Elementary Principles of Carpentry (Paperback)

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1871 edition. Excerpt: ...on each truss being 75 tons, or 168,000 lbs. 100_x 168000 m 17500Q lhB t o X" the strain at the centre of the top and bottom strings, which, divided by 1000 for the top and 700 for the bottom, give 175 square inches and 250 squaro inches respectively for the sectional area of the strings in each truss, and by making the depth 10 inches, the width must bo 17 inches in tho top and 25 inches in tho bottom. It is a good practical rule (and one which is observed in Howe's bridges) to make the upper chord or string consist of T three, and the lower of four timbers to each truss; a joint will then occur in each panel, and the pieces should be sufficiently long to extend over four panels. With this arrangement three of the timbers must be supposed to sustain the whole strain, since that which contains the joint is not capable of opposing any resistance. 364. In a solid beam resting on two supports, the strain at the ends of the strings is nothing, and it increases uniformly towards the centre; but in a bridge truss of a single span there will be a horizontal strain at the end of the brace nearest the abutment, which will equal the weight on the brace multiplied by the co-tangent of the inclination of the brace, which, if 45, the horizontal strain will equal the vertical weight at the end. If the angle with the horizontal is greater than 45;. (which is generally the case) the horizontal strain will be hss than the weight, and consequently it will be safe in practice to assume the horizontal strain at the end of the string, or. more correctly, at the end of the first brace, as equal to the vertical force acting on that brace. This vertical force i equal to one-half of the whole weight on the truss, or in tht example given in Art. 303 it will...