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CE 479: Design of Building Components and Systems Fall 2012 – J. Liu

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1 CE 479: Design of Building Components and Systems Fall 2012 – J. Liu
Wood: Intro, Properties, Grades CE 479 Fall 2009 Properties -- J. Liu

2 OUTLINE Introduction to Wood Properties Design Specifications
Sizes, Grading CE 479 Fall 2009 Properties -- J. Liu

3 Introduction to Wood Wood Members Species and Species Groups
CE 479 Fall 2009 Properties -- J. Liu

4 Wood Members Sawn lumber or solid sawn lumber Glued laminated timbers
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 CE 479 Fall 2009 Properties -- J. Liu

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 Oriented strand board, or OSB, or waferboard, or Sterling board (UK), is an engineered wood product formed by layering strands (flakes) of wood in specific orientations. In appearance it may have a rough and variegated surface with the individual strips (around 2.5 by 15 cm (approx. 1 in by 6 in) each) lying unevenly across each other. CE 479 Fall 2009 Properties -- J. Liu

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 Note: thickness is smaller cross-sectional dimension; width is the larger dimension Availability of lumber in wider widths varies with species; not all sizes are available in all species. CE 479 Fall 2009 Properties -- J. Liu

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 Different properties may be result of the trees being grown in different geographical locations Also may be different individual species included in combinations with similar names. Choice of species is usually economics For a given location, only a few species groups will be available. Grading stamps and hammer (images) CE 479 Fall 2009 Properties -- J. Liu

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 Different properties may be result of the trees being grown in different geographical locations Also may be different individual species included in combinations with similar names. Choice of species is usually economics For a given location, only a few species groups will be available. CE 479 Fall 2009 Properties -- J. Liu

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 CE 479 Fall 2009 Properties -- J. Liu

10 Typical Commercial Hardwoods
Canadian Conseil Wood canadien Council du bois Maples Oaks Birches Elms Walnut There are more than 100 species of broadleaved trees in Canada, but only a small percentage of these are used commercially. The leaves change colour and shed from the tree before winter. CE 479 Fall 2009 Properties -- J. Liu

11 Typical Commercial Softwoods
Canadian Conseil Wood canadien Council du bois Spruces Pines Firs Cedars Hemlocks Larches There are fewer species of conifers - only about 30. These trees bear cones and most of them have needle-like leaves all year round. Douglas Fir is the largest conifer in Canada. Sometimes it grows over 90 metres high and four and one half metres in diameter. Softwoods comprise more than 80 percent of the total volume of standing timber in Canada, and account for most of Canada's commercial lumber and wood products. CE 479 Fall 2009 Properties -- J. Liu

12 Properties Cellular Makeup
Growth Characteristics (+ Guest Lecture R. Kristie) Moisture Content Shrinkage Specific Gravity Strength Other Properties, Decay (+ Guest Lecture R. Kristie) CE 479 Fall 2009 Properties -- J. Liu

13 Interior of a Tree Age Conditions of growth Structures Some properties
Canadian Conseil Wood canadien Council du bois Age Conditions of growth Structures Some properties The interior is also beautiful, and complex. It reveals the complete history of the tree: its age, the conditions of growth, its structure and even some of its properties. CE 479 Fall 2009 Properties -- J. Liu

14 Cellular Makeup Canadian Conseil Wood canadien Council du bois
There are several easily visible layers in the tree. The first layer, the outer bark, provides protection for the tree. For some species, such as Poplar, it is very thin. For others, such as Douglas Fir, it can be very thick, sometime more than 10 centimetres. Moving into the tree, the next layer is the inner bark which stores the nutrients and transports them down through the tree. Just inside this layer is a very thin, invisible layer, the cambium. It produces new growth in girth of the tree. It produces inner bark on the outside... CE 479 Fall 2009 Properties -- J. Liu

15 Cellular Makeup Canadian Conseil Wood canadien Council du bois
…and sapwood on the inside. The sapwood transports the sap from the roots to the leaves where it is turned into nutrients. Width of the sapwood may vary from a few centimetres in Douglas Fir and Spruces to more than 30 centimetres in Ponderosa Pine. The heartwood is the inactive layer underneath the sapwood. It is often darker than the sapwood although there is not much difference in colour for many species. CE 479 Fall 2009 Properties -- J. Liu

16 Cellular Makeup Canadian Conseil Wood canadien Council du bois
The central core of the tree is the pith. It is formed by the stem, which pushes growth upward each year. It also starts new growth upward and downward, at twigs and roots. CE 479 Fall 2009 Properties -- J. Liu

17 Cellular Makeup Canadian Conseil Wood canadien Council du bois
Branches cause the knots inside trees. As the trees grow, the lower branches gradually die due to lack of sunlight and fall off. That stops knot formation at that level. Later, those knots become covered with clear, knot-free wood. It is used to produce clear grades of lumber. CE 479 Fall 2009 Properties -- J. Liu

18 Cellular Makeup Canadian Conseil Wood canadien Council du bois
There are many thin layers inside the inner bark. Each indicates a year of growth, commonly called an annual ring. The total number of layers at ground line tells the tree's age. Thickness of the layers varies with species, tree age and growth conditions. A thin layer, for example, could indicate a year of low rainfall. CE 479 Fall 2009 Properties -- J. Liu

19 Cellular Makeup Canadian Conseil Wood canadien Council du bois
Each layer usually has two distinct zones, a fast growth area on the inside, the earlywood, and a slower growth area, the latewood. CE 479 Fall 2009 Properties -- J. Liu

20 Cellular Makeup Canadian Conseil Wood canadien Council du bois
The wood consists of rows of tube-like cells, or fibres, bonded together along their outer surfaces with lignin, a glue-like substance. The earlywood cells are usually broader because they carry more water in the spring for the leaves and new shoots. The thicker-walled latewood cells are denser and stronger. Most cells are vertically oriented in the tree and provide mechanical support for the tree. CE 479 Fall 2009 Properties -- J. Liu

21 Cellular Makeup Canadian Conseil Wood canadien Council du bois
In softwoods, typical cells are 3 to 5 millimetres in length and four one hundredths of a millimetre in width. The cells in hardwoods are shorter, only about one millimetre long. CE 479 Fall 2009 Properties -- J. Liu

22 Cellular Makeup Canadian Conseil Wood canadien Council du bois
In hardwood trees, such as oak, the larger more pronounced cells are called vessels. They are used primarily to transport sap. Ray cells run radially across the tree in both hardwoods and softwoods, but are easier to see in hardwoods. They store and transport sap. CE 479 Fall 2009 Properties -- J. Liu

23 Cellular Makeup Canadian Conseil Wood canadien Council du bois
Each cell, in a magnified view, has two main zones, a thin primary wall that forms first, followed by a three-layer secondary wall that provides most of the strength. Cells are bonded to other cells with a binder called lignin. CE 479 Fall 2009 Properties -- J. Liu

24 Cellular Makeup Canadian Conseil Wood canadien Council du bois
Cell walls made up of cellulose, cells bound together by material known as lignin CE 479 Fall 2009 Properties -- J. Liu

25 Cellular Makeup Canadian Conseil Wood canadien Council du bois
The cell walls are framed with string-like microfibrils, which in turn are composed of millions of parallel close-packed cellulose molecules. The microfibrils are bound together in the cell wall by a hemicellulose and lignin matrix, similar to fibreglass construction. CE 479 Fall 2009 Properties -- J. Liu

26 Cellular Makeup Canadian Conseil Wood canadien Council du bois
The three layers in the secondary wall are identified by orientation of the microfibrils. The thick S2 layer is almost parallel with the major axis of the cell, providing strength in the longitudinal direction. The thinner S1 and S3 layers are almost perpendicular to the cell axis, acting mainly perpendicular to grain. CE 479 Fall 2009 Properties -- J. Liu

27 Cellular Makeup Canadian Conseil Wood canadien Council du bois
Most woods contain extractives, such as resin. In this pine lumber some extractives are toxic to fungi, providing natural durability. For example, tannin in Oak and thujaplicin in Western Red Cedar make these two species relatively durable. CE 479 Fall 2009 Properties -- J. Liu

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 Will first discuss ones in orange text CE 479 Fall 2009 Properties -- J. Liu

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 Classified by form, size, quality, occurrence CE 479 Fall 2009 Properties -- J. Liu

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 Checks usually happen due to seasoning (discuss later) Shakes happen in growing tree Splits can be due to seasoning or shakes CE 479 Fall 2009 Properties -- J. Liu

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 CE 479 Fall 2009 Properties -- J. Liu

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 Forms to help the tree withstand slopes, etc. CE 479 Fall 2009 Properties -- J. Liu

33 Moisture Content Canadian Conseil Wood canadien Council du bois
Moisture content is the weight of water contained in the wood, expressed as a percentage of the weight of oven-dry wood . The weight of water equals the weight of wet wood minus the weight of oven dry wood. CE 479 Fall 2009 Properties -- J. Liu

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 This moisture content in living trees comes from sap, which consists of water and dissolved mineral salts. Moisture content in living trees can vary from 200 per cent in the sapwood of some trees to 30 percent in the heartwood of others. CE 479 Fall 2009 Properties -- J. Liu

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) CE 479 Fall 2009 Properties -- J. Liu

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 CE 479 Fall 2009 Properties -- J. Liu

37 Moisture Content Breyer example 4.1
Lumber was manufactured (point 1) at an MC lower than FSP Additional drying occurred before lumber used in construction (point 2) EMC is less than MC at time of construction – typical for most buildings CE 479 Fall 2009 Properties -- J. Liu

38 Fiber Saturation Point (FSP)
When the cell walls are completely saturated, and there is no free water in the cell cavity, the wood is at its fibre-saturation point. This occurs when the moisture content is between 25 and 32 per cent, depending on species. CE 479 Fall 2009 Properties -- J. Liu

39 Above FSP Canadian Conseil Wood canadien Council du bois
Above the fibre-saturation point, the cell walls are saturated and there is additional water in the cell cavity. This condition exists in living trees and in freshly cut logs or sawn lumber. Volume of wood does not change above the fibre-saturation point. CE 479 Fall 2009 Properties -- J. Liu

40 Below FSP Canadian Conseil Wood canadien Council du bois
Below the fibre-saturation point, moisture starts to leave the cell walls, resulting in shrinkage proportional to the water removed. As the cells dry out they become stronger and stiffer. CE 479 Fall 2009 Properties -- J. Liu

41 Shrinkage Canadian Conseil Wood canadien Council du bois
That is why there are two sets of sizes for lumber -- green sizes and dry sizes. Lumber finished green is dressed oversize so that when it dries it will shrink to the same size as lumber dressed dry. CE 479 Fall 2009 Properties -- J. Liu

42 Moisture Content Canadian Conseil Wood canadien Council du bois
Wood is a hygroscopic material. It evaporates or absorbs moisture until its moisture content is in equilibrium with the surrounding air. This is called the equilibrium moisture content of wood. It is a function of temperature and relative humidity. CE 479 Fall 2009 Properties -- J. Liu

43 Shrinkage Canadian Conseil Wood canadien Council du bois
Shrinkage is not equal in all directions. For example, the greatest shrinkage occurs tangential to growth rings, reaching 7% if the wood is dried to 0% moisture content. In practice this never occurs because most lumber in housing stabilizes at an equilibrium moisture content between 10 and 15 per cent. This results in about 3 percent shrinkage. Millwork, however, will go below 10 percent moisture content in most homes. CE 479 Fall 2009 Properties -- J. Liu

44 Shrinkage Canadian Conseil Wood canadien Council du bois
Radial shrinkage is also substantial. About two thirds of tangential shrinkage, or 2 percent for most framing lumber uses. This usual 2 to 3 percent range of shrinkage in radial and tangential directions is used to arrive at differences between green and dry sizes for lumber. CE 479 Fall 2009 Properties -- J. Liu

45 Shrinkage Canadian Conseil Wood canadien Council du bois
Longitudinal shrinkage is insignificant in most cases, only about 10 percent of radial shrinkage. CE 479 Fall 2009 Properties -- J. Liu

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 On one hand, shrinkage causes reduction in section properties (reduction in size) But reduction in MC to approx 15% increases most structural properties. Net effect of decrease of MC in % range is an overall increase in structural properties. Note: Drying of lumber in order to increase its structural properties is known as SEASONING (usually refers to a controlled drying process by air or kiln drying) Both increase the cost of lumber CE 479 Fall 2009 Properties -- J. Liu

47 Shrinkage Tangential shrinkage greater than radial
Promotes the formation of radial cracks known as END CHECKS CE 479 Fall 2009 Properties -- J. Liu

48 Seasoning Checks Seasoning checks may occur in the wide side of the member at or near the neutral axis These cracks form because the wood near the surface dries and shrinks first In larger pieces of lumber, the inner core of the member loses moisture and shrinks much more slowly. Checking relieves the stresses caused by nonuniform drying. Causes a reduction in shear strength – taken into account in lumber grading rules and reference design values More common in thicker members CE 479 Fall 2009 Properties -- J. Liu

49 Shrinkage Canadian Conseil Wood canadien Council du bois
These differences in shrinkage affect the way lumber reacts during drying, depending on how it was cut out of the log. Skilled sawyers at the sawmill turn large logs during sawing to produce lumber least susceptible to warping, thus minimizing effects of differential shrinkage. CE 479 Fall 2009 Properties -- J. Liu

50 Moisture Content Canadian Conseil Wood canadien Council du bois
Moisture content of lumber needs to be controlled after manufacture. For example, codes state that framing lumber must have a moisture content of 19 percent or less. Usually this is achieved by natural air drying, on site or in a storage yard. CE 479 Fall 2009 Properties -- J. Liu

51 Kiln Drying Canadian Conseil Wood canadien Council du bois
But sometimes the products are kiln dried to speed up the process. This results in quicker availability of products, and improved quality control. CE 479 Fall 2009 Properties -- J. Liu

52 Rate of Drying Canadian Conseil Wood canadien Council du bois
Rate of drying should be controlled to minimize shrinkage problems. Checking is usually the first indication of drying lumber too fast. Lumber should also be piled properly with spacers between layers, to promote uniform moisture evaporation. This minimizes drying defects such as bow, twist, crook and cup. Generally, as moisture content decreases, strength increases because of stiffening of the cell walls and an increase in compactness of the wood. CE 479 Fall 2009 Properties -- J. Liu

53 Equilibrium Moisture Content
For design purposes, no adjustment need be made in allowable stress if the average equilibrium moisture content of wood over a year is l5 percent or less, that is considered to be a dry service condition. CE 479 Fall 2009 Properties -- J. Liu

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 Shrinkage is variable – what occurs in an individual member may be considerably different than average values given Orientation of rings unknown, sides of member probably not parallel or perpendicular to growth rings CE 479 Fall 2009 Properties -- J. Liu

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 “Multistory Wood-Frame Structures: Shrinkage Considerations and Calculations,” Proceedings of 1992 ASCE Structures Congress CE 479 Fall 2009 Properties -- J. Liu

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. Figure 4.7 in Breyer Framing is typical platform construction with 2-12 floor joists resting on bearing walls. Wall framing is conventional 2x studs with a typical single 2x bottom plate and double 2x top plates. To carry out this type of calc – must be able to establish reasonable values for initial and final MC (given here) Initial MC is defined to some extent by specification of lumber for job. General MC range at time of manufacture is shown in the grade stamp. CE 479 Fall 2009 Properties -- J. Liu

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 DMC = x d x DMC CE 479 Fall 2009 Properties -- J. Liu

58 Estimating Shrinkage – Example, cont’d.
Shrinkage in depth of 2x12 floor joist: Sfloor = in/in x in x (15-8) = in Shrinkage in thickness of one 2x wall plate: Will explain dimensions (1.25 and 11.25) later Splate= in/in x 1.5 in x (15-8) = in Shrinkage in length of a stud; longitudinal shrinkage is small: Sstud ≈ 0 in CE 479 Fall 2009 Properties -- J. Liu

59 Estimating Shrinkage – Example, cont’d.
Total S = 3 Sfloor + 12 Splate Total S = 3 (0.158 in) + 12 (0.021 in) Total S = in ≈ ¾ in Note: even if shrinkage calcs not performed Designer should allow for movement (shrinkage or swelling) that may occur Primary concern is potential for splitting (see later – weak in tension perpendicular to grain) Can also get differential movement (e.g. truss supported on wood frame on one end and concrete/masonry on other) Or, masonry stairwell in middle of wood frame (distress of ceiling, floor, wall sheathing) Plumbing, piping, electrical, mechanical systems must also be able to accommodate movement. CE 479 Fall 2009 Properties -- J. Liu

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) Timbers are not produced in a dry condition because excessive amount of time needed to season Therefore, cross-sectional dimensions based on a green condition Reference design values also adjusted for higher MC of Timbers CE 479 Fall 2009 Properties -- J. Liu

61 Specific Gravity Canadian Conseil Wood canadien Council du bois
Most seasoned commercial softwoods in Canada, and even most hardwoods, are less than two thirds the weight of water for comparable volumes. Variations in size of cell cavities, and thickness of cell walls, cause some wood species to have more wood substance than others and therefore higher specific gravities. CE 479 Fall 2009 Properties -- J. Liu

62 Specific Gravity and Strength
Because specific gravity indicates the amount of solid matter, it is an excellent indicator of strength. The denser the wood, the stronger it is. All strength properties are not affected equally but that is not important to the designer because listed stresses already take that into consideration. CE 479 Fall 2009 Properties -- J. Liu

63 Strength Canadian Conseil Wood canadien Council du bois
It was shown earlier that wood is anisotropic, meaning it has different properties in different directions. For example, wood is much stronger parallel to grain than perpendicular to grain. CE 479 Fall 2009 Properties -- J. Liu

64 Compressive Strength Canadian Conseil Wood canadien Council du bois
Wood is very strong in compression parallel to grain because the wood cells act as tiny columns or tubes bonded together, giving and receiving support from neighbouring cells. CE 479 Fall 2009 Properties -- J. Liu

65 Compressive Strength Canadian Conseil Wood canadien Council du bois
Strength in compression perpendicular to grain is difficult to measure. Compressive strength increases with deformation, reaching a maximum when the wood is compressed to about one third of its original thickness. CE 479 Fall 2009 Properties -- J. Liu

66 Strength Canadian Conseil Wood canadien Council du bois
Strength at angle to grain is somewhere between values for parallel and perpendicular to grain. Formulas and charts are used to determine strength values for angle to grain loading. CE 479 Fall 2009 Properties -- J. Liu

67 Tensile Strength Canadian Conseil Wood canadien Council du bois
Wood is also strong in tension parallel to grain. Knots reduce the strength, but this is already considered in setting design strength properties. CE 479 Fall 2009 Properties -- J. Liu

68 Tensile Strength Canadian Conseil Wood canadien Council du bois
Wood is relatively weak in tension perpendicular to grain. However, it is rarely required to take much load in that direction except for secondary stresses in some curved members. CE 479 Fall 2009 Properties -- J. Liu

69 Radial Stress in Curved Members
For example, tension or compression perpendicular to grain can be induced by radial stress in curved members. CE 479 Fall 2009 Properties -- J. Liu

70 Bending Canadian Conseil Wood canadien Council du bois
Wood is very strong in bending. Shallow beams have relatively greater resistance to bending in comparison to proportionately deeper beams. Therefore, depth effect is considered in setting design properties. CE 479 Fall 2009 Properties -- J. Liu

71 Longitudinal Shear Canadian Conseil Wood canadien Council du bois
Longitudinal or horizontal shear is often a controlling factor in beam design. It is caused by bending loads, creating maximum longitudinal shear stresses parallel to grain at the neutral axis. CE 479 Fall 2009 Properties -- J. Liu

72 Fatigue Loading Canadian Conseil Wood canadien Council du bois
Wood is very resistant to fatigue caused by cyclic loading in bridges or structures subject to high wind loads or vibrating machinery. CE 479 Fall 2009 Properties -- J. Liu

73 Temperature Canadian Conseil Wood canadien Council du bois
Strength of wood is little affected by temperatures and for normal construction applications is considered to be the same, from the arctic to the equator. At low temperatures, strength increases slightly. Up to 37° celcius, strength is hardly affected, even with occasional exposures up to 51°. Above that, stresses should be reduced. In actual practice, however, these extremes would seldom occur. CE 479 Fall 2009 Properties -- J. Liu

74 Preservative Processes, Fire-retardant Chemicals
Codes and design standards sometimes require strength values to be adjusted for preservative treating processes, especially when the wood is incised. Also, when wood is treated with fire-retardant chemicals, the design values must be reduced. CE 479 Fall 2009 Properties -- J. Liu

75 Thermal Expansion Canadian Conseil Wood canadien Council du bois
Thermal expansion of wood is usually insignificant, especially along the grain. For very long spans, such as in bridges and wood stave pipes, expansion should be calculated taking the offsetting effect of moisture shrinkage into account. CE 479 Fall 2009 Properties -- J. Liu

76 Insulation, Acoustics Canadian Conseil Wood canadien Council du bois
Wood is a natural insulator against heat and cold because of the tiny air pockets within its cellular structure. Wood is also a good acoustical material. Wood converts sound energy into heat energy by frictional and viscous resistance in the wood cells. CE 479 Fall 2009 Properties -- J. Liu

77 Pressure-Treating Canadian Conseil Wood canadien Council du bois
When decay conditions cannot be avoided, decay can be prevented by treating the wood with preservatives in pressure treating cylinders. This treatment makes the wood toxic to fungi and eliminates food as one of the main factors of fungal growth. Pressure treating takes place in large steel cylinder, typically Cylinder is closed and filled with preservative. Cylinder is subjected to pressure which forces chemical into the wood. CE 479 Fall 2009 Properties -- J. Liu

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 The chemical does not saturate the complete cross section, so field cutting and drilling of holes for connections after treating should be minimized. CE 479 Fall 2009 Properties -- J. Liu

79 Design Specifications
NDS for Wood Construction CE 479 Fall 2009 Properties -- J. Liu

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 CE 479 Fall 2009 Properties -- J. Liu

81 Sizes, Grading Size Categories Commercial Grades
Grading Structural Lumber Grade Marks Machine Grading Basic Design Values CE 479 Fall 2009 Properties -- J. Liu

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 CE 479 Fall 2009 Properties -- J. Liu

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 Textured surface may be desired for architectural purposes May be specially ordered at smaller sizes Cross-sectional properties of rough and full-sawn not provided in NDS specs because of relatively infrequent use. Full sawn not generally available CE 479 Fall 2009 Properties -- J. Liu

84 Sizes of Structural Lumber
Consider nominal 8 x 12 member (8 in x 12 in) Nominal Size Actual Size Note: used to be 7.5 x 11.5, but changed in NDS 2012 Appears to be more consistent with dressed dimensions for dimension lumber OLD NOTE: Note: this is a timber. Dressed dimension in either direction different than for dimension lumber (next)???? Standard Dressed Size DRESSED ROUGH SAWN FULL SAWN 7 ¼ x 11 ¼ in 7-5/8 x 11-5/8 in 8 x 12 in CE 479 Fall 2009 Properties -- J. Liu

85 Dressed Lumber In general for dimension lumber 1” -- subtract ¼
2” to 6 – subtract ½ 8” and larger, subtract 3/4 CE 479 Fall 2009 Properties -- J. Liu

86 NDS 2012 Supplement Chapter 3 Section Properties
CE 479 Fall 2009 Properties -- J. Liu

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 Timbers 5 in and thicker 5 in and wider Size and use are related CE 479 Fall 2009 Properties -- J. Liu

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) Stress rated boards may be used in structural applications But since relatively thin, not commonly used in structural framing Therefore, we will focus on dimension lumber and timber CE 479 Fall 2009 Properties -- J. Liu

89 Size Categories Name Nominal Thickness Nominal Width Examples 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) 5 in and wider 2 x 6, 2 x 14, 4 x 10 Stud 2 in and wider 2 x 4, 2 x 6, 4 x 6 (lengths 10 ft and shorter) Decking* 4 in and wider 2 x 4, 2 x 8 *stressed about its minor axis CE 479 Fall 2009 Properties -- J. Liu

90 Size Categories Name Nominal Thickness Nominal Width Examples of Sizes
Beams and Stringers (B&S) 5 in and thicker More than 2 in greater than thickness 6 x 10, 6 x 14, 12 x 16 Posts and Timbers (P&T) Not more than 2 in greater than thickness 6 x 6, 6 x 8, 12 x 14 NDS 2012 Section 4.1.3 CE 479 Fall 2009 Properties -- J. Liu

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 CE 479 Fall 2009 Properties -- J. Liu

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 Decking Select Decking, Commercial Decking Beams & Stringers Dense Select Structural, Select Structural, Dense No. 1, No. 1, Dense No. 2, No. 2 CE 479 Fall 2009 Properties -- J. Liu

93 Grading Structural Lumber
Majority of sawn lumber is visually graded As moves through stamping machine, grade mark stamped 18”-24” from one end of the piece. Pieces are then bundled CE 479 Fall 2009 Properties -- J. Liu

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” Stamped by person familiar with the lumber grading rules as it comes out of the mill. CE 479 Fall 2009 Properties -- J. Liu

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) Because designer does not have control over which will be used – should use the lower reference value Higher reference design value justified only if grade stamp associated with value appears on member CE 479 Fall 2009 Properties -- J. Liu

96 Grade Marks Mill number Lumber Grading Agency
Mill number not so important, unless need to identify source because of a problem or question. Note: grading process is voluntary and lumber can be produced that is ungraded However, it will not be acceptable for structural applications under the codes Lumber Grading Agency (e.g. Western Wood Products Association (WWPA)) CE 479 Fall 2009 Properties -- J. Liu

97 Grade Marks Lumber Grade
Moisture content at time of surfacing, or condition of seasoning CE 479 Fall 2009 Properties -- J. Liu

98 Grade Marks S-DRY = “Surface Dry” S-GRN = “Surface Green”
KD = “Kiln Dried” MC = “Moisture Content” Excessive moisture has detrimental effect Therefore lumber is dried before it leaves the mill MC at the time the lumber was manufactured, but actual MC may be different if exposed to moisture after leaving the mill. S-DRY – 19% max, decay won’t begin or continued…. If has not been exposed to moisture since leaving the mill, might be as low as 10% ONLY Smaller lumber sizes may be seasons to 15 percent or less – larger (i.e. Timbers) are not produced in a dry condition, size would require excessive amount of time for seasoning. KD has very low moisture content .. Normally seen on lumber used for finish purposes or engineered wood products, furniture. CE 479 Fall 2009 Properties -- J. Liu

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 Larger size member will dry more slowly EMC between 7 and 14 percent for most buildings. Dry southwestern states – CA, nevada, utah, arizona averages 9 percent, 7 to 12 percent range Remainder of US averages 12 percent, range expected of 9 to 14 CE 479 Fall 2009 Properties -- J. Liu

100 Grade Marks Commercial lumber species (Douglas Fir)
CE 479 Fall 2009 Properties -- J. Liu

101 Grade Marks Some WWPA grade stamps
CE 479 Fall 2009 Properties -- J. Liu

102 Grade Marks A number of western lumber species have similar performance properties and are marketed with a common species designation. These species groupings are used for lumber to which design values are assigned. CE 479 Fall 2009 Properties -- J. Liu

103 Grade Marks 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 Required by some countries before they will permit importation of lumber; not the same as kiln-drying. Exposes wood to relatively high temp for relatively short time and does not typ. Result in drying of wood. KD exposes wood to lower temperatures, but for long periods to dry the wood. CE 479 Fall 2009 Properties -- J. Liu

104 Grade Marks Missing species of wood? Because SPIB is Southern Pine Inspection Bureau – only grades southern pine!! CE 479 Fall 2009 Properties -- J. Liu

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 Various structural properties can be derived from density Typically only used on lumber in which structural properties need to be determined as accurately as possible, such as wood used for engineered wood products Passes through machine, bending load is applied about minor axis, modulus of elasticity is measured X-ray inspection used to measure density CE 479 Fall 2009 Properties -- J. Liu

106 Machine Grading Note the grade is missing
But have fiber stress in bending and modulus of elasticity Identified by design values, strength and stiffness, instead of grade (insert info about finger jointed lumber? From Wood_Grade_Marks.pdf) CE 479 Fall 2009 Properties -- J. Liu


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