Fundamentals of Timber Engineering. Module 1.2.1 Understanding Timber: Growth of Wood

Summary

This video explores the properties of wood as a natural and renewable material, focusing on its cellular structure and its role in structural timber. It covers the benefits of using timber for carbon sequestration, sustainability, and its versatility in construction. The script delves into the differences between hardwoods and softwoods, moisture content and its effects on wood movement, and the concept of creep under compression. The key takeaway is that understanding the behavior of timber cells under load helps engineers design for optimal structural performance, making timber a valuable, eco-friendly material for the built environment.

Takeaways

• 😀 Timber is derived from wood cells, which form the basis of all structural timber.
• 🌱 Trees absorb carbon dioxide through photosynthesis, locking carbon into their trunks, making timber a carbon sink.
• 🌍 Timber is beneficial for reducing atmospheric carbon levels, as it stores carbon for the duration of its use in structures.
• ♻️ Timber is a renewable resource, with trees taking about 30 years to grow in a plantation forest before being used for structural timber.
• 🏠 Timber production in Australia can generate enough material for a house in about 10 minutes of forest growth.
• 🔄 Waste timber products can be recycled into other products, such as chips, ensuring minimal waste.
• 🌳 Hardwoods and softwoods have different structural properties, with softwoods having thinner cell walls and lower density compared to hardwoods.
• 📏 Timber is much stronger and stiffer when loaded parallel to the grain, but weaker when loaded perpendicular to the grain.
• 💧 Wood is hygroscopic, meaning it absorbs moisture from the atmosphere and can shrink or swell based on the surrounding moisture content.
• ⏳ Timber responds to moisture changes by moving in size perpendicular to the grain, with about a 1-2% change in size.
• 🛠️ In bending and compression applications, timber experiences creep under sustained loads, causing the cells to shorten over time.

Q & A

How do trees contribute to reducing atmospheric carbon?

Trees use photosynthesis to absorb carbon dioxide from the atmosphere, which they store in their trunks. As trees grow, they lock away carbon in the form of wood, helping reduce atmospheric carbon levels.

What percentage of the weight of timber is carbon?

About 50% of the weight of timber is carbon, which has been absorbed from the atmosphere during the tree's growth.

Why is timber considered beneficial for the atmospheric carbon cycle?

Timber is beneficial because it removes carbon from the atmosphere when it is used in construction or other applications. The longer timber remains in service, the longer it keeps that carbon locked away.

What is the difference between hardwood and softwood in terms of structure?

Hardwoods have thicker cell walls and a more closed structure, making them denser and often darker in color. Softwoods, on the other hand, have thinner cell walls and a less dense structure.

How does moisture affect the behavior of timber?

Moisture content affects the size and movement of timber. When timber absorbs moisture, the cell walls swell, making the timber expand. Conversely, when it loses moisture, the cell walls shrink, leading to timber contraction.

What is the significance of the direction of loading on timber cells?

Timber cells have high stiffness and strength when loaded parallel to their grain. However, when loaded perpendicular to the grain, they have low strength and stiffness, making the timber more prone to squishing or splitting.

What is 'seasoned timber'?

Seasoned timber is wood that has been dried to a moisture content of around 15%. This process reduces the amount of moisture in the wood, making it more stable for use in structural applications.

What happens to timber when it is exposed to different atmospheric moisture conditions?

Timber will absorb moisture when exposed to a moist atmosphere, causing the cell walls to swell. In a dry atmosphere, timber will lose moisture, causing the cell walls to shrink.

How does atmospheric moisture influence timber in air-conditioned environments?

In air-conditioned environments, the air is typically drier, which leads timber with a lower moisture content (around 8-10%) to be in equilibrium with the dry air, making it stable in such conditions.

What is 'creep' in timber and how does it affect structural performance?

Creep refers to the gradual deformation of timber under sustained load, particularly in compression or bending applications. Over time, this can cause the timber cells to compress slightly, leading to a reduction in the structural strength of the timber under continuous loading.