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all. The rail-timbers are notched down slightly over these cross-beams, and the intermediate spaces between the timbers are filled in with 3-inch flanking laid longitudinally.
Malleable-Iron Bridges of different Constructions-Lattice BridgesTubular Bow-Bridge-Tubular Girder Bridge, with intervening Arches of Brick-work-Compound Wrought-Iron and Concrete Girders— Combinations of Malleable and Cast Iron in Framed BridgesCorrugated Wrought-Iron Girders.
MALLEABLE iron having been applied in several forms of combination-besides that of the tubular girder and tube— to the construction of bridges, we propose, in order to make our historical sketch complete, to devote this section to a succinct description of the principal designs which have been executed or proposed.
To arrive at the earliest of these, we have to go back to the year 1824, when the ingenious Mr. George Smart suggested a combination of wrought-iron bars arranged in a diagonal form, under the title of a " Patent Iron Bridge." This design, which is the parent of the extensive family now known as "lattice bridges," of which our American brethren have erected some gigantic examples, exhibits a vertical framing, perfectly horizontal on its upper and lower lines, and composed of iron bars crossing each other in a diagonal direction, and forming angles of about 18° with the horizon. The framing also comprises vertical or "hanging bars," and "base bars," forming the lower horizontal lines of the framing, and also passing horizontally over each alternate row of intersections of the diagonal bars. The number and dimensions of the several parts are, of course, regulated according to the extent of the structure, and service for which it is destined; but each bar is intended to be forged of enlarged width at the points of intersection, and through which bolts are fixed to connect the whole together. Two of these trussed frames,
erected vertically and parallel to each other, would form the supports of the roadway to be formed between them, the two frames being tied together by transverse connecting-rods, the roadway or flooring being situated near the top of the frames, and never on the lower bars, which Mr. Smart considered a common but very erroneous practice in wooden bridges.* Between the frames, cross-braces, consisting of two light bars, are to be fixed, bolted together, and fitted to the connectingrods. In recommendation of this design, it was urged that it possessed extraordinary simplicity and economy; that the several bars employed being all of different lengths, and the holes all drilled or punched in one uniform manner, none of the parts could be misplaced in erection; and therefore the whole might be put together with great expedition, while, consisting of many small parts, and none of great weight, the bridge might be considered portable, easy of transportation by an army, and put up, when required, in a few days. Mr. Smart proposed to construct the piers of a bridge of a diagonal framing of wrought-iron bars, similar to that adopted in the bridge itself, and he showed the applicability of the same system to the formation of wooden bridges, in which he remarked there would be no necessity to limit the length of the pieces forming the framing, as no expansion or contraction takes place in that direction in wood.
Of the numerous examples in which this diagonal form of construction has been adopted in American wooden bridges, it does not belong to this sketch of iron structures to give any lengthened account, although their remarkable simplicity and strength render them highly interesting studies to the engineer, and will justify a brief notice here, by way of illustrating the value of the principle first suggested as applicable
*To this principle, and its practical value and effects, further inquiry should be devoted; we have here only to record the view entertained by the inventor of the lattice bridge, at a date so long before those experimental inquiries into the forces acting upon loaded beams which have developed the position of the neutral axis and the agency of Compressive and extensive forces.
to iron and wooden bridges, by Mr. Smart, twenty-five years
Some of the principal examples of the lattice bridge, in America, are built over the rivers, supporting roads and rail-ways. One of these, erected over the Susquehannah at Columbia, consists of twenty-nine spans or openings, each two hundred feet wide, the entire bridge being about a mile and a quarter in length. The principle on which this bridge is constructed has been more properly referred to that of the common roof; the two centre and opposite diagonal bars being considered as two rafters meeting at the centre of the bridge, and abutting at their other end on a tie-beam, which is extended longitudinally on each side to the opposite abutments. A series of rafters, parallel with the centre one, is extended on either side, throughout the whole length of the bridge, being secured at their feet, and also connected at the head with a horizontal upper beam, placed vertically over and parallel with the continuous tie-beam. These rafters are placed at such an angle of obliquity, and at such a distance from each other, that vertical posts or ties between them will unite the head of one rafter with the foot of the contiguous one, towards the centre of the bridge. These ties, which, the bridge being loaded on the lower chord, are subject to a tensile strain, have been recently formed of malleable-iron rods, instead of timber, which rods, fitted with screwed nuts, admit of being regulated in length, so that the whole structure may be brought to a perfect degree of tension, and each joint and each member made to bear its due share of the load: they moreover remedy the mischief of shrinkage of the timber, or other derangement, as the equilibrium and perfect form of the structure can by their means be readily restored and maintained. By screwing up these ties, the bridge tends to assume an arched form, rising with a camber in the middle this is prevented by the introduction of the counter-braces, which connect the head of one rafter with the foot of the contiguous one, from the centre toward the extremities of the bridge,
In the American bridges of 200 feet span, the following are the dimensions of the members: span, 200 feet; depth of frame throughout, 20 feet; top and bottom chord timbers, 10 × 25 inches; braces, in pairs, 7 inches square; tie-rods, in pairs, 2 inches diameter; counter-braces, single, each 71⁄2 inches square. One of these frames is placed on each side of the bridge, connected at the bottom by cross-beams, on which the planking of the roadway is laid.
One distinguishing advantage of this mode of construction is its simplicity; the braces and counter-braces being all cut exactly to the same length, and square on the ends, which simply rest in blocks attached to the top and bottom chords, and are without mortising or jointing in the members: the tie-rods pass through these blocks, and the whole structure is so simple, that it may be readily taken down, removed to another site, and re-erected with the utmost facility and precision.
A lattice bridge of wrought iron, erected across the line of the Dublin and Drogheda Railway, is 84 feet in clear span, and built over an excavation 36 feet in depth. The two lattice beams, set parallel to each other, and resting at each end on plain stone abutments built in the slope, are 10 feet deep, and formed of a series of flat bars of iron 2 inches wide and
inch thick, crossing at an angle of 45°. At a height of 5 feet 6 inches above the bottom edge, transverse bearers of angle iron are fixed, and upon these the planking for the roadway is fixed. To provide for deflection, the beams were constructed with a camber or curve upwards, from the ends to the centre, of 12 inches; but it has been found that the passage of heavy weights does not produce any sensible deflection. The total cost of this structure is said to have been £510.
An important distinction between the simple lattice or diagonal framing and the roof framing must, however, be carefully borne in mind. In the former, the strength is obtained by the connection of the bars at each intersection, while the abutting principle of the roof, which equally belongs to the roofmed bridges before described, is disregarded. The strain
is therefore borne wholly by the rivets or pins which pass through the crossing bars, and the effect of this strain is exhibited in the gradual loosening of the pins. The bars, too, it must be observed, are considerably weakened by the holes through the middle of them; and in wooden lattice bridges, fracture and failing of the material have often resulted. By way of remedying these defects, consequent upon the simple lattice principle, many of the large lattice bridges in use in America have been strengthened by the introduction of strong trussed frames within the lattice frames, or of strong arches of timber-work.
The lattice principle has been considerably improved upon in some bridges designed and built by Mr. R. B. Osborne, C.E., which consist of a top and bottom chord of malleable iron, with intermediate braces of cast iron in the form of rectangular tubes. This form of construction was introduced into the United States of America in the year 1844, since which time about a dozen have been constructed, varying in span from 30 to 90 feet. Girders, formed of diagonal bars of wrought iron, abutting against each other, with cast-iron transoms to support the pressure, while the wrought-iron bars are intended to furnish the tensile power, appear to have been introduced into France before the year 1844. By order of the Minister of Public Works, experiments were tried at Paris upon four girders constructed in this manner, and placed side by side, with a bearing of 74 feet 8 inches. With a load of 62 tons, the deflection of these girders was 1 inch; and on the removal of the load, after remaining on them for a month, they resumed their original position without permanent deflection. To try the effect of a sudden shock, a cart loaded with 4 tons of iron was caused to break down suddenly in the middle of the bridge, without producing any injury, except crushing the flooring planks. The weight of these girders was stated to be 201 tons.
A similar combination of cast and malleable iron in the construction of girders for bridges is the subject of a patent