📘 Lecture: Timber Design
1. Introduction
Timber is one of the oldest structural materials used by humans. Even today, despite the rise of
concrete and steel, timber remains a popular choice because it is:
Renewable and sustainable
Lightweight but strong
Easy to work with (cutting, shaping, joining)
Aesthetically pleasing
Timber design refers to the principles and methods of designing structural members made of
wood — beams, columns, trusses, floors, and roof systems.
However, timber has unique characteristics (anisotropy, variability, susceptibility to decay) that
must be considered in design.
2. Properties of Timber as a Structural Material
A. Mechanical Properties
Strength: Depends on species, density, and moisture content.
Anisotropy: Stronger along the grain (longitudinal direction) than across it.
Elasticity: Timber has relatively low modulus of elasticity compared to steel/concrete →
higher deflections.
B. Physical Properties
Density: 400–800 kg/m³ (depends on hardwood vs softwood).
Moisture Content: Strongly affects strength and durability.
Shrinkage & Swelling: Occurs with changes in moisture.
C. Durability
Timber is prone to:
o Rot (due to fungi, moisture exposure)
o Termite and insect attack
o Fire (combustible, though large sections char and can retain strength inside).
3. Types of Timber Used in Construction
1. Softwoods (pine, cedar, fir)
o Fast-growing, lightweight, easy to work with.
o Common in framing, trusses, light structures.
2. Hardwoods (oak, mahogany, narra, molave)
o Dense, durable, stronger.
o Used in flooring, heavy structures, and where appearance matters.
3. Engineered Wood Products
o Plywood, laminated veneer lumber (LVL), glued laminated timber (glulam), cross-
laminated timber (CLT).
o Manufactured to improve strength, consistency, and span capabilities.
4. Loads on Timber Structures
Timber members are designed to resist:
Axial Loads → compression (columns, struts) and tension (ties, trusses).
Flexure (Bending) → beams, joists, rafters.
Shear → horizontal shear in beams, vertical shear at supports.
Combined Loading → members carrying both bending and axial loads (e.g., rafters in
pitched roofs).
5. Limit States in Timber Design
1. Strength Limit State
o Member must resist applied loads without failure.
o Checks for bending strength, shear strength, tension, and compression.
2. Serviceability Limit State
o Member must not deflect or vibrate excessively.
o Consider shrinkage, creep, and long-term deformation.
3. Durability Limit State
o Member must withstand environmental effects (moisture, decay, insects, fire).
6. Design Principles
A. Tension Members (e.g., truss ties)
Governed by tensile strength parallel to the grain.
Net section (after bolt holes) must be checked.
B. Compression Members (e.g., posts, columns)
Governed by compressive strength parallel to the grain.
Buckling (slender columns) is critical.
C. Flexural Members (e.g., beams, joists)
Governed by bending strength (fiber stress in bending).
Shear strength and deflection limits must also be checked.
D. Connections
Timber is often connected with:
o Bolts and dowels
o Steel plates and nails
o Adhesives (in engineered wood)
Connection strength is often the weakest link in timber design.
7. Advantages of Timber Structures
High strength-to-weight ratio.
Natural thermal and acoustic insulation.
Renewable and eco-friendly.
Aesthetic and versatile in architecture.
8. Disadvantages / Challenges
Anisotropic and variable properties.
Prone to decay, insects, and fire.
Limited span and load capacity compared to steel/concrete (unless engineered wood is
used).
Requires proper seasoning and treatment.
9. Modern Applications of Timber Design
Traditional Housing: Wooden houses, nipa huts, log cabins.
Roof Trusses: In schools, warehouses, gymnasiums.
Bridges: Timber pedestrian and rural road bridges.
Engineered Wood Structures: High-rise buildings using CLT (Cross-Laminated Timber) in
Europe, US, and Asia.
10. Summary
Timber design is the process of safely and efficiently using wood as a structural material.
Must consider its anisotropy, variability, and susceptibility to decay.
Members are designed for tension, compression, flexure, shear, and connections.
Serviceability and durability are as important as strength.
Engineered wood has expanded timber’s role into large-span and high-rise
construction.
