170 CHAPTER SIX
The loads apply where the nail or spike penetrates into the member, receiving its
point at least 10 diameters for Group I species, 11 diameters for Group II
species, 13 diameters for Group III species, and 14 diameters for Group IV
species. Allowable loads for lesser penetrations are directly proportional to the
penetration, but the penetration must be at least one-third that specified.
Wood Screws
The allowable withdrawal load per inch (mm) of penetration of the threaded
portion of a wood screw into side grain of seasoned wood that remains dry is
p2,850G^2 D (6.37)wherepallowable load per inch (mm) of penetration of threaded portion into
member receiving point, lb (N)
Ddiameter of wood screw, in (mm)
Gspecific gravity of wood, oven dry (see Table 6.1)
Wood screws should not be loaded in withdrawal from end grain.
The total allowable lateral load for wood screws driven into the side grain of
seasoned wood which remains dry is
pCD^2 (6.38)wherepallowable load per wood screw, lb (N)
Ddiameter of wood screw, in (mm)
Ccoefficient dependent on group number of wood (Table 6.2)
Values of Cfor the four groups into which stress-grade lumber is classified are
Group I: C4,800
Group II: C3,960
Group III: C3,240
Group IV:C2,520TABLE 6.2 Specific Gravity and Group Number for Common
Species of LumberGroup Specific
Species number gravity, GG^2 G5/2Douglas fir II 0.51 0.260 0.186
Pine, southern II 0.59 0.348 0.267
Hemlock, western III 0.44 0.194 0.128
Hemlock, eastern IV 0.43 0.185 0.121
Pine, Norway III 0.47 0.221 0.151
Redwood III 0.42 0.176 0.114
Spruce IV 0.41 0.168 0.108