Fundamentals of wood design and engineering

Fundamentals of wood design and engineering The basis for this seminar is the use and explanation of the 2005 National Design Specification for Wood Construction (NDS). The NDS has been adopted by all model building codes in the United States—it is currently in compliance with the 2006 International Building Code (IBC) and the 2006 International Residential Code (IRC)—and is used for the design of wood structures worldwide. In this interactive seminar, we will apply NDS guidelines to specific examples and sample problems of wood construction in order to determine the adjustment factors for specified design values as they relate to conditions of use (temperature, load duration, moisture), member geometry, and member stability. The ultimate goal of this seminar is to provide civil or structural engineers with general design knowledge of the wood elements typically employed in residential and mixed-use construction.

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Fundamentals of

Wood Design

and Engineering

Wood Design

p Session 3

wIntroduction to Wood Engineering; Codes &

Standards; Load combinations, weights of building materials and tributary area; Simple beam design:

floor/roof joists, beams and girders

p Session 4

wColumn design, stud walls, headers, posts

p Session 5

wConnection design, bolts, lag bolts, screws, nails

p Session 6

wDiaphragms and shearwalls, seismic issues; Options regarding composite panels

Codes and Standards

p Original Model Codes

wUniform Building Code (UBC) - International

Conference of Building Officials (ICBO) - 1997

wNational Building Code (NBC) - Building Officials

and Code Administrators International (BOCA)

-1999

wStandard Building Code (SBC) - Southern

Building Code Congress International (SBCCI)

-1997 and 1999

p Codes (continued)

w One and Two Family Dwelling Code (OTFDC) - Council of American Building Officials (CABO) - 1995

w International Building Code (IBC) - International Code Council (ICC) – 2000 and 2003

w International Residential Code (IRC) - International Code Council (ICC) – 2000 and 2003

w National Fire Protection Association (NFPA) – NFPA Building Code (NFPA 5000) - 2003

w National Earthquake Hazard Reduction Program (NEHRP)

- Federal Emergency Management Administration – 1994,

1997 and 2000

p Jurisdictions

wNational

HNEHRP document, other FEMA publications

wState

HTwo Versions

• State buildings, Schools, Hospitals - Higher requirements

• Minimum requirements for all jurisdictions in the state

wCities, Counties

p National Standards

wNational Design Specifications (NDS) - American Forest & Paper Association, American Wood Council – 1991, 1997 and 2001

HAllowable Stress Design (ASD) of wood sawn and glued laminated members, diaphragms, shearwalls and connections

wLoad and Resistance Factor Design (LRFD) -American Forest & Paper Association, -American Wood Council – 1996

HLoad and Resistance Factor Design of wood members, diaphragms, shearwalls, connections

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February 17, 2005 7

p National Standards

wASCE-7 – American Society of Civil Engineers –

1998 and 2003

wACI-318 - American Concrete Institute (ACI) - 2002

wASD Specification for Structural Steel Buildings

American Institute for Steel Construction (AISC)

-1989

wLRFD Specification for Structural Steel Buildings

-American Institute for Steel Construction (AISC) –

1999/2000

wACI-530/ASCE-5/TMS-402 - American Concrete

Institute (ACI), American Society of Civil Engineers

(ASCE), The Masonry Society (TMS) - Masonry

ASD - 2002

p Industry Associations

wAmerican Forest & Paper Association

wAmerican Wood Council

wAmerican Plywood Association

wAmerican Institute of Timber Construction

wGrading Agencies

HWestern Wood Products Association (WWPA)

HWest Coast Lumber Inspection Bureau (WCLIB)

HOthers - see NDS

Allowable Stress Design

p Load Combinations - 1997 Uniform Building Code

2 D + L + (Lror S)

3 D + (W or E/1.4)

4 0.9D ± E/1.4

5 D + 0.75[L + (Lror S) + (W or E/1.4)]

wNote:

HSeismic force, E, is a strength level force in the 1997

UBC

p Load Combinations 1997 Uniform Building Code -Alternate (1994 UBC Load Combinations)

1 D + L + (Lror S)

2 D + L + (W or E/1.4)

3 D + L + W + S/2

4 D + L + S + W/2

5 D + L + S + E/1.4

wNotes:

Ha 1/3 allowable stress increase is permitted for Load Combinations 2 through 5 for the 1997 UBC Alternate ASD Load Combinations

HSeismic force, E, is a strength level force in the 1997 UBC

p Load Combinations - 2003 International Building

Code

2 D + L

3 D + L + (Lror S or R)

4 D + (W or 0.7E) + L + (Lror S or R)

5 0.6D + W

6 0.6D + 0.7E

wNote:

HSeismic force, E, is a strength level force in the 2000

IBC

p Load Combinations - 2003 International Building Code - Alternate

1 D + L + (Lror S or R)

2 D + L + ( W)

3 D + L + W + S/2

4 D + L + S + W/2

5 D + L + S + E/1.4

6 0.9D + E/1.4

wNotes:

Ha 1/3 allowable stress increase is permitted for Load Combinations 2 through 6 for the 2000 IBC Alternate ASD Load Combinations

HSeismic force, E, is a strength level force in the 2000 IBC

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February 17, 2005 13

Dead Loads

Timber Construction Manual, Fourth Edition, AITC, Page 8-742

Dead Loads

p Floors

Plywood (Sheathing) 3.0 psf

Electrical & Mechanical 2.0 psf to 5.0 psf

Partitions 10.0 psf for residential

20.0 psf for office

Dead Loads

p Roofs

Roofing 5 ply or singles 6.0 psf plus 3.0 psf for reroofing

or Shakes 3.0 psf

Plywood (Sheathing) 2.0 psf

Electrical & Mechanical 1.0 psf to 2.0 psf

Live Loads

p Stairs/Corridors 100 psf

Live Load Reduction

p Old UBC Method - Tributary Area

wR = r (A - 150)

p ASCE 7 Method - Influence Area

wL = L

A i

00 25 + 15











Tributary Area vs Influence Area

p Joist and Purlins

16’

16”

20’

2 x joists at 16” on center

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Tributary Area vs

Influence Area

p Beams and Girders

16’

20’

Tributary Area vs Influence Area

p Columns

16’

20’

Framing Methods

p Platform Framing p Balloon Framing

CABO One and Two Family Dwelling Code, 1995 Edition, Page 67

Types of Wood Buildings

p Residential/Houses

wSingle Family

wTwo Family (Duplexes)

wTownhouses

wApartments

p Commercial

wStores, offices

p Warehouse/Industrial

p Bridges

wVehicular

wPedestrian

p Miscellaneous

wPlay Structures

wGazebos

wDecks

Why Use Wood?

p Economics

p Availability

p High Strength per Weight Ratio

p Simple Construction

p Light Weight

p Fire Resistant

Why Use Wood?

p Strength of Material per Unit Weight

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Wood Products

p Remanufactured Lumber

wPlywood

wGlued Laminated Beams

wMicrolam

wLaminated Decking

p Wood Chips and Fibers

wOSB - Oriented Strand Board

wParticle Board

p Pre-Engineered Products

wI - Joists

wOpen Web Joists

wPre-Manufactured Trusses

Lumber Sizes

p Sawn Lumber

w2 x 4 — 2 x 14

w3 x 4 — 3 x 16

w4 x 4 — 4 x 16

w6 x 6 — 6 x 24

w8 x 8 — 8 x 24

w10 x 10 — 10 x 24

w12 x 12 — 12 x 24

w14 x 14 — 14 x 24

w 16 x 16 — 16 x 24

w 18 x 18 — 18 x 24

w 20 x 20 — 20 x 24

w 22 x 22 — 22 x 24

w 24 x 24

Lumber Sizes

p Sawn Lumber

wDimension Lumber

H2 x 4 — 2 x 12

H3 x 4 — 3 x 16

H4 x 4 — 4 x 16

wBeams & Stringers

H6 x 10 — 6 x 16

H8 x 12 — 8 x 16

wPosts & Timbers

H6 x 6 — 6 x 8

H8 x 8 — 8 x 10

H10 x 10

Lumber Sizes

p Sawn Lumber - Standard Dressed Sizes

wDimension Lumber

HThickness

• 2 x, 3 x, 4 x - nominal thickness minus 1/2”

HWidth

• 2” through 6” - nominal width minus 1/2”

• 8” and wider - nominal width minus 3/4”

wTimbers

HThickness

• 5 x and thicker - nominal thickness minus 1/2”

HWidth

• 5” and wider - nominal width minus 1/2”

Lumber Sizes

p Glued Laminated Lumber

wWestern Species Beams

HWidths

• 3-1/8”, 5-1/8”, 6-3/4”, 8-3/4”, 10-3/4”, 12-1/4”

HLaminations

• 1-1/2”

wSouthern Pine Beams

HWidths

• 3”, 5”, 6-3/4”, 8-1/2”, 10-1/2”

HLaminations

• 1-3/8”

Connections

p Nails

wCommon

wBox

wSinkers

p Bolts

p Staples

p Glue

p Sheet Metal Connectors

w“Simpson Strong-Tie”

w“KC Metals”

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Grading of Lumber

p Knots

p Checks

p Shakes

p Splits

p Slope of Grain

Grading of Lumber

p Visual Grading

wDense Select Structural

wSelect Structural

wDense No 1

wNo 1 and Better

wNo 1

wNo 2

wNo 3

wStud

wStandard

wConstruction

wUtility

Allowable Stresses

p Allowable Stresses (Allowable Design Values)

wTabulated Design Values x Adjustment Factors

HF b ’ = F b x C D C M C t C L C F C V C fu C i C r C c C f

HF t ’ = F t x C D C M C t C F C i

HF v ’ = F v x C D C M C t C i [C H ] (C Hno longer used)

HF c⊥’ = F c⊥x C M C t C i C b

HF c ’ = F c x C D C M C F C i C P

HE’ = E x C M C t C i C T

HF g ’ = F g x C D C t– Not listed in 2001 NDS

p Allowable Stresses (Allowable Design Values)

wTabulated Design Values x Adjustment Factors

p Tabulated Design Values

wTables 4A

HVisually Graded Dimension Lumber except Southern Pine

wTable 4B

HVisually Graded Southern Pine Dimension Lumber

wTable 4C

p Bending Stress Adjustment Factors

wLoad Duration Factor, C D

Design of Wood Structures, Breyer, Donald, Page 4.39

wLoad Duration Factor, C D

HUse shortest duration load in combination

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wWet Service Factor, C M

HC M= 1.0 for moisture content less than or equal to 19

percent for sawn dimension lumber and timber

percent for glued laminated timber

HC M= 0.85 for moisture content greater than 19

percent for sawn dimension lumber with a tabulated

allowable bending stress times the size factor of more

than 1150 psi Otherwise, C M= 1.00

HC M= 1.0 for moisture content greater than 19 percent

for sawn timber

wTemperature Factor, C t

HWet Service Condition

• C t= 1.0 for temperature less than or equal to 100 degrees Fahrenheit.

• C t= 0.7 for temperature greater than 100 and less than or equal to 125 degrees Fahrenheit.

HDry Service Condition

• C t= 1.0 for temperature less than or equal to 100 degrees Fahrenheit.

wBeam Stability Factor, C L

HFor beams which are laterally supported on their

compression flange and braced to prevent buckling

or have shapes which do not buckle under bending,

C L= 1.0

HFor beams which do not meet the above criteria a

stability factor is calculated depending on the

unbraced length of the member

• See NDS Section 3.3.3, Equation 3.3-6

Lateral Stability of Beams

wBeam Stability Factor, C L

Hd/b < 2; no lateral support required

H2 < d/b < 4; the ends shall be held in position

H4 < d/b < 5; the compression edge of the member

shall be held in line for its entire length and ends at

points of bearing shall be held in position

H5 < d/b < 6; bridging, full depth blocking or cross

bracing shall be installed at 8 feet o.c maximum, the

compression edge of the member shall be held in

line for its entire length and ends at points of bearing

shall be held in position

H6 < d/b < 7; both edges of the member shall be held

in line for their entire length and ends at points of

bearing shall be held in position

wSize Factor, C F

HC Ffor sawn dimension lumber, except Southern Pine, ranges from 0.9 to 1.5 depending on the width and thickness of the member

HC Ffor Southern Pine sawn dimension lumber has been incorporated into the design value tables

HC Ffor sawn timber loaded on the narrow face is

calculated by the equation C F = (12/d) 1/9when the depth exceeds 12 inches

HC Ffor sawn timber loaded on the wide face ranges between 0.74 and 1.00

HC Fdoes not apply to glued laminated timbers

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wVolume Factor, C V

HC V = (21/L) 1/x (12/d) 1/x (5.125/b) 1/x

• L = distance between points of zero moment

• d = depth of member

• b = width of member

• x = 10 for all species except Southern Pine (SP = 20)

HC Vdoes not apply to sawn dimension lumber and

timber

HC Vfor glued laminated lumber is calculated for each

size member

HC Vdoes not apply simultaneously with the CLfactor

The lesser values is taken where both factors apply

wFlat Use Factor, C fu

HC fufor sawn dimension lumber ranges between 1.0 and 1.2

HC fufor glued laminated timber ranges between 1.01 and 1.19

wIncising Factor, C i

HIncisions parallel to grain to a maximum depth of 0.4

inches and a maximum length of 3/8 inches with a

maximum density of 1,100 per square foot

HC i= 0.80 for sawn dimension lumber and timber,

when incisions have been made to increase

penetration of pressure preservative treatment

HC iwas 0.85 in previous versions of the NDS

wRepetitive Member Factor, C r

HC r= 1.15 for sawn dimension lumber 2” to 4” thick, when the same members are repeated and spaced at less than or equal to 24 inches on center

wCurvature Factor, C c

HC cfor glued laminated timber is calculated when the

member is curved, such as in arched glued laminated

timbers

wForm Factor, C f

HC f= 1.18 for round wood sections

HC f= 1.414 for square wood sections loaded on the diagonal (diamond shaped wood section)

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p Shear Stress Adjustment Factors

wThe same as bending stress adjustment factors for

the following:

HLoad Duration Factor, C D

HTemperature Factor, C t

HC M= 1.0 for moisture content less than or equal to 19 percent for sawn dimension lumber and timber

HC M= 1.0 for moisture content less than or equal to 16 percent for glued laminated timber

HC M= 0.97 for moisture content greater than 19 percent for sawn dimension lumber

HC M= 1.0 for moisture content greater than 19 percent for sawn timber

HC M= 0.875 for moisture content greater than 16 percent for glued laminated timber

whether or not incisions have been made to increase

wShear Stress Factor, C H– Factor Eliminated in the

2001 NDS

HC Hwas based on the size of splits, checks and shakes on the face of a member

HThe tabulated shear stress values were based on standard sizes of splits, checks and shakes

HIf the sizes of splits, checks and shakes were less than assumed for the tabulated values, then the shear stress value may be increased

HThe values for C Hranged between 1.00 and 2.00

p Bearing Stress (Compression Perpendicular to

Grain) Adjustment Factors

wThe same as bending stress adjustment factors for

the following:

HTemperature Factor, C t

p Bearing Stress (Compression Perpendicular to Grain) Adjustment Factors

HC M= 1.0 for moisture content less than or equal to 19 percent for sawn dimension lumber and timber

HC M= 1.0 for moisture content less than or equal to 16 percent for glued laminated timber

HC M= 0.67 for moisture content greater than 19 percent for sawn dimension lumber

HC M= 0.67 for moisture content greater than 19 percent for sawn timber

HC M= 0.53 for moisture content greater than 16 percent for glued laminated timber

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p Bearing Stress (Compression Perpendicular to

Grain) Adjustment Factors

whether or not incisions have been made to increase

p Bearing Stress (Compression Perpendicular to Grain) Adjustment Factors

wBearing Area Factor, C b

HC b = l b + 0.375/ l bfor bearing lengths less than 6 inches long and greater than 3 inches from the end of the member

HSupports in the middle of the span

HRanges between 1.75 for 0.5 inch bearing length and 1.0 for 6 inch bearing length

p Modulus of Elasticity Adjustment Factors

percent for sawn dimension lumber and timber

for sawn dimension lumber

for sawn timber

wTemperature Factor, C t

HC t= 1.0 for temperature less than or equal to 100 degrees Fahrenheit

HC t= 0.9 for temperature greater than 100 and less than or equal to 125 degrees Fahrenheit

HC t= 0.9 for temperature greater than 125 and less than or equal to 150 degrees Fahrenheit

when incisions have been made to increase

wBuckling Stiffness Factor, C T

HC Tis only used for 2” x 4” or smaller members in sawn lumber truss compression chords

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