Military Bridges; With Suggestions for New Expedients and Constructions for Crossing Streams and Chasms; Including, Also, Designs for Trestle (Paperback)

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: that the dowels will fit well, and enter readily. If no mistake has been made in getting the height of the lower trestles, the upper series will be all alike, and no difficulty will be presented in framing them. PIank caps can be used, except for the top series, where square or flatted timbers should be employed. In all cases, 2-inch dowels are used instead of mortises and tenons. Neither bolts nor braces are required in any part of the structure, excepting that in high trestles plank or poles, spiked as lateral longitudinal bracing between the caps, is an advantage, and the longitudinal poles or ties at the ends of the bridge should be butted against masonry, or dowelled into logs that have been bedded in the bank. The object of these arrangements is to resist motion in a longitudinal direction, and stiffen the posts; the transverse motion being resisted by the shape of the trestles themselves. Raising the Trestles. The trestles having been framed, and placed in convenient positions on the ground, or floated on the water, they are raised to the place which they are to occupy by means of blocks attached to sliding-beams. These sliding-beams consist of long timbers, 35 to 40 feet in length, and about 12 inches square, in cross section. They are trussed with ropes, rods, or chains, as represented in PI. I., figure 3, and on an enlarged scale in PI. II., so as to give them sufficient strength to hold the heaviest trestle, and are projected forward a distance equal to one interval. Large iron pulleys are placed in the ends, over which pass the ropes required in hoisting. The power employed is usually that of a large numberof men, forming lines on each side of each rope, so as to raise or lower, by signal, either side of the trestle at pleasure. The inside ends of the sliding-bea...