3B - WOOD

• This book was produced by the Forest Service of the Department of Agriculture, which is an excellent document describing the characteristics and properties of wood

  The Wood Handbook
• is a natural, renewable product from trees

  Wood
• is a woody plant that attains a height of atleast 6 m, normally has a single self-supporting trunk with no branches for about 1.5 m above the ground, and has a definite crown.

  Tree
• Ex. Palm Trees - grow with intertwined fibers

  -Not generally used for engineering applications in the United States.

  Endogenous Trees
• Grow from the center out by adding concentric layers of wood around the central core.

  Exogenous Trees
• produce hardwood and generally shed their leaves at the end of each growing season.

  Deciduous Trees
• produce softwood and have needlelike leaves and normally do not shed them at the end of the growing season.

  Conifer
• is the process of reducing the moisture content (drying) of timber in order to

  prevent the timber from possible fermentation and making it suitable for use.

  Seasoning of Timber
• The log of wood is sawn into planks of convenient sizes and stacked under a covered shed in cross-wise direction in alternate layers so as to permit free circulation of air.

  Natural or Air Seasoning
• The logs of wood are kept completely immersed in running stream of

  water, with their larger ends pointing upstream. Consequently the sap,

  sugar, and gum are leached out and are replaced by water. The logs are

  then kept out in air to dry.

  Water Seasoning
• Boiling in water or exposing the wood to the action of steam spray is a very quick but expensive process of seasoning.


  Boiling
• is adopted for rapid seasoning of timber on large scale to any moisture content. The circulating air takes up moisture required from wood and seasons it.

  Kiln Seasoning
• 2 TYPES OF KILN

  PROGRESSIVE & COMPARTMENT
• An aqueous solution of certain chemicals have lower vapour pressures than that of pure water. If the outer layers of timber are treated with such chemicals the vapour pressure will reduce and a vapour pressure gradient is setup. The interior of timber, containing no salts, retains its original vapour pressure and, therefore, tends to dry as rapidly as if there had been no treatment.

  Chemical or Salt Seasoning
• The logs are placed in such a way that

  their two ends touch the electrodes.

  Current is passed through the setup, being a bad conductor, wood resists the flow of current, generating heat in the process, which results in its drying.

  The drawback is that the wood may split.

  Electric Seasoning
• Has no adverse effects; it is the best method although most expensive.

  The timber is stacked in a chamber with free air space (1/3rd of its capacity) and containing products of combustion of fuels in the fire place.

  Mc. Neill’s Process
• Defects in lumber

  KNOTS

  SHAKES

  WANE

  SAP STREAKS
• is a distortion of wood from the desired true plane.

  Warp
• four major types of warp

  bow, crook, cup, and twist
• also known as relative density is the ratio

  between the density of an object, and a reference substance

  Specific Gravity
• is a measure of the rate at which heat

  flows through a material.

  Thermal Conductivity
• THE THERMAL CONDUCTIVITY

  OF WOOD DEPENDS ON SEVERAL ITEMS INCLUDING:

  1. grain orientation
  2. moisture content
  3. specific gravity
  4. extractive content
  5. structural irregularities such as knots
• It is the ratio of the quantity of heat required to raise the temperature of the material to one degree to that required

  to raise the temperature of an equal mass of water one degree.

  Specific Heat
• It is a measure of the rate at which a material absorbs heat from its surroundings.

  Thermal Diffusivity
• It is a measure of dimensional changes caused by temperature variance.

  Coefficient of Thermal Expansion
• The stress-strain relationship of wood varies within and between species

  and is affected by variation in moisture content and specific gravity.

  Modulus of Elasticity
• This characteristic is accounted for during design through what is known as a Load Duration Factor (LDF).

  Load Duration
• is the phenomenon in which the

  amplitude of vibration in a material decreases with time.

  Damping
• TWO MAIN TESTING TECHNIQUES FOR

  ESTABLISHING STRENGTH PARAMETERS

  Testing of Timbers of Structural Sizes

  Testing of Representative, Small, Clear Specimens
• presents the testing standards for full-size tests

  ASTM D198
• presents the testing standards for small, clear wood specimens

  ASTM D143
• This test method covers the determination of the flexural properties of structural beams made of solid or laminated wood, or of composite constructions.

  Flexure (Bending)
• This test method covers the determination of the compressive properties of elements taken from structural members made of solid or

  laminated wood, or of composite constructions when such an element has a slenderness ratio of less than 17.

  Compression (Short Column)
• This test method covers the determination of the

  compressive properties of structural members

  made of solid or laminated wood, or of composite constructions when such a member has a slenderness ratio of more than 17, and when such a member is to be evaluated in full size but with lateral supports which are spaced to produce an effective slenderness ratio.

  Compression (Long Column)
• This test method covers the determination of the tensile properties of

  structural elements made primarily of

  lumber equal to and greater than nominal 1 in. (19 mm) thick.

  Tension
• Here, a unique bend fixture is required, along with means to measure displacement at the specimen’s neutral axis

  Static Bending (Flexure)
• defines the brittleness of the wood when stress beyond the proportional limit is

  applied.

  Impact Bending
• Simple compression test of

  rectangular specimen that is

  stood up on its end.

  Compression Parallel to the Grain
• Simple compression test of

  rectangular specimen laid on its

  side.

  Compression Perpendicular to the Grain
• This tension test most clearly represents a standard tensile test on a dog bone

  shaped specimen where the specimen is pulled from its ends. The use of an

  extensometer is common.

  Tension Parallel to the Grain