(f) Shear Perpendicular to Grain . Shear perpendicular to 

 grain is not a design factor as effective control is applied through 

 limits on design stress for shear parallel to grain. 



(g) Fiber Stress in Bending . Fiber stress in bending 

 creates compression in fibers on one side and tension in fibers on 

 the other side of a beam. The higher stresses occur in the fibers 

 most distant from the center. Deviations in the slope of grain and 

 the presence of knots and holes in these outside faces reduces the 

 resistance in the extreme fibers. 



(3) Preservative Treatment . In order to extend the useful life of 

 wood long enough to make it an economical and practical material for use in 

 the coastal zone or other marine environments, it must be protected from its 

 natural enemies, fungi, bacteria, insects and marine organisms. The most 

 effective method of treating wood with preservatives is the pressure-treating 

 process. The pressure-treating process requires placing the wood in an air- 

 tight chamber in which either a vacuum or a pressure can be created while the 

 preservative is introduced into the chamber. The preservative generally will 

 penetrate the wood surface from 1.5 to 4 centimeters (0.5 to 1.5 inches) and 

 coat the walls of the wood cells in this area. Penetration to 10 centimeters 

 (4 inches) is required in some cases. Although two processes, the empty-cell 

 and the full- cell process, have had success in preserving wood structures in 

 marine environment, the full- cell process is most commonly accepted as the 

 preferred treatment for coastal zone use. 



Wood preservatives commonly used are grouped into two broad classes, 

 preservative oils and waterborne preservatives. The preservative oils are 

 considered the best wood protection in a marine environment and include 

 byproducts of petroleum such as creosotes, coal-tar creosotes, and mixtures 

 of these with other oils. They may include solutions of toxic chemicals such 

 as pentachlorophenol or copper naphthenate. Waterborne preservatives include 

 solutions of chromated zinc chloride, fluor- chrome- ar senate-phenol, chromated 

 copper arsenate, and other toxic chemicals. 



(4) Other Protective Methods . In a marine environment wood struc- 

 tures can be protected by other materials which are not strictly preserva- 

 tives. Such protection is in the form of sheet metal, concrete jackets and 

 flexible synthetic sheets such as vinyl and polyethelene films. Because 

 virtually all organisms causing wood deterioration are aerobic, surrounding 



a wood element such as a pile with a jacket that prevents seawater containing 

 free oxygen from coming in contact with the wood creates a hostile environ- 

 ment for the organisms. 



(5) Durability . Wood, when properly treated with appropriate 

 preservatives has a good history of satisfactory service in marine and 

 coastal structures. Wood piles supporting piers and wharves, when not 

 subjected to abrasion, have lasted many years. Wood sheet piles in groins, 

 jetties, bulkheads and like structures will perform satisfactory. Care must 

 be used in installing wood members to ensure that construction joints and 

 connections do not damage the preservative protection or that field repairs 

 are carefully and adequately made. , 



382 



