building tech

• Classification of wood


  • Softwoods (conifers with needles)
  • Hardwoods (broad-leaved or deciduous trees)
• Sapwood


  The softer, younger outer portion of a tree that lies between the cambium and the heartwood. More permeable, less durable and usually lighter in color than the heartwood.
• Heartwood


  The older, harder central portion of a tree. Usually contains deposits of various materials that frequently give it a darker color than sapwood. Denser, less permeable and more durable than the surrounding sapwood.
• Properties of wood


  • Hardness
  • Flexibility
  • Strength
  • Durability
• Defects of wood


  • Decay
  • Checks
  • Shakes
  • Knots
  • Pitchpockets
  • Wane
  • Warping (crook, bow, cup, twist)
• A log of wood can be cut in two different ways to produce lumber
• Softwoods are generally brittle while most hardwoods are flexible
• Sapwood contains living cells and takes an active part in the life process of a tree, while heartwood is composed of inactive cells and serves only as a mechanical support
• Sapwood is more permeable, less durable and usually lighter in color than the heartwood
• Heartwood is denser, less permeable and more durable than the surrounding sapwood
• Sawing methods


  • Plainsawing
  • Quartersawing

  
• Plainsawing


  Lumber cut tangent to the annual rings or growth, or in commercial practice, cut with annual rings at an angle 0 to 45

  
• Quartersawing


  Wood cut radially to the annual rings of growth parallel to the rays, or in commercial practice, cut with the annual growth rings at an angle 45 to 90

  
• Plainsawn lumber is preferable when a pleasing pattern is required, as in wall paneling
• Quartersawn lumber is desirable because it has less shrinkage than plainsawn lumber, and this is important where joints must be kept tight
• Lumber


  Wood used in construction
• Classifications of lumber


  • Strips
  • Board Lumber
  • Dimension Lumber
  • Timbers
• Dimension Lumber


  Pieces more than 2" and less than 5" in any dimension
• Board Lumber


  Pieces less than 2" thick and at least 8" wide
• The two most common classifications are Dimension Lumber and Board Lumber
• Nominal size


  The size of lumber when it is cut from the log
• Actual size


  The finished size of lumber after it is dried and planed on all four sides
• The stated size of lumber is not its actual finished size
• Lumber is sold in lengths from 6' up to 20' in increments of 2'. Special lengths greater than 20' are also available but cost more per board foot than the standard lengths
• Board foot


  The measure of a piece of wood 1" thick, 12" (or 1') wide and 12" (or 1') long
• Calculating board feet


  Thickness (in.) x Width (in.) x Length (ft.) / 12
• All lumber is sized according to its rough dimensions as it comes from the saw, its size, however, is reduced after being surfaced or dressed by a planer
• Dressed lumber costs slightly more than rough
• Free water


  Water found in cell cavities and intercellular spaces of the wood
• Absorbed or hygroscopic water


  Water held in the cell walls
• Drying of wood


  Free water passes out first before the absorbed water in the cell wall vaporizes
• Fiber-saturation point


  When all free water is removed but all absorbed water remains, approximately 30% moisture content (M.C.) for all species
• Shrinkage


  Occurs at moisture content percentages below the fiber saturation point
• Equilibrium moisture content (E.M.C.)


  The condition of dryness of the wood when the vapor pressure in the air just balances the vapor pressure on the wood surface, equivalent to 12 to 16 percent moisture content in the Philippines
• Seasoning


  The process of removing moisture from green wood (wood from freshly-cut logs)
• Seasoning


  1. Air-drying

  2. Kiln-drying
• The ideal condition in seasoning is for the moisture from the interior of the wood to replace the surface moisture which vaporizes
• When the moisture from the surface escapes faster than that which travels from the interior to the surface, then there is equal drying in the board and if the difference in moisture content is big, shrinkage and then stress in the wood develops, causing seasoning checks
• Advantages of seasoned lumber


  • Lessens liability of wood to be attacked by fungi and wood-boring insects
  • Reduction of weight
  • Increased strength
  • Minimum shrinkage after the lumber is in place
  • Reduced checking and warping
  • Increased nail-holding power
  • Improvement for application of paint and wood preservatives, fire retardants
• Advantages of kiln-drying over air-drying


  • Greater reduction in weight
  • Control of moisture content to any desired value
  • Reduction in drying time
  • Killing of any fungi or insects
  • Setting the resins in resinous wood
  • Less degrade