Lecture #19 Structure of Wood

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

WOOD Introduction Definitions Macrostructure Microstructure Molecular Structure Cell Wall Structure

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.

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

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

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

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

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

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

Common Sawing Patterns

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

Microstructure softwoods Figure 14.5

Microstructure softwoods Figure 14.5

Microstructure

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 (0.2-1.2mm long; 0.5mm wide) vertical transport **tracheids parenchyma cells pits interconnect cells

Microstructure hardwoods Figure 14.6

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

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

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 50-700 from vertical S2 layer thick 10-300 from vertical S3 layer very thin similar to S1 layer

Microstructure softwoods Figure 14.4

Cell Wall Structure