Building Design and Construction Handbook - Merritt - Ventech!

WOOD CONSTRUCTION 10.63
P�Q�
N� � (10.32)
2 2
P� sin � � Q� cos �
where N�, P�, and Q� are, respectively, the adjusted design value, lb, or stress, psi,
at inclination � with the direction of grain, parallel to grain, and perpendicular to
grain.
Design values are based on the assumption that the wood at the joint is clear
and relatively free from checks, shakes, and splits. If knots are present in the longitudinal
projection of the net section within a distance from the critical section of
half the diameter of the connector, the area of the knot should be subtracted from
the area of the critical section. It is assumed that slope of the grain at the joint does
not exceed 1 in 10.
The stress, whether tension or compression, in the net area, the area remaining
at the critical section after subtracting the projected area of the connectors and the
bolt from the full cross-sectional area of the member, should not exceed the design
value of clear wood in compression parallel to the grain.
Tables in the NDS list the least thickness of member that should be used with
the various sizes of connectors. The design values listed for the greatest thickness
of member with each type and size of connector unit are the maximums to be used
for all thicker material. Design values for members with thicknesses between those
listed may be obtained by interpolation.
The NDS also lists minimum end and edge distances and spacing for timber
connectors (Table 10.37). Edge distance is the distance from the edge of a member
to the center of the connector closest to that edge and measured perpendicular to
the edge. End distance is measured parallel to the grain from the center of the
connector to the square-cut end of the member. If the end of the member is not cut
normal to the longitudinal axis, the end distance, measured parallel to that axis
from any point on the center half of the connector diameter that is perpendicular
to the axis, should not be less than the minimum end distance required for a squarecut
member. Spacing of connectors is measured between their centers along a line
between centers.
10.18 WOOD SCREWS
The ‘‘National Design Specification for Wood Construction’’ contains design provisions
and design values for wood screws that conform to ANSI/ASME Standard
B18.6.1. When wood screws are loaded in withdrawal, the design value for tension
in the screws should not be exceeded. For single-shear wood-to-wood construction,
the screws should be inserted in the side grain of the main member with axis
perpendicular to the wood fibers.
Edge and end distances and spacing of wood screws should be sufficient to
prevent splitting of the wood. If building-code requirements for such distances are
not available, Table 10.38 may be used to establish wood-screw patterns. Spacing,
or pitch, between fasteners in a row is affected by species, moisture content, and
grain orientation.
Screws may be inserted in lumber or timber that has a specific gravity less than
0.50, without preboring a hole for them. In denser wood, lead holes should be
drilled and screws inserted by turning, not by hammering. Holes for wood screws
loaded in withdrawal should have a diameter of 90% of the screw root diameter in