Wood as Construction Material


Wood is widely used for constructions of buildings, bridges, utility poles, piles, floor, tethers and roofs

Advantages of using wood as construction material

  • It is Natural and found easily in nature
  • Intricately engineered wood products (laminates, plywood, and strand board.)
  • Economic, easy availability, ease of use, and recyclable

Wood is renewable natural products from different types of trees. There are more than 600 varieties of the trees in the United States alone.

Types of woods

  • Exogenous (most extensively used in the U.S.)
  • Growth from the center out by toting up concentric levels of wood
  1. Softwoods – (conifers-evergreens)

Not very dense, grows rapidly, have unvarying stem (20 species)

  1. Hardwood – (deciduous)

These are not used for constructions

  1. Endogenous

These species grow with entwined fibers like bamboo. Not used often in United States

Predominant physical features of tree stem

  1. Cambium
  2. Bark
  3. Pith
  4. Wood
  • The wood part of the trees consist heartwood and sapwood.
  1. Sapwood

It functions as the storehouse for starches and a pipeline to transport the sap

  1. Heartwood

The cells are altered physically and chemically manipulated by mineral deposit. It gives structural strength for trees

Composition of Wood – Growth rings

  • Yearly rings or tree rings are the concentric levels in stem of the exogenous trees
  • Individual yearly ring is made of latewood and earlywood and latewood

– Early wood (grows in spring time)

They have very big cell openings (cavities)

– Latewood (grows in summer)

They consists of dense, dark, and thick cells wall, which produce a stronger wood than early wood

 

The isotropic Nature of the wood

  • the isotropic nature refers to the different properties in diverse directions
  • The 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

 

The isotropic nature alters the physical, chemical, and mechanical properties such as contraction, stiffness, and potency.

  • Wood cells have a rectangular cross section. The center of the tubes is vacant. The tube formation opposes stress similar to its length, however, it will distort when laden 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 force.

  • Hemicellulose

15-20% of softwood

20 % to 30% of hardwood.

Polymeric units made from sugar molecules.

Xylone in hardwoods, and mannose in softwood

  • Extractives

5%-30% of the wood substance

Consist poly-phenolics, coloring material, fats and oils, resins, waxes, gums, starches. Is soluble in the water, alcohol, benzene and acetone

  • Ash-forming material

0.1 to 3.0% of the wood material.

Consists calcium, potassium, phosphate, and silica

Moisture Content

  • Weight of water as a percentage of the oven-dry weight of the wood
  • Oven-dried is achievable in an oven at 100 C to 150 C until the wood attains a stable weight
  • Physical properties, such as weight, contraction and strength is based on the moisture content of wood

 

Types of water in wood:

  • Bound water

Held within the cell wall by absorption forces

  • Free water exist as either condensed water or water vapor

 

Fiber Saturation Point (FSP)

  • The level at which the cell walls are entirely saturated, but no free water exists in the cell cavities
  • FSP differs among species, generally range in 21-32%
  • Physical and mechanical properties are based on the FSP

Contraction

  • If the moisture content is greater than the FSP, the wood is dimensionally constant
  • Construction might affect with the moisture content less than the FSP
  • Happens when moisture is lost from cell walls
  • Engorgement happens when moisture is absorbed in the cell walls
  • Contraction in the radial direction is basicallt one-half the change in the tangential direction
  • Contraction in the longitudinal direction is normally minimal, range varying from 0.1 to 0.2% for a alteration in the moisture content from FSP to oven dry

 

Equilibrium Moisture Content (EMC)

  • The moisture content for standard atmospheric conditions (humidity and air temperature) is the EMC
  • The EMC ranges from

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

– Greater than 20% at temperatures less than 27 C (80 F) and 90% humidity

Wood Production
Logs
  • Logs are harvested at some point in the fall or winter because of fire hazards and regards 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 erases 5-10 mm from the dimensions
  • Sizes consist 2X4, 2X6, 2X8, 2X10, 2X12, 4X4 referring to rough cut dimensions in inches, actual sizes are less
  • Lengths vary from 8′ to 24′
  • Uses consists 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 because of surfacing.

– Uses consist of heavy-frame construction, landscaping, railroad ties, and marine construction

Round Stock

Poles and posts used for building, marine pilings, and utility poles

 

Engineered Wood
  • Created by bonding together wooden strands, veneers, lumber, and other wood to create a composite unit
  • The units are tested for their responses to loading
  • Examples consist 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 then cut into lumber and timber mills
  • Quality is monitored by the angle between the aging rings and sawing blade,

Categorized 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 consist 30 to 200% moisture by oven-dried weight, this is lessened to 7% for dry areas or up to 14% in moist areas, leaving a saw mill, wood is at 15% moisture
  • Air drying (cheap and slow)

– Stack boards with air space between them to allow drying

– In 3 to 4 months, it achieves the local humidity level

– Recurrently needs additional dying to reach satisfactory levels

  • Kiln drying (scientific and expensive)

– Boards dried at 70-120 F for 4-10 days

– Drying may lead to cracks and distorted lumber, and post-process wood is thirsty, so it should be enclosed and cared for appropriately

Surfacing
  • Planning (surfacing) to create a smooth surface
  • Post-drying surfacing results to greater quality lumber because it erases small deforms developed during the drying process
  • 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
  • Normally dependent on number and magnitude of its flaws
  • Flaws consist knots, checks, pitch pockets, shakes, stain
  • Flaws affect potency, stability, workability
  • High degree of variability in grading standards because of the variability of wood sources and types
  • The National Bureau of Standards and the USDA are attempting to create an easy, consistent technique of lumber sizing, common nomenclature, and grading standards
  • There are various agencies (7 listed in the book) that are licensed 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
  • Make sure that all lumber inside a particular graded has at least the bare minimum mechanical or load-carrying capability
  • Graded visually or by machine-stress graded
Machine Stress Grading
  • Depends on bending test. Measurements consist of 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, precise samples are tested to establish basic mechanical properties
  • Properties are specified for each species of softwood
  • Designations are: yard, structural, factory and shop, construction, standard, utility