1. Lecture #19
Structure of Wood

2. WOOD one of oldest construction materials
easily produced & handled
widely used
light construction
forming material (PCC)
only naturally renewable building material

3. WOOD Introduction Definitions Macrostructure Microstructure
Molecular Structure
Cell Wall Structure

4. WOOD comparison w/ other materials
economical
aesthetically pleasing
specific properties (per unit weight)
subject to decay, fire
more complex (natural, not manufactured)
variation in properties
composite
many flaws, imperfections, defects
high degree of anisotropy
manufactured products to overcome deficiencies
1) At least 30,000 species exist.
2) Wood is a composite. Must be examined on a molecular, microscopic, an macroscopic level
3) Wood contains many flaws and imperfections.
4) Wood is anisotropic.
Manufactured wood products are intended to overcome many of the nature defects of wood materials.

5. Comparison of Specific Properties
On a unit weight basis, wood does very well.

6. WOOD vs. TIMBER wood timber small, clear specimens
free of macroscopic defects
timber
sawn structural members
many macroscopic defects

8. Does not indicate actual hardness
Types of Trees
Does not indicate actual hardness
softwood
conifer
needlelike or scale-like leaves
fir
pine
faster growing
most commercial lumber
hardwood
deciduous
broad leaves
oak
maple
walnut
ash

9. Structure of a Tree root system trunk crown anchor
absorption of moisture & minerals
trunk
support crown
transport
* structural timber
crown
produce food, seed
Figure 14.1 right side

10. Macrostructure outer bark inner bark cambium sapwood protection
transport
cambium
growth
sapwood
outside
moisture conduction
food storage
Figure 14.1 left side

11. Macrostructure heartwood annual rings pith inside preserve dead cells
support
annual rings
springwood (light)
large cell, thin walls
summerwood (dark)
small cells, thick walls
pith
center

13. Common Sawing Patterns

14. Microstructure bundle of thin-walled tubes glued together
proportion of cell types determines density; mechanical properties
softwoods
tracheids (90%)
longitudinal (2-5mm long)
l/d = 100
support
vertical transport
parenchyma cells
transverse (200 x 30mm)
in rays, radially
food storage
horizontal transport
pits interconnect cells

15. Microstructure
softwoods
Figure 14.5

16. Microstructure
softwoods
Figure 14.5

17. Microstructure

18. Microstructure bundle of thin-walled tubes glued together
proportion of cell types determines density; mechanical properties
hardwoods
**fibers
longitudinal (0.7-3mm long)
l/d = 100
support
**vessels (pores)
longitudinal ( mm long; 0.5mm wide)
vertical transport
**tracheids
parenchyma cells
pits interconnect cells

19. Microstructure
hardwoods
Figure 14.6

20. Molecular Structure cellulose hemicellulose lignin extractives
linear polymer of glucose units
covalent bonds within & between glucose units
hydrogen & vdW bonding between bundles (OH-)
creating microfibrils
OH- attract water
swelling, shrinking
hemicellulose
polymeric of various sugars
bond cellulose units
lignin
complex structure
stiffness, compressive strength
extractives
not basic component

21. Molecular Structure organic material 50% C; 44% O; 6% H; .1% N
Table 14.3

22. Cell Wall Structure middle lamella different microfibril orientations
join cells
different microfibril orientations
primary wall
formed first
thin
random
secondary wall
S1 layer
thin
4-6 microfibril layers
alternate helices from vertical
S2 layer
thick
from vertical
S3 layer
very thin
similar to S1 layer

23. Microstructure
softwoods
Figure 14.4

24. Cell Wall Structure