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Materials for Civil and Construction Engineers CHAPTER 10 Wood

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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.

38 http://www. extension. umn

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

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