Timber, the primary material used in timber structures, is classified in several key ways based on its intrinsic properties. While the classification of timber structures themselves typically relates to their form or construction system (like post-and-beam or frame systems), understanding the classification of the timber material is essential for determining its characteristics and suitability for various structural applications.
Based on the provided information, timber material is classified according to several important attributes:
Key Timber Classifications
Timber is commonly classified to define its characteristics, performance, and appropriate applications in construction. The reference highlights the following classification methods:
Classification by Growth Mode
One primary way to classify timber is based on how the parent tree grows:
- Exogenous: These trees grow by adding concentric layers of tissue each year, resulting in distinct growth rings. Most structural timbers (e.g., pine, oak) are exogenous.
- Endogenous: These trees grow from within, expanding internally. Examples include palm trees and bamboo. Their structure is typically different from exogenous timbers.
Classification by Species Type
Timber species are broadly categorized based on the type of tree:
- Coniferous: Wood from cone-bearing, usually evergreen, trees. Often referred to as softwood. Generally lighter and less dense than hardwood, commonly used for framing (e.g., spruce, pine, fir).
- Deciduous: Wood from trees that shed their leaves seasonally. Often referred to as hardwood. Generally denser and stronger than softwood, used for demanding structural applications and finishes (e.g., oak, maple, beech).
Classification by Modulus of Elasticity
The modulus of elasticity (MOE) measures timber's stiffness – its resistance to elastic deformation under load. This is a critical property for structural design. It can also be classified based on modulus of elasticity into groups A, B, or C. A simplified grouping often used is:
| Group | Modulus of Elasticity (Stiffness) | Typical Use Considerations |
|---|---|---|
| A | High | Beams, rafters, elements requiring high stiffness |
| B | Medium | General structural framing |
| C | Lower | Less demanding structural elements |
Higher MOE values indicate a stiffer timber that will deflect less under load.
Classification by Durability
Timber durability refers to its natural resistance to biological degradation agents like fungi and insects. This classification helps determine the expected service life of timber, especially in environments conducive to decay. Durability classification is based on tests measuring average timber lifespan buried in soil. This test method provides a standard way to compare the natural resistance of different timber species under severe exposure conditions. Timbers are typically ranked into durability classes (e.g., very durable, durable, moderately durable, non-durable) based on such tests.
Conclusion
While the question specifically asks about timber structures, the provided reference details how timber material is classified. Understanding these classifications is crucial for selecting appropriate timber for structural use, ensuring the resulting structures are safe, durable, and perform as intended. The classifications based on growth mode, species, modulus of elasticity, and durability are fundamental criteria in the field of timber engineering and construction.
