Wood, the primary construction material used in industrial towers, is composed primarily of cellulose fibers held together by lignin. The woods most suited for industrial towers are special grades of California redwood, red cedar, Douglas fir and southern pine.
Industrial tower wood can be attacked in several ways, falling into three basic categories: biological, chemical and physical attack. The first two causes of deterioration are linked to microorganisms. Biological attack is directly caused by microoganisms; they cause chemical attack indirectly in most cases. Physical attack is independent of microbial activity in the tower.
It was first thought that biological attack resulted from using inferior grades of wood. Other explanations include the increased use, after World War II, of mechanical draft towers in preference to atmospheric types. Biological wood deterioration is divided into two subclasses that are based on the area of wood affected, i.e., surface or soft rot and internal decay.
Surface rot occurs in the flooded areas of a tower and is caused by cellulose-destroying fungi of the Ascomycetes and Fungi imperfecti groups. The cellulose fibers of the wood are destroyed, leaving only the lignin binder and, consequently, little structural strength. The wood loses bulk as surface cells are washed away. When still wet, it will appear brash and