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

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

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.

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)

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

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

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.

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

Relationship Longitudinal Compressive Strength to Moisture Content 70 60 50 40 30 20 10 Maximum Compressive Strength (N/mm2) FSP Moisture Content % Data for Scots Pine

Interdependence of T and MC Effects on Stiffness of Wood Relative Stiffness where E20oC 100% Temperature oC % MC 60o 40o 20o 0o -20o 0 97 98 100 102 104 8 89 96 100 103 106 12 84 93 100 104 108 20 73 89 100 107 113 Increases in T and %MC Decrease Stiffness Decrease in T and % MC Increase Stiffness

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.

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.

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.

Strength decreases in proportion to time over which load is applied. Example: bending strength Time Breaking load 1 min. 8500 psi 5 min. 8000 psi 1 year 5350 psi

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.

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

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

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