508
HISTORY OF ENGINEERING.
Book T.
sum of 160/.; its length is 110 feet, and its breadth 4 feet. Columns of cast-iron 9 feethigh, fixed securely in the soil, were placed at each angle of the bridge, and supported thesuspending wires, which were -fa inch in diameter. The floor is carried by five obliquewires, which are secured to an iron bar 10 feet in height, and of a sectional area of 2^ inches,inserted in the top of the column ; the way over is formed of frames of wrought-ironcovered with 11-inch deal ; on each side are chains, which serve as ties, of ^-inch iron; thelength of the links is 5 feet, they are fixed at one end into masonry underground, andat the other to the iron bar inserted in the columns, which prevents them from being pulledover.
Dryburgh Bridges were erected in the year 1817, about 18 feet above the low water of atorrent. The supports were 260 feet from each other ; and about a year after its erectionit was destroyed by a high wind, which gave an undulating motion to the roadway, andat last broke the suspension chains. A new bridge, which cost 720/., was immediatelycommenced, with perpendicular suspension rods ^ inch in diameter, having at the top across-head, by which they were kept between the oval links of the suspension chains;at the lower end they pass through the side beams of the flooring, to which they arescrewed. Chains of nearly Finch in diameter pass under the roadway, and extend from oneabutment to the other. On each side a diagonal trellis firmly joined together forms aparapet, and prevents the effects of any undulatory motion. The two suspension chains areeach 1-| S 5 inches in diameter, and the length of the bars composing them is 10 feet. At eachextremity of the bar is an eye, through which passes an oval ring 9 inches in length, connect-ing the two contiguous bars ; these suspension chains are secured to the head of woodenpillars 28 feet high above the level of the roadway ; the space between them, which formsthe approach to the bridge, is 9 feet. The two pillars at the extremities are connected bybraces forming a St. Andrew’s cross, and by transverse beams, over which the suspensionchains rest; each pair of chains is 12 feet distant from the other where they pass overthe pillars, and only 4 feet 6 inches in the middle of the bridge. By this arrangement theyobtain an oblique power, both horizontally and vertically, and thus prevent, to a certaindegree, any oscillatory motion.
The suspension chains are under the platform, which they support by upright rods ofcast-iron, kept in their places by small arches of the same material, in a horizontal line underthe flooring, also of cast-iron, over which is a layer of small stones. The chains pass overeach abutment, descending behind it, and are secured in masonry where a passage is left toexamine and repair them. The lower extremity of the chains rests upon the base of aconical tube of cast-iron, through which it passes, and the tube being strongly bedded inthe masonry keeps it secure.
Suspension Bridge over the Tweed, at Kelso , in Scotland , was finished in July, 1820, byCaptain Brown. It is 300 feet in length, and 18 feet in width, and has a carriage-roadin the middle; the total cost, including the masonry, timber, and iron-work, was only50,000/.
The roadway, 27 feet above the level of low water, presents a gentle rise towards themiddle, where it is 2 feet higher than at the extremities. There are twelve suspensionchains, arranged in pairs; the diameter of the iron with which they are made is 2 inches.The links are composed of bars 15 feet long, terminated with an eye at each end ; two shortflat links are applied at each side of the two contiguous bars, and bolts well-riveted traversethe two links as well as the eye of the bar. The vertical suspension rods are 1 inch indiameter, and pass through a saddle-cap ; the bolts are cylindrical, having for their base anellipsis, the two axes of which are 2 inches and 2£ inches. The three chains are placed almostperpendicularly over each other, and the suspension rods are fastened first to the upper, andnext to the middle chain, and then to the lower. The chains which correspond with eachother right and left of the bridge have their joints so placed that the suspending rods also>crfectly correspond. The lower ends of the suspension rods traverse a longitudinal pieceof iron, at the extremity of which the timbers for the floor rest, and under which the rod isstrongly riveted. The timbers arc 15 inches deep and "7 inches wide, and are covered withplank 4 inches thick.
The parapets are formed into lozenges, the sides of which are 6 inches; the height is 5 feet.The distance between the points of support for the chains is 437 feet, although that betweenthe abutments is only 360 feet; the angle formed by the chains with the vertical line at thepoints of suspension is equal to 78 degrees. The weight of each chain, with its suspendingrods, bolts, links, &c., is about 900 pounds. The twelve chains and all the iron-work usedin the bridge weighs 13,000 pounds.
The masonry of the abutments against the steep rock which borders the river is 20 feethigh, and the isolated pier on the other side is 60 feet, and of a pyramidal form; its thick-ness is 17 feet 6 inches, and at half its height the width is 36 feet; it is square at the bottomto the height of 10 feet, when the slope commences diminishing about a twelfth part Theroad passes through this pier by an arched gateway 12 feet wide and 17 feet high. Thepairs of chains run through the masonry of the piers, the openings for which are 2 feet wide.