Title: Properties of Wood
1Properties of Wood
- CE A433 Spring 2008
- T. Bart Quimby, P.E., Ph.D.
- University of Alaska Anchorage
- Civil Engineering
2Cellular 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
3Effect 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.
4Grain
- Grain runs along the trunk.
- Grain size is non uniform
Grain Direction
5Principle 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
6Moisture 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)
7More 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
8Shrinkage 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
9Imperfections in Wood
Knots
http//www.vermonttimberworks.com/images/shake.jpg
10Members 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
11Sawn Lumber Visual Grading
- Rules agencies establish grading rules based on
observed wood quality - Inspectors visually grade and mark each piece as
it is manufactured
12Pressure 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
13Standard 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
14Sawn 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
15Glue 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
16Glulams Designed as Beams
- Best material on outer faces
- Butt splice in compression zone
- Scarf splice in tension region
17Glulams Designed as Columns
- Materials more uniform
- Butt joints can be used throughout
18Glulam 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
19The 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
20Basic 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)
21Sawn Lumber Reference Values
22Sawn LumberReference Values
23Glulam Reference Values
24Glulam Reference Values
25Sawn Lumber Design Values
26Glulam Design Values
27The Modifiers
28CM 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)
29Ct 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
30CL 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
31More 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
32CF 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.
33Cfu 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
34Ci 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.
35Cr 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.
36Cp 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.
37CT 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.
38Cb 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.
39CD 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!
40Time 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
41Determining 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
42Example
- Consider a column subjected to the loads shown
43Structural 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.
45f 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
46KF 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
47KF Table
48l 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
49The l Table
50Example
- Consider a column with the loads shown
51(No Transcript)
52Computing 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.
53LRFD Sawn Lumber Example
54ASD Sawn Lumber Example