Timber Structures
Introduction
Timber structures use engineered wood products—solid-sawn lumber, glulam, LVL, CLT, and other mass timber panels—to create efficient, low-carbon buildings and bridges. Designers choose timber for speed, aesthetics, and sustainability; owners value warm interiors, excellent strength-to-weight, and a lighter foundation demand than concrete. This guide lays out what timber systems are, how they behave, and how to design and build them so loads flow reliably along the load path from roof to ground. We tie decisions to credible loads, rigorous analysis, and disciplined field inspections.
Timber succeeds when the system—material + moisture control + connections + fire strategy—is designed as one.
Materials & Engineered Wood Products
Wood is orthotropic: properties differ parallel and perpendicular to grain. Engineered products optimize fiber alignment, grading, and adhesives to deliver predictable performance at scale. Selecting the right product and grade is step one.
- Solid-Sawn Lumber: Dimensional lumber for light framing; visually or machine graded.
- Glulam (GLT): Laminated lumber beams/arches; deep sections and curved geometries are possible.
- LVL/PSL/LSL: Veneer- or strand-based members for headers, beams, and rim boards with high reliability.
- CLT (Cross-Laminated Timber): Orthogonal panel product for floors, roofs, and walls; acts as a two-way or one-way plate depending on layup and support.
- NLT/DLT: Nail- or dowel-laminated timber—rapid to fabricate; often used with a topping for acoustics and mass.
- HSS & Steel Hybrids: Steel connectors and braces complement timber’s compression/tension performance in long spans and lateral systems.
Choosing a Product
For long-span beams → glulam; for speed and flat plates → CLT; for heavy headers in light-frame → LVL/PSL; for cost and availability in residential → solid-sawn lumber.
Did you know?
Because timber is much lighter than concrete or steel, reducing seismic mass can shrink brace or shear wall demands—see seismic design.
Structural Systems: Gravity & Lateral
Select the system that best matches spans, openings, and MEP routing. Timber often shines as a hybrid with steel or concrete for cores and foundations.
- Light-Frame (Platform): Stud walls with joists/trusses; economical and fast for low- to mid-rise.
- Post-and-Beam: Glulam or LVL columns and beams with panelized floors/roofs (CLT, NLT, DLT).
- Mass Timber Plates: CLT diaphragms spanning to beams/walls; two-way action with proper support.
- Arches & Trusses: Glulam arches or timber–steel trusses for long spans and expressive roofs.
- Lateral Systems: Wood shear walls, CLT walls, or steel braced frames; diaphragms typically CLT or sheathed wood panels.
Coordination Tip
Lay out beam grids to align with panel sizes and trucking lengths. Early module planning avoids field cuts and preserves factory-rated fire and acoustics.
Design Fundamentals
Timber design checks strength and serviceability with adjustment factors for load duration, temperature, and moisture. Members are verified in tension, compression, bending, shear, and bearing; columns require stability checks; panels act as plates with rolling-shear considerations in cross-laminated products.
Bending Capacity (Concept)
Deflection (Serviceability)
- Panels: For CLT, consider rolling shear in cross-layers and verify panel layup against span and load.
- Columns: Slenderness and end fixity govern; check combined axial + bending from eccentricities and drift.
- Diaphragms: For CLT or sheathed wood, verify shear transfer, chord forces, and collector detailing.
Workflow
Establish loads → choose system → size beams/columns/panels → check deflection/vibration → design connections and diaphragms → coordinate fire and moisture protection → document fabrication tolerances.
Connections: Fasteners, Plates & Ductility
Connections often govern timber performance. Ductile failure modes (fastener yielding, plug shear with capacity design) are preferred over brittle splitting or withdrawal. Steel plates, self-tapping screws (STS), dowels, bolts, and concealed connectors are common.
- Fastener Behavior: Bearing in wood + yielding in steel creates ductility; withdrawal is brittle—avoid relying solely on pullout where possible.
- Screws & Rods: Inclined STS for high-capacity shear/compression transfer; threaded rods with epoxy for uplift and diaphragm chords (detail for creep and fire).
- Moisture Detailing: Keep plates and end grain dry; vent and drain concealed cavities; avoid water traps at knife plates or hangers.
- Fire Detailing: Recess plates and provide char cover, or use tested proprietary hardware with protection collars.
Important
Pre-drill where required, respect edge/end distances, and orient fasteners to favor ductile bearing/yielding. Field substitutions for connectors require engineering review.
Moisture, Durability & Fire Strategy
Moisture is the primary durability driver for timber. Control wetting during construction and in service. Fire safety relies on predictable charring that preserves a structural core or on rated assemblies that protect members.
Moisture Content (Concept)
- Moisture Barriers: Air/water barriers, overhangs, drips, and ventilated cavities; avoid direct ground contact unless preservative-treated.
- Interfaces: Separate timber from concrete/steel with membranes, gaskets, or vented stand-offs; detail for wind-driven rain.
- Fire: Size members with sacrificial char layers or use gypsum/encasement. Protect connections—steel heats quickly.
- Inspection: Plan access to concealed zones; monitor MC during construction; include timber-specific inspection checkpoints.
Did you know?
Mass timber’s inherent charring can maintain load-bearing capacity during fire; the char layer insulates the core when sized correctly.
Vibration, Acoustics & Serviceability
Timber’s lower density means floors can be lively without careful tuning. Serviceability often governs panel thickness, beam depth, and bay size, especially for office, lab, and residential uses.
First-Mode Frequency (Concept)
- Vibration Control: Increase depth, use composite toppings, reduce bay lengths, or add tuned stiffness (secondary beams).
- Acoustics: Floating toppings, resilient mounts, sealed joints, and airtight layers; staggered seams on CLT reduce flanking.
- Deflection: Check immediate and long-term (creep) deflection; set camber or construction sequences to protect finishes.
Comfort-Focused Bay
Target frequency > 8–10 Hz for offices; use thicker CLT layups, composite screws to beams, and continuous topping for damping and mass.
Construction, Logistics & QA/QC
Prefabrication drives timber’s schedule advantage. Protecting members from moisture, aligning tolerances, and verifying connection installations keep performance on track.
- Submittals: Shop drawings/connection calcs, panel layups, fire/acoustic assemblies, coatings, and erection sequencing tied to the project’s analysis.
- Logistics: Just-in-time delivery; laydown areas with dunnage and covers; lifting points coordinated with panel design.
- Temporary Protection: Edge seals, taped seams, and sloped temporary roofs; monitor moisture with pins.
- QA/QC: Torque checks for screws/bolts, verification of recess/cover for fire, and documentation of membrane continuity at wet zones.
- Closeout: Moisture readings at turnover, as-built connection photos, and an inspection plan for the owner.
Important
Do not field-route or notch primary members without engineering review—local fiber cuts can trigger brittle failures or reduce fire cover.
Codes, Standards & Trusted References
Anchor your design to authoritative, stable resources:
- American Wood Council (AWC): NDS® for Wood Construction, SDPWS® for shear walls/diaphragms, and mass timber resources. Visit awc.org.
- APA – The Engineered Wood Association: Panel ratings, diaphragms, and field guides. Visit apawood.org.
- ICC: Building codes and tall wood provisions. Visit iccsafe.org.
- NIST: Research on timber fire, connections, and resilience. Visit nist.gov.
- USDA Forest Products Laboratory: Technical reports and handbooks. Visit fpl.fs.usda.gov.
For system context, see our pages on building materials, compare steel design and concrete design, confirm loads and analysis, and finalize foundations and inspections.
Frequently Asked Questions
When should I choose timber instead of steel or concrete?
Choose timber for reduced embodied carbon, lighter foundations, fast installation via prefabrication, and warm exposed finishes. Use hybrids—e.g., concrete cores—with timber floors to balance stiffness, cost, and schedule.
How high can mass timber go?
Modern codes permit tall mass timber under specific types with fire-resistance ratings. Heights depend on jurisdiction and occupancy—coordinate early with the AHJ and fire engineer.
Is timber durable in wet climates?
Yes—if detailed to stay dry. Provide overhangs, ventilated cavities, membranes, and drainage paths. Use preservative-treated material where contact or splash is unavoidable and plan routine inspections.
Do I need composite action with concrete toppings?
Composite toppings improve vibration and acoustics and can add strength with shear connectors, but they add mass and reduce deconstruction options. Decide based on performance targets and lifecycle plans.
How are penetrations handled?
Coordinate MEP openings in shop drawings; reinforce around large holes with added laminations, screws, or steel frames; protect edges for fire and acoustics; avoid cutting after installation.
Key Takeaways & Next Steps
Timber structures deliver speed, beauty, and sustainability when a holistic strategy unites product selection, moisture control, connection detailing, and fire performance. Start with realistic loads, choose a system that suits spans and routing, size members with proper adjustment factors, and confirm serviceability with vibration and deflection checks. Protect against moisture from day one and document QA/QC for connectors, membranes, and fire details.
Continue with our guides on structural analysis, refine the load path into foundations, and plan proactive inspections. For standards and research, begin at AWC, APA, ICC, NIST, and USDA FPL. Thoughtful system selection + disciplined detailing + moisture-first construction = timber structures that perform for decades.