Shrinkage of normal wood in the longitudinal direction is usually about 0.1 percent and considered
2.4.4 Wood Drying Process. As lumber dries, a moisture gradient is set up because the
shell (wood near the surface of the board) and the core of the board dry at different rates. If this
moisture gradient becomes too steep, serious defects can develop.
In kiln drying of lumber, the moisture gradient is controlled so the wood will dry rapidly without
developing serious defects.
2.4.5 Weathering. The natural weathering process results from a complex combination of
chemical, mechanical, biological, and light-induced changes. In general, within two months of ex-
posure to sunlight, freshly cut woods begin to change color. Lightcolored woods generally become
darker, while dark-colored woods become lighter. Subsequently, surface checks, then cracks, may
develop and the wood becomes gray.
The gray color of the surface layer of weathered wood usually results from growth of dark-colored
fungi. A silvery cast or the bleaching of dark woods exposed to sunlight results from degradation of
the dark colored lignin, but not the cellulose (which is white) on wood surfaces. After prolonged ex-
posure to sunlight, surfaces of unpainted wood will have a covering of loose, white cellulose fibers.
If wood is repeatedly wetted and dried, boards may cup and warp and wood surfaces may become
friable, with fragments separating from the surface.
Wood treated with an oil based preservative will weather at a slower rate than untreated wood.
Wood treated with water borne salts, such as chromated copper arsenate, will slowly weather to a
reduce the degrading effect of sunlight.
2.5 MECHANICAL PROPERTIES. Mechanical, or strength properties, refer to those charac-
teristics which an architect or engineer uses in the design of a structure (utility line, wharf, bridge,
housed etc.). These properties are expressed numerically, i.e., pounds per square inch (psi) The four
most common ways in which any member of a structure can be loaded or stressed are: Tension--it
may be stretched. Compression--it may be compressed. Bending--it may be bent as in a floor joist
which is supported on a foundation at its ends and loaded along its length. Shear--it may be loaded
such that one surface tries to slide past an adjoining surface.
A complicating factor arises with wood because it has different strength values across the gram than
it has along the grain. Most species are four to five times as strong in compression along the grain
as they are across the grain.
The question "How strong is one wood compared to another ?" must be qualified by indicating how
the wood will be used. It is important to recognize which mechanical properties should be con-