This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The various strains that can come upon any member of a structure are - (1) Tension : stretching or pulling; (2) Compression : crushing or pushing; (3) Transverse strain: cross strain or bending; (4) Torsion: twisting or wrenching; (5) Shearing : cutting. But in woodwork, when the last-named force acts along the grain, it is generally called "detrusion," the term shearing being limited to the action across the grain. The first 3 varieties are the strains which usually come upon ties, struts, and beams respectively. The transverse strain, it must be observed, is resolvable into tension and compression, the former occurring on the convex side of a loaded beam, and the latter on the concave side, the 2 being separated by the neutral axis or line of no strain. The shearing strain occurs principally in beams, and is greatest at the point of support, the tendency being to cut the timber through at right angles to the grain; but in nearly all cases, if the timber is strong enough to resist the transverse strain, it is amply strong for any possible shearing strain which can occur.