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CE 479: DESIGN OF BUILDING COMPONENTS AND SYSTEMS FALL 2012 – J. LIU Wood: Intro, Properties, Grades.

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Presentation on theme: "CE 479: DESIGN OF BUILDING COMPONENTS AND SYSTEMS FALL 2012 – J. LIU Wood: Intro, Properties, Grades."— Presentation transcript:

1 CE 479: DESIGN OF BUILDING COMPONENTS AND SYSTEMS FALL 2012 – J. LIU Wood: Intro, Properties, Grades

2 Introduction to Wood Properties Design Specifications Sizes, Grading OUTLINE

3 Wood Members Species and Species Groups Introduction to Wood

4 Wood Members  Sawn lumber or solid sawn lumber  Wood members manufactured by cutting a member directly from a log  Glued laminated timbers  a.k.a. “glulams”  Laminated stock, glued and laid up to form larger wood members

5 Wood Members  Wood poles/timber piles  Manufactured products  Plywood  Oriented strand board (OSB)  Structural composite lumber (laminated veneer or parallel strand lumber)  Fabricated components  Trusses  Wood I-joists  Box beams

6 Sawn lumber – Basic size classifications  Dimension lumber  Smaller (thinner) sizes of structural lumber  Ranges from 2x2 through 4x16  Any material with nominal thickness of 2 to 4 inches  Timbers  Larger sizes  5 inch minimum nominal dimension  Practically speaking, smallest timber size is a 6x6

7 Species and Species Groups  Structural designer uses lumber from a commercial species group rather than a specific species  Same grading rules, reference design values, grade stamps are applied to all species in a species group

8 Species and Species Groups  Note: some groups have similar names; each is separate and distinct – different sets of reference design values  Douglas Fir-Larch and Douglas Fir-Larch (N)  Hem-Fir and Hem-Fir (N)  Spruce-Pine-Fir and Spruce-Pine-Fir (S)  (N) indicates a Canadian species group; (S) indicates USA species

9 Species and Species Groups  Hardwoods and Softwoods  Hardwoods - broadleafed deciduous trees  Softwoods – narrow, needle-like leaves, generally evergreen, also known as conifers  “C is for Conifers”  Large majority comes from Softwoods  Note: Douglas Fir-Larch and Southern Pine are classified as softwoods, but are relatively dense and have structural properties exceeding those of many hardwoods

10 Typical Commercial Hardwoods  Maples  Oaks  Birches  Elms  Walnut Canadian Conseil Wood canadien Council du bois

11 Typical Commercial Softwoods  Spruces  Pines  Firs  Cedars  Hemlocks  Larches Canadian Conseil Wood canadien Council du bois

12 Cellular Makeup Growth Characteristics (+ Guest Lecture R. Kristie) Moisture Content Shrinkage Specific Gravity Strength Other Properties, Decay (+ Guest Lecture R. Kristie) Properties

13 Interior of a Tree  Age  Conditions of growth  Structures  Some properties Canadian Conseil Wood canadien Council du bois

14 Cellular Makeup Canadian Conseil Wood canadien Council du bois

15 Cellular Makeup Canadian Conseil Wood canadien Council du bois

16 Cellular Makeup Canadian Conseil Wood canadien Council du bois

17 Cellular Makeup Canadian Conseil Wood canadien Council du bois

18 Cellular Makeup Canadian Conseil Wood canadien Council du bois

19 Cellular Makeup Canadian Conseil Wood canadien Council du bois

20 Cellular Makeup Canadian Conseil Wood canadien Council du bois

21 Cellular Makeup Canadian Conseil Wood canadien Council du bois

22 Cellular Makeup Canadian Conseil Wood canadien Council du bois

23 Cellular Makeup Canadian Conseil Wood canadien Council du bois

24 Cellular Makeup Canadian Conseil Wood canadien Council du bois

25 Cellular Makeup Canadian Conseil Wood canadien Council du bois

26 Cellular Makeup Canadian Conseil Wood canadien Council du bois

27 Cellular Makeup Canadian Conseil Wood canadien Council du bois

28 Growth Characteristics  Include density, moisture content, knots, checks, shakes, splits, slope of grain, reaction wood, decay  Affect strength of lumber  Limits on size and number of defects permitted in a given stress grade

29 Knots  Portion of a branch or limb that has been incorporated into the main body of the tree  Displace clear wood, cause slope of grain to deviate around them, therefore decreasing mechanical properties  Can cause stress concentrations and/or checking  Effect on tension and compression; greater effect on tension

30 Checks, Shakes, Splits  Separations of wood fibers  (A) Checks = radial cracks  (C) Shakes = separation parallel to annual rings  (B) Splits = complete separation of wood fibers

31 Slope of Grain  Deviation of wood fibers from a line that is parallel to edge of piece of lumber  Expressed as a ratio (e.g., 1:8, 1:15, etc.)  Measured over sufficient area to be representative of general slope of fibers; local deviations around knots disregarded

32 Reaction Wood  Known as compression wood in softwood species  Abnormal wood that forms on underside of leaning and crooked trees  Hard and brittle  Unbalanced structure in wood  Not permitted in stress grades of lumber

33 Moisture Content Canadian Conseil Wood canadien Council du bois

34 Moisture Content  Moisture content in living trees comes from sap (water and dissolved mineral salts)  Can be as high as 200% in sapwood of some tress  May be 30% in heartwood of others  Held in wood in two ways:  Free water in the cell cavity First to be driven off as wood dries  Bound water in the cell walls

35 Moisture Content  Moisture content of lumber in service is much less than that of a living tree (can be 200 percent)  Equilibrium moisture content (EMC)  Average moisture content in service  Ranges between 7 – 14%  MC at time of construction will be higher than EMC of a building (perhaps 2 times higher)

36 Moisture Content  Fiber Saturation Point (FSP)  Moisture content that corresponds to complete loss of free water  100% of bound water remaining  No loss of bound water occurs above FSP  No volume changes or other changes in structural properties associated with change in MC above FSP

37 Moisture Content

38 Fiber Saturation Point (FSP)

39 Above FSP Canadian Conseil Wood canadien Council du bois

40 Below FSP Canadian Conseil Wood canadien Council du bois

41 Shrinkage Canadian Conseil Wood canadien Council du bois

42 Moisture Content Canadian Conseil Wood canadien Council du bois

43 Shrinkage Canadian Conseil Wood canadien Council du bois

44 Shrinkage Canadian Conseil Wood canadien Council du bois

45 Shrinkage Canadian Conseil Wood canadien Council du bois

46 Shrinkage  Shrinkage causes reduction in section properties, but reduction in MC increases structural properties  Drying of lumber in order to increase structural properties is known as “Seasoning”  “Seasoning” usually refers to a controlled drying process such as air or kiln drying

47 Shrinkage

48 Seasoning Checks

49 Shrinkage Canadian Conseil Wood canadien Council du bois

50 Moisture Content Canadian Conseil Wood canadien Council du bois

51 Kiln Drying Canadian Conseil Wood canadien Council du bois

52 Rate of Drying Canadian Conseil Wood canadien Council du bois

53 Equilibrium Moisture Content

54 Estimating Shrinkage  Wood Handbook (Forest Products Laboratory) provides values of tangential, radial, and volumetric shrinkage from clearwood samples, for different species  Values given from 0 at nominal FSP to full shrinkage at zero MC; intermediate values are interpolated  Other methods exist, but a simpler method recommended for following reasons:  Shrinkage is a variable property  Orientation of annual rings in a real piece of lumber unknown  Designer will probably only know species group, not individual species

55 Estimating Shrinkage  Simple method (Rummelhart and Fantozzi, 1992)  Constant shrinkage of 6 percent used for both width and thickness of a member  Shrinkage taken as 0 at an FSP of 30 percent, and the full 6 percent shrinkage assumed to occur at an MC of zero.  Linear interpolation used for MC values between 30 and 0.  Method based on western species lumber, but method shown to give reasonable estimates for most species

56 Estimating Shrinkage - Example Estimate the shrinkage that will occur in a four- story wood-frame wall that uses Hem-Fir lumber. Consider a decrease in moisture from 15 to 8 percent.

57 Estimating Shrinkage – Example, cont’d. A shrinkage of 6 percent is assumed to occur between MC=30% and MC=0%. Use linear interpolation. Shrinkage value SV = 6/30 = 0.2% per 1 % change in MC = in/in per 1% change in MC Shrinkage S that occurs in the dimension, d, of a piece: Shrinkage S = SV x d x  MC = x d x  MC

58 Estimating Shrinkage – Example, cont’d. Shrinkage in depth of 2x12 floor joist: S floor = in/in x in x (15-8) = in Shrinkage in thickness of one 2x wall plate: S plate = in/in x 1.5 in x (15-8) = in Shrinkage in length of a stud; longitudinal shrinkage is small: S stud ≈ 0 in

59 Estimating Shrinkage – Example, cont’d. Total S = 3 S floor + 12 S plate Total S = 3 (0.158 in) + 12 (0.021 in) Total S = in ≈ ¾ in

60 Moisture Content and Lumber Sizes  Moisture content of lumber affects cross-sectional dimensions  No need to adjust section properties to account for initial MC and EMC and resulting shrinkage  Grading practices for dimension lumber have established the dry size (MC≤19 percent) of a member as basis for structural calculations  Manufacturing adjusted to MC of wood at time of manufacturer (i.e., lumber from green wood is larger at time of manufacture)

61 Specific Gravity Canadian Conseil Wood canadien Council du bois

62 Specific Gravity and Strength

63 Strength Canadian Conseil Wood canadien Council du bois

64 Compressive Strength Canadian Conseil Wood canadien Council du bois

65 Compressive Strength Canadian Conseil Wood canadien Council du bois

66 Strength Canadian Conseil Wood canadien Council du bois

67 Tensile Strength Canadian Conseil Wood canadien Council du bois

68 Tensile Strength Canadian Conseil Wood canadien Council du bois

69 Radial Stress in Curved Members

70 Bending Canadian Conseil Wood canadien Council du bois

71 Longitudinal Shear Canadian Conseil Wood canadien Council du bois

72 Fatigue Loading Canadian Conseil Wood canadien Council du bois

73 Temperature Canadian Conseil Wood canadien Council du bois

74 Preservative Processes, Fire-retardant Chemicals

75 Thermal Expansion Canadian Conseil Wood canadien Council du bois

76 Insulation, Acoustics Canadian Conseil Wood canadien Council du bois

77 Pressure-Treating Canadian Conseil Wood canadien Council du bois

78 Pressure-Treating  Chemical does not saturate the complete cross section; minimize field cutting and drilling of holes  Many species (e.g. southern pines) readily accept treatment  Others require incising (small cuts or incisions on all four sides)  Modification of modulus of elasticity and bending, tension and compression parallel to grain must be made  No modification required for pressure-treated lumber without incising

79 NDS for Wood Construction Design Specifications

80 NDS for Wood Construction  2012 National Design Specification (NDS) for Wood Construction  All or part of NDS usually incorporated into the International Building Code (IBC)  Integration of new Load and Resistance Factor Design (LRFD) and traditional Allowable Stress Design (ASD) provisions  NDS Supplement Contains numerical values of design stresses

81 Size Categories Commercial Grades Grading Structural Lumber Grade Marks Machine Grading Basic Design Values Sizes, Grading

82 Sizes of Structural Lumber  Dressed lumber  Surfaced to standard net size  Net size is less than nominal size  Most structural lumber is dressed  Dressed on a planing machine for smooth surfaces and uniform sizes  Typically surfaced four sides (S4S)  Other finishes include S2S1E – surfaced 2 sides 1 edge

83 Sizes of Structural Lumber  Rough Sawn  Large timbers are commonly rough sawn  Dimensions close to standard net sizes  Textured surface  Approximately 1/8 in larger than standard net sizes  Full Sawn  Less common  Actual size of lumber same as the specified size

84 Sizes of Structural Lumber  Consider nominal 8 x 12 member (8 in x 12 in) DRESSEDROUGH SAWN FULL SAWN 7 ¼ x 11 ¼ in 7-5/8 x 11-5/8 in8 x 12 in Standard Dressed Size Nominal SizeActual Size

85 Dressed Lumber

86 NDS 2012 Supplement Chapter 3 Section Properties

87 Size Categories – Nominal Size Ranges  Boards  ¾ to 1-1/2 in thick  2 in and wider  Dimension Lumber  2 to 4 in thick  2 in and wider  Timbers  5 in and thicker  5 in and wider

88 Size Categories – Subdivisions  Boards  Stress-Rated Board (SRB)  Dimension Lumber  Structural Light Framing (SLF)  Light Framing  Studs  Structural Joists and Planks (SJ&P)  Decking  Timbers  Beams and Stringers (B&S)  Posts and Timbers (P&T)

89 Size Categories NameNominal Thickness Nominal WidthExamples of Sizes Light Framing (LF) and Structural Light Framing (SLF) 2 to 4 in 2 x 2, 2 x 4, 4x4 Structural Joist and Plank (SJ&P) 2 to 4 in5 in and wider2 x 6, 2 x 14, 4 x 10 Stud2 to 4 in2 in and wider2 x 4, 2 x 6, 4 x 6 (lengths 10 ft and shorter) Decking*2 to 4 in4 in and wider2 x 4, 2 x 8 *stressed about its minor axis

90 Size Categories NameNominal Thickness Nominal WidthExamples of Sizes Beams and Stringers (B&S) 5 in and thickerMore than 2 in greater than thickness 6 x 10, 6 x 14, 12 x 16 Posts and Timbers (P&T) 5 in and thickerNot more than 2 in greater than thickness 6 x 6, 6 x 8, 12 x 14 NDS 2012 Section 4.1.3

91 Commercial Grades  Vary within various size and use categories  Different design values apply to same grade name in different size categories  For example, Select Structural is available in SLF, SJ&P, B&S, and P&T  Lumber grading rules reflect anticipated use of wood member based on size, but no restriction on actual use  Reference design values given for tension, compression and bending for ALL size categories

92 Commercial Grades – Examples  Structural Light Framing (SLF)  Select Structural, No. 1 and Better, No. 1, No. 2, No. 3  Light Framing (LF)  Construction, Standard, Utility  Stud  Stud  Decking  Select Decking, Commercial Decking  Beams & Stringers  Dense Select Structural, Select Structural, Dense No. 1, No. 1, Dense No. 2, No. 2

93 Grading Structural Lumber  Majority of sawn lumber is visually graded

94 Grading Structural Lumber  Grade stamp includes:  Grade  Species or species group  Other pertinent information  Stress grade  If lumber grade has recognized mechanical properties for use in structural design, referred to as a “stress grade”

95 Grading Structural Lumber  More than one set of grading rules can be used to grade some commercial species groups  For example, Douglas Fir-Larch can be graded under Western Wood Products Association (WWPA) rules or under West Coast Lumber Inspection Bureau (WCLIB) rules  Tables in NDS supplement clearly identify grading rules (e.g. WWPA and/or WCLIB)

96 Grade Marks Lumber Grading Agency (e.g. Western Wood Products Association (WWPA)) Mill number

97 Grade Marks Moisture content at time of surfacing, or condition of seasoning Lumber Grade

98 Grade Marks  S-DRY = “Surface Dry”  S-GRN = “Surface Green”  KD = “Kiln Dried”  MC = “Moisture Content”

99 Grade Marks, Moisture Content  S-GRN (MC greater than 19 percent at time manufacture)  Assumed to have 19 percent initial moisture content  S-DRY or KD (MC of 19percent or less at time of manufacture)  Assumed to have 15 percent initial moisture content  These assumptions appropriate for relatively thin material (i.e., 2 x floor joists and wall plates)  Final moisture content can be taken as equilibrium moisture content (EMC) – between 7 to 14 percent

100 Grade Marks Commercial lumber species (Douglas Fir)

101 Grade Marks

102

103  HT – heat-treated  Sometimes heat-treated to kill insects for international shipments  Not the same as KD – kiln dried  Relatively high temperatures for relatively short times

104 Grade Marks

105 Machine Grading  Machine evaluation  Lumber moves through a machine that non- destructively tests for a given property of the lumber such as density; other structural properties measured or derived  Typically only used on lumber for which very accurate structural properties needed  Also visually checked

106 Machine Grading


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