This document discusses the elastic properties and defects of wood. It describes how wood exhibits ductility and has a fairly straight stress-strain curve over a considerable range before curving off, as wood does not have a defined elastic limit. It also discusses the modulus of elasticity, compressive strength, tensile strength, and bending strength of wood in relation to grain direction. Finally, it outlines several common defects in wood like checks, shakes, knots, heart shake, and star shake that can occur during growth or processing.

1. Engineering construction
Prof. Syed Ali Hussnain
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2. Elastic Properties Of Wood
• There is no sharply defined elastic limit in
wood.
• However, the stress-strain diagram in any
direction is fairly straight over a considerable
range before it gradually curves off.
• It is a ductile material.
• The relative stress-strain curves for direct
tension, direct compression and bending
stress intensities parallel to the grain in the
figure which is given at the next page.
• Modulus of elasticity of the grain is
practically the same in direct tension, direct
compression and bending.
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3. Elastic Properties Of Wood
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Graph of Stress Strain

4. Elastic Properties Of Wood
o COMPRESSIVE STRENGTH
• When subjected to compressive force
acting parallel to the axis of growth, wood is
found to be one of the strongest structural
material.
• However, compressive strength
perpendicular to fibers of wood is much lower
than that parallel to fibers of wood.
• When wood is subjected to compression
parallel to grain, it may fail through collapsing
of the cell walls or through bending of cells
and fibers.
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5. Elastic Properties Of Wood
o TENSILE STRENGTH
• When a properly shaped wooden stick is
subjected to tensile forces acting parallel to the
grain it is found to have greater strength that can
be developed under any other kind of stresses.
• The tensile strength parallel to the grain is two
to four times the compressive strength, the
latter governs the strength of beams.
• The tensile strength parallel to the grain is
influenced to some extent by nature of the wood
elements and their arrangement, but principally by
the straightness of the grain and the thickness of
the walls of the longitudinal elements.
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6. Elastic Properties Of Wood
o BENDING STRENGTH
• Wood well withstands static bending, owing
to which it is widely employed for elements of
buildings, e.g. beams, slabs, trusses, etc.
• The initial failure of long beams of uniform
width is indicated by a wrinkling of the
overstressed compression fibers.
• Very dry specimens sometimes fail very
suddenly in tension before any wrinkling of the
compression fibers is noticeable.
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7. Decay of Timber
• Timber does not deteriorate by natural,
physical and chemical changes or by pure
ageing.
• It is, however, affected by destructive
elements, such as weathering, chemical attack,
fungi, insects or rodents.
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8. Defects In Timber
• Defects can occur in timber at various
stages, principally during the growing period
and during the conversion and seasoning
process.
• Defects affect the quality, reduce the
quantity of useful wood, reduce the strength,
spoil the appearance and favour its decay.
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9. Defects In Timber
• Following are the some of the important
defects commonly found in wood due to abnormal
growth or rupture of tissues due to natural
forces.
o CHEEKS
• cheeks is the longitudinal crack which is usually
normal to the annual rings.
• These adversely affect the durability of timber
because they readily admit moisture and air.
o SHAKES
• Shakes are longitudinal separations in the wood
between the annual rings.
• These lengthwise separations reduce the
allowable shear strength without much effect on
compressive and tensile values.
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10. Defects In Timber
• The separations make the wood undesirable
when appearance is important.
o KNOTS
• Knots are bases of twigs or branches buried
by cambial activity of the mother branch.
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11. Defects In Timber
o HEART SHAKE
• Heart shake occurs due to shrinkage of
heart wood, when tree is over matured.
• Cracks start from center
And run towards sap wood.
• These are the wider at the
Centre and diminish outwards.
o STAR SHAKE
• Star shakes are radial splits
or cracks wide at
Circumference and diminishing
Towards the centre of the tree. 11