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Wood and steel, if compared pound for pound are similar in strength. Steel is more dense, requiring a smaller amount of material to do the job, but it weighs nearly the same as wood. Steel is manufactured in different grades with different strength characteristics and is homogenous throughout.
Wood consists of directional fibers, like grains of rice that lay together in the same direction. The fibers are strongest in their long direction and therefore the design of wood members requires evaluation of how the member is oriented to the loads. Wood has more attributes that refer to the direction of the wood fibers. The direction of fibers is especially important when designing fasteners and connections. Also, the design properties of wood vary with the wood species, how it is graded at the lumber mill, its moisture content, and how it will be used within the structure.
When a force is acting in the same direction as the fibers it is said to be parallel to the grain. When a force is acting across the grain it is said to be perpendicular to the grain.
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