Figure 54. The principal axes of wood: L, longitu- 

 dinal; R, radial; T, tangential (American 

 Institute Timber Construction, 1974). 



increase of wood in length due to a rise in temperature is negligible, and, 

 as a result, the secondary stresses due to temperature changes may, in most 

 cases, be neglected. This increase in length is important only in certain 

 structures that are subjected to considerable temperature changes, or in 

 members with very long spans. 



The increase in length per unit of length for a rise in temperature of 

 1 is designated the coefficient of linear thermal expansion. It differs in 

 the three structural directions of wood. Radially and tangentially (perpen- 

 dicular to grain), the coefficient of linear thermal expansion varies 

 directly with the specific gravity of the species. It is in the range of 45 

 x 10" 6 meters per meter per ° Celsius (25 x 10 6 feet per foot per 

 Fahrenheit) times specific gravity for a dense hardwood such as sugar 

 maple to 81 x 10 6 meters per meter per ° Celsius (45 x 10 6 feet per foot 

 per Fahrenheit) times specific gravity for softwoods such as Douglas 

 fir, Sitka spruce, redwood, and white fir. Radial or tangential dimensional 

 changes for common sizes of wood structural members are relatively small. 

 Longitudinally (parallel to grain), the coefficient is independent of 

 specific gravity and varies from 3.08 x 10 6 meters per meter per Celsius 

 (1.7 x 10^ feet per foot per ° Fahrenheit) to 4.5 x 10 6 meters per meter 

 per ° Celsius (2.5 x 10 6 feet per foot per Fahrenheit) for different 

 species. This is from one-tenth to one- third of the values for other 

 common structural materials and glass. For this reason, consideration must 

 be given to the different thermal expansion coefficients of various materials 

 used in conjunction with wood. The average coefficient of linear thermal 

 expansion for plywood is 6.12 x 10 6 meters per meter per Celsius (3.4 x 

 10 6 feet per foot per ° Fahrenheit) . The coefficient of thermal expansion 

 for thickness is essentially the same as for solid lumber. 



(2) Effect of Moisture Content . Between zero moisture content and 

 the fiber saturation point, wood shrinks as it loses moisture and swells as 



234 



