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• Mechanical properties of wood refer to its strength and resistance to deformation
• Mechanical properties are essential when considering wood or wood products for structural building applications or in the design of wooden structures such as beams, columns, trusses, and joists
• Knowledge on mechanical properties of wood is also essential in the selection of species for non-structural applications such as for sporting instruments, tool handles, bookshelves, and ladders
• MOE in bending


  Measure of the stiffness of wood as a beam
• MOR


  Accepted criterion for the maximum carrying capacity of a beam
• Maximum crushing strength


  Maximum stress sustained by compression parallel to the grain
• Compression strength perpendicular to the grain


  Required in applications such as railways, sleepers, rollers, wedges, bearing blocks, bolted timbers and other applications where resistance to crushing is an important property
• Shear strength parallel to grain


  Measure of ability to resist internal slipping of one part upon another along the grain
• Tensile strength


  Due to the difficulties of testing wood in tensile loading, limited data are available for tensile strength parallel or perpendicular to the grain
• Impact bending


  Ability to absorb shocks that cause stresses beyond the proportional limit
• Hardness


  Resistance to wear and marring
• Cleavage


  Indication on how easily wood will split
• Load


  • External forces characterized by having magnitude, direction, and speed of application
• Types of forces


  • Tensile
  • Compressive
  • Bending
  • Shear
  • Torsion
• Axial forces


  Collinear with the longitudinal axis of the member, causing the member to either be stretched or be shortened
• Shear forces


  Unaligned forces pushing one part of a body in one specific direction, and another part of the body in the opposite direction, causing one section to slide past its adjacent section
• Transverse forces


  Applied perpendicular to the longitudinal axis of a member, causing the member to bend and deflect from its original position
• Types of loads based on speed

  • Dead load
  • Static load
  • Rolling load
  • Impact load
• Stress


  Resultant of internal forces within a body that resist change in size and shape
• Strain


  Reduction or elongation in length per unit length resulting from the action of an applied load
• Elasticity


  Property of a material to resist deformation and recover its original shape after removal of the deforming force
• Hooke's law


  Deformation of an elastic body is directly proportional to the applied stress, provided that the elastic limit is not exceeded
• Modulus of elasticity (E)

  Ratio of stress to strain within the elastic limit
• The deformation is directly proportional to the applied load and length and is inversely proportional to the MOE and cross sectional area
• Comparative Elastic Properties of Selected Material

  • Steel
  • Aluminum
  • Glass
  • Concrete
  • Wood
  • Bone
  • Polystyrene
• The steepness of the slope of the elastic line in the stress-strain diagram is a measure of the magnitude of the elastic modulus
• Proportional limit (PL)


  Point wherein the stress-strain curve deviates from a straight line, beyond which the stress is no longer proportional to the strain
• Ultimate stress


  Highest stress registered in a wooden member
• Breaking stress


  Stress at the time the material undergoes failure
• Columns are structural members designed to support loads applied at its end
• Columns are subjected to compressive, tensile, and shear stresses
• Compressed material


  • Retains a permanent deformation or set when force is removed
  • Breaks and stress-strain diagram is no longer meaningful
• Slope of straight line below proportional limit

  Ratio of stress to strain, can be used to calculate MOE
• MOE


  Modulus of Elasticity = tan θ = σ/ε
• Stress-strain diagram


  • Provides information on ultimate stress, breaking stress, proportional limit, and elastic limit
• Compressive stress (σc)


  Caused by forces that tend to shorten the length (compress) of the body
• Tensile stress (σt)


  Caused by forces that tend to increase the length of the member, i.e. pull the member apart
• Shear stress (τ)


  Caused by forces that tend to cause one section of a body to slide past its adjacent section
• Calculating stress, strain and MOE

  1. Stress = P/A

  2. Strain = ΔL/L

  3. MOE = σ/ε
• Calculating compressive and shear stresses

  1. σc = Pc/A

  2. τ = V/A