Journal of the Franklin Institute of the Pennslbania (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. 1842 edition. Excerpt: ...from the piers towards the centre of the span, in the same manner as in a single long piece of timber, supported at two ends, when it sags in the middle: consequently, when the bridge settles, as it always will, more or less, the head of any one post is moved a greater distance towards the centre of the span than the post behind it, that is, between it and a pier. Therefore, the opening, p', behind the post, Q', must be a little wider than the opening p, behind the post Q; and, consequently, the inner end of the straining-piece, T, cannot be forced up into contact with the head of the post Q, but must remain distant from it an amount equal to the difference of the compression which takes place in that part of the pole between Q' and the centre of the span, and that part which extends from p' to p. This difference in the amount of compression between any two consecutive posts, is very perceptible in all large bridges, being generally about one-eighth of an inch, that is if there be seven spaces in the truss, between a pier and a king post, the opening at the inner one will generally be about seven-eighths of an inch, at the next one six-eighths, at the next five-eighths, and so on to the queea post near the pier, where it will diminish to nothing. In some bridges, and those excellent ones, I have seen the openings behind the queen posts much greater than this, at least double; but, I believe, only in such bridges as have no chords to confine the feet of the ribs. Of course some portion of these openings, in every case, is due to the compression which takes place in the heads of the posts themselves. This is-frequently very perceptible. I could just detect it in a few of the queen posts of the aqueduct. But it may be objected that if the...