WOOD AS A CONSTRUCTION MATERIAL

Wood as a Construction Material:

· Used extensively for buildings, bridges, utility poles, piles, floor, trusses, roofs

- Natural, and

- Engineered wood products (laminates, plywood, strand board.)

· 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

· Trees are classified into 2 types based on growth

- Exogenous (most widely used in the U.S.)

==> Growth from center out by adding concentric layer of wood

- Softwoods – (conifers-evergreens)

Less dense, grow faster, have uniform stem (20 species)

- Hardwood – (deciduous)

Not used for construction

- Endogenous

==> Growth with intertwined fibers, such as bamboo. Not used widely in U.S.

Predominant physical features of tree stem

1. Bark

2. Cambium

3. Wood

4. Pith

· The wood section of a tree is made up of sapwood and heartwood

1. Sapwood

Function as storehouse for starches and as a pipeline to transport sap

2. Heartwood

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

Structure of Wood – Growth Rings

· Annual rings or tree rings are the concentric layers in the stem of exogenous trees

· Each annual ring is composed of earlywood and latewood

- Earlywood (growth during spring time)

Have large cell openings (cavities)

- Latewood (growth during summer)

Consists of dense, dark, and thick cells wall, which produce a stronger wood than earlywood

An isotropic Nature of Wood

· An isotropic ==> different properties in various directions

· Three-axis orientation in wood are

1. Longitudinal or parallel to the grain

2. Radical or across the growth rings (perpendicular to the grain)

3. Tangential or tangent to the growth rings

· An isotropic nature affects physical and mechanical properties such as shrinkage, stiffness, and strength

· The wood cells have a rectangular cross section. The center of the tubes are 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

· Cellulose

50% by weight, HMW linear polymer

· 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 material

0.1 to 3.0% of the wood material.

Include calcium, potassium, phosphate, and silica

Moisture Content

· Weight of water as a percentage of the oven-dry weight of the wood

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

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

Equilibrium Moisture Content (EMC)

· The moisture content for average atmospheric conditions (humidity and air temperature) is the EMC

· The EMC ranges from

- less than 1% at temperatures greater than 55 C (130 F) and 5% humidity to

- over 20% at temperatures less than 27 C (80 F) and 90% humidity

Wood Production

Logs

· Logs are harvested during the fall or winter due to fire hazards and respect for non-tree plant life in the forest

· Logs are transported to a sawmill where they are cut into useful dimensional shapes

Lumber

Dimensional Lumber

· 50 – 125 mm (2 – 5 inch) thick, sawing and surfacing on all four sides remove 5-10 mm from the dimensions

· Sizes include 2X4, 2X6, 2X8, 2X10, 2X12, 4X4 referring to rough cut dimensions in inches, actual sizes are less

· Lengths range from 8′ to 24′

· Uses include studs, sill, and top plates, joists, beams, rafters, trusses, and decking

Heavy Timber

Rough sawn dimensions of 4X6, 6X6, 8X8 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

Engineered Wood

· Produced by bonding together wooden strands, veneers, lumber, and other wood to produce a composite unit

· The units are tested for their responses to loading

· Examples include plywood, oriented strand-board, composite panels

· Glued laminated lumber (glulam), structural composite lumber, and wood I-joists

Wood Production Processes

Sawing into the desired shape

· Harvested wood is cut into lumber and timber at sawmills

· Quality is governed by the angle between the aging rings and the saw blade,

categorize by:

- flat sawn (< 45 degrees)

- rift sawn (45 to 80 degrees)

- vertical or edge sawn (80+ degrees)

· Vertical sawn are considered to have the highest structural value while flat sawn look nice

Seasoning

· 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

Surfacing

· 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

Preservative Treatment (optional)

Superficial (surface) treated or pressure treated by fluid penetration

Grading Systems

· Usually based on number and magnitude of its flaws

· Flaws include knots, checks, pitch pockets, shakes, stain, and many more (see below)

· Flaws affects strength, durability, workability

· High variability in grading standards due to variability of wood sources and types

· The National Bureau of Standards and the USDA is trying to produce a simple, uniform method of lumber sizing, common nomenclature, and grading standards

· There are several agencies (7 listed in the book) that are certified by the American Lumber Standards Committee

Hardwood Grades:

· The National Hardwood Lumber Association based grading of the hardwood on the amount of usable lumber each piece of standard-length lumber

· Grades are given the classification of Firsts, Seconds, Selects, and Common (No. 1, No. 2, No. 3A, or 3B)

Software Grades

· Purpose of grading

· Ensure that all lumber within a specific graded has at least the minimum mechanical or load-carrying capability

· Graded visually or by machine-stress graded

Machine Stress Grading

· Based on bending test. Measurements include bending stress, tension parallel to grain, compression parallel to grain, modulus of elasticity

· Grade is based on mechanical responses (Table 10.2)(text)

Visual Grading

· Small, clear specimens are tested to determine basic mechanical properties

· Properties are defined for each species of softwood

· Designations are: yard, structural, factory and shop, construction, standard, utility

- Sub-grades are select, select B, select C, and No.1, No. 2, and No. 3 commons, appearance and studs

Structural lumber is graded according to its intended use

- Designations are light framing, joists and plans, beam and stringers, post and timbers