A good example of east-iron compound girder bridge trussed 0 Rh iron bars. was erected some years since over the river Lea, on the line of the Northern and Eastern railway. This bridge is formed with girders, each girder being 70 feet long, and made of two castings, joined at the centre by bolts passing through vertical flanges. Upon the meeting ends of the two castings, are cast dove tailed projections, or bosses, with wrought-iron clips fixed over them.
"These bosses give additional security to the ,joinings of the castings. Each girder is perfectly horizontal from end to end, and the top and bottom lines parallel. The girders are 36 inches in depth, with bearings on the abutments of 2 feet at each end ; the clear span between bearings being 66 feet. The sections of the castings shows a vertical rib, and 1 rejecting flanges at top and bottom. The truss bars are arranged in sets of 4 bars on each side of the girder, and pass obliquely downward from the top of the girder over the bearings at either end, to the under side of the girder, at a distance of about 11 feet short of the centre. The space between of 22 feet has horizontal truss-bars passing beneath; Loth horizontal and oblique bars are secured by bolts or pins 3 inches in diameter, passing through projecting saddles under the lower flange of the girders. The bars at their HITher extrenlities pass through sockets cast upon the girders, and are keyed them." It would be easy to multiply examples of iron bridges, many of them possessing great merit ; but the lamentable faihne of sonic. large bridges of this construction, has, within the last few years, led to the conclusion, that sonic radical defect existed in these compound constructions.
This defect evidently consists in the impossibility. in st•netnres of this kind, of connecting wrought and cast iron in such a manner as to bear equally the strain of such a load as a railway train, passing over at, a high velocity. This became fatally apparent in the failure of the largest bridge of this kind. erected over the river Dee, near Chester, and on the line of the Chester and llolvhead railway.
This bridge, which crosses the Dee at an angle of -18 (kgr•eti, CI AliiiStS of 3 spans or bays, each 95 feet wide in the clear, the three series of girders forming the bridge being supported on two abutments of masonry. one at either end, and two intermediate piers. The width of the bridge is formed by 4 of these girders, placed parallel to each other, in 2 pairs, one roadway or railway being supported between each pair of girders. and formed of 4-inch planking laid upon transverse balks of totter, xi hh-h rest upon the bottom flan ,re of the girders. The girders ;ire secured transversely from moving outward or away from each other by tension-hars, fitted at the ends to dove-tailed sockets, cast upon the girders.
The t mile bridge thus comprises 12 girders, each having a clear span of 9S feet, and a total length of 109 feet ; that is, including, a bearing at each end of 5 feet 6 inches in length. Each of these girders, 109 feet long, is composed of three castings, or lengths, having a uniform vertical depth of 3 feet 9 inches. The dimensions of the section as :—ve•tical rib or web. 21-, inches thick ; top flange, 7r inches wide, and LI inch thick ; bottom flange, 2 feet w ide and 21- inches thick. The sectional area of the top flange, including, the moulding, is equal to 14 square inches; of the bottom flange. including the moulding, 66 square inches; and of the rib. 80 square. incites; makiug a total uniform sectional area of 160 square inches. The joints; of the three castings, in each girder. seemed by iron bolts. passing through flanges. are strengthened by additional east-iron joint plates, 3 feet deep at the centre, over the joint. and 13 feet in bolted to and searped over the top flanges of the eastinii.s. a length of 6 feet 6 inches upon each; dove-tailed bosses. east upon the lower flanges, are also secured with clips of •rought-iron.
" The total depth of the girders, at each joint, is thus increased to 0 feet 0 inches-. Similar plates of half the length of those over the joints, are also bolted over the ends 411 each compound girder ; and the vertical inclination of the truss-bars, from the top of the girder at each end of the hottmn of it at the joints, is thus increased to about 6 feet. The malleable iron timss-bars are arranged in sets of 4 each, 1 set on each side of the girder. each bar being 6 inches wide and 1+ inch thick. put together in lengths or long links, similar to those used fir suspension bridges. and secured by at the joints of the girders, passing through the cast-iron girder and the S wrought .iron liars. The upper ends of the bars are seemed with wrought-iron keys, driven the bars and the casting, so as to tighten them well np in their position. By the great length of the girders, and the comparatively small depth thus affhrded for the trussing, the action of the bars is reduced to nearly a horizontal direction, and their power to avert deflection in the girders is thus touch diminished. 'Besides this, it must be remarked, that the sectional area of the bars is much less compared with the total length of each girder, than in all smaller structures on this principle ; and the relative effect of any increase of temperature in extending their length, and thus reducing the eflixitiveness of their assistance, is similarly angmented."—Dempscy, Tre«lise on Iron Girder Bridges.