Glulam (GLT) Design for AS1720
Launch the free Glulam Beam Calculator below & verify your design in seconds!
Designing Glulam In Australia
Glue Laminated Timber (Glulam) is the workhorse of the Australian mass timber industry. Glulam offers superior dimensional stability and span capabilities compared to sawn timber.
The SPEC Toolbox Glulam Calculator unifies the complex landscape of Australian & European supply. Whether you are specifying local ASH, or imported Binderholz product – the tool with Eurocode 5 & AS1720 included, has got you covered!
The Australian Engineering Platform for Glulam Design
This tool performs a comprehensive structural analysis of Glulam beams and columns under gravity and wind loads. It verifies:
About : Glulam Design
Technical Guide: Designing to AS 1720.1
1. The “Grade Confusion” (GL17 vs. GL24)
Australian engineers often face a choice between local and imported products, which use different grading logic.
- GL17 (Local): Stiffness-driven. Typically Radiata Pine. High MOE (16,700 MPa) but lower bending strength (f’b approx 30-33 MPa).
- GL24/GL28 (Imported): Strength-driven. Typically European Spruce. Lower MOE (11,000-12,500 MPa) but high bending strength.
Our calculator automatically adjusts the E-values and characteristic strengths based on your selection, ensuring you don’t inadvertently use a “stiff” GL17 modulus for a “flexible” GL24 beam.
2. Beam Stability Factor (k12)
Unlike steel beams which check “LTB,” AS 1720.1 uses the k12 Factor to reduce capacity based on slenderness.
The calculator analyzes your Restraint Conditions (e.g., “Top edge restrained by flooring”) to calculate the effective length (Lay).
- Low Risk: Beams with continuous lateral restraint (k12 = 1.0).
- High Risk: Deep, narrow beams (Fly-braced or free-standing) where k12 can drop significantly below 1.0.
3. Column Stability (k12 for Columns)
For columns, the stability factor (k12) accounts for buckling about the x-axis or y-axis.
The tool calculates the Slenderness Coefficient (S3 or S4) based on the effective buckling length (g13 • L) and applies the interaction equation for combined axial and bending loads.
Key Formulas & Parameters
Capacity Factors (Φ):
We apply the standard AS 1720.1 capacity factors:
- Φ = 0.95 (Typical for Glulam, which is a controlled engineered product).
- Note: This is higher than Sawn Timber (Φ=0.90), reflecting the reliability of lamination.
Modification Factors:
- k1 (Duration of Load): Critical for timber. A beam that works for “Wind” (k1=1.14) might fail under “Dead Load” (k1=0.57). The tool checks all load cases.
- k4 (Partial Seasoning): Typically 1.0 for Glulam (manufactured dry).
- j2 (Creep): For long-term deflection limits (e.g., Span/300), the tool applies the creep factor (j2=2.0 for unseasoned, 1.5-2.0 for Glulam depending on exposure).
Bearing Area:
Nd = Φ • k1 • k4 • k6 • k7 • f’p Ap
- k7: The bearing length factor. The tool automatically increases bearing capacity if the bearing length is short (<150mm) and away from the beam end.
Frequently Asked Questions
Does this check Hardwood Glulam?
Yes. You can select Australian Hardwood grades (e.g., GL18, GL21 Vic Ash) which are popular for high-end architectural trusses requiring high strength and stunning appearance.
How do I check fire ratings?
This module covers “Cold” structural design. For FRLs (e.g., 60/60/60), please use our Char Rate Calculator to determine the residual sacrificial timber section.
Can I design curved beams?
This module currently handles straight (prismatic) members. Curved beams require a specialized check for radial tension ($k_{radial}$), which is covered in our advanced geometry module.


CLT
Dowels
GLT
Viti
Staffe
Light-frame