| SOUTHERN PINE ADJUSTMENT
FACTORS
|
Based on 1994 SPIB Grading Rules
and 1991 AF&PA National Design Specification |
Tabulated design values are based on normal loading
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 1991 National Design Specification® (NDS®) for wood
Construction published by the American Forest & Paper Association. The latest
connection design information is also provided in the 1991 NDS.
Tabulated design values (Fb Ft
Fv Fc* Fc E Fg) in Tables 1 thru 3
shall be multiplied by all applicable adjustment factors to determine allowable design
values (Fb' Ft' Fv' Fc* Fc'
E' Fg').
Table A-1 is excerpted from the 1991 NDS and summarizes the
applicability of adjustment factors for solid-sawn lumber.
Table A-1 Summary of Adjustment
Factors
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 1991 NDS.
Table
A-2 Wet Service Factor, CM
For lumber 2" to 4" thick |
Applies to all values
except Fg |
When lumber is used where moisture content will
exceed 19% for an extended time period, tabulated design values shall be multiplied by the
appropriate wet service factors from the following table:
| Fb |
Ft |
Fv |
Fc* |
Fc |
E |
| 0.851 |
1.00 |
0.97 |
0.67 |
0.802 |
0.90 |
| (1) When Fb x CF * 1150 psi, CM = 1.00 |
(2)
When Fc*750 psi, CM=1.00 |
Table A-3 Duration of Load Factor, CD
For all solid wood products |
Applies to Fb,
Ft, Fv, Fc, and Fg values
Does not apply to Fc* and E values |
Wood has the property of carrying substantially
greater maximum loads for short durations than for long durations of loading. Tabulated
design values apply to normal load conditions, and may be multiplied by the appropriate
load duration factors from the following table:
| Load Duration |
CD |
Load Duration |
CD |
| Permanent |
0.9 |
Seven days (construction) |
1.25 |
| Ten years (normal) |
1.0 |
Ten minutes (wind or
earthquake) |
1.6 |
| Two months (snow) |
1.15 |
Impact1 |
2.0 |
(1) The impact load duration factor shall
not apply to structural members pressure treated with waterborne preservatives to the
heavy retentions required for "marine" exposure, not to structural members
pressure treated with fire retardant chemicals.
Table
A-4 Flat Use Factor, Cfu
For lumber 2" to 4" thick |
Applies to Fb values
only |
Tabulated bending design values, Fb, are based on
edgewise use (load applied to narrow face). When such lumber is used flatwise (load
applied to wide face), Fb may be multiplied by the following flat use factors:
| Flat Use Factor, Cfu |
| Width |
Thickness |
|
2" or 3" |
4" |
| less than 4" |
1.00 |
- |
| 4" |
1.10 |
- |
| 6" |
1.10 |
1.00 |
| 8" |
1.15 |
1.05 |
| 10" and wider |
1.20 |
1.10 |
|
