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Mechanical Properties of Timber

Swan Lumber:

Lumbers that cut from tree are normally dressed and dried before they can be used for construction. Most construction lumber has 19% or less moisture content. Their dressed sizes are smaller than their nominal size.  Most timber designs in U.S. are based on NDS (National Design Specification for Wood Construction) by American Forest & Paper Association.  The actual sizes and section properties can be found in NDS supplement Table 1B.  Examples are

Nominal size

Actual size

Nominal size

Actual size

2”x4”

1-1/2”x3-1/2”

4”x4”

3-1/2”x3-1/2”

2”x8”

1-1/2”x7-1/4”

5”x5”

4-1/2”x4-1/2”

2x10”

1-1/2”x9-1/4”

6”x6”

5-1/2”x5-1/2”

2x12”

1-1/2”x1-1/4”

8”x8”

7-1/2”x7-1/2”

Because timbers are sawn from tree, its properties vary a lot.  They are graded for their mechanical properties.

Visually Graded structural lumber – mechanical properties of visually graded lumber are listed based on their species, qualities of lumber, sizes etc.  Design values for visually graded dimension lumber are listed in NDS supplement Table 4A and 4B.  Examples are

Species & grade

Size

Classification

Bending

Fb

Tension parallel to grain

Ft

Shear parallel to grain

Fv

Compression perpendicular to grain

Fc^

Compression parallel to grain

Fc

Modulus of Elasticity

E

Douglas Fir-Larch (design value in psi)

No. 1

 2”-4” thick

2” & wider

1000

675

95

625

1450

1,700,000

No. 2

875

575

95

625

1300

1,600,000

No. 3

500

325

95

625

750

1,400,000

v     From NDS supplement Table 4A.

Species & grade

Size

Classification

Bending

Fb

Tension parallel to grain

Ft

Shear parallel to grain

Fv

Compression perpendicular to grain

Fc^

Compression parallel to grain

Fc

Modulus of Elasticity

E

Southern Pine (design value in psi)

Select structure

2”-4” thick

2”-4” wide

2850

1600

100

660

2250

1,900,000

No. 1

1850

1050

100

565

1850

1,700,000

No.2

1500

825

90

565

1650

1,600,000

No. 3

850

475

90

565

975

1,400,000

Stud

875

500

90

565

975

1,400,000

v     From NDS supplement Table 4B.

These values need to be adjusted by size factor, Cf, repetitive member factor, Cr, flat use factor, Cfu, wet service factor, CM and shear stress factor, whenever they are applicable.

Machine Stress-Rated (MSR) structural Lumber – the grading is based on relationship between elastic modulus and bending strength, tensile strength etc.  Examples from NDS supplement Table 4C are

Species and commercial grade

Size classification

Bending

Fb

Tension parallel to grain

Ft

Compression parallel to grain

Fc

Modulus of elasticity

E

1800f-1.5E

2” and less in thickness

2” & wider

1800

1300

1750

1,500,000

2000f-1.6E

2000

1300

1825

1,600,000

2250f-1.6E

2250

1750

1925

1,600,000

2400f-1.8E

2400

1925

1975

1,800,000

  • All design values are in psi.

These values need to be adjusted by size factor, Cf, repetitive member factor, Cr, flat use factor, Cfu, wet service factor, CM and shear stress factor, whenever they are applicable.

For timbers, 5”x5” or larger, their design values and adjustment factors are listed in NDS Table 4D.

For timbers used for decking, their design values and adjustment factors are listed in NDS Table 4E.

Structural Glued Laminated Lumber

Structural Glued Laminated Lumbers are wood laminations glued together to form larger dimension.  The thickness of lamination is usually less than 2”.  The grain of all laminations is approximately parallel.  Their dimensions are based on the number and thickness of laminations. Examples from NDS supplement Table 1C are

Southern pine (based on 1-3/8” thick laminations)

Western Species (based on 1-1/2” thick laminations)

Net dimensions

# of laminations

Net dimensions

# of laminations

6-3/4”x6-7/8”

5

6-3/4”x7-1/2”

5

6-3/4”x18-1/8”

35

6-3/4”x48”

32

8-1/2”x8-1/4”

6

8-3/4”x9”

6

8-1/2”x63-1/4”

46

8-3/4”x63”

42

Mechanical properties of glued laminated lumbers are depended on the species of laminations used.  Because lamination, the properties in X axis and Y axis, axial direction are different.  Design values for structural glued laminated soft wood timber for member stressed primarily in bending are listed in NDS Table 5A.  Examples are

Symbol

Lamination

/core combination

Bending about Y-Y axis (load perp. to wide face of lamination)

Bending

Compression perp. to grain

Strear perp. to grain

Fvxx

Modulus of elasticity

Exx

 

Tension zone stress in tension

Fbxx

Compression zone stress in tension

Fbxx

Tension face

Fc^xx

Compression face

Fc^xx

Visually graded western species

16F-V1

DF/WW

1600

800

560

560

140

1300000

20F-V1

DF/WW

2000

1000

650

560

140

1400000

22F-V1

DF/WW

2200

1100

650

560

140

1600000

24F-V1

DF/WW

2400

1200

650

560

140

1700000

E-Rated western species

 

16F-E1

WW/WW

1600

800

255

255

140

1300000

20F-E1

WW/WW

2000

1000

255

255

140

1600000

22F-E1

DF/DF

2200

1100

650

650

165

1700000

24F-E1

DF/DF

2400

1200

650

560

165

1800000

Visually graded southern pine

16F-V1

SP/SP

1600

800

560

560

200

1400000

20F-V1

SP/SP

2000

1000

650

560

200

1500000

22F-V1

SP/SP

2200

1100

650

560

200

1600000

24F-V1

SP/SP

2400

1200

650

560

200

1700000

E-Rated southern pine

16F-E1

SP/SP

1600

800

560

560

200

1600000

20F-E1

SP/SP

2000

1000

560

560

200

1700000

22F-E1

SP/SP

2200

1100

650

560

200

1700000

24F-E1

SP/SP

2400

1200

650

560

200

1800000

These values need to be adjusted with volume factor Cv, flat use factor, Cfu, and west service factor, CM.

Design values for structural glued laminated softwood timber member stressed primarily in axial tension or compression are listed in NDS Table 5B.

Design values for structural glued laminated hardwood timber are listed in NDS Table 5C.

 

 

Related Topics

Timber beams

Timber columns