Building Techoclogy » knots http://www.building-tech.com The Building Technology Resource Tue, 25 Jan 2011 23:11:06 +0000 en hourly 1 http://wordpress.org/?v=3.3.1 Wood Formation http://www.building-tech.com/wood-formation/ http://www.building-tech.com/wood-formation/#comments Tue, 02 Jun 2009 07:01:39 +0000 blogtopia http://www.building-tech.com/reference/definitions/wood-formation/ wood_fraxinus_excelsior Wood, in the strict sense, is yielded by trees, which increase in diameter by the formation, between the existing wood and the inner bark, of new woody layers which envelop the entire stem, living branches, and roots. Technically this is known as secondary growth; it is the result of cell division in the vascular cambium, a lateral meristem, and subsequent expansion of the new cells.

  • Growth rings
  • Knots
  • Heartwood and sapwood

 

Source: Wikipedia

]]> http://www.building-tech.com/wood-formation/feed/ 0 Knots in Wood http://www.building-tech.com/knots-in-wood/ http://www.building-tech.com/knots-in-wood/#comments Tue, 02 Jun 2009 03:48:15 +0000 blogtopia http://www.building-tech.com/reference/definitions/knots-in-wood/ A knot is a particular type of imperfection in a piece of wood; it will affect the technical properties of the wood, usually for the worse, but may be exploited for artistic effect. In a longitudinally-sawn plank, a knot will appear as a roughly circular "solid" (usually darker) piece of wood around which the grain of the rest of the wood "flows" (parts and rejoins). Within a knot, the direction of the wood (grain direction) is up to 90 degrees different from the grain direction of the regular wood.

In the tree a knot is either the base of a side branch or a dormant bud. A knot (when the base of a side branch) is conical in shape (hence the roughly circular cross-section) with the tip at the point in stem diameter at which the plant’s cambium was located when the branch formed as a bud.

During the development of a tree, the lower limbs often die, but may persist for a time, sometimes years. Subsequent layers of growth of the attaching stem are no longer intimately joined with the dead limb, but are grown around it. Hence, dead branches produce knots which are not attached, and likely to drop out after the tree has been sawn into boards.

In grading lumber and structural timber, knots are classified according to their form, size, soundness, and the firmness with which they are held in place. This firmness is affected by, among other factors, the length of time for which the branch was dead while the attaching stem continued to grow.

Knots materially affect cracking (known in the industry as checking) and warping, ease in working, and cleavability of timber. They are defects which weaken timber and lower its value for structural purposes where strength is an important consideration. The weakening effect is much more serious when timber is subjected to forces perpendicular to the grain and/or tension than where under load along the grain and/or compression. The extent to which knots affect the strength of a beam depends upon their position, size, number, and condition.

A knot on the upper side is compressed, while one on the lower side is subjected to tension. If there is a season check in the knot, as is often the case, it will offer little resistance to this tensile stress. Small knots, however, may be located along the neutral plane of a beam and increase the strength by preventing longitudinal shearing. Knots in a board or plank are least injurious when they extend through it at right angles to its broadest surface. Knots which occur near the ends of a beam do not weaken it. Sound knots which occur in the central portion one-fourth the height of the beam from either edge are not serious defects.

Source: Wikipedia

]]> http://www.building-tech.com/knots-in-wood/feed/ 0 US plywood grades http://www.building-tech.com/us-plywood-grades/ http://www.building-tech.com/us-plywood-grades/#comments Mon, 01 Jun 2009 08:46:53 +0000 blogtopia http://www.building-tech.com/reference/definitions/us-plywood-grades/ Plywood grades are determined by a veneer quality on the face and back of each panel. The first letter designates quality of face veneer (best side), while the second letter denotes the surface quality of the back of the panel. The letter "X" indicates the panel was manufactured with scrap wood as the center plies, not "exterior" as is commonly thought. The A-D rating is only good for construction (softwood) plywood, not for hardwood plywoods such as oak or maple.

"A": Highest grade quality available. Can be defect free or contain small knots, providing they are replaced with wooden plugs (the fillers having a "boat" or an "American football" shape) or repaired with synthetic patch. This grade may contain occasional surface splits that are repaired with synthetic filler. The surface is always sanded and provides for smooth paintable face quality.

"B": Second highest quality veneer grade. Normally a by-product of downgraded "A" quality veneer. Solid surface, but may contain small diameter knots and narrow surface splits. Normally repaired with wooden plugs or synthetic filler. The surface is normally sanded smooth.

"C": Considered to be a lower end face quality, but a reasonable choice for general construction purposes. May contain tight knots up to 1½ inches diameter, some open knot holes, some face splits, and discoloration. Some manufactures may repair the defects with synthetic filler. Panels are typically not sanded.

"D": Considered to be the lowest quality veneer and often used for the back surface for construction grade panels. Allows for several knots, large and small, as well as open knots up to 2½ inches diameter. Open knots, splits, and discoloration are acceptable. "D" grade veneers are neither repaired nor sanded. This grade is not recommended for permanent exposure to weather elements.

 

Source: Wikipedia

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]]> http://www.building-tech.com/us-plywood-grades/feed/ 0 Drying defects http://www.building-tech.com/drying-defects/ http://www.building-tech.com/drying-defects/#comments Mon, 01 Jun 2009 08:15:41 +0000 blogtopia http://www.building-tech.com/reference/definitions/drying-defects/ Drying defects are the most common form of degrade in timber, next to natural problems such as knots (Desch and Dinwoodie, 1996). There are two broad categories of drying defects (some defects involve both causes):

  • defects that arise due to the shrinkage anisotropy. This leads to warping: cupping, bowing, twisting, spring and diamonding.
  • defects that arise due to uneven drying. This leads to the rupture of the wood tissue: checks (surface, end and internal), end splits, honey-combing and case-hardening. Another such defect is collapse, often seen as a corrugation, or "washboarding" of the wood surface (Innes, 1996). Collapse is a defect that results from the physical flattening of fibres, above the fibre saturation point (thus not a form of shrinkage anisotropy).

Australian and New Zealand Standard Organisations (AS/NZS 4787, 2001) have developed a standard for timber quality. Their five criteria for measuring drying quality:

   1. moisture content gradient and presence of residual drying stress (case-hardening);
   2. surface, internal and end checks;
   3. collapse;
   4. distortions;
   5. and discolouration caused by drying.

This standard also indicates how to assess each of these drying quality criteria and provides a classification to express drying quality.

 

Source: Wikipedia

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