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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.
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