TABLE OF CONTENTS Timber is among the oldest and most widely available construction materials. Unlike steel, concrete, and masonry, timber grows naturally with natural defects and is renewable. It is used in a variety of building structural elements: beams, columns, girders, trusses, wall and floor panels, pilings, poles, and temporary forms in concrete construction. Most light structures built in North America, including schools, single-family homes, and two- to three-story commercial and apartment buildings, are built with wood and wood products. Many large existing institutional and industrial buildings, not to mention transportation and marine structures, have timber framing as well.
A mention of wood's anatomy and terminology is in order. A piece of lumber is essentially an assembly of fibers held together by lignin, a natural binder of cellulose and hemicellulose. The "structural" part of a tree that is responsible for its support is called xylem. Xylem carries water and nutrients (sap) from the roots to the branches and leaves; it is enclosed by bark (cambrium) outside, and contains pith in the center. The younger cells carrying sap are porous and light; they are called sapwood. As the cells get older, they gradually become darker and clogged with various deposits. These older cells form the center of the tree (heartwood) and are enveloped by new sapwood cells. Heartwood and sapwood are similar in strength, but heartwood is more resistant to decay. The tree's annual rings reflect different rates of growth: fast in the spring, slow in the summer, and almost zero...
