Properties of Wood

7/21/2019 Properties of Wood

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Properties of Wood

http://www.tekniksipil.org/

http://www.tekniksipil.org/



Cellular Makeup

• Cells areelongated,tube likecells

• Cell wallsare madeof cellulose

• Cells areboundtogether bylignin

http://concise.britannica.com/ebc/art-66141/Cross-section-of-a-tree-trunk



Effect of Cell Structure

• Since the cells are elongated, the wood

has different strength properties when

stress is transverse or parallel to the cell

longitudinal axis.

• Shrinkage properties are also different in

each direction.



Grain

• Grain

runs

along thetrunk.

• Grain size

is nonuniform



Principle Directions

• “With the Grain” =

Longitudinal

• “Cross Grain” or

“Perpendicular to

Grain” = Radial or

Tangential

• Strength and

Shrinkage

Properties areDIFFERENT IN

EACH DIRECTION

http://concise.britannica.com/ebc/art-66141/Cross-section-of-a-tree-trunk

RadialTangential

Longitudinal



Moisture Content & Shrinkage

MC = (moist wt – oven dry wt)/ (oven dry weight) x 100

• Living trees may have MC up to 200%.

• Lumber in service has MC less than 20%

• The loss of moisture results in wood shrinkage

– Shrinkage is most pronounced perpendicular to grain

• Moisture is found within the wood cell cavities(free water) and the cell walls (bound water)



More Moisture

• Fiber Saturation Point (FSP):The MC where all the free water islost, leaving only the bound water.There is no shrinkage when theMC is above the FSP

• Volume changes take place as theMC varies below the FSP.

• Typically, MC continues todecrease after both manufacturingand installation.

• Equilibrium Moisture Content(EMC): The in service moisturecontent – This can vary with occupancy and/or

season

MC vs. Shrinkage

0

20

40

60

80

100

120

140

160

180

200

0 0.5 1

Shrinkage

M C

Manufacture

Installation



Shrinkage Example

• 5 Story Condoin Juneau, AK

• It rained all but3 days during

the 4-5 weeksof framing.

• In the first yearafterconstruction

there wasconsiderableshrinkage



Imperfections in Wood

http://www.vermonttimberworks.com/images/shake.jpg

Knots



Members

Cut froma Log

• Perpendicularto graindirection maybe eithertangential,radial, or acombinationof each

http://web.utk.edu/~grissino/images/zuni%20doug-fir.jpg

Cross Grain

Cross Grain

Cross Grain



Sawn Lumber Visual Grading

• Rules agencies establish grading rules

based on observed wood quality

• Inspector’s visually grade and mark each

piece as it is manufactured



Pressure Treated Wood

• Wood is often chemicallytreated to increase it’sdurability – Minimizes decay and mold

– Discourages insect infestation – Fire treatments also available

• Moist, dark (or nearly dark)locations with minimal air

circulation are prime locationsfor decay and mold

• High moisture conditions thatare variable are alsoproblematic



Standard Sizes for

Sawn Lumber • NDS Supplement Section 3, Table 1B

• Nominal Sizes are larger than the actual

sizes!

– Check out the sizes! Not all are readily

available

– Some available sizes:

• 2x2 to 2x12

• 4x4 to 4x12

• 6x6 to 6x12



Sawn Lumber Size

Classifications• See text pg 4.30

• Structural Joists and Plank (SJ&P)

– 2 to 4 inches thick

– 2 inches and wider

• Beams & Stringers (B&S)

– 5 inches & thicker

– Width > thickness + 2 inches• Posts & Timbers (P&T)

– 5 inches & thicker

– Width <= thickness + 2 inches



Glue Laminated Timbers

• Laminations can be

strategically used to

make efficient use of the

best materials.• Members can be

fabricated for particular

uses

• Larger, Longer, &

Curved sections are

possible

http://www.lamisellbeams.com/images/cover-image/lamisell-image03c-shadow04.jpg



Glulams Designed as Beams

• Best material on outer faces

• Butt splice in compression zone

• Scarf splice in tension region



Glulams Designed as Columns

• Materials more uniform

• Butt joints can be used throughout



Glulam Standard Sizes

• See NDS Supplement

Table 1C

• Common Widths for

Western Species GL – 3.1/8”, 5.1/8”, 6.3/4”,

8.3/4”, 10.3/4”

• Common depths:

– 6” and larger, in 1.1/2”

incrementshttp://www.woodnet.org.uk/wec/images/gluelam2.jpg



The National Design Specification

• The “model code” for timber design in the

US

• The NDS Specification tells us what we

can do with timber

• The NDS Supplement provides material

data for the various types of timber



Basic Design Inequality

• As with all structural codes:Req’d Strength < Available Capacity

• In the NDS this takes the general form:

f < F’ or U < fN

– Where:

• f = stress caused by internal forces

• F’ = adjusted design stress = F * modifiers• F = Reference design stresses

• U = the LRFD factored internal force

• N = nominal capacity = F’ * (Section Property)



Sawn Lumber

Reference Values• NDS Supplement Table 4A



Sawn Lumber

Reference Values• NDS Supplement Table 4D



Glulam Reference Values

• NDS Supplement Table 5A



Glulam Reference Values

• NDS Supplement Table 5B



Sawn Lumber Design Values

• NDS

Table

4.3.1



Glulam Design Values

• NDS

Table

5.3.1



The Modifiers



CM: Wet Service Factor

• Applies to all reference values

• Applies to both Sawn Lumber and Glulams

• Specified in EACH NDS SupplementReference Value Table

• This factor generally reduces strengths for

wood that is used in a high moisture

environment (EMC > 19%)



Ct: Temperature Factor

• Applies to all reference values

• Use for timber used in environments with

sustained temperatures up to 150 deg F

• NDS 4.3.4 for Sawn Lumber & NDS 5.3.4

for Glulams

– References NDS Table 2.2.3



CL: Beam Stability Factor

• Applies only to bending stress, Fb

• Applies to both Sawn Lumber and Glulams

• Found in NDS 3.3.3

– This factor accounts for instability in laterallyunsupported beams (i.e. lateral torsional buckling)

• Glulams: This factor is

NOT simultaneously

applied with the VolumeFactor, CV

• More on this factor when

we cover beam design



More CL

• See NDS Equation 3.3-6

• LTB is a function of both the laterally unbraced (buckling)

length AND the variation in the moment diagram.

• First check the slenderness ratio

– RB must not exceed 50

• Then compute CL

• Note that CL is a function of the beam size!

– This means that you must know the beam size before computing

this factor

– When designing, this may lead to iterative computations



CF: Size Factor

• Applies to Sawn Lumber Fb, Ft, and Fc

bent about the strong axis


– 4.3.6.1: For SJ&P see NDS Supplement

Tables 4A and 4B

– 4.3.6.2: For B&S with d > 12”, CF =

(12/d)1/9

– 4.3.6.3: For beams of circular cross section

• The reference values are normalized toa 12” deep member. This factor

accounts for the difference.



Cfu: Flat Use Factor

• Applies only to Fb for both Sawn Lumber and Glulams

when the member is bent about it’s minor axis.

• Found in NDS 4.3.7 and 5.3.7

• For Sawn Lumber, values for Cfu are found in the NDSSupplement Tables 4A, 4B, 4C, and 4F

• For Glulams, values for Cfu

are found in the NDS

Supplement Tables 5A,5B, 5C, and 5D



Ci: Incising Factor

• Applies to all reference values for Sawn

Lumber.


• Accounts fordamage tomember due to

incisions madefor chemicalpressuretreatment.



Cr : Repetitive Use Factor

• Applies to Sawn

Lumber Fb


• Read criteria in NDS

• Intended to account for

the community effort of

a repetitive series ofbending members such

as joists, rafters, studs,

etc.



Cp: Column Stability Factor

• Applies to Fc for both Sawn Lumber

and Glulams

• Found in NDS 3.7 via 4.3.10 and

5.3.9• This factor accounts for column

stability as a function of slenderness.

• There will be more on this factor

when column design is discussed.



CT: Buckling Stiffness Factor

• Applies to Emin for Sawn Lumber, which has an

impact on Cp

• Found in NDS 4.4.2 via 4.3.11

• Only applies to truss top chord members,subject to combined bending and axial

compression, and made of 2x4 or smaller

sections that meet certain criteria.



Cb: Bearing Area Factor

• Applies to Fcp for bothSawn Lumber andGlulams

• Found in NDS 3.10.4 via4.3.12 and 5.3.10

• Accounts for increasedstrength of bearing

areas which are, in part,aided by adjacent wood.

• More on this one when

we cover beams.



CD: Load Duration Factor

• ASD ONLY!!!!

• Applies to Fb, Ft, Fv, Fc for both Sawn Lumber

and Glulams. Also applies to Frt for Glulams.

• Found in NDS 2.3.2 via 4.3.2 and 5.3.2• The value of CD is based on the actual shortest

duration load in the load combination being

considered. This has the unfortunate affect of

making it very difficult to determine the

controlling load combination!

Ti Eff



Time Effects on

Timber Strength• “Wood has the

property of

carrying

substantiallygreater maximum

loads for short

durations than for

long durations ofloading.” NDS

2.3.2.1

D t i i C t lli



Determining Controlling

Load Case• Consider ASD tensile limit state:

Ta/A = f t < F’t = FtCDCMCt

for ALL LOAD COMBINATIONS

• CD is different for every considered ASCE 7 loadcombination

– This makes it tough to see which Ta to use.

– To determine the controlling load case divide both sides by CD

(Ta/CD)/A = f t / CD < F’t / CD = FtCMCt

– The load combination that gives the largest Ta/CD is the

controlling load combination



Example

• Consider a

column

subjected

to theloads

shown

St t l El t ith



Structural Elements with

Multiple Load Sources• Each source has a different make up

• Which ASD LC controls the design of the

member?

D, L

D, Lr , S, L

W, E



- Determine CD for each source and each combination.- Pick the controlling CD for the combination and apply to

all loads in the combination.



f: Resistance Factor

• LRFD Only!!!!!

• Applies to all reference values except for E

• Found in NDS Appendix N.3.2 via 4.3.15

and 5.3.13



KF: Format Conversion Factor

• LRFD Only!!!!!

• Applies to all reference values except E

• Found in NDS Appendix N.3.1 via 4.3.14

and 5.3.12

• Need to know f to determine KF



KF Table



l: Time Effect Factor

• LRFD Only!!!!!

• Applies to all reference values except E and Emin.

• Found in NDS Appendix N.3.3 via 4.3.16 and 5.3.14

• This is the LRFD equivalent to CD. l accounts for timeeffects on strength.

• l is easier to apply than CD since it’s value is specified

by ASCE 7 LRFD load combination instead of by actual

shortest duration load

• Still need to divide load by l to find controlling cases



The l Table



Example

• Consider

a column

with the

loadsshown



Comp ting Adj sted Val es



Computing Adjusted Values

for a Member • Once the modifiers have been determined,

then you can compute the modified

stresses for a member.

• Member design often requires more than

one strength limit state, so you will have to

compute adjusted values for several types

of stress.



LRFD Sawn Lumber Example



ASD Sawn Lumber Example

Properties of Wood

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