Week 9 Timber Structure Moisture Control Stress Grading

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Week 9 Timber Structure Moisture Control Stress Grading Powered By Docstoc
					Timber Structure, Moisture
Control & Stress Grading

   Week 9
   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
   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
Environmental Considerations
   Availability      Widespread ( not
                      always well managed)
   Extraction        Environmental
   Energy used       Low
   Health & safety   Few problems (wood
                      dust & treatments)
   Recyclability     Many options
Structural Variation
  Four orders:
1.) Macroscopic
2.) Microscopic
3.) Ultra-structural
4.) Molecular
   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
   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
   Heartwood advances to include former sapwood
   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
   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
   Cells in the outer side of the cambium
    develop into new phloem (inner bark)
   Radial growth of the trunk must
    accommodate existing branches – Knot
   Live branches fuse with truck – live knots
   Dead branches – trunk grows around
    branch – dead knots
                  Middle Lamella

Cell Structure
   Primary Wall

                                   Secondary Wall


   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
   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
   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
Diagram of Timber Wedge
   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
   Secondary function – mechanical support of the
    tree as a whole
   Tertiary function – serves as a food store during
    the winter to supply spring growth
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
   Wood cells are of three main types
    according to adaptations for conduction,
    mechanical support & food storage
   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
   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
   Therefore in softwoods water conduction &
    mechanical strength are provided by
    modifications of a single type of cell
   Examples include pine, spruce, larch &
    Douglas fir
   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
   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
   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
Characteristic Differences
   Hardwoods – water conducting elements
    distributed throughout the annual ring
   Softwoods – water conducting &
    strengthening elements segregated in the
    springwood & summerwood respectively
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
Resin Ducts
   Common softwoods are characterised by
    special ducts or passages containing resin
   Similar ducts are also found in some of the
   Gum or resin ducts can be formed as a
    result of injury to the living tree
Molecular & Ultra-structure
  Chemical analysis – four constituents:
1.) Cellulose – fibre
2.) Hemicelluloses – matrix
3.) Lignin – matrix
4.) Extractives – extraneous (compounds)
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
Flat-sawn & Quarter-sawn
   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)
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
   First - loses a large proportion of the free water in
    the cell cavities
   Second – loses moisture absorbed in the cell
   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
   Beyond this point the timber starts to shrink as
    the moisture contained in the cell wall begins to
   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
   Conditions indoors generally require the use of
    kiln dried timber – typically 12% moisture
   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)
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
Importance of Moisture
   Dimensional stability
   Ability to work with
   Strength
   Durability
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%
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
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).
   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.
   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.
   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.
   Moisture changes – before the timber is
    installed, the environment should be
    controlled to avoid wetting or severe
    drying to prevent distortion.
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.
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).
   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.
   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 of Creep
   Reversible component – reverses with time
    – delayed elastic behaviour.
   Irreversible component – results in plastic
    or viscous flow.
Timber Deformation Behaviour
   Three forms:
   Elastic
   Delayed elastic
   Viscous
   Hence timber is termed viscoelastic.
Elastic Deformation
   Load – deflection plots for stressed timber
    samples assume a limit of proportionality
    below which the relationship between the
    load & deformation is linear.
       Defromation  Applied load
       Load / Deformation  Constant
  Elastic Modulus
Stress  Load/Area
Strain  Deformation/Original length
Stress / Strain  Modulus of Elasticity, ' E'
E  
Other Constants
    Modulus of Rigidity, ' G'
    G  
    Poisson's Ratio, ' '
       y  x
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
   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 &
   a.) Strengths are variable even in visually perfect
   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.
   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.
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)
Special & General Structural

   SS - Strength between 50 & 60% of that of
    a clear (perfect) timber of the same species.
   GS – Strength 30 – 50%.
   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
   Edge knots are more
    serious & a margin
    condition is used when >
    half the top or bottom
    quarter of any section is
    occupied by knots.
   Fissures - size of fissure equal to depth of
    crack – result from seasoning problems –
    can cause problems if the length of defect
    is excessive.
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.
   Wane - corner of timber cut to close to the
    outside of the tree – reduces the bending
    strength across spans or bearing strength at
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.
   Distortion – cupping, bowing, springing
    and twisting.
   Wall plates – bowing & twisting problems
   Rafters – Springing & twisting
   Floorboards - Cupping
Resin Pockets
   These are one of a number of other defects
    including wormholes, fungal decay sap
    stain etc..
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
   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
Simplified Experiment

 Left reaction                 Right reaction
                  Dial gauge
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.
Strength Classes (BS 5268)
   Strength classes are used:
       Classes 1-5 generally used for
       Classes 6-9 used for denser hardwoods
   SC3 & SC4 are most common:
       SC3 – European redwood (GS)
       SC4 – European whitewood (SS)
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
Service Classes (Euro Code 5)
   Allowable stresses for structural timber:
       Internal heated (service class 1)
       Internal unheated (service class 2)
       External & (service class 3)
   Class 1- based on moisture content at 20°C
    & relative humidity of 65% - average
    moisture content not to exceed 12%
   Class 2 - moisture content at 20°C &
    relative humidity of 85% - average
    moisture content not to exceed 20%
   Class 3 – allows for higher moisture
    contents due to variations in climate.
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.

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