Wood differs in several significant ways from other building materials. Its cellular structure is responsible, to a considerable degree, for these differences. Because of this structure, structural properties depend on grain orientation. While most structural materials are essentially isotropic, with nearly equal properties in all directions, wood has three principal grain directions longitudinal, radial, and tangential.
Loading in the longitudinal direction is referred to as parallel to the grain, whereas transverse loading is considered to be across the grain. Parallel to the grain, wood possesses high strength and stiffness characteristics. Across the grain, strength and stiffness are much lower. In tension, wood stressed parallel to the grain is 25 to 40 times stronger than when stressed across the grain. In compression, wood loaded parallel to the grain is 6 to 10 times stronger than when loaded perpendicular to the grain. Furthermore, a wood member has three moduli of elasticity, with a ratio of largest to smallest as large as 150:1.
Wood undergoes dimensional changes from causes different from those in most other structural materials. For instance, thermal expansion of wood is so small as to be unimportant in ordinary usage. Significant dimensional changes, however, occur because of gain or loss in moisture. Swelling and shrinkage caused by moisture changes vary in the three grain directions; these size changes are about 5 to 11% tangentially, 3 to 7% radially, but only 0.1 to 0.3% longitudinally. Table 10.1 gives shrinkage values for some commonly used species of wood.
Wood offers numerous advantages in construction applications warmth and beauty, versatility, durability, workability, low cost per pound, high strength-toweight ratio, good electrical insulation, low thermal conductance, and excellent strength at low temperatures. It has high shock-absorption capacity. It can withstand good wearing qualities, particularly on end grain. It can be bent easily to relatively sharp curative. A wide range of finishes can be applied for decorative or protective purposes. Wood can be used in both wet and dry applications. Preservative treatments are available for use when necessary, as are fire retardants (not appropriate for all wood products). Also, there is a choice of a wide range of species with a range of unique properties.
In addition, a wide variety of wood framing systems is available. The intended use of a structure, geographical location, configuration required, cost, and many other factors determine the best framing system to be used for a particular project.
Wood is naturally resistant to many chemicals that are highly corrosive to other materials. It is superior to many building materials in resistance to mild acids, particularly at ordinary temperatures. It has excellent resistance to most organic acids, notably acetic. However, wood is seldom used in contact with solutions that are more than weakly alkaline. Oxidizing chemicals and solutions of iron salts, in combination with damp conditions, should be avoided.
Wood is composed of roughly 50 to 70% cellulose, 25 to 30% lignin, and 5% extractives with less than 2% protein. Acids such as acetic, formic, lactic, and boric do not ionize sufficiently at room temperature to attack cellulose, and thus do not harm wood.
When the pH of aqueous solutions of weak acids is 2 or more, the rate of hydrolysis of cellulose is small and dependent on the temperature. A rough approximation of this temperature effect is that, for every 20F increase, the rate of hydrolysis doubles. Acids with pH values above 2, or bases with pH below 10, have little weakening effect on wood at room temperature, if the duration of exposure is moderate.
Design Recommendations. The following recommendations aim at achieved economical designs with wood framing:
Use standard sizes and grades of lumber. Consider using standardized structural components, whether lumber, stock glued-laminated beams, or other framing members designed for structural adequacy, efficiency, and economy.
Use standard details wherever possible. Avoid specially designed and manufactured connecting hardware.
Use as simple and as few joints as possible. Place splices, when required, in areas of lowest stress. Do not locate splices where bending moments are large, thus avoiding design, installation, and fabrication difficulties.
Avoid unnecessary variations in cross section of members along their length.
Use identical member designs repeatedly throughout a structure, whenever practicable.
Keep the number of different arrangements to a minimum.
Specify required design stresses to permit the widest range of products that can be used for a given design situation.
Use wood products pressure treated with preservatives where service conditions dictate. Such treatment need not be used where decay or insect attack hazards do not exist. Fire-retardant treatments may be used to meet a specific flamespread rating for interior finish, but are not necessary for large-cross-sectional members that are widely spaced and have a natural resistance to fire because of their relatively large size.
Instead of long, simple spans, consider using continuous or suspended spans or simple spans with overhangs.
Select an appearance grade best suited to the project. Do not specify the highest quality appearance grade available for all members if it is not required.
Table 10.2 may be used as a general guide to typical ranges of spans for roof and main floor framing members (excluding repetitive member joist and rafter applications).