IRON BRIDGES.-Augustus Canfield of Plainfield, N. J., took out papers for a "tension iron bridge' on June 29, 1833, which is believed to be the first iron bridge patented in this country. It is represented as having a span of 80 feet, and two stringpieces-one above the other in a straight line, or rather arching a little upward. The upper and lower stringpieces, as shown in the drawing, are ten in number, and the limbs are 8 feet in length, of wrought iron. The distance between the upper and lower string-pieces is also represented as 8 feet. Vertical bars connect the strings at each joint, dividing the whole into 8-feet squares. Diagonal braces of cast iron, pointing downward from the centre toward the abutments, are placed in each of the squares. The inventor believed this to be the best arrangement of the materials that could be made, and remarked that "in this construction the stress upon each part by any given pressure is a matter of simple calculation;" also, that spiral springs formed in the chords, as seen in the drawing (fig. 23),

FIG. 23.-Canfield's Iron Bridge.

may be used if desired, so as to leave every part free to expand or contract without any strain, the chords yielding about an inch to every two tons' tension.

The earliest patent for trussed beams is that of Richard T. L. Witty of Lowell, Mass., dated Oct. 14, 1835, which is described as follows:

"I combine the lateral thrust or pressure of a beam to the catenary curve, which said curve may be formed of rods of wrought iron or other metal, or of chain or rope, etc. But I prefer rods of wrought iron. The curve of iron must be stretched from end to end of a beam of wood, and firmly bolted thereto, and passing over stanchions or other supports of wood or iron fastened to the beam; which said supports may be placed at suitable distances under the beam to form the catenarian curve, and the more firmly to retain it in its place. By this arrangement a very considerable strength of beam may be obtained for the making of bridges over rivers, canals, etc., and for the formation of viaducts to carry over railways, in the construction of piers for docks or harbors, and for public buildings where considerable extent of beam may be required."

The invention is shown by the annexed diagram (fig. 24).

FIG. 24.-Witty's Trussed Beam. Whipple Iron Bow-string.-On April 24, 1841, Squire Whipple of Utica, N. Y., patented a modification of an iron arch bridge as follows:

"I claim the method of sustaining the flooring of bridges by iron trusses containing cast-iron arches, formed in sections or segments, in combination with diagonal ties or braces to sustain the form of the arch against the effect of unequal pressure (with or without vertical posts or rods), and wrought-iron arch-strings or thrust-ties to sustain the thrust and prevent the spreading of the arch in case the abutments and piers be not relied on for that purpose; also the divergence or horizontal expansion of the arch from the middle portion to the ends thereof in wooden trusses or arches, as well as in those composed of iron."

A large number of these bridges (shown in fig. 25) having a span of 100 feet, rise of 12, were built by Mr. Whipple for common road-travel in the State of New York. The breadth was 19 feet from centre to centre, and there were two footwalks of 6 feet each outside the ribs. In cross-section the arches were of the form of an inverted channel-iron, having a net

FIG. 26.-Whipple's Trapezoidal Truss.

The form generally known as the Whipple is a trapezoidal truss having parallel chords and inclined endposts (fig. 26). It is adapted chiefly to iron bridges.

Aside from its trapezoidal form, the Whipple bridge is distinguished by the introduction of trussed cast-iron posts or verticals, and long wroughtiron links, extending from panel to panel, forming the lower chords, and by several other minor peculiarities. (See figs. 27, 28.)

Lattice Iron Bridges. - About 1824, George Stuart patented a wrought-iron lattice truss in England, and in 1843, Sir John MacNeill erected an open - work girder of wrought iron of 84 feet span.

An iron lattice truss was invented by Nathan Rider of Massachusetts, Nov. 26, 1845. It is composed of an upper chord of cast T-iron, lower chord, upright posts, and diagonal ties. The upper chord is of cast iron of the form shown in fig. 29. The lower chord and ties of wrought iron are secured to each other and the upper chord by bolts. The posts are cast-iron I-beams, slotted at the ends to embrace the chords. A wedge is inserted on top of each post, under the top chord, to keep the diagonal ties in tension. These ties have a run of three panels, making three

Posts

FIG. 27.-Posts.

FIG. 28.-Early Form of Pin-Connection.

independent systems, which may be brought into greater or lesser action by the driving of the wedges, producing

sockets at intervals of 12 inches; the two trusses are connected by lateral bracing, which is composed of 4-inch round iron, set diagonally and bound together at the crossing by two cast-iron plates about 4 inches in diameter, the sides next to the bracing being cut in such a manner that when the two g-inch bolts passing through them were screwed up, it holds them firmly together. There is also a bolt passing through both truss-frames and through the heels of the lateral bracing, at right angles to the bridge, which secures the heels of the lateral braces and by means of a socket in the centre make a lateral tie to the bridge, giving it stability. The lower chords are of hammered iron, there being some difficulty at that time to get rolled iron of proper size, and are in one entire piece, being welded from bars 12 feet long. There are eight of them, 5 x inches, one on either side of each piece of boiler iron, and fastened to it with -inch iron rivets 6 inches distant from each other. There are but four top chords, and of the same size as the bottom, two on each truss near the top, the timber for the rail making up the deficiency for compression. The entire weight of the bridge is 14 gross tons, and cost $2200." The span is 55 be reduced to $30. The centre of each girder was placed feet, or $40 per linear foot, which the inventor thinks can exactly under the rails, which were spiked to the timber forming the top chord. The breaking strain of each girder was 125 tons of distributed load.

The tubular bridge does not seem to have met with favor in the United States, but in England and Canada several very expensive structures of this class have been erected-notably, the Britannia and Victoria bridges. which are remarkable for the boldness of their designs and for the energy and ability displayed by their projectors, Messrs. Fairbairn & Stephenson, in conducting the work.

Such were the principal bridges in common use in America, both for highways and railroads, until the middle of the nineteenth century, when the researches into the strength of materials by numerous experimenters, and the investigations into the theory of strains in bridge and roof members by competent writers, gave a fresh impetus to the subject of framed structures and transformed an art into a science.

This transformation, and the substitution of iron for wood, were, however, very gradual. The resulting structures of wood and iron were known as "combina tion" bridges.

The Bollman Truss.-Wendel Bollman obtained a patent Jan. 6, 1852, for an iron truss bridge, which was adopted by the Baltimore and Ohio Railroad Co., and erected at numerous points on its line.

The peculiar features of this form of bridge (fig. 32) consist in the suspension of the load at the foot of each post by the tie-rods running directly to the ends of the top chord, and in the details of the connections to admit of an automatic adjustment for the great differences of length in the ties near the ends of the bridge. It is, in fact, a suspension

the large bridge at Quincy, over the Mississippi River. The bridge at Harper's Ferry, across the Potomac, 124 feet span and 17 feet 6 inches deep, was tested June 1, 1852, by running over it three first-class engines weighing in the aggregate 136 7 tons, being over a ton for each foot in length of the bridge, at a speed of about eight miles per hour, giving deflections at the first panel post of of an inch, and at the centre of 1%.

The lenticular girder, as constructed at Saltash by Brunel in 1859, of 455 feet span, was doubtless anticipated by the patents of Messrs. H. L. Hervey of Illinois and R. E. Osborn of Ohio, issued Aug. 21, 1855.

FIG. 38.-Post-and-Chord Connections, Fink Truss. This pioneer type of long-span bridges was soon followed by similar trusses over the Monongahela at Pittsburg, span of 260 feet; over the Ohio for the Baltimore and Ohio Railroad at Bellaire and Parkersburg, with spans of 350 feet; and the great span of 420 feet on the Newport and Cincinnati Railroad at Cincinnati. (See fig. 40.) The Parkersburg bridge has two spans of 348 feet each, four of 200 feet, with numerous shorter spans; that at Bellaire has one span of 348 feet, one of 250, four of 200, and a number of 107 feet. The approaches consist of forty-three stone arches of 28 feet 4 inches span on a five-degree curve. The cost was about $1,000,000.

FIG. 37.—Suspension Link at end of Top Chord, Fink Truss. posts, and ties are pin-connected, the diameter of pins being 4 inches. The upper chords each contain two similar tubes of cast iron, having an external diameter of 15 inches at centre and sloping slightly to ends of panel. The posts are also twin tubes of cast iron 8 inches in diameter, combined by intermediate webs and rails, and made in two sections with flanged ends, which are bolted together. The parting-plate at the middle of each post has four projecting pieces, with concave bearings to receive the truss-rods, which are secured near the tops and bases of the posts. These rods deflect 18 inches, and are adjustable by sleevenuts, the arrangement being precisely that known as "Whipple's post.' The diagonal ties are square bars, having an eye at lower end, and are swaged round at top to admit a screw-thread and nut, by which they bear against angle-bosses cast on the chords. There is the usual lateral bracing and rolled-plate bearings at end for changes of temperature. This channel-span

This same general design has been adopted for the great bridge over the Hudson at Poughkeepsie (sec fig. 41), having five spans of 525 feet each and a clear height of 130 feet above high water. The grade will be 190 feet above the same level. The depth of water, varying from 50 to 60 feet, will make the piers of unusual height. The river at this point is 2430 feet wide. requiring four piers to be placed in the bed of the stream. These are founded on caissons-one at a depth of 122 feet, in which case the dredging was carried to the extreme depth of 130 feet and a concrete foundation put in place. The caisson of No. 4 is filled with concrete to a height of 77 feet, at which level, 20 feet below low tide, the granite masonry begins. The superstructure of this bridge is not yet (Nov., 1882) erected, in consequence of the delays occasioned by the great depth and volume of water, extreme height of piers, and length of spans, all of which combine to make it one of the grandest and most difficult pieces of bridge-construction yet undertaken.

The stiffened triangular girder which replaced the wooden Howe truss spanning the Susquehanna River at Rockville was erected in 1877, under the supervision of the Messrs. Wilson Bros., without interruption to travel, at a cost of $326,614.10. The contractors were the Delaware Bridge Co., who completed the work between July 1 and December 1. The iron-work was constructed and assembled at the Edgmoor Iron Works