metrically through, or by rivets passing through sleeves or separating pieces, leaving an open column. (See fig. 65.) The arrangements for "deck" and "through" bridge connections are clearly shown in figs. 66, 67. Similar connections of the Phoenix Bridge Co. are shown in figs. 68, 69, 70, 71, 72, 73. The Baltimore Bridge Co. uses for compression-members in Fink trusses a combination of an I-beam with the Phoenix quarter-section column. (See figs. 74, 75.) The joint-boxes for a Fink truss are shown in figs. 37, 38. The "Kellogg" column is composed of concave or fluted longitudinal segments, with flanges on the edges. Various sizes are rolled, from 120° are FIG. 74. are merely channel-bars bent to the proper angle. Another form consists of I-beams bent longitudinally at right angles and riveted back to back (c, c), (fig. 78). The American Bridge Co.'s column is composed of several "channels," with flat flanges united by a plate placed at right angles to the centre of their backs and connected by angle-irons, or an I-beam may be used instead, thus (dd) (fig. 63, 6, 8). Fig. 79 shows the connections at the foot and head of the American column when a rolled I-beam is used to support the roadway, as well as to serve the purpose of lateral bracing. When the upper chord is composed of three I-beams, the pin connections are made by a casting filling the space between the flanges, as shown in fig. 81. In the stiffened triangular truss the compressionmembers are generally rectangular troughs, the sides of which are channels, the top, plates, and the bottoms lattice bars. Where the main struts are perforated by pins, as for the feet of sub-panel posts, they are reinforced by stiffening plates riveted on the webs. The connections for end-posts and laterals are shown in fig. 42, for panels and sub-panels of a deck-bridge in fig. FIG. 75.-Section of Bridge as built by the Baltimore Bridge Co. to 60°, making a column of from three to six such segments, with rivets under the flanges (figs. 76, 77). They 42, and the connections for road-bearers, ties, and lower chords in fig. 82. The same joints for a combination bridge pin-connection are shown in figs. MOVABLE BRIDGES. Movable or draw bridges are of very ancient date, and were used chiefly as bascules or portcullises to span the moats surrounding castles or fortresses. Subsequently, the necessity for keeping open navigable waters led to numerous modifications in the forms of this class of bridges, as well as to the methods of operating them. They may be composed of one or two arms, which may revolve about a horizontal or vertical axis, or they may slide in or out on a level, or revolve on a centre pin or pintle, thus forming the subdivisions known as swing, lift, rolling, or pivot bridges. An early form of a lift bridge is shown in fig. 83 at a, FIG. 79.-Pin-Connections at Ends of Post for a Through Bridge, American Bridge Co. where a portion of the floor is raised into an upright position by a windlass fastened to the fixed part of the span, in connection with a rope and pulleys as shown. This bridge was built on the post-route from Philadelphia to New York about 1810-14. Lift bridges are most frequently operated by a counterpoise weight running over a pulley attached to a framework at or over the hinge, but in such cases, as the arms rise from their horizontal position and the moment of resistance becomes less in consequence of the reduced leverage, the velocity becomes accelerated, and the motion is only arrested with a shock. Better devices consist of cams or eccentrics attached to an arbor containing the lifting chains. The counterpoise O о FIG. 81. Detail, showing case. mer or conical for the latter Swing bridges are ordinarily subject to a reversal of the strains, due to their becoming ordinary trusses when closed and semi-girders when open, requiring a large amount of counterbracing, but the Menomonee and Kinnikinnic draw beams. bridges, erected by Mr. C. S. Smith, are typical of a class of frames so arranged as to admit of no reversion of strains in the several members, and so supported that the load bears uniformly on the live ring by reason of a rocker link (a, fig. 84) at the head of the middle postframe, eliminating any ambiguity as to the strains. The draw at Sabula, 370 feet span, is probably the most complete in all its appointments. The ends are raised or lowered in forty-five seconds, and the draw swung open 90° in sixty-five seconds. The Passaic drawbridge (fig. 85) is a wrought-iron double-track structure crossing the Passaic River at Newark, N. J., on the Morris and Essex Railroad. There are three trusses sustaining the tracks upon iron beams suspended from the lower chords. It is thus described in Trans. Am. Soc. Civil Engineers (1876: "General Dimensions.-Length of trusses from centre to centre of end piers........ Height from centre to centre of chords........ Width of bridge from centre to centre...... Height of rail above water....... ....... Weights adopted for calculation: 220 ft. 6 in. 22" 30" 45" .2000 pounds. ..5000 .........7000 66 "When the bridge is open, all the dead load is transmitted to the central support, and the strains are determined as in the case of a beam supported in the middle. When the bridge is shut and the wedges adjusted, the extremities are brought to rest only, and the strains due to dead load are not materially changed; the effects produced by the rolling loads are now determined under the assumption that the bridge is supported at the centre and extremities; and finally, the maximum strains are obtained by adding the strains due to fixed and rolling loads with their proper sign: Weight of iron in trusses....... Total....... .350,770 lbs. .107,690 " .458,460 lbs. "The trusses are of the 'Whipple' type, with the endposts inclined, and with two inclined posts and a vertical at the central point of support. The chords are, for the most part, made up of two 10-inch channels and a coverplate on one side, with open lattice-work on the other, the upper chord, where tension alone exists, eye-bars are forming a trough-shaped section. In the centre panels of used; all other connections in the chords are designed to resist both tension and compression. The lower chords of FIG. 82.-Details of Pin-Connection at Roadway-bearer of Stiffened Triangular Truss, Pennsylvania R. R. the centre truss abut against a casting which forms the upper portion of the turn-table, and are secured to it by plates, making a continuous wrought-iron connection. The connections at the abutment ends are also riveted. Vertical posts are made up of channels latticed on both sides, forming a rectangular section open on both sides to inspection. The ends are reinforced, and pin-holes bored for connection with the chords. No joint-boxes are used. "The dead weight of the bridge is transmitted to the revolving surface of the turn-table through the casting at the centre of the lower chord of the middle truss; that portion is borne by the middle truss directly, while the outer trusses rest upon the extremities of two plate-girders connected on each side the centre by diaphragm plates and FIG. 83.-Drawbridge over Neshaminy Creek, near Phila- lugs. By this means the entire weight of the outer trusses delphia, Pa. (Built by Wernwag.) angle-iron lugs. The lower chords and central posts of the outer trusses are riveted at their intersection to splice and cross-girders may be suspended upon twelve vertical bolts passing through holes in the sides of the central casting. This casting rests upon conical steel rollers enclosed between steel rings grooved to fit the coning of the rolls, and having half an inch play between: upon these rollers the bridge | adjusted so as to bear lightly upon the track. When one revolves. The lower ring is fitted to a cast-iron seat having line of rails is loaded, these wheels sustain a portion of the a hemispherical cavity on the under side to receive a cor- weight, but not when the bridge is turning. Attached to the lower side of the base-girders forming the ends of the bridge are castings containing movable wedges having 14 inches vertical play; they are bevelled to fit bed-castings attached to piers, so that when they are driven to a bearing horizontal displacement becomes impossible. The free ends of the track are lifted simultaneously with the wedge-movement. FIG. 84. responding cast bearing-block which distributes the weight upon a wrought-iron pintle extending to the masonry. Under each end of the cross-girders are four trailing-wheels "The bridge may be operated either by steam or by hand, as desired. A doublecylinder engine drives a vertical shaft to which are attached two friction-clutches. By throwing the upper one into gear, a horizontal line of shafting is made to actuate the wedges and rail-lifters. By FIG. 85.-Passaic Drawbridge, Delaware, Lackawanna, and Western R. R. shifting to the lower friction, motion is communicated to a driving pinion working into cast toothed segments which are bolted to the masonry through the track. A vertical shaft for working by hand is placed between the rails; it communicates with the wedge gear by a chainwheel and with the driving pinion by a small intermediate. The time required to withdraw the wedges and swing the bridge does not exceed two minutes. This structure was erected in place of an old wooden bridge during the winter of 1876-77, and the traffic of the railroad was not seriously interrupted during the progress of the work." One of the simplest forms of a floating drawbridge has long been in use at Rouse's Point, N. Y., where the Central Vermont Railroad crosses an arm of Lake Champlain. It consisted of a large bateau or scow 300 feet long, 30 wide, and 7 deep, floored over with an arched deck, and containing two rows of posts in the hold under the stringers which carried the rails. The sides were stiffened by Pratt trusses of seven panels, each 10 feet high. The diagonal ties were rods of 1 inches diameter, which frequently FIG. 86. Railroad Drawbridge over the Mississippi River, at Quincy, Ill. (Bollman System.) broke from the twisting of the float by the waves. This floating span was operated by an engine placed on one side of the centre, which at first moved it in and out in the direction of the length of the bridge by chains fastened to an anchorage, but as it was found difficult to keep it in line in high winds, it was subsequently changed to swing around. The engine was taken from an old locomotive, and the whole was worked by one man, who opened or closed the draw in from two and a half to three minutes in fair weather and in five minutes in storms. When empty the scow drew about 12 inches of water, and with a train 300 feet long about 4 inches more. This draw formed but a small part of the centre of the bridge, which was 5000 feet long. It was built about 1850 by Henry R. Campbell, engineer, at a cost, exclusive of the draw, of about $300,000. The railway was single track. The permanent portion of the bridge is believed to have been an ordinary pile structure. An adaptation of the Bollman truss to picot bridges is shown in fig. 86, representing a portion of the celebrated bridge over the Mississippi at Quincy, Ill., erected in 1867-68. This draw-span is 190 feet long, VOL. I.-2Q having two bays of 95 feet cach. The main portion of the bridge is composed of quadrangular girders of the Whipple type in spans of 200 and 157 feet. Its total length is 3189 feet, and of the bay branch 552 feet; total, 3741. Cost, $1,500,000. The pivot span in the main branch is 360 feet long, 26 feet high at ends, and 34 feet at centre. The clear width is 14 feet. The pivot span of the Rock Island drawbridge over the Mississippi River is 368 feet long, and weighs 1,560,000 pounds above the rollers. The travelling weight on each wheel is 44,250 pounds. The span is rotated by two hydraulic rams worked by a doublecylinder engine. This bridge-span is peculiar in that it is proportioned without dead-weight reactions under the end-supports, that the top chord is composed entirely of eye-bars, that every panel-point in both chords is hinged vertically, and that all lateral connections are hinged horizontally. It carries two roadways. C. Shaler Smith was the engineer. So great have the requirements of commerce become that in the construction of proposed structures over frequented waterways the draw-spans are now required to be 500 feet. The table below, compiled by Messrs. T. C. Clarke and Thomas Griffin of the Phoenixville Bridge Works. gives the necessary data for determining the linear units of live load for all bridges. Suspension Bridges.-Aside from the purely temporary pendent bridges of ropes, thongs, or fibres, the earliest designs of suspension bridges making any pretensions to permanency in America were those erected by James Finley of Fayette co., Pa., who patented his improvements in 1808, although he built bridges as early as 1797. The cable in these structures was composed of 7-feet links of square bar iron. The level roadway was attached to this by iron pendants of different lengths. The versed sine of the are was a full oneseventh of the span, and the chain made equal angles with the vertical line through the tops of the piers, which were generally of wood. The first bridge of this description, of 70 feet span, 124 feet width, warranted for fifty years (except the woodwork), was built over Jacob's Creek at a cost of $600. In 1810 there were eight of these bridges in existence, the largest of which was at the Falls of Schuylkill, Philadelphia, 306 feet span, aided by an intermediate pier. The first bridges were destitute of a parapet, but this want was soon supplied, and added much to the stiffness, as well as safety, of the bridge. The form in use until 1841-42, when Charles Ellett, Jr., substituted wire cables for chains, is clearly shown in fig. 88. The first bridge of wire, which was erected at Fairmount, Philadelphia, in 1842, had a span of 358 feet. supported upon four obelisks by ten cables of 3 inches diameter. It served its purpose admirably for about thirty years, when it was removed to make room for a more commodious structure. The beautiful wire suspension bridge at the crossing of the National Road over the Ohio River at Wheeling, Va., was also built by Ellett, in 1848-49. It was destroyed by a hurricane in 1854, which, being deflected Actual Weights of Engines, Tenders, Cars, etc. Driving wheels. 12 None 102,000 19 7 5204 tender. Ft.In. 10 20882 12 None 82,200 15 8 5268 80,000 22 0 3636 84,000 12 6 6720 60,480 8 O 7560 11 Erie broad-gauge, special freight................. 72,000 12 0 6000 53,000 9 6 5578 54,500 12 5 4360 71,500 12 0 5948 15 6 4193 14 6 4976 Class No. 4-Loaded cars: From this it appears that the greatest strain per lineal foot of driving-wheel base would be produced by No. 5, an English pattern seldom used in this country, which amounts to 7560 pounds on both rails, or 3780 pounds-say 13 tons -on each truss per foot. from the water-surface below, raised the centre of the bridge 20 feet. The span was 1010 feet, or 960 feet clear, being the longest single-span bridge then in existence. The twelve cables contained 6600 wires, having a net section of 93 square inches, and weighing 313 pounds per foot of span. The total weight between the towers was but 440 tons. The deflection was the span; width of roadway, 24 feet in the clear; and height above low water 93 feet at centre of bridge, with a grade of 4 feet per 100 towards the ends. After FIG. 88.-Finley's Chain Bridge. but in consequence of some litigation operations were suspended. The Clifton bridge at Niagara Falls, intended for foot-passengers only, has a span of 1268 feet 4 inches between towers. The deflection varies from 89 feet in winter to 92 in summer. The Niagara Suspension Bridge was completed under Mr. John A. Roebling in 1852-55. For a description of this, the only bridge of its kind, see 80 and Plate XIX. in article on BRIDGES in ENCYCLOPEDIA BRITANNICA. The boldness of the design can more readily be seen by reference to fig. 89. Mr. Roebling built the long span between Cincinnati and Covington, of which the main bay measures 1057 feet between towers, and the end bays 281 feet each; total length, 1619; deflection of cables, 89 feet; factor of safety, 4; elevation of floor at centre above low water, 103 feet, and at tower 91 feet. The total length, including approaches, is 2252 feet. There are only two cables, 12 inches in diameter, each of which contains 5200 No. 9 wires; ultimate strength of each, 4212 tons; weight of main span, 1500 tons. This bridge was completed in 1867. Even this magnificent span has since been excelled in are perforated by two pointed arches at the height of the the Brooklyn Bridge (fig. 90), connecting that city with floor, forming entrances to the main span. Each arch, New York by a single reach of 1595 feet. The approach 32 feet wide, admits a railroad-track, carriage- and footon the New York side is 24923 feet long, and that on the way. The piers each contain over 900.000 cubic feet Brooklyn side 1901 feet, making the total length 5989 of granite, weighing over 70,000 tons. The weight of feet. The height of the bridge at the centre above high the bridge will be 3600 tons, and that of the maximum water is 135 feet, and at the ends 120 feet. The width load, when covered with people, cars, and vehicles, is 85 feet. There are four cables, each 15 inches in 1400 tons, making the total load about 5000 tons. diameter, containing 5434 parallel wires. The cables To stiffen it against wind-pressure, the outer cables each have an ultimate strength of 11,200 tons. The are inclined inward at the centre of the span, whilst piers are 1620 feet apart between centres, 280 feet high the inner cables are drawn in at the piers and spread and 134 feet long, by 56 wide at the water-line. They at the centre, neither hanging in a vertical plane, |