|The density of wood varies a great deal within a single species, and even within a single tree. Increased density is the most reliable indicator of a species’ higher strength. Density is principally a function of the amount of wood present, any extractives, and the amount of moisture present.
Amount of Wood
The specific gravity of the basic wood material, the cell wall, is a nearly constant 1.5, regardless of species. What does vary is the amount of that material present in the tree. Oak cells have relatively thick walls, while balsa cell walls are very thin.
Within a species, growth rate is the major variable in establishing density. The slower a softwood grows, the higher its percentage of the denser summer wood, and subsequently. Its density. A tree’s growth rate is affected by local environmental conditions more than anything else.
Only cells within several inches of the bark are alive and transfer fluids within the tree. As the tree grows, this sapwood gradually transforms into heartwood. That transformation process involves the deposition on the cell walls of chemicals known as extractives or infiltrates. There is wide variety in types and amounts of extractives. Silicas in tropical woods, for example, dull saw blades rapidly. The most naturally rot-resistant timbers result from the presence of certain extractives, such as resins and waxes. The densest tropical woods, some of which sink in water, have large amounts of extractives.
Amount of Moisture
The amount of water present is the major influence on the density of a given piece of wood. Cellulose is a hygroscopic material, meaning it absorbs and gives up water readily. Since wood is primarily made of cellulose, it will change moisture content in response to changing conditions.
Moisture content varies widely with species and time. Fresh-cut cedar can have a moisture content as high as 250 percent. Water in wood exists in two forms. The majority of the water is known as free water and simply fills the hollow cells while the tree is alive. This is the first water to go after the tree is cut, and only the weight of the wood changes as it does. There is about five times as much free water as there is bound water. The bound water is chemically bonded in the walls of the wood cells. As the bound water leaves, the wood shrinks and generally gets stronger.
There are certain moisture contents that of interest:
Green: When the tree is cut, its moisture content starts to drop as the free water is lost. The term green wood can mean the fresh-cut state. Green wood is also defined in allowable stress tables as having a moisture content level of 19 percent and above.”
Fiber Saturation Point (FSP): When the free water is mostly gone, but the bound water is still intact, the moisture content of the wood is defined as being at the fiber saturation point. The fiber saturation point is about 30 percent for all wood species. It is only below FSP that wood starts to shrink.”
Equilibrium Moisture Content (EMC): The wood continues to dry by losing bound water until it stabilizes at the equilibrium moisture content. The equilibrium moisture content of wood varies with species, and ambient temperature and humidity. In the United States, equilibrium moisture content can range from 5 percent to 25 percent, and 10 percent to 15 percent is the common range.”
Oven-Dry: In order to lower the moisture content below its equilibrium level, the remaining bound water must be driven off. Wood is hearted in a ventilated oven at a temperature just above water’s boiling point, and the weight is monitored. There is no absolutely dry wood, but one a baking specimen has reached a relatively stable weight, it is said to be oven-dry.
The amount of wood shrinks as it passes from its fiber saturation point to oven-dry varies with fiber orientation. The shrinkage along the fibers (longitudinally) is very small—0.1 to 0.2 percent. This means that a ten foot long timber will typically shorten less than an eighth of an inch.
Since it is the cell wall that shrinks with decreasing moisture content, it is not surprising that wood shrinks much more transverse to the cells than along them. There is a further difference in the shrinkage across the fibers, which depends on the orientation of the growth rings. Along the rings (the tangential direction), wood shrinks about eight percent from fiber saturation point to oven-dry. Across the rings (the radial direction), wood shrinks only three or four percent. An eight inch wide timber might be expected to shrink as much as a quarter inch or more as it passes from fiber saturation point to its equilibrium moisture content. This change in dimension can cause problems for unsophisticated designers, particularly in the area of connection detailing.
The difference in shrinkage between radial and tangential directions can also cause difficulties. The differential shrinkage means that the circumference shrinks more than the radius. The resultant hoop residual tensions in the wood make longitudinal splits in dry timber virtually inevitable. Since lumber is typically cut from green wood and then dried, the differential shrinkage also causes some distortion of the cross sections.
Since both volume and weight are functions of the amount of water present in wood, comparing species on the basis of density can be misleading and confusing. Specific gravity is the measure commonly used for comparing species. Specific gravity is the ratio of the wood’s density to that of water. Wood density must be derived from weights and volumes evaluated at a specified moisture content. Unfortunately, several different benchmark conditions have been used over the years. Since the structural codes generally use the oven-dry condition for both weight and volume, that condition is assumed throughout this book.