AS 1720.1 is the Australian Standard for timber structures, covering the design of sawn timber, glue-laminated timber (glulam), and laminated veneer lumber (LVL) members.
Timber design differs from steel and concrete in one key respect: characteristic strengths are heavily modified by duration of load, moisture content, and member size, reflecting timber's biological, anisotropic nature.
Stress Grades
Timber is assigned a stress grade based on characteristic strength and stiffness, either through visual grading or machine (mechanical stress) grading.
| Grade | Typical Use |
|---|---|
| F5–F8 | Light framing, low-stress applications |
| F11–F17 | General structural framing |
| F22–F34 | Engineered/glulam beams, high-load applications |
| MGP10, MGP12, MGP15 | Machine graded pine: common in residential framing |
Design Basis
$$ \phi N_d \geq N^* $$Design capacity is derived from characteristic strength modified by a series of factors:
$$ N_d = k_1 k_4 k_6 k_9 \ldots \phi N_u $$Key Modification Factors
| Factor | Accounts For |
|---|---|
| $k_1$ | Duration of load (strength reduces under sustained load) |
| $k_4$ | Moisture content in service |
| $k_6$ | Temperature (tropical environments) |
| $k_9$ | Load sharing between parallel members (e.g. closely spaced joists) |
| $k_{12}$ | Stability factor (lateral torsional buckling of beams) |
Duration of Load Factor ($k_1$)
| Load Duration | $k_1$ |
|---|---|
| 5 seconds (e.g. wind gust) | 1.0 |
| 5 days | 0.94 |
| 5 months | 0.80 |
| 50+ years (permanent) | 0.57 |
This is the single biggest difference from steel/concrete design: a timber member sized for permanent load can carry significantly more short-duration load.
Bending Members
$$ M_d = \phi k_1 k_4 k_6 k_9 k_{12} f'_b Z $$Where $f'_b$ = characteristic bending strength, $Z$ = section modulus.
Lateral stability ($k_{12}$) reduces capacity for beams without adequate lateral or torsional restraint, similar in concept to lateral torsional buckling in steel.
Compression Members (Columns)
$$ N_d = \phi k_1 k_4 k_6 k_{12} f'_c A_c $$ $k_{12}$ here is a function of slenderness coefficient $S$: $$ S = \frac{L_{ay}}{d} \sqrt{\frac{d}{b}} \quad \text{(for rectangular sections)} $$Connections
| Connector Type | Typical Application |
|---|---|
| Nails | Light framing, sheet bracing |
| Bolts | Beam-column connections, moment splices |
| Timber rivets / screws | Engineered glulam connections |
| Metal connector plates | Prefabricated trusses |
Serviceability
| Member | Deflection Limit |
|---|---|
| Floor joists | Span / 300 (total), Span / 400 (live) |
| Roof beams (no ceiling) | Span / 150 |
| Roof beams (with brittle ceiling) | Span / 250 |
Durability
| Hazard Class | Exposure | Typical Treatment |
|---|---|---|
| H1 | Inside, above ground, protected | None/low |
| H3 | Outside, above ground | Preservative treated |
| H4 | In-ground contact | Preservative treated |
| H5/H6 | In-ground critical/marine | Heavy treatment |
Practical Notes
- $k_1$ (duration of load) is the factor most often mishandled: always check which load case (permanent vs short-term wind gust) governs a given member.
- Load sharing ($k_9$) can meaningfully increase capacity for closely spaced joists and rafters: don't ignore it in residential framing.
- Connections, not member sizing, are frequently the governing design check in light timber framing.