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Properties of Wood

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Title: 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
3
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.

4
Grain
  • Grain runs along the trunk.
  • Grain size is non uniform

Grain Direction
5
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
6
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)

7
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
8
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

9
Imperfections in Wood
Knots
http//www.vermonttimberworks.com/images/shake.jpg
10
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
11
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

12
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

13
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

14
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

15
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
16
Glulams Designed as Beams
  • Best material on outer faces
  • Butt splice in compression zone
  • Scarf splice in tension region

17
Glulams Designed as Columns
  • Materials more uniform
  • Butt joints can be used throughout

18
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
19
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

20
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)

21
Sawn Lumber Reference Values
  • NDS Supplement Table 4A

22
Sawn LumberReference Values
  • NDS Supplement Table 4D

23
Glulam Reference Values
  • NDS Supplement Table 5A

24
Glulam Reference Values
  • NDS Supplement Table 5B

25
Sawn Lumber Design Values
  • NDS Table 4.3.1

26
Glulam Design Values
  • NDS Table 5.3.1

27
The Modifiers
28
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)

29
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

30
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

31
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

32
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.

33
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

34
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.

35
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.

36
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.

37
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!

40
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

41
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

42
Example
  • Consider a column subjected to the loads shown

43
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
44
- Determine CD for each source and each
combination. - Pick the controlling CD for the
combination and apply to all loads in the
combination.
45
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

46
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

47
KF Table
48
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

49
The l Table
50
Example
  • Consider a column with the loads shown

51
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52
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.

53
LRFD Sawn Lumber Example
54
ASD Sawn Lumber Example
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