Properties of Wood

1
Properties of Wood

• CE A433 Spring 2008
• T. Bart Quimby, P.E., Ph.D.
• University of Alaska Anchorage
• Civil Engineering

2
Cellular Makeup

• Cells are elongated, tube like cells
• Cell walls are made of cellulose
• Cells are bound together by lignin

http//concise.britannica.com/ebc/art-66141/Cross-
section-of-a-tree-trunk
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Effect of Cell Structure

• Since the cells are elongated, the wood has
  different strength properties when stress is
  transverse or parallel to the cell longitudinal
  axis.
• Shrinkage properties are also different in each
  direction.

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Grain

• Grain runs along the trunk.
• Grain size is non uniform

Grain Direction
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Principle Directions

• With the Grain Longitudinal
• Cross Grain or Perpendicular to Grain
  Radial or Tangential
• Strength and Shrinkage Properties are DIFFERENT
  IN EACH DIRECTION

Radial
Tangential
Longitudinal
http//concise.britannica.com/ebc/art-66141/Cross-
section-of-a-tree-trunk
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Moisture Content Shrinkage

• MC (moist wt oven dry wt)/ (oven dry weight)
  x 100
• Living trees may have MC up to 200.
• Lumber in service has MC less than 20
• The loss of moisture results in wood shrinkage
• Shrinkage is most pronounced perpendicular to
  grain
• Moisture is found within the wood cell cavities
  (free water) and the cell walls (bound water)

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More Moisture

• Fiber Saturation Point (FSP) The MC where all
  the free water is lost, leaving only the bound
  water. There is no shrinkage when the MC is
  above the FSP
• Volume changes take place as the MC varies below
  the FSP.
• Typically, MC continues to decrease after both
  manufacturing and installation.
• Equilibrium Moisture Content (EMC) The in
  service moisture content
• This can vary with occupancy and/or season

Manufacture
Installation
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Shrinkage Example

• 5 Story Condo in Juneau, AK
• It rained all but 3 days during the 4-5 weeks of
  framing.
• In the first year after construction there was
  considerable shrinkage

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Imperfections in Wood
Knots
http//www.vermonttimberworks.com/images/shake.jpg
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Members Cut from a Log
Cross Grain
Cross Grain

• Perpendicular to grain direction may be either
  tangential, radial, or a combination of each

Cross Grain
http//web.utk.edu/grissino/images/zuni20doug-fi
r.jpg
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Sawn Lumber Visual Grading

• Rules agencies establish grading rules based on
  observed wood quality
• Inspectors visually grade and mark each piece as
  it is manufactured

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Pressure Treated Wood

• Wood is often chemically treated to increase its
  durability
• Minimizes decay and mold
• Discourages insect infestation
• Fire treatments also available
• Moist, dark (or nearly dark) locations with
  minimal air circulation are prime locations for
  decay and mold
• High moisture conditions that are variable are
  also problematic

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Standard Sizes for Sawn Lumber

• NDS Supplement Section 3, Table 1B
• Nominal Sizes are larger than the actual sizes!
• Check out the sizes! Not all are readily
  available
• Some available sizes
• 2x2 to 2x12
• 4x4 to 4x12
• 6x6 to 6x12

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Sawn Lumber Size Classifications

• See text pg 4.30
• Structural Joists and Plank (SJP)
• 2 to 4 inches thick
• 2 inches and wider
• Beams Stringers (BS)
• 5 inches thicker
• Width gt thickness 2 inches
• Posts Timbers (PT)
• 5 inches thicker
• Width lt thickness 2 inches

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Glue Laminated Timbers

• Laminations can be strategically used to make
  efficient use of the best materials.
• Members can be fabricated for particular uses
• Larger, Longer, Curved sections are possible

http//www.lamisellbeams.com/images/cover-image/la
misell-image03c-shadow04.jpg
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Glulams Designed as Beams

• Best material on outer faces
• Butt splice in compression zone
• Scarf splice in tension region

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Glulams Designed as Columns

• Materials more uniform
• Butt joints can be used throughout

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Glulam Standard Sizes

• See NDS Supplement Table 1C
• Common Widths for Western Species GL
• 3.1/8, 5.1/8, 6.3/4, 8.3/4, 10.3/4
• Common depths
• 6 and larger, in 1.1/2 increments

http//www.woodnet.org.uk/wec/images/gluelam2.jpg
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The National Design Specification

• The model code for timber design in the US
• The NDS Specification tells us what we can do
  with timber
• The NDS Supplement provides material data for the
  various types of timber

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Basic Design Inequality

• As with all structural codes
• Reqd Strength lt Available Capacity
• In the NDS this takes the general form
• f lt F or U lt fN
• Where
• f stress caused by internal forces
• F adjusted design stress F modifiers
• F Reference design stresses
• U the LRFD factored internal force
• N nominal capacity F (Section Property)

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Sawn Lumber Reference Values

• NDS Supplement Table 4A

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Sawn LumberReference Values

• NDS Supplement Table 4D

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Glulam Reference Values

• NDS Supplement Table 5A

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Glulam Reference Values

• NDS Supplement Table 5B

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Sawn Lumber Design Values

• NDS Table 4.3.1

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Glulam Design Values

• NDS Table 5.3.1

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The Modifiers
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CM Wet Service Factor

• Applies to all reference values
• Applies to both Sawn Lumber and Glulams
• Specified in EACH NDS Supplement Reference Value
  Table
• This factor generally reduces strengths for wood
  that is used in a high moisture environment (EMC
  gt 19)

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Ct Temperature Factor

• Applies to all reference values
• Use for timber used in environments with
  sustained temperatures up to 150 deg F
• NDS 4.3.4 for Sawn Lumber NDS 5.3.4 for Glulams
• References NDS Table 2.2.3

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CL Beam Stability Factor

• Applies only to bending stress, Fb
• Applies to both Sawn Lumber and Glulams
• Found in NDS 3.3.3
• This factor accounts for instability in laterally
  unsupported beams (i.e. lateral torsional
  buckling)

• Glulams This factor is NOT simultaneously
  applied with the Volume Factor, CV
• More on this factor when we cover beam design

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More CL

• See NDS Equation 3.3-6
• LTB is a function of both the laterally unbraced
  (buckling) length AND the variation in the moment
  diagram.
• First check the slenderness ratio
• RB must not exceed 50
• Then compute CL
• Note that CL is a function of the beam size!
• This means that you must know the beam size
  before computing this factor
• When designing, this may lead to iterative
  computations

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CF Size Factor

• Applies to Sawn Lumber Fb, Ft, and Fc bent about
  the strong axis
• Found in NDS 4.3.6
• 4.3.6.1 For SJP see NDS Supplement Tables 4A
  and 4B
• 4.3.6.2 For BS with d gt 12, CF (12/d)1/9
• 4.3.6.3 For beams of circular cross section
• The reference values are normalized to a 12 deep
  member. This factor accounts for the difference.

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Cfu Flat Use Factor

• Applies only to Fb for both Sawn Lumber and
  Glulams when the member is bent about its minor
  axis.
• Found in NDS 4.3.7 and 5.3.7
• For Sawn Lumber, values for Cfu are found in the
  NDS Supplement Tables 4A, 4B, 4C, and 4F

• For Glulams, values for Cfu are found in the NDS
  Supplement Tables 5A, 5B, 5C, and 5D

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Ci Incising Factor

• Applies to all reference values for Sawn Lumber.
• Found in NDS 4.3.8

• Accounts for damage to member due to incisions
  made for chemical pressure treatment.

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Cr Repetitive Use Factor

• Applies to Sawn Lumber Fb
• Found in NDS 4.3.9
• Read criteria in NDS
• Intended to account for the community effort of a
  repetitive series of bending members such as
  joists, rafters, studs, etc.

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Cp Column Stability Factor

• Applies to Fc for both Sawn Lumber and Glulams
• Found in NDS 3.7 via 4.3.10 and 5.3.9
• This factor accounts for column stability as a
  function of slenderness.
• There will be more on this factor when column
  design is discussed.

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CT Buckling Stiffness Factor

• Applies to Emin for Sawn Lumber, which has an
  impact on Cp
• Found in NDS 4.4.2 via 4.3.11
• Only applies to truss top chord members, subject
  to combined bending and axial compression, and
  made of 2x4 or smaller sections that meet certain
  criteria.

38
Cb Bearing Area Factor

• Applies to Fcp for both Sawn Lumber and Glulams
• Found in NDS 3.10.4 via 4.3.12 and 5.3.10
• Accounts for increased strength of bearing areas
  which are, in part, aided by adjacent wood.
• More on this one when we cover beams.

39
CD Load Duration Factor

• ASD ONLY!!!!
• Applies to Fb, Ft, Fv, Fc for both Sawn Lumber
  and Glulams. Also applies to Frt for Glulams.
• Found in NDS 2.3.2 via 4.3.2 and 5.3.2
• The value of CD is based on the actual shortest
  duration load in the load combination being
  considered. This has the unfortunate affect of
  making it very difficult to determine the
  controlling load combination!

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Time Effects on Timber Strength

• Wood has the property of carrying substantially
  greater maximum loads for short durations than
  for long durations of loading. NDS 2.3.2.1

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Determining ControllingLoad Case

• Consider ASD tensile limit state
• Ta/A ft lt Ft FtCDCMCt
• for ALL LOAD COMBINATIONS
• CD is different for every considered ASCE 7 load
  combination
• This makes it tough to see which Ta to use.
• To determine the controlling load case divide
  both sides by CD
• (Ta/CD)/A ft / CD lt Ft / CD FtCMCt
• The load combination that gives the largest Ta/CD
  is the controlling load combination

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Example

• Consider a column subjected to the loads shown

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Structural Elements with Multiple Load Sources

• Each source has a different make up
• Which ASD LC controls the design of the member?

D, Lr, S, L
D, L
W, E
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- Determine CD for each source and each
combination. - Pick the controlling CD for the
combination and apply to all loads in the
combination.
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f Resistance Factor

• LRFD Only!!!!!
• Applies to all reference values except for E
• Found in NDS Appendix N.3.2 via 4.3.15 and 5.3.13

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KF Format Conversion Factor

• LRFD Only!!!!!
• Applies to all reference values except E
• Found in NDS Appendix N.3.1 via 4.3.14 and 5.3.12
• Need to know f to determine KF

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KF Table
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l Time Effect Factor

• LRFD Only!!!!!
• Applies to all reference values except E and
  Emin.
• Found in NDS Appendix N.3.3 via 4.3.16 and 5.3.14
• This is the LRFD equivalent to CD. l accounts
  for time effects on strength.
• l is easier to apply than CD since its value is
  specified by ASCE 7 LRFD load combination instead
  of by actual shortest duration load
• Still need to divide load by l to find
  controlling cases

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The l Table
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Example

• Consider a column with the loads shown

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Computing Adjusted Valuesfor a Member

• Once the modifiers have been determined, then you
  can compute the modified stresses for a member.
• Member design often requires more than one
  strength limit state, so you will have to compute
  adjusted values for several types of stress.

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LRFD Sawn Lumber Example
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ASD Sawn Lumber Example