Many timber requirements specify a particular FIBER STRESS IN BENDING or (Fb). To explain what this means, some fundamental understanding of the mechanical properties of wood is required.

Wood behaves in relation to stresses and strains. A STRESS is a distributed force (expressed as lbs/sq inch) upon the material, and the STRAIN is the distortion or deformation (expressed in linear units such as inches). Every stress produces a corresponding strain, and within certain limits the strain is directly proportional to the stress.

Within wood's elastic range, a wood beam will return to its original shape upon release of the stress imposed on it. Eventually, as the stress (load) is increased, a point will be reached where the recovery of the beam is incomplete upon removal of the stress. This point is known as the PROPORTIONAL LIMIT (also known as the elastic limit). The stress (load) upon the beam at this point is known as FIBER STRESS AT PROPORTIONAL LIMIT, or commonly called FIBER STRESS IN BENDING (Fb).

Timber Fb values are published in the lumber grading rule books for all of the structural species of wood and are affected by such factors as moisture content, the size and quantity of knots, slope of grain, and density. Fiber stress values are also published for loads in compression, tension, and shear, but the bending stresses are generally of the greatest interest.

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