Lesson 9 - Timber Flashcards
comes from a deciduous tree which loses its leaves annually
Hardwood
tend to be slower growing, and are therefore usually more dense.
Hardwood
an angiosperm, a plant that produces seeds with
some sort of covering such as a shell or a fruit. Angiosperms usually form flowers to reproduce. Birds and insects attracted to
the flowers carry the pollen to other trees and when fertilized
the trees form fruits or nuts and seeds. It includes
eucalyptus, yakal, apitong
Hardwood
comes from a conifer, which usually remains evergreen.
Softwood
known as a gymnosperm. Gymnosperms reproduce by forming cones which emit pollen to be spread by the wind to other trees. Pollinated trees form naked seeds which are dropped to the ground or borne on the wind so that new trees can grow elsewhere. Some examples of it includes
pine, agoho
Softwood
do tend to be less dense than deciduous trees, and
therefore easier to cut, while most hardwoods tend to be more
dense, and therefore sturdier. In practical terms, this denseness
also means that the wood will split if you pound a nail into it. Thus
you need to drill screw or bolt holes to fasten hardwood together.
But structural lumber is soft and light, accepts nails easily without
splitting and thus is great for general construction.
Evergreens
wood that has been recently cut and therefore has not had an
opportunity to season (dry) by evaporation of the internal
moisture. It is considered to have 100% moisture
content relative to air-dried
Green wood
Natural Characteristics Affecting Mechanical Properties of Wood
- Knots
- Reaction Woods
- Juvenile Wood
- Pitch Pocket
- Bird Peck
- Wane
- Warp
- Check
- Shake
- Split
- Stain
- Spalt
- Dry Rot
- Wormholes
that portion of a branch that has become incorporated in the bole
of a tree.
Knots
abnormal woody tissue is frequently associated with leaning boles
and crooked limbs of both conifers and hardwoods.
Reaction woods
the wood produced near the pith of the tree, it has considerably different physical and anatomical properties than that of mature wood
Juvenile wood
a well-defined opening that contains free resin. The pocket
extends parallel to the annual rings; it is almost flat on the pith
side and curved on the bark side. Accumulation of resinous
material (gum or sap)
Pitch pocket
damage caused by woodpeckers, most of the time small holes
around the wood.
Bird peck
the presence of bark or absence of wood on the corner of along
the length of a lumber
Wane
distortion in wood due to shrinkage and swelling( eg. Twist, cup,
bow, kink and crook/spring)
Warp
A crack in the wood structure of a piece, usually running
lengthwise, usually restricted to the end of a board and do not
penetrate as far as the opposite side of a piece of sawn timber.
Check
separation of grain between the growth rings, often extending
along the boards face and sometimes below its surface
Shake
longitudinal separation of the fibers which extends to the opposite
face of the lumber
Split
discoloration that penetrates the wood fiber, can be any color
other than the natural color of wood (blue or brown)
Stain
any form of discoloration caused by fungi (found in dead trees)
Spalt
fungus breaks down wood fibers and renders the wood weak and
brittle
Dry rot
small holes caused by insects (beetles)
Wormholes
Strength of wood
- Tensile strength
- Compression strength
- Flexural strength
- Elastic properties
// to the grain is much higher (~3x) than compressive strength
Tensile strength
Limiting factor for tension members is compression or shear at the
point of concentration
Tensile strength
perpendicular to the grain is ~(1/3) of the strength // to the grain
Tensile strength
// to the grain is 3~4x than compressive strength when load is perpendicular to the grain
Compression strength
Failure when load is perpendicular to the grain: crushing of wood fiber
Compression strength
Failure when load is // to the grain: bending or buckling of wood fibers
Compression strength
critical factors are:
• comp strength // to the grain
• shear strength // to the grain
Flexural strength
shear strength is very low but if wood is free of defect, initial failure will be compressive
Flexural strength
shear strength is very low but if wood is free of defect, initial failure will be compressive
Flexural strength
Factors affecting Strength of Wood
- direction of wood fibers
- moisture
- weight
- Rate of growth
(strength depends heavily of the direction of load with respect to the grain)
Direction of wood fibres
(drying beyond the FSP (Fiber saturation point), leads to increase in strength)
Moisture
(denser wood is stronger)
Weight
- for air-dry timber
- for green timber
Respectively
- modulus of rupture = 26,200 x (specific gravity)1.25
- modulus of rupture = 18,500 x (specific gravity) 1.25
greater number of annual rings per unit length gives higher strength
Rate of growth
