1. Materials for Civil and Construction Engineers CHAPTER 10 Wood

2. Introduction
Wood is the earliest construction material used by mankind.
easy to use
durable
high strength
low weight
widely available
low cost
Still very widely used today for:
building frames
bridges
utility poles
floors
roofs
trusses
piles, etc.

3. Classification of Trees
Endogenous (intertwined growth): e.g., palm trees
very strong and lightweight
not generally used for engineering applications in U.S.
Exogenous (outward growth): e.g., most other trees
Fibers grow from the center outward by adding concentric layers (annual rings) which gives more predictable engineering properties.
Deciduous (broad leaf) = hardwood (ash, oak, maple, walnut, etc.) – expensive slow growing
Coniferous (cone bearing, evergreens) = softwood (Douglas fir, pine, spruce, cedar, etc.)

4. 10.1 Structure of Wood
Each annual ring of exogenous tree is composed of:
Earlywood (light ring): rapid spring growth of hollow thin-walled cells
Latewood (dark ring): dense summer growth of thick-walled cells which are much harder & stronger

5. Main Structural Features of Tree Stem
From center axis outwards:
Pith – center stem
Heartwood (darker) – provides structural support
Sapwood (lighter) – transports the sap
Cambium (very thin layer) – location of wood growth
Inner bark
Outer bark

6. – properties change with direction:
Wood is Anisotropic
Longitudinal
parallel to the long axis (grain)
strongest and least shrinkage
Radial
perpendicular to the growth rings (out from center)
Tangential
tangent to the growth rings
weakest and most shrinkage
directions influence strength, modulus, thermal expansion, conductivity, shrinkage, etc.

7. 10.2 Chemical Composition Cellulose Hemicellulose 50% by weight
polymer that forms strands (fibrils) that make up cell walls (wood fibers)
Gcellulose = 1.5
High density indicates higher strength
Lignin
23-33% of softwood
16-25% of hardwood by weight
glue
Hemicellulose
15-20% of softwood
20-30% of hardwood
Extractives
5-30% by weight
tannins, coloring matters, essential oils, fats, resins, waxes, starches
Ash-forming (minerals)
0.1-3% by weight
calcium, potassium, phosphate, silica

8. 10.3 Moisture Content
shrinkage, strength, & weight depend on moisture content
depends on air temperature and humidity:
slow changing so it tends to adjust near the average
Equilibrium Moisture Content (EMC)
moisture content for average atmospheric conditions
1% when hot & dry >130oF & 5% humidity
20% when warm & humid <80oF & 90% humidity

9. Fiber Saturation Point (FSP)
moisture content when cells are completely saturated with bound water but no free water inside cell cavities
FSP = 21-32%
Above FSP
changes affect only wet weight
Below FSP
small changes strongly affect all physical and mechanical properties
water inside cell cavities doesn't affect shrinkage
held tightly in cell cavities, wood shrinks on removal

10. Shrinkage largest shrinkage is in the tangential direction
Fiber Saturation
Point
Shrinkage
largest shrinkage is in the tangential direction
smallest shrinkage is in the longitudinal direction
zero shrinkage above FSP regardless of direction
For glulam (varying growth ring orientations)
assume 6% shrinkage in 30% change in m/c below FSP (or 1% shrinkage per 5% change in m/c)

11. 10.4 Wood Production
Production Steps: 1. Harvesting 2. Sawing 3. Seasoning (drying) 4. Surfacing (Planing) (optional) 5. Grading 6. Preservative Treating (optional)

12. Wood Products for Construction
Dimensional lumber – 2” to 5" thick – 2x4,s etc.
used for light framing – studs, joists, beams, rafters, trusses, decking
Heavy timber – 4x6, 6x6, 8x8 and larger
usually rough sawn (actual sizes)
used for heavy framing, railroad ties, landscaping
Round stock
posts and poles – used for marine piling, utility poles, etc.
Specialty items
handrails, spindles, radius edge decking, turned posts, lattice, etc.
Engineered wood products
bonding wood strands, veneers, lumber or other wood fibers
large integral composite unit 12 12

13. Chapter 10: Wood

14. Step 1. Harvesting minimal sap concerns of fire hazard
other plant growth and underbrush is minimal

15. Step 2. Sawing Live (plain) sawing – most rapid and economic
Quarter sawing – maximum amount of prime (vertical) cuts
Combination – most typical
Live (Plain) Sawing
Quarter Sawing
Combination

16. Three types of board cut
Flat-sawn (grain is <45o from flat side)
worst quality, most problems and defects
Rift-sawn (45o-80o)
Quarter-sawn (vertical- or edge-sawn) (80o-90o)
best quality, least shrinkage problems
Flat-Sawn
Rift-Sawn
Quarter-Sawn

17. Step 3. Seasoning (Drying)
Green wood has % moisture content
~15% when it leaves the mill
Methods of Seasoning
air drying (cheap & slow)
kiln drying (fast & expensive)
usually a combination
Uneven shrinkage in different directions during seasoning causes warping, checks, shakes, etc.
Type of cut controls these problems (vertical is the best)

18. Step 4. Surfacing (Planing)
Surfacing takes 1/4" (or more) from each side
S4S = surfaced 4 sides = “dressed”
Nominal sizes refer to the rough-sawn (unsurfaced) dimensions of the lumber in inches
For example, the actual dimensions of a 2 x 4 are 1 ½ in. x 3 ½ in.

19. 10.5 Lumber Grades (Step 5)
Several agencies for different regions and species
Graded according to number of defects that affect strength & durability (knots, checks, pitch pockets, shakes, stains)
Visual (appearance) grading
Stress (structural) grading – Table 10.3
Hardwood grades – visual (also stress) grading
Softwood grades – visual & machine stress grading
For civil engineering applications, appearance grades are less important than structural grades

20. 10.6 Lumber Defects Affect both appearance & mechanical properties
Caused by:
natural wood growth
seasoning too fast
wood diseases
animal parasites
faulty processing
Knots – branch base that degrades mechanical properties
sound, tight knots may be good in compression but don’t count on it

21. Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.

22. Shakes – wood separations between annual rings
Wane – bark or other soft wood left on the edge of the board
Sap Streak – colored streak of sap accumulated in wood fibers
Reaction Wood – extra dense woody tissue that can cause warping and cracking
Pitch pockets – opening between annual rings that contain resin
Bark Pockets – small patches of bark embedded in the wood
Checks – ruptures along the grain from drying

23. Splits – lengthwise separations caused by mishandling or seasoning
Warping – (several types) from uneven drying of internal tree stress
Bowing – lengthwise curvature from end to end
Crooking – lengthwise curvature from side to side
Cupping – edges roll up
Twisting – one corner lifts
Raised, Loosened, or Fuzzy Grain
Chipped or Torn Grain
Machine Burn – from worn saw blades

24. 10.7 Physical Properties of Wood
Specific Gravity & Density
Specific gravity of the cell walls (cellulose) = 1.5 regardless of species
excellent indicator of the amount of material (and properties) in dry wood
closer to 1.5 means more cell walls which is denser & stronger
Dry density = usually lb/ft3 ( kg/m3)

25. 2. Thermal Properties
Thermal conductivity
The rate that heat flows through (inverse of thermal resistance R value)
Good R value (R = 1 / conductivity)
much better than metals
slightly worse than insulation
reduces loss of heat and cold
delays fire
Specific Heat
Ratio of the quantity of heat required to raise the temp. of the material 1o to that required to raise the temp. of an equal mass of water 1o

26. Thermal Diffusivity
Rate that material absorbs heat from surroundings
Much better (lower) than most other building materials
Thermal Expansion
Anisotropic: 5-10x greater across grain than parallel to it
Applying heat to wood:
first expands the wood from thermal expansion
then it shrinks from moisture loss (when below FSP)
3. Electrical Properties
Good electrical insulator which decreases with moisture content – more water is a better electrical conductor

27. 10.8 Mechanical Properties of Wood
Wood is extremely anisotropic
1. Modulus of Elasticity
1-2 x 106 psi – for compression parallel to the grain
linear up to proportional limit, then small non-linear curve
Depends on:
species variation
moisture content
specific gravity
direction of grain

28. 2. Strength Properties
Vary widely because of anisotropy, moisture content, defects, etc.
Tensile strength is greater than compressive strength
Tensile strength parallel to grain is 20x greater than perpendicular

29. Wood can support higher loads of short duration than sustained loads
3. Load Duration
Wood can support higher loads of short duration than sustained loads
Under sustained loads wood continues to deform
Design values assume 10 year loading and/or 90% of full maximum load throughout life of the structure
Multiply design values by load duration factors for short- duration loads
Load Duration Factors
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.

30. 4. Damping Capacity
Vibration damping (like shock absorbers) increases with moisture content up to FSP
10x greater damping than structural metals
wood structures dampen vibrations much better than metal

31. strength of wood structures is usually controlled by the joints and connections, which is the main concern of structural wood design classes
we have lots of experience with smaller structures (residential, light commercial) so design is usually empirical

32. 10.9 Mechanical Testing Wood is tested to predict performance
two main techniques
testing of timbers of structural sizes (ASTM D 198)
testing of representative, small, clear specimens (ASTM D 143)
Testing of structural-size members is more important –more applicable to design values
Tests include flexure, compression, tension, etc.
Flexure test is more commonly used than the other tests
Two-point, third-point, or center-point loading

33. Third-point bending test on a 4 x 6 wood lumber

34. Testing Representative, Small, Clear Specimens
Compression parallel to grain
Hardness perpendicular to grain
Tension perpendicular to grain
Compression perpendicular to grain
Tension parallel to grain
Hardness parallel to grain
Bending

35. 10.10 Design Considerations
For design of wood structures, strength properties (Tables 10.3 &10.4) must be adjusted for the following factors
Load duration Wet service
Temperature Beam stability
Size Volume (glulam only)
Flat use Curvature (glulam only)
Column stability Bearing area
Repetitive member (lumber only)

36. 10.11 Organisms that Degrade Wood
Fungi caused dry rot
Spruce Ips Beetle
Marine-borer damage to a buried pile
Bacteria damage black heartwood
Termite damage
Dry rot
Spruce Ips beetle
Bacteria damage
Termite

37. 10.12 Wood Preservation Petroleum-based Solutions
Waterborne Preservatives (Salts)
Application Techniques
Superficial treatment: generally not effective
Liquid penetration (pressure treating at high temp., heat, & moisture)
Structural members need to be fabricated as much as possible before treatment in order not to expose untreated wood by cutting, drilling holes, etc.
If not possible, treat cuts and holes with a liberal application of field applied preservative
Chapter 10: Wood
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.

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39. 10.13 Engineered Wood Products
Made by bonding together wood strands, veneers, lumber, or other forms of wood fibers to produce large units
engineered to produce specific and consistent mechanical properties that are better than natural large pieces
very difficult and expensive to find high quality large natural pieces

40. Plywood
thin sheets (plies) glued together with the grain at right angles to each other so it has the same properties in both directions
veneer is peeled from a soaked log on a giant lathe

41. Particle & strand board
glue together wood scraps with resin to form sheets:
particle board = sawdust sized particles
chip board = randomly oriented wood chips
OSB = wood chips & strands oriented in specific direction

42. Floor joists
made with two 2x4s or 2x6s as flanges and an OSB web

43. Glue-Laminated Timbers (Glulam)
lumber glued together with the parallel grain
used for structural members, furniture, sports equipment, and decorative wood finishes
preferred because:
ease of manufacturing large
members from standard commercial lumber
can vary the cross section along
the length
special architectural designs
can use lower wood grade
in less stressed areas
minimizes shrinkage defects