wood defects for civil engineering students

No matter what species of timber you use, you will undoubtedly come across some pieces that contain defects.
Defects may be naturally occurring or can be man-made. Natural defects can be due to many reasons such as environmental factors, growth patterns, soil composition, etc. Man-made defects can occur at many points ... from the felling of the tree, transport, storage, sawing, drying, etc.

1. WOOD DEFECTS

2. •No matter what species of timber you use,
you will undoubtedly come across some
pieces that contain defects.
•Defects may be naturally occurring or can
be man-made. Natural defects can be due
to many reasons such as environmental
factors, growth patterns, soil composition,
etc. Man-made defects can occur at many
points ... from the felling of the tree,
transport, storage, sawing, drying, etc.

3. Hardwood
This term is usually used for wood which comes
from dicotyledonoustrees of both temperate
and tropical origin.
The xylem of these trees contains vessels and
fibresas well as tracheids.
The rays ofhardwoods are more diverse
than those of softwoods and seldom contain
resin canals.
The fibresmake the wood both stronger,
denser and more durable than softwoods. It is
difficult to drive a nail into these woods and so
they are not usually used for construction.
Hardwoods are often used for making fine
furniture. There are however many exceptions
to this generalisationas the wood of some
dicotyledonous trees is softer than some
softwoods e.g. poplar wood. Balsa wood, which
is obtained from the tree Ochroma lagopusis
one of the lightest woods known.
Examples of hardwoods include oak, maple, ash,
walnut and stinkwood of temperate origin, and
ebony and kiaatof tropical origin.
Sofwood
Softwoods come from non-flowering cone
bearing trees.
Their xylem only contains mainly (90%)
tracheidsand has no vessels.Theirwood is
therefore homogeneous. Many softwoods have
resin canals which are orientated vertically and
filled with resin which is secreted by living
parenchyma cellswhich line the canals.
Their wood contains vertically orientated
rays of one cell thickness.
Softwoods are relatively light and easily
penetrated by nails and are therefore widely
used in construction. Many commercial wood
plantations grow pines and firs as they grow
faster than hardwoods and are in many cases
more profitable. Much of the paper pulp and
plywood comes from softwoods.
Examples of softwoods include pines, fir,
redwoods and yellowwood.

4. WOOD DEFECTS
•Warp -Bow
–A curve along the face of a board that
usually runs from end to end.

5. WOOD DEFECTS
•Warp -Cupping
–Warping along the face of a board
across the width of the board. This
defect is most common of plain-sawn
lumber.

6. WOOD DEFECTS
•Warp -Crook
–Warping along the edge from one end to
the other. This is most common in wood
that was cut from the centre of the
tree near the pith.

7. WOOD DEFECTS
•Warp -Twist
–Warping in lumber where the ends twist
in opposite directions.

8. WOOD DEFECTS
•Checking
–A crack in the wood structure of a piece,
usually running lengthwise. Checks are
usually restricted to the end of a board
and do not penetrate as far as the
opposite side of a piece of sawn timber.

9. WOOD DEFECTS
•Split
–A longitudinal separation of the fibres
which extends to the opposite face of a
piece of sawn timber.

10. WOOD DEFECTS
•Wane
–The presence of bark or absence of
wood on corners of a piece of lumber.

11. WOOD DEFECTS
•Blue Stain
•A discoloration that penetrates the
wood fibre. It can be any colour
other than the natural colour of the
piece in which it is found. It is
classed as light, medium or heavy and
is generally blue or brown.

12. WOOD DEFECTS

13. WOOD DEFECTS
•Machine Burn
–A darkening of the wood due to
overheating by the machine knives or
rolls when pieces are stopped in a
machine.

14. WOOD DEFECTS
•Pitch
–An accumulation of resinous material on
the surface or in pockets below the
surface of wood. Also called gum or sap.

15. WOOD DEFECTS
•Loose Knot
–A knot that cannot be relied upon to
remain in place in the piece. Caused by a
dead branch that was not fully
integrated into the tree before it was
cut down.

16. WOOD DEFECTS
•Tight Knot
–A knot fixed by growth or position in
the wood structure so that it firmly
retains its place in the surrounding
wood.

18. WOOD DEFECTS
•Wormholes
–Small holes in the wood caused by
insects and beetles.

19. DESIGN
CONSIDERATIONS

20. Design Considerations
•Measurement of wood properties in
the lab does not reflect all the
factors which affect the behavior of
the material in engineering
application

21. Design Considerations
•For design of wood structures, the
strength properties must be
adjusted for the ff:
–Load duration
–Wet service
–Temperature
–Beam stability

22. Design Considerations
–Size
–Volume
–Flat use
–Repetitive member
–Curvature
–Column stability
–Bearing area

23. Organisms That
Degrade Wood

24. Organisms that Degrade
Wood
•Fungi
–Fungi need four essential conditions to
exist: food, proper range of
temperature, moisture, and oxygen
–Fungi attack produces stains and/or
decay damage
–To protect against fungal attack, one of
the four essential conditions for growth
must be removed.

25. Organisms that Degrade
Wood
•Construction procedures that limit
decay in buildings
–Building with dry lumber
–Using designs that keep wood
components dry

26. Organisms that Degrade
Wood
•Construction procedures that limit
decay in buildings
–Using a heartwood from decay-resistant
species or pressure-treated wood is
sections exposed to above-ground decay
hazards
–Using pressure-treated wood for
components in contact with the ground

27. Organisms that Degrade
Wood
•Insects
–Beetles and termites are the most
common wood-attacking insects
–Storage of logs in water or a water
spray prevents the parent beetle from
boring
–Quick drying or early removal of barks
also prevent beetle attack

28. Organisms that Degrade
Wood
•Insects
–Termites are one of the most
destructive insect that attacks wood
–Prevention is partially achieved by using
pressure-treated wood and prohibiting
insect entry into areas of unprotected
wood through the use of screening, sill
plates, and sealing compounds.

29. Organisms that Degrade
Wood
•Marine organisms
•Bacteria
–Bacteria cause “wet wood” and “black
heartwood” in living trees and a general
degradation of lumber
–Wet wood –water-soaked condition that
occupies the stem centers of living
trees

30. Organisms that Degrade
Wood
•Bacteria
–Black heartwood causes the center of
the stem to turn dark brown or black
–Bacterial growth is sometimes fostered
by prolonged storage in contact with
soils
–This type of bacterial activity produces
softening of the outer wood layers
which results in excessive shrinkage
when redried.

31. Wood Preservation
Petroleum-Based and Waterborne
Solutions

32. Petroleum-Based
Solutions
•Coal-tar creosote, petroleum
creosote, creosote solutions, and
pentachlorophenol solutions are some
of the oil-based preservatives
•These preservatives are very
effective, but some are
environmentally sensitive

33. Petroleum-Based
Solutions
•They are commonly used where a high
degree of environmental exposure
exists and human contact is not a
concern
•Applications include utility poles,
railroad ties, and retaining walls

34. Waterborne
Preservatives
•Ammoniacal copper arsenate,
chromated copper arsenate, and
ammoniacal copper zinc arsenate
•The advantage over oil-based are
cleanliness and its ability to be
painted

35. Waterborne
Preservatives
•The disadvantage is their removal by
leaching when exposed to moist
conditions over long periods of time
•They are also environmentally
sensitive and must be applied under
carefully controlled conditions

36. Application Techniques

37. Application Techniques
•Superficial treatments are generally
not effective
•Pressure-treated wood has greater
resistance to degradation
•Pressure treatment does not
thoroughly treat the entire cross-
section but provide a “shell” of
treatment

38. Application Techniques
•It is recommended that structural
members be fabricated as much as
possible before treatment
•With proper treatment, decay can be
eliminated for an extended period of
time

39. Construction
Precautions

40. Construction Precautions
•Care should be taken to avoid inhaling
saw dust during cutting of the wood
•Hands should be washed after
handling pressure-treated wood
•Clothes should be washed separately
from other items
•Waste material must be disposed of
properly; it should not be burned.

41. Engineered Wood
Products

42. Engineered Woods
•Engineered wood includes a variety of
products manufactured by bonding
together wood strands, veneers,
lumber, or other forms of wood
fibers to produce large and integral
units

43. Engineered Woods
•These products are “engineered” to
produce specific and consistent
mechanical behavior and thus have
consistent design properties
•An engineered product consists of
wood stock material glued together
with an appropriate adhesive

44. Engineered Woods
•These products are “engineered” to
produce specific and consistent
mechanical behavior and thus have
consistent design properties
•An engineered product consists of
wood stock material glued together
with an appropriate adhesive

45. Engineered Woods
•Need to increase efficiency of using
wood products
•Engineered woods are frequently more
economical wood particularly when
large dimensions are required.
•Engineered woods can have
characteristics superior to natural
wood