1. Tree Design. Structure - Function
“Trees Grow Tall to Intercept Sunlight”
“Problems” of Height:
must support large weight:
biomass and HOH
move water/sap over long distances against gravity
structure exposed to strong winds 1

2. Engineering Requirements Imposed on Tree Structure
prevent cell collapse under large water tensions
be flexible enough to bend to reduce
wind resistance area
have sufficient bending stiffness
resist crack propagation
be able to bear crown’s weight on the stem,
as well as be able to resist forces generated
by wind Booker 1996 1

3. Stress & Strain Stress can produce change in size and shape
Strain = Stretch / Original Length
If stress applied for a short time and strain induced is small, generally fully recoverable.

4. Three Primary Stresses
Wood Subjected to
Three Primary Stresses
Compression stress that would cause shortening of dimension
Tension stress that would cause lengthening of dimension
Shear slippage of two parts of body past one another (the San Andreas fault is a shear fault)

5. Stress / Strain Curve [ Compression Parallel-to-Grain Test]
Stress at Failure
Stress *
Proportional Limit
Slope of line = Stress/Strain
MOE
Strain

6. Stress / Strain Curve [ Compression Parallel-to-Grain Test]
Maximum Load, wood fails
Stress *
P.L. Past this point, permanent deformation
Elastic Line, straight line portion. Strain is recoverable, when remove stress material recovers original shape.
Strain

7. MODULUS OF ELASTICITY (MOE)
Measure of resistance to bending, related to stiffness of a beam
Factor in the strength of a long column
MOE parallel to the grain (Young’s modulus)
Measure of resistance to elongation or shortening of a specimen under uniform tension or compression.

8. Direction stress applied Duration stress applied
Factors Affecting Strength
Specific gravity
Moisture content
Temperature
Direction stress applied
Duration stress applied

9. Relationship Longitudinal Compressive Strength to Moisture Content70 60 50 40 30 20 10
Maximum Compressive Strength
(N/mm2)
FSP
Moisture Content %
Data for Scots Pine

10. Interdependence of T and MC Effects
on Stiffness of Wood
Relative Stiffness where E20oC 100%
Temperature oC
% MC 60o o 20o 0o o
Increases in T and %MC Decrease Stiffness
Decrease in T and % MC Increase Stiffness

11. TEMPERATURE Extremes especially a problem
Heat above 150o F lose strength from hydrolysis of cellulose.
Frozen wood with High M.C. more likely to develop longitudinal splits and fractures.
“Weakness” of wood at High To and Wood at High M.C. taken advantage of -- Steam Bending.

12. Higher parallel to grain than to perpendicular.
Compressive Strength
And Tensile Strength
Higher parallel to grain
than to perpendicular.
_______________________
Related to anatomy, esp.
longitudinally oriented cells
and S2 microfibrils.
Parallel to the Grain
Photo Courtesy W.C. Brown Center, SUNY.

13. Wood is Anisotropic
Strength and elastic properties drastically different
parallel versus perpendicular to grain
_______________________
Mechanical properties
in radial vs. tangential direction usually do not differ greatly.
Perpendicular to Grain Property
Parallel to the Grain
Photo Courtesy W.C. Brown Center, SUNY.

14. Strength decreases in proportion to time over which load is applied.
Example: bending strength
Time Breaking load
1 min psi
5 min psi 1 year psi

15. Wood can deform or deflect slowly
The longer a load is supported, the lower the load that can be carried.
Wood can deform or deflect slowly
if under constant stress [Creep]
Usual example:
bookshelf sagging under full load of books.

16. SPIRAL GRAIN Longitudinal elements (tracheids, fibers, vessels)
are not parallel to
the long axis of the tree,
but in a right-handed
or left-handed spiral. 1

17. SPIRAL GRAIN More common in juvenile wood
Angles are higher near the pith. 1

18. INTERLOCKED GRAIN Orientation of the longitudinal elements
changes from right-handed
to left-handed,
or left-handed to right-handed,
and back & forth. 1