Quantcast Section 2. Factors to Consider

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Section 2. FACTORS TO CONSIDER
1.2.1 STRUCTURAL PROPERTIES OF WOOD. In appraising the
structural value of old lumber, the three principal things to consider are species.
quality. and condition. While identification of these items is not expected from
this inspection. an understanding of how they influence structural properties may
be useful.
1.2.1-A SPECIES.  If a strength value is to be assigned. the species must
be identified. This can often be done from visual examination. while in other
instances it may require a microscopic examination by a trained specialist.
1.2.1-B QUALITY. Within a species, a high-quality piece may have twice
the strength of a low-quality piece of comparable size. The principal features
affecting the strength of lumber are knots, cross grain. and checks or splits.
The effect of a knot depends on its size and location together with the type
of strength property being considered. The strength reducing effect is generally
in direct proportion to the portion of the cross section occupied by the knot or
to the portion of the width of the surface in which the knot appears. Knots near
the bottom edges of members loaded in bending have about twice the strength-
reducing effect of those located elsewhere. Knots near the ends of beams have
little effect on the strength. Compressive strength is reduced only about half as
much as are tensile or bending strength. Knots have only a small effect on lumber
stiffness. Since the principal effects from knots are in the distorted grain around
the knot, loose knots or knotholes are generally considered to have about the
same effect on strength properties as intergrown knots of the same size. Knots
have no significant effect on bearing strength (compression perpendicular to grain)
or shear strength.
Cross grain is of two kinds, spiral or diagonal. Spiral grain is a growth
characteristic in which the wood fibers are inclined spirally around the tree instead
of parallel to the axis of the trunk. It is easily detected by the inclination of season-
ing checks with respect to the direction of length of the piece. Diagonal grain
results from sawing a crooked tree or from not sawing parallel to the bark. and
can be detected by the trace of the annual rings on a radial surface. Cross grain
is shown in Figure 1-2.
Splits or deep checks reduce the resistance of beams to longitudinal shear.
Only those appearing in the sides of beams need be considered.
1.2.1-C CONDITION. It is hard to make more than the most general
observations about condition as a factor in appraising structural members.
Broadly, if its condition is good. it can be used with working stresses near those
for new lumber of its species and grade; if its condition is not good. it should
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