ce 479: design of building components and systems fall 2012 – j. liu

CE 479: DESIGN OF BUILDING COMPONENTS AND SYSTEMSFALL 2012 – J. LIU

Wood: Intro, Properties, Grades

Introduction to WoodPropertiesDesign SpecificationsSizes, Grading

OUTLINE

Wood MembersSpecies and Species Groups

Introduction to Wood

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

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

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

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

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

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” http://

www.youtube.com/watch?v=FijQbZeTGNc 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

http://www.youtube.com/watch?v=FijQbZeTGNc



Typical Commercial Hardwoods

Maples Oaks Birches Elms Walnut

Canadian Conseil Wood canadienCouncil du bois

Typical Commercial Softwoods

Spruces Pines Firs Cedars Hemlocks Larches


Cellular MakeupGrowth Characteristics (+ Guest Lecture R. Kristie)Moisture ContentShrinkageSpecific GravityStrengthOther Properties, Decay (+ Guest Lecture R. Kristie)

Properties

Interior of a Tree

Age Conditions of

growth Structures Some properties


Cellular Makeup Canadian Conseil Wood canadienCouncil du bois

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

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

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

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

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

Moisture Content Canadian Conseil Wood canadienCouncil du bois

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

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)

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

Moisture Content

Fiber Saturation Point (FSP)

Above FSP Canadian Conseil Wood canadienCouncil du bois

Below FSP Canadian Conseil Wood canadienCouncil du bois

Shrinkage Canadian Conseil Wood canadienCouncil du bois

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

Shrinkage

Seasoning Checks

Kiln Drying Canadian Conseil Wood canadienCouncil du bois

Rate of Drying Canadian Conseil Wood canadienCouncil du bois

Equilibrium Moisture Content

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

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

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.

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

= 0.002 in/in per 1% change in MCShrinkage S that occurs in the dimension,

d, of a piece:Shrinkage S = SV x d x DMC = 0.002 x d x DMC


Shrinkage in depth of 2x12 floor joist:Sfloor = 0.002 in/in x 11.25 in x (15-8) = 0.158 inShrinkage in thickness of one 2x wall plate:Splate= 0.002 in/in x 1.5 in x (15-8) = 0.021 inShrinkage in length of a stud; longitudinal shrinkage is small: Sstud ≈ 0 in


Total S = 3 Sfloor + 12 Splate

Total S = 3 (0.158 in) + 12 (0.021 in)Total S = 0.725 in ≈ ¾ in

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)

Specific Gravity Canadian Conseil Wood canadienCouncil du bois

Specific Gravity and Strength

Strength Canadian Conseil Wood canadienCouncil du bois

Compressive StrengthCanadian Conseil Wood canadienCouncil du bois

Strength Canadian Conseil Wood canadienCouncil du bois

Tensile Strength Canadian Conseil Wood canadienCouncil du bois

Radial Stress in Curved Members

Bending Canadian Conseil Wood canadienCouncil du bois

Longitudinal Shear Canadian Conseil Wood canadienCouncil du bois

Fatigue Loading Canadian Conseil Wood canadienCouncil du bois

Temperature Canadian Conseil Wood canadienCouncil du bois

Preservative Processes, Fire-retardant Chemicals

Thermal Expansion Canadian Conseil Wood canadienCouncil du bois

Insulation, Acoustics Canadian Conseil Wood canadienCouncil du bois

Pressure-Treating Canadian Conseil Wood canadienCouncil du bois

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

NDS for Wood Construction

Design Specifications

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

Size CategoriesCommercial GradesGrading Structural LumberGrade MarksMachine GradingBasic Design Values

Sizes, Grading

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

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

Sizes of Structural Lumber Consider nominal 8 x 12 member (8 in x

12 in)

DRESSED ROUGHSAWN

FULLSAWN7 ¼ x 11 ¼ in

7-5/8 x 11-5/8 in 8 x 12 in

Standard Dressed Size

Nominal Size Actual Size

Dressed Lumber

NDS 2012 Supplement Chapter 3 Section Properties

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

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)

Size Categories

Name Nominal Thickness

Nominal Width

Examples of Sizes

Light Framing (LF) and

Structural Light Framing

(SLF)

2 to 4 in 2 to 4 in 2 x 2, 2 x 4, 4x4

Structural Joist and Plank

(SJ&P)

2 to 4 in 5 in and wider 2 x 6, 2 x 14, 4 x 10

Stud 2 to 4 in 2 in and wider 2 x 4, 2 x 6, 4 x 6

(lengths 10 ft and shorter)

Decking* 2 to 4 in 4 in and wider 2 x 4, 2 x 8

*stressed about its minor axis

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)

5 in and thicker

Not more than 2 in greater

than thickness

6 x 6, 6 x 8, 12 x 14

NDS 2012 Section 4.1.3

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

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

Grading Structural Lumber Majority of sawn lumber is visually

graded

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”

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)

Grade Marks

Lumber Grading Agency (e.g. Western Wood Products Association (WWPA))

Mill number

Grade Marks

Moisture content at time of surfacing, or condition of seasoning

Lumber Grade

Grade Marks

S-DRY = “Surface Dry”

S-GRN = “Surface Green”

KD = “Kiln Dried” MC = “Moisture

Content”

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

Grade Marks

Commercial lumber species (Douglas Fir)

Grade Marks

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

Grade Marks

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

Machine Grading

ce 479: design of building components and systems fall 2012 – j. liu

Documents

choice of species

species groupshardwoods

species groupsintroduction

different individual

species groupsnote

commercial species group

canadian species group

fir sn