Based on SPIB Grading Rules and AF&PA National Design Specification®
Reference design values are based on normal load duration and dry service conditions. Because
the strength of wood varies with conditions under which it is used, these
design values should only be applied in conjunction with appropriate design
and service recommendations from the National Design Specification®
(NDS®) for Wood Construction published by the American Forest
& Paper Association. The latest connection design information is also
provided in the NDS.
Reference design values
(Fb Ft Fv Fc⊥ Fc E Emin)
in Tables 1 thru 3 shall be multiplied by
all applicable adjustment factors to determine allowable design values
(Fb' Ft' Fv' Fc⊥' Fc' E' Emin').
Table A-1 is excerpted from the NDS and summarizes the
applicability of adjustment factors for solid-sawn lumber.
Table A-1 Applicability of Adjustment
Factors for Sawn Lumber
Tables A-2 thru A-4 highlight the most common adjustment factors as
they apply to Southern Pine. In addition, Table 1 and 2 footnotes provide
information about the Size Factor, CF, while
Tables 1 and 3 include the Repetitive Member Factor, Cr,
in the tabulated Repetitive Member Use bending design values. For complete
information on adjustment factors, see the NDS.
Table A-2 Wet Service Factor, CM
For lumber 2" to 4" thick
Applies to all values
When dimension lumber is used under conditions where the moisture
content of the wood in service will exceed 19% for an extended time
period, reference design values shall be multiplied by the appropriate
wet service factors from the following table:
Table A-3 Load Duration Factor, CD
For all solid wood products - Allowable Stress Design Only
Applies to Fb , Ft , Fv , and Fc values
Does not apply to Fc⊥, E, and Emin values
Wood has the property of carrying substantially greater maximum loads
for short durations than for long durations of loading. Reference design
values apply to normal load duration, meaning a load that fully stresses a member to its allowable design value by the application of the full design
load for a cumulative duration of approximately ten years. When the
cumulative duration of the full maximum load does not exceed the specified
time period, all reference design values (except Fc⊥, E, and Emin) shall be multiplied by the appropriate load duration factor.
Frequently used load duration factors are provided in the following
table:
| Load Duration (Typical Design Loads) |
CD |
Load Duration (Typical Design Loads) |
CD |
Permanent (dead load)
Ten years (occupancy live load)
Two months (snow load) |
0.9
1.0
1.15 |
Seven days (construction load)
Ten minutes (wind/earthquake load)
Impact1 (impact load) |
1.25
1.6
2.0
|
(1) Load duration factors greater than 1.6 shall not apply
to structural members pressure treated with waterborne preservatives,
or fire-retardant chemicals. The impact load duration factor shall not
apply to connections.
Table A-4 Flat Use Factor, Cfu
For lumber 2" to 4" thick
Applies to Fb values only
Reference bending design values, Fb, are based on edgewise
use (load applied to narrow face). When dimension lumber is used flatwise
(load applied to wide face), Fb shall also be multiplied
by the following flat use factors:
| Flat Use Factor, Cfu |
| Width |
Thickness
|
|
2" & 3"
|
4"
|
2" & 3"
4"
5"
6"
8"
10" and wider
|
1.0
1.1
1.1
1.15
1.15
1.2
|
--
1.0
1.05
1.05
1.05
1.1
|
|
|
Disclaimer:
The Southern Pine Council™ does not test lumber or establish design values. The purpose of the Use Guide is to collect and organize data available from other sources for the convenience of builders, engineers, architects and other professionals. The Southern Pine design values contained herein are taken from the Standard Grading Rules for Southern Pine Lumber, published by the Southern Pine Inspection Bureau, and the adjustment factors are taken from the National Design Specification® (NDS®) for Wood Construction, published by the American Forest & Paper Association. Neither the Southern Pine Council™, nor its members, warrant that the data from such sources on which the recommended uses of Southern Pine lumber contained herein are based is correct, and disclaim responsibility for injury or damage resulting from the use of such design values.
The conditions under which lumber is used in construction may vary widely, as does the quality of workmanship and construction methods. Neither the Southern Pine Council™, nor its members, have knowledge of the quality of the workmanship or construction methods used on any construction project, and, accordingly, do not warrant the design or performance of the lumber in completed structures.
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