Drying and control of moisture content and dimensional changes

In the living tree, wood contains large quantities of water. As green wood dries, most of the water is removed. The moisture remaining in the wood tends to come to equilibrium with the relative humidity of the surrounding air. Correct drying, handling, and storage of wood will minimize moisture content changes that might occur after drying when the wood is in service and such changes are undesirable. If moisture content is controlled within reasonable limits by such methods, major problems from dimensional changes can usually be avoided. The discussion in this chapter is concerned with moisture content determination, recommended moisture content values, drying methods, methods of calculating dimensional changes, design factors affecting such changes in structures, and moisture content control during transit, storage, and construction. Data on green moisture content, fiber saturation point, shrinkage, and equilibrium moisture content are given with information on other physical properties in Chapter 3. Wood in service is virtually always undergoing at least slight changes in moisture content. Changes in response to daily humidity changes are small and usually of no consequence. Changes that occur as a result of seasonal variation, although gradual, tend to be of more concern. Protective coatings can retard dimensional changes in wood but do not prevent them. In general, no significant dimensional changes will occur if wood is fabricated or installed at a moisture content corresponding to the average atmospheric conditions to which it will be exposed. When incompletely dried material is used in construction, some minor dimensional changes can be tolerated if the proper design is used. 1999 This article is in PDF format (file size: 1331 kb). To download this article, right click on the link immediately below and choose "save target as". To view the article, left click the link immediately below.
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Drying and control of moisture content and dimensional changes   (1999)

In the living tree, wood contains large quantities of water. As green wood dries, most of the water is removed. The moisture remaining in the wood tends to come to equilibrium with the relative humidity of the surrounding air. Correct drying, handling, and storage of wood will minimize moisture content changes that might occur after drying when the wood is in service and such changes are undesirable. If moisture content is controlled within reasonable limits by such methods, major problems from dimensional changes can usually be avoided. The discussion in this chapter is concerned with moisture content determination, recommended moisture content values, drying methods, methods of calculating dimensional changes, design factors affecting such changes in structures, and moisture content control during transit, storage, and construction. Data on green moisture content, fiber saturation point, shrinkage, and equilibrium moisture content are given with information on other physical properties in Chapter 3. Wood in service is virtually always undergoing at least slight changes in moisture content. Changes in response to daily humidity changes are small and usually of no consequence. Changes that occur as a result of seasonal variation, although gradual, tend to be of more concern. Protective coatings can retard dimensional changes in wood but do not prevent them. In general, no significant dimensional changes will occur if wood is fabricated or installed at a moisture content corresponding to the average atmospheric conditions to which it will be exposed. When incompletely dried material is used in construction, some minor dimensional changes can be tolerated if the proper design is used.

Author: Simpson, William T.

Source: Wood handbook : wood as an engineering material. Madison, WI : USDA Forest Service, Forest Products Laboratory, 1999. General technical report FPL ; GTR-113: Pages 12.1-12.20

Citation: Simpson, William T.  1999.  Drying and control of moisture content and dimensional changes  Wood handbook : wood as an engineering material. Madison, WI : USDA Forest Service, Forest Products Laboratory, 1999. General technical report FPL ; GTR-113: Pages 12.1-12.20.