Properties of Wood & Timber

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Wood is the oldest material used by humans for construction after stone. Despite its complex chemical nature, wood has excellent properties which lend themselves to human use.
It is readily and economically available; easily machinable; amenable to fabrication into an infinite variety of sizes and shapes using simple on-site building techniques;

• Exceptionally strong relative to its weight
• A good heat and electrical insulator;
• of increasing importance
• It is a renewable and biodegradable resource.

However, it also has some drawbacks of which the user must be aware. It is a “natural” material and is available in limited amount.

Physical properties:

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Following properties of wood makes it good for use in consruction

Specific Gravity (SG):

Generally, specific gravity (SG) and the major strength properties of wood are directly related. SG for the major , usually used structural species ranges from roughly 0.30 to 0.90. Higher allowable design values are assigned to those pieces having narrower growth rings (more rings per inch) or more dense latewood per growth ring and, hence, higher SG.

Moisture Content (MC) and Shrinkage:

Undoubtedly, wood’s reaction to moisture provides more problems than any other factor in its use. Wood is hygroscopic ; that is, it picks up or gives off moisture to equalize with the relative humidity and temperature in the atmosphere. As it does so, it changes in strength; bending strength can increase by about 50% in going from green to a moisture content (MC) found in wood members in a residential structure, for example. Wood also shrinks as it dries, or swells as it picks up moisture, with concomitant warpage potential. Critical in this process is the fiber saturation point (fsp) , the point (about 25% moisture content, on oven-dry basis) below which the hollow center of the cell has lost its fluid contents, the cell walls begin to dry and shrink, and wood strength begins to increase. The swelling and shrinkage processes are reversible and approximately linear between fiber saturation point and 0% MC.

Wood decay or fungal stain do not occur when the MC is below 20%. There is no practical way to prevent moisture change in wood; most wood finishes and coatings only slow the process down. Thus, vapor barriers, adequate ventilation, exclusion of water from wood, or preservative treatment are absolutely essential in wood construction.

Thermal Properties/Temperature Effects:

Although wood is an excellent heat insulator, its strength and other properties are affected adversely by exposure for extended periods to temperatures above about 100°F. The combination of high relative humidity or MC and high temperatures, as in unventilated attic areas, can have serious effects on roof sheathing materials and structural elements over and above the potential for attack by decay organisms. Simple remedies and caution usually prevent any problems.
At temperatures above 220°F, wood takes on a thermoplastic behavior. This characteristic, which is rarely encountered in normal construction, is an advantage in the manufacture of some reconstituted board products, where high temperatures and pressures are utilized.

Environmentally friendly

Timber is the most environmentally responsible building material. Timber has low production energy requirements and is a net carbon absorber. Timber is a renewable resource. Well-managed forests produce timber on a sustained continuous basis, with minimal adverse effects on soil and water values.

In plentiful and growing supply

Timber is readily available. Australia has significant forest resources including a plantation estate covering more than 1.6 million hectares, and the area is growing rapidly.

Strong and lightweight

Timber is strong, light and reliable making timber construction simpler and safer than steel or concrete construction. A comparison with steel and concrete shows that radiata pine structural timber, for example, has a strength for weight ratio 20 percent higher than structural steel and four to five times better than unreinforced concrete in compression.
The lightweight structures possible in wood confer flow-on advantages in terms of reduced foundation costs, reduced earthquake loading and easier transport. Building components and complete constructions are simple and safe to erect, and cheaper to deconstruct or reuse at the end of a buildingís useful life.
Additionally, timber is:

Safe

Timber has low toxicity and therefore requires no special safety precautions to work with it, other than normal protection from dusts and splinters. Timber frame construction requires little in the way of heavy lifting equipment making building sites safer work places. Timber being non-conductive has obvious benefits in terms of electrical safety. Modern timber construction has increased fire resistance due to incombustible linings protecting light frames.

Easy to install

Increasingly specialist timber frame and truss manufacturers use high tech prefabrication enabling accurate and speedy installation. Recyclable -Timber is a forgiving material that can be easily disassembled and reworked. If demolition or deconstruction of a wooden building is necessary, many wood-based products can be recycled or reused. Timber trusses and frames, factory fabricated from sawn timber and toothed metal plate connectors, have come to dominate roof construction for small buildings such as houses and large industrial buildings where clear spans up to 50 metres are required. Timber trusses compete with other roof structural systems on cost, high performance, versatility and ready availability, supported by design software packages supplied by the plate manufacturers to the fabricators.

Cost effective Comparative studies of the economics of different wall framing systems indicate that, in terms of direct building expenses, timber frames are consistently the most cost-effective solution. There are many factors to consider when comparing the economics of different construction systems including the complexity of the layout, site, builder experience, and relative material prices at the time of building. However, comparative studies of the economics of different wall framing systems indicate that, in terms of direct building expenses, timber frames are consistently the most cost-effective solution. In the medium to long term, the forecasts for the Australian wood supply indicate a stable and growing supply. This means that prices for framing timber are likely to be more stable for builders in the long term. However, this price stability is questionable for materials such as steel, which consume considerable amounts of fossil fuels in their manufacture. The smelting of steel is heavily reliant on the continued availability of cheap sources of fossil fuels, a scenario which is becoming highly uncertain in an increasingly energy and security conscious world. Durable Good detailing, coating and maintenance ensure that timber structures last for lifetimes. Although many buildings become obsolete and are demolished long before the end of their natural lives, timber buildings correctly designed and maintained can have an indefinite life. The key to long life is protection from weather, insect attack and decay, through well-established design detailing, surface coating systems, selection of durable species, and preservative treatment processes. In all countries of the world, and Australia is no exception, historic timber buildings testify to these principles. In termite-prone areas, all buildings are vulnerable to termite attack of contents, so protection is needed regardless of construction materials. Protection systems rely on physical or chemical barriers, or both, and their effectiveness depends on the quality of the design, construction, inspection and maintenance. The risk of termite attack should be assessed after consulting with local building authoritiesand an appropriate termite management system should be implemented. The system may include physical or chemical barriers or in higher risk areas, a termite resistant treated timber or naturally termite resistant frame may also be chosen. In any case any management system should include regular inspection to ensure that barriers have not been breached. It is therefore critical that the system type and inspection schedule are understood by all future householders. Importantly, termites are an integral part of the ecology of Australia, however, with awareness, planning and using cost effective systems, they can be effectively managed. Comfortable Well-designed Timber structures are comfortable to live in all year round no matter where you are. Flexible A particular feature of timber is the flexibility of design forms and finishes that can be used. This flexibility also extends to the ease with which existing buildings can be added to or modified to suit changing circumstances. User friendly versatile timber gives building designers creative freedom providing homeowners with flexible design choices.Timber is simply the best building material for builders, designers and homeowners and can be used to construct the homes we love, structures we admire and warehouses, commercial buildings and other structures. The timber frame method of building gives designers flexibility in both layout and external appearance. High levels of thermal insulation are incorporated within the construction, reducing heating costs and conserving energy. Compression Strength

Chemical Properties

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Chemical Effects

Though, would is chemically inert as compared to other materials but is affected by some acids and bases. Some species have proven very useful for food containers (berry boxes and crates) because they are nontoxic and impart no taste to the foods contained therein. Wood structures have also found widespread use as storage facilities for salt and fertilizer chemicals.

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