AS 4100 is the Australian Standard for the design of steel structures, covering the strength, stability, and serviceability of steel members and connections in buildings and other structures.
Like AS 3600, it uses limit state design principles, requiring the design capacity of a member or connection to exceed the design action effect under factored loads.
Design Basis
$$ \phi R_u \geq N^*, \ V^*, \ M^* $$| Action | $\phi$ |
|---|---|
| Member in tension | 0.9 |
| Member in compression | 0.9 |
| Member in bending | 0.9 |
| Bolted/welded connections | 0.8 – 0.9 |
Section Classification
Steel sections are classified by their slenderness to determine whether local buckling limits capacity before yield:
| Class | Behaviour |
|---|---|
| Compact | Reaches full plastic moment, sustains rotation for redistribution |
| Non-compact | Reaches yield moment but limited rotation capacity |
| Slender | Local buckling occurs before yield is reached |
Member Capacity
Tension Members
$$ N_t = \min(A_g f_y,\ 0.85 k_t A_n f_u) $$Where $A_g$ = gross area, $A_n$ = net area (deducting bolt holes), $k_t$ = correction factor for shear lag.
Compression Members
$$ N_c = \alpha_c N_s $$Where $N_s = k_f A_n f_y$ is the section capacity, and $\alpha_c$ is a member slenderness reduction factor from the AS 4100 column curves, a function of the modified slenderness ratio:
$$ \lambda_n = \frac{L_e}{r}\sqrt{k_f}\sqrt{\frac{f_y}{250}} $$Flexural Members (Beams)
Nominal section moment capacity:
$$ M_s = f_y Z_e $$Where $Z_e$ = effective section modulus (accounting for local buckling class).
Lateral torsional buckling reduces capacity for unrestrained or partially restrained beams:
$$ M_b = \alpha_m \alpha_s M_s \leq M_s $$Where $\alpha_m$ accounts for moment distribution along the segment and $\alpha_s$ accounts for the segment's slenderness.
Connections
| Connection Type | Key Check |
|---|---|
| Bolted (bearing type) | Bolt shear, ply bearing, block shear |
| Bolted (friction type, high-strength) | Slip resistance under service loads |
| Welded (fillet) | Weld throat shear capacity |
| Welded (full penetration butt) | Base metal capacity governs |
Bolt shear capacity (single shear, one interface):
$$ V_f = 0.62 f_{uf} k_r \left(n_n A_c + n_x A_o\right) $$Deflection Limits
| Element | Limit (typical) |
|---|---|
| Beams supporting brittle finishes | Span / 500 |
| Roof purlins/girts (elastic) | Span / 150 – Span / 250 |
| Crane runway beams (vertical) | Span / 600 |
Fabrication and Corrosion Protection
AS 4100 works alongside:
| Standard | Scope |
|---|---|
| AS/NZS 5131 | Structural steelwork fabrication and erection |
| AS/NZS 2312 | Corrosion protection of steel structures |
| AS 1554 | Structural steel welding |
Practical Notes
- Section classification (compact/non-compact/slender) drives whether full plastic capacity is available: check local buckling before assuming $M_s = f_y Z$.
- Lateral restraint spacing is often the governing factor in beam capacity, more so than the section's bending strength itself.
- Connection design frequently governs overall member utilisation in portal frame and truss structures: don't leave it as an afterthought.