Lecture on Timber Design
Good day, everyone. Today, we will be discussing Timber Design—one of the oldest yet still
highly relevant areas of structural engineering. Despite the rise of concrete and steel, timber
remains an essential construction material due to its renewability, versatility, and aesthetic
appeal. However, designing with timber requires careful consideration of its unique properties,
limitations, and behavior under load.
1. Introduction to Timber as a Structural Material
Timber is a natural, anisotropic material, meaning its strength and mechanical properties vary
depending on the direction of the grain. Historically, timber has been used in houses, bridges,
temples, and even ships, showing that it can provide both strength and durability when used
properly.
In structural engineering, timber is typically used for:
Beams and columns in residential and light commercial buildings,
Roof trusses and purlins,
Floor joists and decking,
Bridges and boardwalks, and
Architectural finishes where aesthetics are also important.
2. Types of Structural Timber
Solid-Sawn Lumber
Cut directly from logs.
Classified as dimension lumber (e.g., 2×4, 2×6) or large timber.
Engineered Wood Products
Plywood – thin veneers glued in layers with grains at right angles.
Laminated Veneer Lumber (LVL) – parallel veneers bonded with adhesives.
Glulam (Glued Laminated Timber) – multiple layers of lumber glued together to form beams or
arches.
Oriented Strand Board (OSB) – wood strands compressed with resin.
Treated Wood
Chemically treated to resist decay, insects, and fire.
3. Mechanical Properties of Timber
Since timber is a natural material, its strength is not uniform. Key properties include:
Compressive Strength (parallel to grain) – important for columns and posts.
Tensile Strength (parallel to grain) – governs members in tension, like bottom chords of trusses.
Flexural Strength – governs beams and joists.
Shear Strength – resistance to sliding along grain planes, often critical in short beams.
Modulus of Elasticity (E) – measures stiffness, used in deflection calculations.
Anisotropy in timber means:
Parallel to grain: stronger in tension and compression.
Perpendicular to grain: weaker, prone to crushing or splitting.
4. Design Considerations in Timber Structures
(a) Strength and Safety Factors
Design codes such as the National Structural Code of the Philippines (NSCP 2015, Vol. 1,
Wood Structures) or the National Design Specification (NDS) for Wood Construction provide
allowable stresses or strength reduction factors for timber members.
(b) Load Duration
Timber can resist higher loads for shorter durations (e.g., wind or earthquake) than for sustained
loads (e.g., dead and live loads). Codes apply load duration factors.
(c) Moisture Content
Wood strength decreases as moisture content increases. Dry wood is stronger, while wet wood
is prone to shrinkage, swelling, or decay.
(d) Connections
Nails, bolts, screws, and metal plates are used.
Connections are often the weakest link in timber structures, so their design is critical.
(e) Serviceability
Deflection limits are applied to timber beams and joists to ensure functionality and avoid
excessive sagging.
5. Common Timber Design Problems
Bending of Beams
Use flexural formula:
σ
=
M
y
I
σ=
I
My
Strength is checked against the allowable bending stress.
Column Buckling
Timber columns can fail due to buckling. Slenderness ratio (length vs. radius of gyration) is
critical.
Shear Parallel to Grain
Important in short beams and connections.
Bearing Stresses at Supports
Crushing of fibers under concentrated loads (e.g., beam ends on supports).
Connection Failures
Splitting, withdrawal of nails, or bolt crushing of wood fibers.
6. Advantages of Timber
Renewable and sustainable when sourced responsibly.
Lightweight, easy to transport and assemble.
High strength-to-weight ratio.
Aesthetic appeal.
Good thermal properties compared to steel and concrete.
7. Limitations of Timber
Susceptible to fire, decay, and termites if untreated.
Strength varies depending on species, moisture, and defects like knots or splits.
Limited span capacity compared to steel or concrete.
8. Applications of Timber Design
Residential Housing – framing, floor joists, rafters.
Bridges and Boardwalks – especially glulam beams for medium spans.
Long-Span Roofs – sports arenas, churches, or auditoriums using laminated timber arches.
Hybrid Systems – timber combined with steel or concrete for both strength and aesthetics.
9. Conclusion
Timber design blends traditional building techniques with modern engineering principles. To
design effectively with wood, an engineer must understand its unique anisotropic properties,
apply the appropriate design codes, and anticipate challenges such as moisture, connections,
and durability.
In sustainable construction, timber is making a resurgence, particularly with engineered wood
products and mass timber construction, which can rival steel and concrete in strength while
offering environmental benefits.
As engineers, you must see timber not just as a traditional material, but as a modern,
sustainable, and adaptable option when designed and detailed properly.
