Timber Structure, Moisture Control & Stress Grading Week

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Timber Structure, Moisture Control Stress
Grading

• Week 9

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Introduction

• Timber is a product cut machined from trees
  is a natural material
• A tree is capable of supporting its crown,
  conduct mineral solutions store food material
• There are about 30,000 different species of tree
• Timber is an extremely variable material

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Definition

• Timber as a material can be defined as a
  low-density, cellular, polymeric composite
• It has high strength performance is relatively
  low cost
• Timber is the worlds most popular fibre composite

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Environmental Considerations

• Availability Widespread ( not always well
  managed)
• Extraction Environmental implications
• Energy used Low
• Health safety Few problems (wood dust
  treatments)
• Recyclability Many options

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Structural Variation

• Four orders
• 1.) Macroscopic
• 2.) Microscopic
• 3.) Ultra-structural
• 4.) Molecular

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Macroscopic

• Increasing crown diameter linked to diameter of
  trunk
• Conduction storage restricted to the outer
  region of the trunk sapwood
• Area in which this function is no loner carried
  out heartwood

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Sapwood

• Width varies from species to species, rate of
  growth age of tree
• Except for very young trees (sapwood whole
  radius) sapwood typically represents 20 to 50 of
  the total radius

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Heartwood

• Heartwood advances to include former sapwood
  cells
• The acidity of the heartwood increases
  extractives are formed colouration changes take
  place
• Resistance to fungal insect attack increases
• Many timbers develop gums resins in the
  heartwood

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Growth

• Trunk grows by division of cambial cells
  immediately beneath the bark crown size
  increases enlargement of branches production
  of new ones
• When cambium cells divide into two the new cell
  formed on the inner side increases in size
  thickness of its wall until it is a fully
  developed wood (xylem) element
• Process continues through the growing season

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Contd

• Cells in the outer side of the cambium develop
  into new phloem (inner bark)
• Radial growth of the trunk must accommodate
  existing branches Knot formation
• Live branches fuse with truck live knots
• Dead branches trunk grows around branch dead
  knots

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Cell Structure
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Contd

• Temperate climates growth begins in spring
  continues until a month or so before the fall of
  the leaves in autumn
• A complete sheath of new wood appears all over
  the tree between the bark the old wood
• Viewed in cross-section this zone of new wood
  appears as a complete annual ring around the trunk

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Springwood

• In British trees the structure of the wood formed
  in the early part of the season is known as
  springwood
• Springwood is more open porous than the later
  formed wood

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Summerwood

• The later formed wood is known as summerwood
  the contrast in density of these two layers marks
  the growth of successive years
• Note in tropical climates seasonal growth does
  not always correspond with annual periods are
  normally termed growth rings rather than annual
  rings

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Diagram of Timber Wedge
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Microscopic

• The structure of timber is connected to the
  functions performed within the growing tree
• Primary function conduction of water dissolved
  mineral salts from the roots to the leaves
• Secondary function mechanical support of the
  tree as a whole
• Tertiary function serves as a food store during
  the winter to supply spring growth

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Cellular Structure

• Cellular structure specially designed to perform
  the functions listed above
• Each different kind of tree has evolved its own
  individual way of performing these functions
• Wood cells are of three main types according to
  adaptations for conduction, mechanical support
  food storage

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Softwood

• Typically, in a temperate climate softwood the
  water conduction is carried mainly by the
  springwood, which is formed at the time when the
  new leaves are developing need large quantities
  of water
• Scots pine (redwood) springwood honeycomb
  structure square or hexagonal cells (tracheids)
  with thin walls large open cavities tubular
  cells

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Contd

• Ends of tracheids overlap with the pits of
  adjoining elements opposite to one another form
  pathways for solutions to pass
• Pits are found in all types of wood cells
• The structure of summerwood is similar to that of
  spring wood, but thicker cell walls and narrower
  cavities are adapted to provide strength

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Contd

• Therefore in softwoods water conduction
  mechanical strength are provided by modifications
  of a single type of cell
• Examples include pine, spruce, larch Douglas fir

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Hardwood

• Hardwoods have two distinct types of cell for
  water conduction mechanical support
• Water conduction vessels (pores) vertical
  series of open ended cells arranged one above the
  other (similar to rainwater pipes) continuous
  for long lengths
• Pores generally distributed uniformly through the
  wood can exist in groups depending upon species

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Contd

• Woods in which the pores of the springwood are
  larger than those of the summerwood form a well
  defined ring are termed ring-porous ash,
  chestnut, elm oak
• Woods with scattered pores no distinct
  differences between springwood summerwood
  diffuse-porous beach, sycamore birch

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Contd

• Cells concerned with mechanical support are the
  fibres long, narrow, thick walled elements
  contributing to the bulk of the timber tissue
• Fibres similar to summerwood tracheids in
  softwoods, but shorter in length

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Characteristic Differences

• Hardwoods water conducting elements distributed
  throughout the annual ring
• Softwoods water conducting strengthening
  elements segregated in the springwood
  summerwood respectively

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Food Storage

• Performed by parenchyma (soft tissue) thin
  walled brick shaped cells occur in the form of
  rays (horizontal strands of cells running across
  the grain in a radial direction)
• Parenchyma cells also occur as vertical strands
  scattered among the pores fibres, or in the
  form of a sheath surrounding the pores or grouped
  in bands at right angles to the rays

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Resin Ducts

• Common softwoods are characterised by special
  ducts or passages containing resin
• Similar ducts are also found in some of the
  hardwoods
• Gum or resin ducts can be formed as a result of
  injury to the living tree

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Molecular Ultra-structure

• Chemical analysis four constituents
• 1.) Cellulose fibre
• 2.) Hemicelluloses matrix
• 3.) Lignin matrix
• 4.) Extractives extraneous (compounds)

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The Saw Mill

• The growth structure of wood are connected with
  practical methods of converting timber in the
  saw-mill
• Flat-sawn or plain-sawn timber is cut in a plane
  tangential to the growth rings
• Quarter-sawn or rift-sawn refers to the method of
  cutting in a plane at right angles to the growth
  rings

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Flat-sawn Quarter-sawn
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Comparisons

• Flat-sawn quicker, cheaper to cut involves
  less waste
• Quarter-sawn more stable (shrinks less in width
  is less likely to warp split during seasoning)

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Moisture in Wood

• Present in two forms
• 1.) As free water cell cavities
• 2.) As absorbed water in the substance of the
  cell walls
• Total weight of water in green timber may
  amount to more than 100 of the dry weight

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Drying

• First - loses a large proportion of the free
  water in the cell cavities
• Second loses moisture absorbed in the cell
  walls
• Timber reaches an important stage when all the
  free water has evaporated leaving the cell walls
  saturated fibre-saturation point moisture
  content approx 30 of the dry weight

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Contd

• Beyond this point the timber starts to shrink as
  the moisture contained in the cell wall begins to
  evaporate
• The process of seasoning reducing moisture
  content to a point where it will be in
  equilibrium with the surrounding atmosphere in
  Britain this is typically 16 to 22 depending on
  the time of year
• Conditions indoors generally require the use of
  kiln dried timber typically 12 moisture content

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Contd

• Timber is hygroscopic well-seasoned timber
  absorbs moisture swells when exposed to damp
  air
• Dimensional changes are not the same in all
  directions due to the structure of the timber
  timber is anisotropic (different properties in
  different directions)

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Strength Moisture

• Changes in moisture content affect strength
• Seasoning increases the hardness stiffness, but
  reduces toughness or resistance to shock
• Thus timber exposed to high humidity will absorb
  moisture the strength will fall with increase
  in moisture content

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Importance of Moisture

• Dimensional stability
• Ability to work with
• Strength
• Durability

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Values Not to be Exceeded

• Floor joists 22
• Floors (intermittent heating) 12-15
• Floors (continuous heating) 9-12
• Floors (under-floor heating) 10
• Internal joinery (continuous heating) 12
• Other internal door 15
• Other internal joinery 17

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Changes in a New Building

• Condition Temp(C) Equilibrium Moisture()
• Roofed,glazed 5-15 19
• walls completed
• Temporary heating 15-21 21-15
• Initial occupation 18-27 15-10
• Building dried out 18-21 10-6

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Distortion of New Timber

• Species some timber species have similar
  movements of moisture in both radial tangential
  directions are less likely to distort (Douglas
  fir). Others that have different relative rates
  of moisture movement are prone to distortion
  (Beech).

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Contd

• Method of conversion timber that is quarter
  sawn is less likely to distort than flat sawn
  timber. Sections including the pith are prone to
  springing, other distortions include sections
  that bow, twist or cup.

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Contd

• when the grain is not straight or when there are
  density variations due to unequal growth the
  section is likely to twist. Uniform, straight
  grain is much less prone to distortion.

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Contd

• Stacking Proper support is important prior to
  during seasoning in order to prevent the self
  weight from causing distortion. The correct use
  of stacks during seasoning should prevent
  twisting of the timber sections.

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Contd

• Moisture changes before the timber is
  installed, the environment should be controlled
  to avoid wetting or severe drying to prevent
  distortion.

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Timber Deformation Under Load

• Timber does not behave in a truly elastic way
  movement is time dependent.
• The magnitude of the strain depends on the
  density of the timber, the angle of the grain
  relative to the direction of the applied load as
  well as temperature relative humidity.

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Timber in Service

• Once in service timber has to withstand loading
  for many years. Initially the deformation is
  reversible is truly elastic, but with
  maintained load the deformation increases,
  although at a decreased rate with time (creep).

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Contd

• On removal of the load after this period of
  continued movement, an instantaneous reduction in
  deformation occurs which is almost the same in
  magnitude to the initial elastic deformation.

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Contd

• As time progresses , the remaining deformation
  decreases until a point is reached where no
  further reduction will take place.
• The creep can thus be divided into two components.

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Components of Creep

• Reversible component reverses with time
  delayed elastic behaviour.
• Irreversible component results in plastic or
  viscous flow.

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Timber Deformation Behaviour

• Three forms
• Elastic
• Delayed elastic
• Viscous
• Hence timber is termed viscoelastic.

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Elastic Deformation

• Load deflection plots for stressed timber
  samples assume a limit of proportionality below
  which the relationship between the load
  deformation is linear.

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Elastic Modulus
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Other Constants
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Timber for Structural Use

• The compressive strength of timber is much lower
  than its tensile strength, due to the buckling of
  the fibres in compression.
• Timber is of particular use in bending
  applications.
• In terms of self weight timber is extremely
  strong is suited to structures where dead load
  accounts for the major proportion of the load
  floors roofing.

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Strength

• a.)  Strengths are variable even in visually
  perfect samples.
• b.) Long term strengths are very much lower than
  short term strengths due to creep.
• c.) Strengths of individual sections of bulk
  timber are susceptible to large variations due to
  the presence of defects such as knots, fissures,
  rate of growth and slope of grain.

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Grading

• Timber is graded according to its anticipated
  performance and the variability leads to the use
  of separate and distinct grades of material
  these grades were initially derived from testing
  of small clear test pieces and later evolved to
  include grade stresses derived from actual
  structural size sections.

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Visual Stress Grading

• Visual stress grading (BS 4978) a visual
  assessment of the quality of a piece of
  structural timber use of permissible defect
  limits.
• One of three grades
• Special Structural (SS)
• General Structural (GS)
• Reject

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Special General Structural Grades

• SS - Strength between 50 60 of that of a clear
  (perfect) timber of the same species.
• GS Strength 30 50.

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Knots

• Knots - knot area ratio (KAR) used to determine
  the weakening effect of knots.
• KAR the proportion of any cross-sectional area
  which is occupied by knots.
• Edge knots are more serious a margin condition
  is used when gt half the top or bottom quarter of
  any section is occupied by knots.

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Fissures

• Fissures - size of fissure equal to depth of
  crack result from seasoning problems can
  cause problems if the length of defect is
  excessive.

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Slope of Grain

• Slope of grain - plane of weakness where grain
  intersects the surface results from growth
  which is not straight or when the trunk is not
  cut parallel to the direction of growth can
  cause shear failure.

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Wane

• Wane - corner of timber cut to close to the
  outside of the tree reduces the bending
  strength across spans or bearing strength at
  supports.

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Rate of Growth

• Rate of growth - faster growing softwoods tend to
  be lower in strength stiffness growth
  variable depending on climate, pollution levels
  etc..
• Hardwoods are not affected.

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Distortion

• Distortion cupping, bowing, springing and
  twisting.
• Wall plates bowing twisting problems
• Rafters Springing twisting
• Floorboards - Cupping

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Resin Pockets

• These are one of a number of other defects
  including wormholes, fungal decay sap stain etc..

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Machine Stress Grading

• Machine stress grading many of the
  disadvantages of visual stress grading are
  removed by a process of machine testing
  correlation between modulus of elasticity and
  modulus of rupture relationship varies with
  different species and the machine has to be set
  up for each species group

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Contd

• Deflections are measured under load for
  successive portions of each timber piece as it is
  passed through the machine machine stress
  grading is widely accepted for timber used in
  structural elements such as roof trusses and
  laminated structural sections

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Simplified Experiment
Load
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Permissible Stresses (BS 5268)

• Safe working stresses are determined for various
  species based on SS experimental sample
  sections of timber 200mm deep - uses section
  depth factor, duration of load factor general
  safety factor.

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Strength Classes (BS 5268)

• Strength classes are used
• Classes 1-5 generally used for softwoods
• Classes 6-9 used for denser hardwoods
• SC3 SC4 are most common
• SC3 European redwood (GS)
• SC4 European whitewood (SS)

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New Strength Classes

• Introduced to match up with Euro Code 5
• C16 European redwood (GS)
• C24 European whitewood (SS)
• C softwood, TR trussed rafter (softwood)
• D - hardwood

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Service Classes (Euro Code 5)

• Allowable stresses for structural timber
• Internal heated (service class 1)
• Internal unheated (service class 2)
• External (service class 3)

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Contd

• Class 1- based on moisture content at 20C
  relative humidity of 65 - average moisture
  content not to exceed 12
• Class 2 - moisture content at 20C relative
  humidity of 85 - average moisture content not to
  exceed 20
• Class 3 allows for higher moisture contents due
  to variations in climate.

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Design of Structural Sections

• Both BS 5268 EC 5 cover the design of
  structural sections include the design of
  engineered timber products such as glued
  laminated timber sections.