Wood members are considered to be permanent without pressure treatment if located in enclosed buildings where good roof coverage, proper roof maintenance, good joint details, adequate flashing, good ventilation, and a well-drained site assure moisture content of the wood continuously below 20%.
Where wood is in contact with the ground or with water, where there is air and the wood may be alternately wet and dry, a preservative treatment, applied by a pressure process, is necessary to obtain an adequate service life. In enclosed buildings where moisture given off by wet-process operations maintains equilibrium moisture contents in the wood above 19%, wood structural members must be preservatively treated. So must wood exposed outdoors without protective roof covering and where the wood moisture content can go above 19% for repeated or prolonged periods.
Where wood structural members are subject to condensation by being in contact with masonry or concrete, preservative treatment may be necessary.
Design values for wood structural members apply to products pressure-treated by an approved process and with an approved preservative. (The ‘‘AWPA Book of Standards,’’ American Wood Preservers Association, Granbury, TX, describes these approved processes.) Design values for pressure-preservative treated lumber are modified with the usual adjustment factors described in Art. 10.5 with one exception.
Load duration factors greater than 1.6 (Table 10.5) do not apply to structural members pressure treated with waterborne preservatives or to structural members treated with fire-retardant chemicals.
Each type of preservative and method of treatment has certain advantages. The preservative to be used depends on the service expected of the member for the specific conditions of exposure. The minimum retentions given in the applicable American Wood Preservers Association (AWPA) standards for specific products and end-use applications may be increased where severe climatic or exposure conditions are involved.
Creosote and creosote solutions have low volatility. They are practically insoluble in water, and thus are most suitable for severe exposure, contact with ground or water, and where painting is not a requirement or a creosote odor is not objectionable.
Oilborne chemicals are organic compounds dissolved in an approved petroleum carrier oil, and are suitable for outdoor exposure or where leaching may be a factor, or where painting is not required. Depending on the type of oil used, they may result in relatively clean surfaces. While there is a slight odor from such treatment, it is usually not objectionable.
Waterborne inorganic salts are dissolved in water or aqua ammonia, which evaporates after treatment and leaves the chemicals in the wood. The strength of solutions varies to provide net retention of dry salt required. These salts are suitable where clean and odorless surfaces are required. The surfaces are paintable after proper seasoning. See also Art. 4.36.
(‘‘Design of Wood-Frame Structures for Permanence,’’ WCD No. 6, American Forest & Paper Association, Washington, D.C.)
Fire-retardant treatment with approved chemicals can make wood highly resistant to the spread of fire. Although wood will char where exposed to fire or high temperatures, even if it is treated with a fire retardant, chemicals will retard transmission of heat and rate of destruction. Treated with adequate quantities of an approved chemical, wood will not support combustion nor contribute fuel to a fire and will cease to burn after the ignition source is removed. The fire retardant may be applied as a paint or by impregnation under pressure. The latter is more effective.
It may be considered permanent if the wood is used where it will be protected from the weather.
The effects of fire-retardant impregnation treatments on strength should be considered in design. Design values, including those for connections, for lumber and structural glued-laminated timber pressure treated with fire-retardant chemicals should be obtained from the company providing the treatment and redrying service.
Load duration factors greater than 1.6 (Table 10.5) should not be applied to structural members pressure-treated with fire-retardant chemicals.