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					WOOD
 -One of the oldest known construction
 material (light construction)
 -The only one that is naturally renewable

STRUCTURE OF WOOD
  -a cross section of log has certain parts
Bark – a thin, rough covering that surrounds
 the tree
Cambium – layer of wood cells, it exists inside
 the bark
Annual ring – ring like structure in the log that
 determines the rate of growth of the tree
 Old growth soft wood/(slow growing trees) –
 have thinner rings
 Plantation trees (rapidly growing) – have
  wider rings
Pith – the center of the log and it is surrounded
  by annual rings. These determines the age of
  the tree (250 rings means 250 years old)
Annual rings have to layers: (inner & outer)
Spring wood / early wood – inner layer represents
 more rapid spring growth
Summerwood – outer layer represents a heavier,
 harder, stronger material
(the proportion of these affects the wood density
  which affects the strength)
Heartwood – made up of dead cells and provides
 mechanical support to tree
Sapwood – part of the tree which contains living
 cells (this is less durable than the heartwood)
CHEMICAL COMPOSITION OF WOOD
Trees are made up of cells.
Dry wood cells – empty or partly filled with gums
  and resins.
Dry wood is made up of:
Cellulose – a carbohydrate, major constituent of
  wood substance
Hemicellulose – built up of one or more units of
  sugar that are found in the tree
Lignin – natural cementing material that binds cells
  and cell walls
Extractives – large variety of substances includes a
  wide variety of chemicals
TYPES OF WOOD
Hardwood – trees with broad leaves that are
 shed in winter (Oak, Maple, Aspen)
Softwood – any species that have needlelike
 leaves and are generally evergreen (conifers)
 (Pine,Larch,Cedar,Douglas Fir)
 this terms only refer to their origin and do
 not necessarily indicate their relative
 hardness
PHYSICAL PROPERTIES OF WOOD
2 most important physical properties of wood
 that affect the strength and durability of
 wood are:
MOISTURE CONTENT – the ability of wood to
 absorb or lose moisture depends on
 temperature and humidity. Different species
 differ in MC.

 MC (%) = (Wt. of water / oven-dry wt.) x 100
DENSITY AND SPECIFIC GRAVITY
Weight of lumber depends on the species,
 growth, moisture content and part of the log.
*Greenwood (fresh cut) is heavier than dry
 wood
Density of the wood- mass or weight per unit
 volume
Wood density = oven-dry wt. / volume of
 green wood
Specific gravity = (Ws / Ww (V)):density of
 wood and density of water
Specific Gravity is determined in 3 conditions:
-Green (moisture content in excess of 19%)
-Air-dry ( moisture content about 12%)
-Oven-dry

DEFECTS
-strength and durability are affected by defects
-these are natural irregularities in the structure
 of the wood
SOME DEFECTS ARE:
CRACKS – occur in various parts of tree and are
  given names (checks,shakes,splits)
     - most cracks are the result of shrinkage
CHECK – a lengthwise separation of wood,
  occurs across the annual rings
SHAKE - a lengthwise separation of wood,
  occurs between and parallel to annual rings
SPLIT – complete separation of wood fibers
(checks and shakes absorbs moisture which
  leads to decay)
KNOT – cross sectional / longitudinal section
 of the branch that was cut w/ the lumber
     - allow stress concentration to occur
SHRINKAGE AND SEASONING
Shrinkage – when wood dries further, water is
 driven off the cell walls which leads to the
 reduction in dimensions
Seasoning – controlled drying of lumber to
 improve it’s structural properties
 2 methods of drying:
 Air drying and Kiln Drying
TREATMENT AND DURABILITY
Durability – long term performance of material
Lumber in contact to the ground should be
  protected through chemical treatment. Wood
  needs to be chemically treated because:
- To prevent destruction from fungi and insects
- To inhibit combustion
DECAY AND DESTRUCTION
Wood that remain dry are durable. Those in contact
  with the ground should be treated with
  preservative. Untreated lumber may decay or be
  destroyed
DECAY – decomposition of wood substance caused
  by attack from fungi
Conditions necessary for fungal growth:
-proper temperature
-moisture content over 19%
-oxygen
-food (wood fiber)
LUMBER SIZES
Can be classified as:
1. Framing Lumber – includes grades for structural
  applications (joists, posts, trusses)
2. Appearance Lumber – non-structural grades
  intended for application where strength are not
  the primary consideration (paneling, siding)
3. Industrial lumber – broad category of structural
  and non-structural intended for specific
  applications (mining, scafolding, foundations)
Softwood lumber – manufactured wood, derived
  from softwood log in a mill
Rough Lumber – has surface imperfections due to
  sawing
Dressed lumber – planed or sanded of
  combinations of sides and edges
Board foot – unit of measurement for most
  lumber items, piece 1 inch thick by 1 foot
  wide by 1 foot long
Board feet = (txwxL)/12
Framing Lumber is divided into 3 classes:
Dimension Lumber – any piece of rectangular
  cross section
Decking – refers to lumber 2-4 in thick and 4-
  12 in wide, intended for use in floor, roof and
  walls
Timbers
Dimension Lumber is divided into 4 use
  categories:
Light framing – (2-4 in wide)
Structural Light Framing – (2-4 in wide)
Studs – (2-6 in wide) used in load bearing walls
Structural Joists and planks – (5 in and wider)
  fit for use as floor joists, rafters, headers,
  beams and truss members
MOISTURE CONTENT CLASSIFICATION AND
  GRADE STAMP
3    softwood      lumber Moisture    Content
  Classification :
Surfaced dry (s-dry)
Surfaced green (S-grn)
15% maximum moisture content (MC15)
Grading – uniform grading follow the American
  Softwood Lumber Standards PS20-70
6 allowable properties are associated with
 stress grade;
Modulus of elasticity
Tensile stress parallel to grain
Compressive stress parallel to grain
Compressive stress perpendicular to grain
Shear stress parallel to grain
Bending stress
Grade Stamp – grade name or number of a
 piece of lumber
MECHANICAL PROPERTIES AND ALLOWABLE VALUES
Modulus of Elasticity – depends on the moisture
 content and decreases by 1-3 % for every 1%
 increase in the moisture content
Compressive Strength – parallel to the grain is
 higher than perpendicular to the grain (parallel to
 the grain 2000-4000 psi)
Bending Strength – wood has very good bending
 strength
Tensile Strength – parallel to the grain is about 2 -
 4 times the compressive strength parallel to the
 grain. Very small tensile strength perpendicular
 to the grain
Shear Strength – lumber has small shear
  strength 700-1500psi
EFFECT OF SLOPE OF GRAIN
-refers to the angle between the direction of
  the fibers and of the edge of a piece of
  lumber
Strength properties can be approximated using
  Hankinson-type formula
N=PQ/(Psinθ+Qcosθ) or
N=P/((P/Q)sin θ+cos θ)
N-strength property at an angle θ
Q- strength perpendicular to the grain
P – strength parallel to the grain
n – empirical constant
STRENGTH RATIO AND IN-GRADE TESTING
In-grade testing - comprehensive re evaluation of
  the mechanical properties based on tests done
  on full sized members is the basis of
  recommended allowable properties of lumber
Strength Ratio – hypothetical ratio of the strength
  of a piece of lumber with visible strength
  reducing growth characteristics to its strength if
  those characteristics were absent
Strength ratio = ((h-k)/h)²
                  = (1-(k/h))²
K- height of the knot
H- beam depth
(ASTM D245)
WOOD PRODUCTS
Panel Products
  Veneered – plywood , thin sheets of wood
  Non-Veneered – manufactured from wood
  particles or fibers
2 types of Nonveneered panels
Particle boards – wood particles combined with
  a synthetic resin or glue, bonded together
Fiber boards – homogenous panel made from
  wood fibers that has not been consolidated
  under heat and pressure
Glulam – glue laminated timber, consists of
  sawn lumber laminations bonded with an
  adhesive
Plywood- panel product with an odd number of
  layers glued together
Veneer – thin sheet of wood, peeled from a log
 from which plywood is made.
Manufacture – involves placing heated lumber
 blocks into a lathe, peeling the blocks, clipping
 the resulting sheets to size and stacking the
 clipped sheets. Sheets are then dried to a target
 moisture content. Adhesives are applied and
 assembled. Panels are consolidated on a cold
 press and later loaded on a hot press where heat
 and pressure are applied. Then, it’s set aside for
 the adhesive to cure, then trimmed. After
 sanding and patching, panels are strapped in
 bundles for shipping.
Types of Plywood
- Softwood plywood
- Hardwood plywood
Plywood is manufactured in 3 exposure
  classification classes:
Exterior type – has the exterior glue and the
  lowest Veneer grade is Grade C.
Interior type – manufactured using the interior
  glue and may include D grade veneer
Exposure-1 type – when interior plywood is
  manufactured with the exterior glue

				
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posted:3/8/2012
language:English
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