This section is from the book "The American House Carpenter", by R. G. Hatfield. Also available from Amazon: The American House Carpenter.

To ascertain what weight may be sustained safely by the resistance of a given area of surface, when the weight tends to split off the part pressed against by causing, in case of fracture, one surface to slide on the other, we have -

Rule III. - Multiply the area of the surface by the value of H, in Table I. divide by the factor of safety, and the quotient will be the weight required in pounds; or -

w=AH/a (3.)

Example. - The foot of a rafter is framed into the end of its tie-beam, so that the uncut substance of the tie-beam is 15 inches long from the end of the tie-beam to the joint of the rafter; the tie-beam is of white pine, and is 6 inches thick: what amount of horizontal thrust will this end of the tie-beam sustain, without danger of having the end of the tie-beam split off? Here the area of surface that sustains the pressure is 6 by 15 inches, equal to 90 inches. This multiplied by 480, the value of H, set opposite to white pine, Table I., and divided by 3, as a factor of safety, gives a quotient of 14400, and this is the required weight in pounds.

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