The various coatings we put on wood, such as paint, varnish, oil, etc., are intended not only to beautify but to preserve it, which they do by filling up the pores and excluding moisture, preventing fungous growths, etc. All of these coatings should be put only on dry wood, else they prevent evaporation of the sap and may hasten decay.
Fig. 234. Strains
Drying lumber increases its strength, as it has been found by experiment, even as much as 400 per cent. if no checking occurs. When this happens it counteracts much of the gain, and if the wood absorbs moisture once more the strength will decrease. The strength of the various timbers varies greatly, and sap wood is usually weaker than heart wood.
The strains that may be brought to bear on timbers are illustrated in [Fig. 234], the arrows indicating the directions in which the forces operate. At a the wood is under tension, the forces at work on it tending to pull it apart. At b the piece is under compression, the forces tending to reduce its length by forcing its fibres together. A pillar supporting a weight is under compression. At c the weight tends to bend the beam. The upper part is under tension, the lower part under compression. This is known as beam action, and depends on whether the beam is supported at one end, as shown, or on both ends. Also it is important to know whether the beam has a uniformly distributed load or whether the weight is at one point only. The problems relative to beam action are largely of an engineering character and involve considerable mathematics.
Shearing is the sliding of one part of the timber along the grain. If a piece of wood is cut to the form shown at d and a weight applied at e, the tendency will be for this upper part to slide down as shown at f. When this occurs, shearing has taken place.
The strength of wood differs in resisting these various strains, the tensile strength being greater than the crushing or compressive strength. Ash, for example, has a tensile strength of 16,000 pounds to the square inch, but its crushing strength is only 6800 for the same size. The tensile strength of dry white pine is 10,000 pounds, its crushing strength 5400 pounds, and its shearing strength varies from 250 pounds to 500 pounds, showing that its weakest point is along the grain. If the young woodworker becomes ambitious enough to think of designing a bridge or large building he can find these figures in any engineers' hand-book. There are so many important factors to be considered that the amateur will do well to go ahead with great caution. Knots and other defects reduce the proportionate strength of large beams greatly, so that it would not be safe to assume that a beam 6 inches square would be 36 times as strong as a piece 1 inch square.
In upright posts of considerable length, not alone the crushing strength must be considered, but a bending action enters into the problem. Wherever the question of danger to life enters, as in a bridge or a house, it is wise to leave a large margin for safety. We realize this fully when we read of a grand stand holding hundreds or thousands of people collapsing under the weight. The architect has also to reckon with still other elements, such as wind pressure and vibration.