Page:The New International Encyclopædia 1st ed. v. 03.djvu/564

* BRIDGES AND DOCKS. 492 BRIDGES AND DOCKS. tion engines or other unusually heavy loads, additional precautions may be necessary to en- able it to carry them with safety. In the United States Army, where the pontoon bridge was extensively and effectively employed during the Civil War, the bridge material of the Corps of Engineers is included in reserve and advance-guard trains. Where a large army is operating in the held and wide and swiftly llow- ing rivers are to be crossed the reserve train must accompany the troops. Such a train is divided into four' pontoon divisions and one supply di- vision. Each pontoon division is complete in itself, containing all the material necessary for constructing a bridge 225 feet in length. Eacli division is divided into four sections — two pon- toon and two abutment sections — each pontoon section consisting of three pontoon-wagons and one chess-wagon. These four wagons contain the material for three bays (or 60 feet) of the bridge. If it is desired to vary the length of a single division, it can be done by diminishing or increasing the number of pontoon sections in the train. The pontoons are flat-bottom boats .31 feet in length, and with a maximum breadth of 5 feet S inches, of about OVo tons burden, and with sufficient capacity to transfer 40 men fully armed and equipped/ besides its crew of pon- toniers. Pontoon bridges may be constructed either by the method of successive pontoons, by rafts, or by conversion. The first mentioned is the most usual method. In following it, the pontoons are placed 20 feet apart, at right angles to the axis of a bridge, and anchored up-stream and down- stream to hold them in position. Five lines of floor-beams, known technically as 'balks,' 5 inches square, rest on the pontoons and are firmly lashed to them. On these floor-planks, 1% inches thick and kno^vn as chess, are laid: these are held in position by side rails lashed through notches in the chess "to the balk beneath. The width of bridge between side rails, when finislied. is 10 feet. Bridges of this type received the hard- est kind of usage during the Civil War. and proved clearly their great value. In the month of February, "l802, a pontoon bridge conqwsed of 60 boats of a reserve train was thrown across the Potomac at Harper's Ferry. The river was then a perfect torrent, the water being 15 feet above the summer level, and filled with drift- wood and floating ice. Under these unfavor- able circumstances, the bridge was completed in eight hours, and General Banks's corps, with all its trains and artillery, passed over it with- out accident or delay. During the year of 1863 the pontoon trains accompanied the army in all its marches through Virginia, frequently bridging the Poto- mac, Rapidan. and Rappahannock. The headquarters train in 1864 was followed by an advance-guard (q.v.) train, which, when a crossing was to be made by surprise, was sent forward with the cavalry" to cover the con- struction of the bridge and hold the position until the main body arrived. The advance-giiard equipment is similar, in general principles, to the reserve. The jiimtoon, however, instead of being made entirely of wood, is made of canvas held in shape by wooden frames, which can be knocked down for trans- portation. For transportation purposes, the material is so packed that the pontoon-wagon contains all the materials for a single bay of 20 feet. The division may, therefore, be in- creased or diminished by one or more pontoon- wagons, if it is desired to increase or diminish the length of the bridge in units of 20 feet. The width of the bridge is two feet less than that of the wooden pontoon bridge. The canvas pontoon, while possessing the advantage over the wooden pontoon of being more easily transport- ed, will of course not stand as hard service. At the same time, it has shown its usefulness on many occasions. The pontoon-trains with the Army of the Cimiberland, in its march from Chattanooga to Atlanta, Savannah, and Wash- ington, in the latter part of the Civil War, were of canvas, and jrave excellent satisfaction. Where pontoon bridges are constructed at night, with a view to making a passage and surprising the enemy, the floor of the bridge should be covered with straw, in order to reduce the noise of construction to a minimum. JIany substi- tutes for pontoons, such as floating supports, have been devised. Among the xnore impor- tant are rafts made of casks lashed to frames, rafts of timber, and boats of commerce. Spar bridges are made with round timber usually cut near the site of the bridge. For narrower spaces of 25 feet or less, the balks consist of single trees stretching across the river. The roadway is made of small saplings smoothed oft' on top or covered with dirt. For spaces of 30 to 35 feet, two trapezoidal frames are constructed of timbers lashed together and braced by diagonal braces lashed to the legs of the frame. These are put in position on the opposite banks, and then lowered so as to rest their tops on eacli other and form an intermedi- ate support above the middle of the stream for the balk or roadway-bearers. For a still wider stream, double-lock bridges can be used. In these the frames, instead of resting on each other, rest on cross-timbers held apart by straining beams. These cross-timbers furnish two intermediate supports over the gap for the balk. For still wider openings, the frames are made consider- ably longer, so that they support each other at a point not less than 10 feet above the roadway. From their junction an additional roadway- bearer is suspended by ro])es. This, together with the points where the roadway crosses the two frames, gives three jioints of support. One of the most generally useful of all the types of 'oridge is the trestle bridge. Vertical frames are constructed, on whidi the balks carry- ing the roadway are placed. In cases where a single trestle is not high enough to come from the bottom of the gap to the desired height of roadway, trestles can be so braced that they will hold others on top of them. This type of bridge particularly lends itself to railroad work, one of the most notable examples of the kind being the bridge across the Potomac Creek, ^'a., dur- ing the Civil War. This bridge was 80 feet high and 400 feet long. Suspension bridges have not been of general application in the United States military ser- vice. The English have made some use of them. They are suitable for wide spans, where it is difliciilt to secure intermediate supports. An army traveling in a country where its ser- vice will probably require the construction of this type of bridge should make provision for