Wooden bridges are regarded to-day as temporary expedients, to be replaced sooner or later by structures of stone or of iron. In America, how ever, where timber is abundant and cheap, wooden bridges are numerous, and the art of building them has been developed to a high state of perfec tion.
Early Wooden Bridges.. TI'llSS 1317.dgCS. —Among the earliest examples of scientific bridge-building as applied to wooden structures was the famous bridge over the Rhine at Schaffhausen (pi, 3s, fig. 4). This was one of the most celebrated wooden bridges ever built. It was planned and con structed in r757 by a common carpenter, Ulric Grubeninann, and was burned by the French in 1799. A bridge over the Liminat near the Abbey of Wettingen, in Switzerland, was built by the brothers Grubenmann, and was also destroyed by the French in 1799. This bridge had a clear span of 390 feet—the greatest span ever executed with timber. Among the ear liest and best examples of wooden bridges in America was the arch-truss erected over the Schuylkill at Fairmount, Philadelphia, in rS r2 by Louis Wernwag (pi. 42, fig. 14; 44, fig. i). This bridge had a clear span of 34oI.1. feet. In IS3S it was burned, and its place was supplied by a single span wire bridge (1). 269). Thomas Burr, who patented several forms of wooden-arch truss bridge, erected a number of such structures, nearly all of which exhibited remarkable durability. A peculiarity of inanv of these was that they were covered or enclosed, which gave them a somewhat sing-ular appearance. A notable structure of this kind was the Columbia Bridge across the Schuylkill at Philadelphia, built in 1834 for the use of the Columbia Railroad, a State work, and subsequently sold to the Read ing- Railroad Company. This bridge, after having withstood more than fifty years of continuous service, has been replaced by an iron structure.
Wooden Trestle Bridges.. Cosar's the famous historic wooden bridg-es is the one built by Julius Coesar over the Rhine about 55 B. c., of which he has left a description in his Commentaries. This account does not appear to be precise enough to permit of the reconstruction of the work without some doubt as to minor details. It is known, however, that it was a trestle bridge founded on piles driven into the bed of the river. These were joined by a beam on which joists were laid in the axis of the bridge. On these joists were laid hurdles, which supported the road-bed. Inclined beams protected the piers on the up-stream side, and each pier was staved below by a group of piles. A reconstruction from the descrip tion left of it is shown in Figure (N. 33).
American Trestle are several remarkable wooden tres tle bridges in America. One of these, over the Potomac Creek, in Virginia, was built in 1862, in nine days, by the Federal soldiers, during the civil war, under the direction of General Haupt, chief of the Bureau of Con struction and Transportation of United States Military Railroads. This structure, built of round sticks of timber cut from the adjoining, woods, is 40o feet long by So feet high. It is in four stories, three of trestles and one of crib-work, and is still in service, carrying a heavy railway traffic. Wor thy of notice, historically, is the great wooden trestle bridge known as the Portage Bridge (fig. 8). It was built to carry the traffic of the Erie Railway over the great gorge of the Genesee River, and was 1600 feet long and 234 feet high. Constructed in 1852, it was destroyed by fire twenty three years later, and was at once replaced by an iron-trestle bridge. (See p. 264.) Primzitive primitive structures, the bridge shown in Figure 2 is worthy of notice as exhibiting considerable origi nality and ingenuity. It is described by J. Foster Flagg in the Trans actions qf Me zinzerican Society of Civil Engineers, where it is credited as the production of a Mexican peon. It was built over the river Armeria, in the State of Colima. As will be perceived, it is a combination of the suspension and cantilever principles. This structure was put together without nails or metal of any kind, the suspension cable being made of twisted vines and all joints of lighter vines. The piers were made of poles driven into the river-bed in the form of a square and tied together with other poles, the interstices being filled in with stones. The stringers of the main span, in two pieces, were tied together at the centre, and the spliced stick was supported near the joint by the suspension cable. The towers were formed of natural forked sticks, which supported both the cable and the corbels, and which aided in shortening the main and lateral spans. Finally, the long stringers were again supported midway between the ends of the corbels and the central-cable attachment by cantilevers of crude construction, which were loaded with stone near their shore-ends, to balance the weight of the central span. The length of this span was about 7o feet, and that of the entire structure about 175 feet. Mr. Flagg reports that this primitive structure was strong and rigid enough to pass mounted men and loaded animals.