LECTURE NO. 20 (Handout)TIMBER

Objectives:• To explain the engineering properties and usages of

timber

INTRODUCTION

• The "wood" used for building, carpentry or various other engineering purposes is termed as "timber".

• Wood is used extensively for buildings, bridges, utility poles, piles, floor, trusses, roofs

• Wood is used in the following forms: – Natural form, and – Engineered wood products (laminates, plywood, strand board,

etc)

• Low cost, availability, ease of use, and renewable

• Wood is natural, renewable product from trees. There are more than 600 species of trees in the U.S alone

INTRODUCTION

• Trees are classified into two types based on growth– Exogenous: Growth from center out by adding concentric layer of wood– Endogenous: Growth with intertwined fibers, such as bamboo

• Predominant physical features of tree stem– Bark– Cambium– Wood (sap wood and heartwood)– Pith

Sapwood functions as storehouse for starches and as a pipeline to transport sap

Heartwood are cells that are chemically and physically altered by mineral deposit. It provides structural strength for the tree

Annual rings or tree rings are the concentric layers in the stem of exogenous treesEach annual ring is composed of earlywood and latewood. Earlywood grows during spring time and has large cell openings (cavities). Latewood grows during summer and consists of dense, dark, and thick cells wall, which produce a stronger wood than earlywood

ANISOTROPIC NATURE OF WOOD

• Wood is an anisotropic material because it has different properties in various directions

• Three-axis orientation in wood are– Longitudinal or parallel to the grain

– Radical or across the growth rings (perpendicular to the grain)

– Tangential or tangent to the growth rings

• An isotropic nature affects physical and mechanical properties of wood such as shrinkage, stiffness, and strength

• The wood cells have a rectangular cross section. The center of the tubes is hollow. The tube structure resists stresses parallel to its length, but it will deform when loaded on its side

• Tubes are 100/1 (length to diameter)

CHEMICAL COMPOSITION OF WOOD

• Cellulose: 50% by weight • Lignin:

– 23-33% in softwoods, 16-26% in hardwoods– It is the glue for the cells. It controls the shear strength.

• Hemicellulose:– 15-20% of softwood and 20-30% of hardwood.– Polymeric units made from sugar molecules. Xylone in

hardwoods, mannose in softwood. • Extractives:

– 5-30% of the wood substance– Include poly-phenolics, coloring material, oils and fats, resins,

waxes, gums, starches.– Soluble in water, alcohol, acetone, and benzene

• Ash-forming materials:– 0.1to 3.0% of the wood material– Include calcium, potassium, phosphate, and silica

WATER IN WOOD

Types of water in wood:• Bound water: held within the cell wall by absorption forces

• Free water exists as either condensed water or water vapor

Fiber Saturation Point (FSP):• The level at which the cell walls are completely saturated, but no free water

exists in the cell cavities • FSP varies among tree species, typically range in 21-32% • Physical and mechanical properties are dependent on the FSP

Shrinkage:• If the moisture content is higher than the FSP, the wood is dimensionally

stable • Shrinkage may result with the moisture content less than the FSP • Occurs when moisture is lost from cell walls • Swelling occurs when moisture is gained in the cell walls • Shrinkage in the radial direction is generally one-half the change in the

tangential direction • Shrinkage in the longitudinal direction is usually minimal, ranging from 0.1

to 0.2% for a change in the moisture content from FSP to oven dry

WOOD PRODUCTION PROCESSES

Wood is produced through following processes: • Harvesting• Sawing into desired shapes and sizes• Seasoning• Surfacing• Preservation

WOOD PRODUCTION PROCESSES:Harvesting of wood

Logs of wood are harvested during the fall or winter due to fire hazards and also cutting the trees at the end of

their non-tree plant lives in the forest

WOOD PRODUCTION PROCESSES:Sawing of wood into desired shapes and sizes

• Harvested logs are transported to sawmill where they are cut into following useful dimensional shapes:

Lumber• 50 - 125 mm (2 - 5 inch) thick, sawing and surfacing on all four sides

remove 5-10 mm from the dimensions • Sizes include 24, 26, 28, 210, 212, 44 referring to rough cut

dimensions in inches, actual sizes are less • Lengths range from 8 to 24 ft• Uses include studs, sill, and top plates, joists, beams, rafters,

trusses, and decking

Heavy timber• Rough sawn dimensions of 46, 66, 88 reduced by 10 mm per side

due to surfacing.• Uses include heavy-frame construction, landscaping, railroad ties,

and marine construction

Round stock • Poles and posts used for building, marine pilings, and utility poles

WOOD PRODUCTION PROCESSES:Seasoning of wood

• Seasoning is the process of removing moisture from a harvested wood

• Green wood contains 30 to 200% moisture by oven-dried weight, this is lowered to 7% for dry areas or up to 14% in damp areas, leaving a saw mill, wood is at 15% moisture

Air drying (inexpensive and slow)• Stack boards with air space between them to allow drying• After 3 to 4 months, it reaches the local humidity level• Often requires further dying to reach acceptable levels

Kiln drying (scientific and expensive)• Boards dried at 70-120 F for 4-10 days• Rapid drying may result in cracks and deformed lumber, and post-

process wood is thirsty, so it must be covered and cared for properly

WOOD PRODUCTION PROCESSES:Surfacing of wood

• Planning (surfacing) to produce a smooth surface• Post-drying surfacing yields higher quality lumber

because it removes small defects developed during the drying

• In case of pre-surfacing, the dimensions are slightly increases to compensate for shrinkage during seasoning

WOOD PRODUCTION PROCESSES:Wood preservation

• The wood needs to be preserved against the degradation caused by various organisms such as: fungi, bacteria, insects, and marine organisms.

• The quality of preservation depends on the following: The type of preservative The degree of penetration by preservative The amount of the chemical retained in the wood

Types of preservatives• Paints • Petroleum-based solutions

• These are very effective but environmentally sensitive. • Used where a high degree of environmental exposure exists and human contact is not a

concern such as utility poles, railroad ties, retaining walls

• Waterborne preservatives (salts)• Ammoniacal copper arsenate• Chromated copper arsenate• Ammoniocal copper zinc arsenate• Advantages: Cleanliness and ability to be painted• Disadvantages: Their removal by leaching when exposed to moist conditions over long

periods of time. Environmentally sensitive.

• Used for wood structure such as residual decks and fences

WOOD PRODUCTION PROCESSES:Wood preservation---contd.

Preservative application techniques

• Superficial treatment– Techniques include coatings applied by painting, spraying, or

immersion

• Fluid penetration process– Occurs by capillary action and is a function of surface tension,

angle of contact,– time, temperature, and pressure

• Pressure-treated wood has greater resistance to degradation than surface-treated wood because the preservative is forced into the entire structure of the wood

ENGINEERED WOOD PRODUCTS

Following engineered wood products are manufactured by bonding wood strands, veneers, lumber, or fibers. • Glued - laminated timber (glulam) • Structural composite lumber (same dimensions as sawn wood

dimensional lumber) • Structural strand panels (plywood, orientated strand board,

and composite panels) • Wood I-Joists

Quality and serviceability depend on:• Gluing properties and wood preparation • Type of adhesive • Quality control in the gluing process

Adhesives used for gluing:• Natural (casein, vegetable protein, and blood protein glues) • Synthetic (phenol, urea, resorcinol, polyvinyl, and epoxy

resins)

ENGINEERED WOOD PRODUCTSGlued-laminated timbers

Douglas-fir and southern pine are the most common

Advantages:– Ease of manufacturing large members – Can design large members whose cross-sections vary along

their length – Can use low grade wood for less stressed areas – Minimal seasoning defects

Factors that affect strength:– Cross-grain – Knots – Effect of end joining

ENGINEERED WOOD PRODUCTSPlywood

Thin sheets of wood (plies) that are glued in layers

Production of Plywood:– Logs are saturated. Six hours before processing are moved into

boiling water. – Bark is removed and logs are cut into eight-foot sections – A continuous sheet of veneer is peeled from the log – Veneer is seasoned and dried – Assemble, glue, and press veneer

Classification based on:– Type of wood used – Number of plies – Grade of plies – Type of adhesive

Properties of Plies:– Adjacent sheets have grain that runs perpendicular to each other – The middle plies in even - plied panels have the same grain orientation – Plies are 1.6 mm to 7.9 mm thick – Plywood panels are 3.2 to 29 mm thick

ENGINEERED WOOD PRODUCTSParticle Board and Strand Board

• Manufactured by gluing together "scraps" produces

• Particle board is made from sawdust-sized particles

• Strand board is made from flat chips

• These products are replacing plywood in many applications because it is cheaper

PHYSICAL PROPERTIES OF WOOD:Moisture content (MC)

• MC is the weight of water as a percentage of the oven-dry weight of the wood

• Oven-dried is attained in an oven at 100ºC to 150ºC until the wood attains a constant weight

• Physical properties, such as weight, shrinkage and strength depend on the moisture content of wood

weight of waterMC (%) = 100

oven-dry weight

PHYSICAL PROPERTIES OF WOOD:Specific gravity (G)

• Specific gravity of wood is determined in the oven-dry condition, as:

• G is a good indicator of mechanical properties

• G depends on cell size, cell-wall thickness, and the number and type of cells.

• G for cell material is 1.5

oven-dry weightG =

density of water volume of green wood

PHYSICAL PROPERTIES OF WOOD:Density or unit weight ()

• Following equation is recommended to calculate density of wood at any moisture content:

Where: G is the specific gravity in oven-dry condition and MC is the moisture content in percent.

• The dry density of wood can range from 160 kg/m3 (10 lb/ft3) to 1000 kg/m3 (65 lb/ft3) depending on tree species

• The range of the majority is 320 - 720 kg/m3 (20 to 45 lb/ft3

100

100

)(009.01

4.62)/( 3 MC

MCG

Gftlb

PHYSICAL PROPERTIES OF WOOD:Thermal conductivity, Thermal diffusivity and Electrical resistivity

Thermal Conductivity: • It is the rate of heat flow• Thermal conductivity for wood is a fraction of most metals and 3 to 4 times greater

than most common insulating material • It depends on grain orientation, moisture content, specific gravity, extractive

content, and irregularities • Heat flow parallel to grain is 2.0 to 2.8 times greater than in the radial direction • As the moisture content increases from 0 to 40%, the thermal conductivity

increases by about 30% • It has linear correlation with specific gravity meaning heavier wood conduct heat

faster

Thermal Diffusivity: • It is a measure of the rate at which a material absorbs heat from its surroundings• For wood, it is much smaller than that of other common building materials• Thermal diffusivity value of wood averages 0.006 mm/s (0.00025 inch/s)

Electrical Resistivity:• Air-dry wood is a good electrical insulator • Resistivity decreases by a factor of three for each percentage increase in moisture

content• Wood has the resistivity of water when it reaches the fiber saturation point (FSP)

PHYSICAL PROPERTIES OF WOOD:Specific heat and Coefficient of thermal expansion

Specific Heat:• It is the ratio of the quantity of heat required to raise the

temperature of a material 1 degree to that required for raising the temperature of an equal mass of water by 1 degree

• For wood, it is dependent on moisture and temperature• Species and density have little or no effect on specific heat

Coefficient of Thermal Expansion: • It is a measure of dimensional changes caused by a temperature

variance• Longitudinal (parallel to grain) coefficient values range from 0.009

to 0.0014 mm/m/°C (0.0000017 to 0.0000025 inch/inch/°F)• Coefficients are 5 to 10 times greater across the grain• Moist wood that is heated, expands due to thermal expansion and

shrinks due to moisture loss (below FSP) which usually results in a net shrinkage

MECHANICAL PROPERTIES OF WOOD:Strength

• Vary widely depending on the tree species and direction of the grain relative to the direction of the force

• Compressive and tensile strengths in the direction parallel to grain are found to be several times more than that in the direction perpendicular to grain

• Columns, posts, and members of a truss subjected to axial loads are the examples of loads parallel to grain.

• A vertical member supported to a horizontal member is an example of load perpendicular to grain

• The compressive strength in the direction parallel to the grain is around more than 10 times the strength perpendicular to the grain

• The tensile strength in the direction parallel to the grain is about more than 30 times the strength perpendicular to the grain

• The tensile strength parallel to the grain is larger than the compressive strength in the same direction

MECHANICAL PROPERTIES OF WOOD:Modulus of elasticity, Creep, and Damping capacity

Modulus of Elasticity:• Wood starts off having a linear stress-strain curve, then a small non-

linear curve occurs before it fails • Important factors are:

– Tree species,– Variation in moisture content, and– Specific gravity

• Wood is an isotropic material (different stress-strain relations exist for different directions)

Creep:• Wood continuously deforms and creeps under sustained loads

Damping Capacity:• Reduction in amplitude of vibration over time due to internal friction

within material and resistance to support system • Higher moisture content means a proportional increase in damping up to

FSP • Wood structures dampen vibrations more quickly than metal structures• Damping capacity of wood parallel to grain is 10 times that of structural

metals

MECHANICAL PROPERTIES OF WOOD

SOME COMMON DEFECTS IN WOOD