Glulam Beam & Column Design

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.

Capacity Factors ($phi$):

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.