 1.0*

where phi = 0.90 and Ns = kf * An * fy. The form factor kf accounts for local buckling of slender plate elements within the cross-section, consistent with the approach used in AS 4100.

Member buckling capacity

Compression member buckling capacity follows NZS 3404.1:1997 Section 6:

utilization = N / (phi * Nc) ≤ 1.0*

Nc is derived from the member slenderness reduction factor alphac, which depends on the modified slenderness lambda_n and the member section constant alphab from NZS 3404 Table 6.3.3. Major- and minor-axis effective lengths are evaluated separately to identify the governing buckling axis.

Flexural capacity

Moment member capacity (lateral-torsional buckling) follows NZS 3404.1:1997 Section 5:

utilization = |M| / (phi * Mb) ≤ 1.0*

Mb is determined from the reference buckling moment Ms and the slenderness reduction factor alphas per Section 5.6. Flange and web compactness classifications (compact, non-compact, slender) govern whether full plastic or reduced section capacity applies.

Combined axial and bending

Combined axial compression and biaxial bending is checked per NZS 3404.1:1997 Section 8 interaction equations. Both major- and minor-axis moment demands are included. The amplified moment approach is applied consistent with the first-order analysis assumption.

Deflection

Lateral deflection under service loads is calculated and reported alongside strength checks for serviceability review.

Outputs

Results are displayed as colour-coded utilization ratios for each limit state with NZS 3404.1:1997 clause references. Section properties, capacity factors, governing demands, and design actions are tabulated for straightforward report documentation.