To ascertain the weight or pressure that may be safely applied to a beam or rod as a tensile strain, we have -
Rule XII. - Multiply the area of the cross-section of the beam or rod in inches by the value of T, Table II.; divide the product by the factor of safety, and the quotient will be the required weight in pounds; or -
w= AT/a. (15.)
The cross-section here intended is that taken at the smallest part of the beam or rod. A beam, in framing, is usually cut with mortices; the area will probably be smallest at the severest cutting; the area used in the rule must be that of the uncut fibres only.
Example. - The tie-beam of a roof-truss is of white pine, 6 x 10 inches; the cutting for the foot of the rafter reduces the uncut area to 40 inches: what amount of horizontal thrust from the foot of the rafter will this tie-beam safely sustain? Here 40 times 12000, the value of T, equals 480000; this divided by 6, as a factor of safety, gives 80000, the required weight in pounds.