Topic 4.2b Timber

Guiding Questions and Tasks

1. Outline why softwoods are generally cheaper than hardwoods 2. Compare the compressive and tensile strength of natural timber 3. Outline in which direction ­ with reference to the grain ­ natural timber is stronger 4. State which lasts longer, hardwoods or softwoods 5. Outline the advantages of manmade boards over natural timber 6. Outline the disadvantages of manmade boards over natural timber 7. Explain why using softwoods is generally considered more environmentally friendly than using hardwoods. 8. Outline why it is necessary to season natural timber 9. Outline how the longevity of natural timber can be improved 10. Outline how timber can be recovered. 11. State the limitation of disposing of timber

Topic 4.2b Timber:

Designers have great influence over the materials that they specify for products. The move towards using timber from sustainably managed forestry gives consumers confidence that rare species found in rainforests have an opportunity to recover.

Nature of design:

Timber is a major building material that is renewable and uses the Sun’s energy to renew itself in a continuous cycle. While timber manufacture uses less energy and results in less air and water pollution than steel or concrete, consideration needs to be given to deforestation and the potential negative environmental impact the use of timber can have on communities and wildlife.

Concepts and principles:

• Characteristics of natural timber: hardwood and softwood • Characteristics of man­made timbers • Treating and finishing timbers • Recovery and disposal of timbers

Guidance:

• Characteristics include tensile strength, resistance to damp environments, longevity, aesthetic properties • Design contexts in which different timbers would be used

Characteristics of natural timber: hardwood and softwood

Timber is made of fibres and is classified into two basic groups: hardwoods and softwoods. However, the groups do not relate to a physical property such as the hardness of the timber: Balsa, which is a hardwood, has one of the lowest ratings of hardness. On the other hand, Yew, which is a softwood, has a relatively high rating of hardness.

The distinction between hardwood and softwood has to do with their structure and explains why softwoods grow faster and hence are usually cheaper.

Softwoods have a simple structure, shown below left, consisting of one type of cell called tracheids. These cells have openings to other cells and it is these cells that are the nutrient conduits. In softwoods, the cells (tracheids) are connected in a way that allows the movement of nutrients throughout the length of the tree. because of this softwoods grow much faster than hardwoods.

Hardwoods have these cells but also have vessels, as shown below right. However, the cells are significantly smaller, and more importantly the cells are closed and cannot function as conduits. In hardwoods it is the vessels that serve as conduits for water and nutrients.

Hardwood trees are angiosperms, plants that produce seeds with some sort of covering This might be a fruit, such as an apple, or a hard shell, such as an acorn. They have broad leaves, as shown in the image on the right. It is common to think of hardwoods as deciduous, but this can be misleading as tropical and subtropical hardwoods keep their leaves all the year round: it is the hardwoods found in temperate regions such as Britain, Japan, USA and New Zealand, that are deciduous.

Softwoods are gymnosperms. They have thin, needle like leaves. They are classed as coniferous, which means they do not lose their leaves in the winter. These trees let seeds fall to the ground as is, with no covering.These seeds are released into the wind once they mature. This spreads the plant's seed over a wide area.

Other Characteristics: ● Resistance to damp environments is very resistant. ● Longevity: Hardwoods very good. Softwoods good. ● Aesthetic properties: natural colours range (red, purple, cream and brown). The grain can add to its aesthetics. Greater variety of grains and colours in hardwoods

Factors affecting strength of timber

Some of the factors that affect the strength of timber include: ● moisture content ● duration of loading (how long the load / stress is applied) ● defects ­ knots, splits, etc. These reduce mechanical properties. ● chemical treatment ­ many chemical treatments have an adverse effect on mechanical properties, although this is greatest for water­based preservatives rather than oil.

Natural timber is what is known as an anisotropic material, which means it behaves differently when loads are applied in different directions. Timber is strongest when loads are applied parallel to the grain (with the grain).

The tensile strength of wood is on the average about three times as great as the compressive strength

Characteristics of man­made wood Man­made wood also referred to as Engineered wood, composite wood, manufactured board or mechanical beams; includes a range of wood products which are manufactured by binding the strands, particles, fibres, or veneers of wood, together with adhesives to form composite ​ materials. ​ Sawmill scraps and other wood waste can be used for engineered wood composed of wood particles or fibres, but whole logs are usually used for veneers. [http://www.shropshirebs.com/timber/engineered­wood­products/] ​ ​

The main characteristics of man­made timber are: ● dimensional stability ● uniformity of properties ● greater availability of sizes ● lower cost compared to natural timber

Man­made timber include plywood, particle board (chipboard) and MDF. Generally made from pieces of wood that are glued together under pressure.

Characteristics of man­made timber ● Tensile strength: depends on the man­made timber ○ Plywood – high tensile strength in all directions ○ Particle board and MDF – very low ● Resistance to damp environments: depends on the man­made timber. ○ Exterior plywood – excellent. Interior plywoods very low ○ Particle board and MDF – very low ● Longevity: ○ Plywood is high ○ Particle board and MDF is low to medium ● Aesthetic properties: ○ Plywood, if the top layer is of a nice timber like Beech, will be good ○ Particle board and MDF requires finishing.

Treating and finishing timbers both manmade and natural

The reasons for treating or finishing wood include: ● Reducing attack by organisms ● Improving chemical resistance ● Enhancing aesthetic properties ● Modifying other properties

Seasoning Timber Either: air drying ­ done in natural atmosphere (usually hardwoods) or kiln drying (usually softwoods. This achieves equilibrium moisture content with the timber’s environment. Green timber contains about 85% moisture. This must be reduced to about 20% in a controlled way, otherwise the wood is subject to decay ­ attack by fungus, or will split and warp, which affects structural integrity.

Timber Treatment Protects timber from weather, insects, fungus and moisture. Can be done by: Oil, paint, polish or varnish.

Recovery and disposal of timbers

Recovering timber Has become popular due to its image as an environmentally friendly product. Common belief among consumers is that by purchasing recovered wood, the demand for "green timber" will fall and ultimately benefit the environment. Greenpeace also view recycled timber as an environmentally friendly product, citing it as the most preferable timber source on their website. [https://en.wikipedia.org/wiki/Timber_recycling] ​ ​

The image on the right shows recovered railway sleepers as a final product. Recovered timber most commonly comes from old buildings, bridges and wharfs, where it is carefully stripped out and put aside by demolishers.

Disposal of Timber Must use clean wood, which is defined as solid wood, lumber, and pallets that are unpainted, unstained, untreated, and free of glue. The wood may be pierced with nails or other metal fasteners, such as screws and staples. Clean wood can be used for landscaping mulch, composting or alternative industrial fuel. Criteria for Selecting Timber

Design Context Selection Criteria

Buildings (construction) Resistance to moisture

Resistance to pest infestation

Durability

Cost

Hardness

Availability

Furniture Dimensional stability

Strength

Colour

Grain

Defect free

Children’s toys Free from toxic chemicals

Resistance to moisture

Durable