Glulam Beam Design
- nick8284
- 6 hours ago
- 3 min read
Introduction
Glulam (glued-laminated timber) is a versatile and high-performing material used in wood-framed structures where longer spans or higher loads are required. Whether it's a roof beam, floor girder, or a header over a large opening, glulam offers strength, dimensional stability, and architectural appeal.
In this post, we walk through the essentials of glulam beam design using Allowable Stress Design (ASD) in accordance with the National Design Specification (NDS 2018). We'll cover design inputs, adjustment factors, and both flexural and shear strength checks — using a straightforward example to guide the process.
The Essentials of Glulam Beam Design
Glulam is an engineered wood product made by bonding layers of lumber together with durable adhesives. This laminated structure gives glulam beams higher strength and stiffness compared to solid-sawn lumber of the same dimensions. But with that performance comes the need for careful design — accounting for specific adjustment factors and code provisions that apply to glulam members.
Key Factors in Glulam Beam Design
Load Identification: The design starts with identifying all relevant loads — dead, live, snow, and any other applicable loading such as wind uplift. Load combinations per ASCE 7 and NDS must be applied using ASD provisions.
Material Selection: Glulam is available in various layup combinations and appearance grades. Strength properties — including allowable bending, shear, and modulus of elasticity — are specified by the manufacturer and tabulated in the NDS Supplement. Common layups like 24F-V4 or 24F-1.8E will drive design capacity.
Adjustment Factors: Several NDS-defined factors modify the allowable stresses for glulam based on real-world conditions:
Load Duration Factor (CD): Adjusts for the duration of the applied load (e.g. 1.0 for dead load, 1.6 for snow).
Wet Service Factor (CM): Reduces strength for members exposed to moisture during service.
Temperature Factor (Ct): Reduces strength in elevated temperature environments.
Curvature Factor (Ccurv): Applies when glulam beams are cambered or curved.
Volume Factor (CV): Reduces allowable bending stress for deep or long-span glulam members.
Beam Stability Factor (CL): Adjusts bending capacity for laterally unbraced members.
Deflection Criteria: Serviceability must be verified by checking beam deflection against span-based limits. Long-term deflection is addressed by applying the deflection modification factor for creep (Kcr).
Code Compliance: The NDS provides the governing procedures for combining adjustment factors, checking design limits, and verifying both strength and serviceability. Interaction equations and unbraced length limitations also apply, particularly for deep or heavily loaded beams.
Structural Analysis: The beam must be analyzed for shear, moment, and deflection under applied loads. Glulam flexural design typically governs, but shear capacity — especially near supports — must be checked as well. For members without full lateral support, lateral-torsional buckling and the CL factor must be considered.
Practical Applications in Glulam Beam Design
Glulam is often used where conventional dimensional lumber isn't sufficient — spanning over garage doors, supporting heavy floor or roof loads, or replacing steel beams in wood-framed buildings.
Engineers use glulam design principles to:
Select a beam size that satisfies both strength and deflection requirements.
Apply environmental and volume-based reductions accurately.
Optimize for cost and performance with clear span and unbraced length considerations.
Other Considerations in Timber Beam Design
Slenderness Ratio and Stability: Deep glulam beams with long unbraced lengths may be vulnerable to lateral-torsional buckling. The slenderness ratio (Le/d) helps determine whether stability factors must be applied. For unbraced beams, the beam stability factor (CL) is required to reduce allowable bending stresses.
Critical Buckling Values: The NDS provides equations to calculate the critical moment (Mcr) for unbraced beams. These calculations involve elastic buckling theory and account for geometry, load application, and lateral bracing.
Volume Factor (CV): Unlike sawn lumber, glulam is subject to volume effects. Longer or deeper beams are statistically more likely to contain defects, and CV is applied to reduce the allowable bending stress accordingly.
Example Problem
(Solutions Provided Using CalcBook) Problem Statement:
Design Inputs:
Moment, Shear & Deflection Diagrams:
Deflection Check:
Flexural Adjustment Factors:
Design Flexural Strength:
Shear Adjustment Factors:
Design Shear Strength:
Conclusion
Glulam beam design combines the efficiency of engineered wood with the flexibility of long-span framing — but it also requires careful attention to code-defined factors, serviceability checks, and member stability. By understanding the full range of adjustment factors, unbraced length effects, and environmental considerations, engineers can design safe, code-compliant glulam beams for a wide range of applications.
With CalcBook, you can streamline glulam design using NDS 2018 ASD provisions — with clear, code-aligned calculations and documentation that’s ready to deliver. From input to output, every step is shown transparently.
Try CalcBook today: click the link below for accurate and efficient timber design calculations.