Structural Engineer's Guide: Wood Product Selection
This guide provides a technical framework for selecting wood members for Beams, Columns, Joists, Rafters, and Diaphragms. All designs must comply with the NDS 2024 (National Design Specification for Wood Construction) for strength and ASCE 7-22 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures) for serviceability.
1. Engineering Profiles of Structural Wood Products
Sawn Lumber (Solid Timber)
- Primary Structural Role: Wall studs, floor joists, and roof rafters.
- Engineering Profile: Visually Graded or Machine Stress Rated (MSR). Performance is governed by NDS Chapter 4.
- Structural Behavior: Performance is limited by natural characteristics (knots, slope of grain).
Structural Glued Laminated Timber (Glulam)
- Primary Structural Role: Long-span gravity beams, headers, and Mass Timber columns.
- Engineering Profile: Manufactured per ANSI A190.1. Performance is governed by NDS Chapter 5.
- Structural Behavior: High depth-to-span efficiency and dimensional stability.
Prefabricated Wood I-Joists
- Primary Structural Role: Repetitive floor joists and roof rafters.
- Engineering Profile: Composite section with SCL (Structural Composite Lumber) or Solid flanges and OSB (Oriented Strand Board) web. Performance is governed by NDS Chapter 7.
- Structural Behavior: Optimized for high Moment of Inertia (I) to weight ratio.
Structural Composite Lumber (SCL: LVL & PSL)
- Primary Structural Role: High-stress headers (LVL: Laminated Veneer Lumber) and high-axial-load columns (PSL: Parallel Strand Lumber).
- Engineering Profile: Manufactured per ASTM D5456. Performance is governed by NDS Chapter 8.
- Structural Behavior: Highest Reference Design Values for Fb (Bending) and Fc (Compression) due to material homogenization (removing natural defects).
Wood Structural Panels (Plywood & OSB)
- Primary Structural Role: Floor/roof sheathing and shear wall/diaphragm cladding.
- Engineering Profile: Manufactured per PS 1 (Plywood) or PS 2 (OSB: Oriented Strand Board). Performance is governed by NDS Chapter 9.
- Structural Behavior: Provides lateral stability and transfers loads to framing members via sub-floor or roof decks.
Cross-Laminated Timber (CLT)
- Primary Structural Role: Floor slabs and roof panels in Mass Timber buildings.
- Engineering Profile: Manufactured per ANSI/APA PRG 320. Performance is governed by NDS Chapter 10.
- Structural Behavior: Enables Two-Way Slab action and acts as a rigid diaphragm.
2. Technical Definitions & Symbols
To understand the technical specifications in the following table, refer to these definitions:
- 24F-V: A specific Glulam grade. 24F means the allowable bending stress (Fb) is 2,400 psi. The V stands for "Visually graded," meaning the wood layers were sorted by human inspection.
- CLT (V2): This refers to a specific "Layup" or recipe of wood layers. V2 is a technical designation for a panel made specifically from Douglas Fir-Larch wood species.
- psi: Pounds per square inch (unit of pressure or stress).
- E (Modulus of Elasticity): Stiffness. A higher number means the wood is stiffer and will deflect (sag) less under load.
- Fb: Allowable Bending Stress.
- Fc: Allowable Compression Stress parallel to the grain.
- Fv: Allowable Shear Stress.
3. Master Technical Comparison Table
Reference design values below reflect general industry ranges. All values must be adjusted by applicable NDS Adjustment Factors (e.g., Cd, Cm, Ct).
| Property | Sawn Lumber (No. 2) | Glulam (24F-V) | I-Joist | SCL (LVL/PSL) | Wood Structural Panels | CLT (V2) |
|---|---|---|---|---|---|---|
| Bending (Fb) | 700 - 1,200 psi | 2,400 psi | High (Moment Cap) | 2,600 - 3,100 psi | 500 - 1,500 psi* | 500 - 1,000 psi |
| Compression (Fc) | 600 - 1,300 psi | 1,650 psi | N/A (Web Buckling) | 2,400 - 3,000 psi | 400 - 900 psi | 500 - 750 psi |
| Modulus (E) | 1.1 - 1.6 × 10⁶ | 1.8 × 10⁶ | High EI | 1.9 - 2.2 × 10⁶ | 1.2 - 1.8 × 10⁶ | 1.5 - 1.8 × 10⁶ |
| Shear (Fv) | 150 - 180 psi | 210 - 265 psi | High (Web-Ltd) | 285 - 400 psi | 150 - 250 psi (Rolling) | 150 - 200 psi |
| Typical Spans | Up to 18' - 20' | Up to 100'+ | Up to 30' | Up to 40' | 16" - 48" (o.c.) | Up to 25' (Slab) |
*Panel values vary significantly based on orientation (Strength Axis).
4. Industry Selection Logic (Decision Matrix)
| Design Priority | Primary Selection | Secondary Selection | Technical Rationale |
|---|---|---|---|
| Maximum Axial Load | PSL (Parallel Strand Lumber) | Glulam Column | PSL provides the highest Fc and is highly resistant to splitting. |
| Lateral Resistance | Wood Structural Panels | CLT | OSB/Plywood create high-strength diaphragms for wind/seismic. |
| Long-Span Floor Beam | Glulam | LVL (Laminated Veneer Lumber) | Optimized for depth and Beam Stability (Cl). |
| Floor Rigidity | I-Joist | CLT | I-joists maximize Moment of Inertia (I) to control vibration/bounce. |
| Lowest Material Cost | Sawn Lumber | I-Joist | Most economical for short repetitive spans. |
| Rapid Site Assembly | CLT | Glulam | Prefabricated panels reduce total labor hours significantly. |