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that estuary the towns of Bangor and Chester. It was commenced in 1822, and completed in 1826, and differs but little in construction from that of the Menai Bridge. The chains, however, were put up in a different manner. A temporary rope bridge was made with the ropes used for the hoisting tackle at the Menai Bridge; and on that the chains were put together in the places intended for them; after which the ropes were slacked and the chains adjusted to their proper tension. The distance between the points of support is 327 feet, the deflection 22^ feet, and the height of the under side of the roadway above high-water of spring tides is 15 feet.
About the time when the Conway Bridge was begun, the Brighton Chain Pier was commenced by Captain Brown in October 1822, and was opened in November 1823. It runs out into the sea 1,014 feet from the face of the esplanade wall. The entire length of the bridge is 1,136 feet, in four openings, each of 255 feet span and 18 feet deflection. The extreme breadth of the platform is 13 feet, and the clear breadth 12 feet 8 inches. The suspension towers consist of pyramidal cast-iron frames, one on each side of the bridge, united by an arch at the top. They are 25 feet high, 10 feet apart, and weigh each about 15 tons. They stand on clumps of piles driven about 10 feet into the chalk rock that forms the bed of the sea, and rising 13 feet above high water. The clump of piles at the outer end or pier head is in the form of the letter T, and contains 150 perpendicular piles besides the diagonal piles, framed strongly together with walings and cross braces. On them a platform is laid 80 feet by 40, paved with a course of Purbeck granite. The three other clumps contain each 20 piles, besides the diagonal piles. The platform is supported by 4 chains on each side of the bridge, arranged two in breadth and two in depth. They are of wrought-iron round eyebolts about 2 inches in diameter, 10 feet long, and united by open coupling links and bolt-pins. They rest upon saddles on the upper part of the suspension towers. The chains on the land-side are carried over a suspension pier of masonry, and the back stays arc carried through two tunnels in the cliff and properly secured. The back stays at the pier head are fastened to the diagonal piles. The platform is suspended from the main chains by vertical suspending rods 1 inch in diameter and 5 feet apart, viz. one at every coupling of the chains. At their upper ends they are formed with a cross or T head, Figs. 321, 322, which is supported by a cap resting upon the ends of
rod is put upwards, and being turned round in it across the slit, is borne up by the cap as shown in Fig. 322. The lower ends of the vertical rods spread out into forks, which clasp over and are fastened by cross pins to a longitndinal side bearer made of bars of flat iron, bolted together and extending from tower to tower. On the side bearers arc laid cross joints 7\ inches deep by 3£ thick, and over these is laid a course of planking for the roadway. The platform is guarded and stiffened by an iron railing or parapet. Considerable difficulty was experienced in driving the piles, on account of the hardness of the bed of the sea, and the storms which are common on that coast. The work, however, was completed, and the pier continued during many years to render its useful services. It withstood the shock of many a violent tempest; but at length, in November 1836, it yielded to a gale of wind. The roadway of the pier gave way half an hour after mid-day of the 29th, about which time Osier's anemometer recorded the pressure caused by the wind's force at Birmingham as equal to 11£ pounds on the square foot. The barometer at Greenwich had sunk to 29.24; the wind's force there also being denoted by 11 £. There was a double motion in the pier, for both chains and roadway oscillated laterally, and undulated longitndinally; but the latter movement increased greatly, whilst the former diminished, just before the fracture took place.
tnc main chains and coupling links, one cap at every coupling. A square cavity or chamber is cast in the lower part of each of the caps, with an oblong slit or entrance, through which the T head of the vertical
/'' /. 873, TUR UR1GHTON l.UAiN-ribH, AfTkH THE kTUHU UV THE - 'iii NOV. 1836.
It was probably owing to this double motion that half the upper part of the roadway, and half the under part, were visible to the spectators at the same instant. As soon as the side-rails gave way, the undulations greatly increased, and almost immediately afterwards the roadway broke. It was remarked at the time that, had the side-railing been a trussed rail, the pier would probably have withstood the force of the storm. Fig. 323 is a sketeh of the ruins of the pier after the accident.
The suspension bridge at Montrose was blown up by the hurricane of the 11th October, 1838. Colonel Pasley, wno was sent to inspect the bridge, remarks, that "it was blown up from below; it being, like our English roofs, rather resting by its own weight than secured agamst hurricane action. The bridge at Montrose bad nothing to stiffen it longitudinally in a vertical direction. Iron transverse beams, supported by the rods, had two tiers of planking over them, and a light railing on each side, like that of a common baleony. The suspension bridge at Hammersmith, on the contrary, has railing of strong iron posts, and the rest of wood, on each side; and two longitudinal sets of king-post trusses on each side of the carriageway, and between it and the footpaths."
Mr. Ilendell was employed by Government to repair the chain bridge at Montrose. The manner in which he proposed to truss the bridge, to prevent a recurrence of the same misfortune, is here shown. Fig. 321 is a longitudinal section of the trussed rail, which,
Our limited space will not admit of any details respecting the many beautiful suspension bridges which have been erected in this country and elsewhere since the opening of the Menai Bridge. The Hammersmith Suspension Bridge, by Mr. W. T. Clarke, and the Hungerford Bridge, will repay an attentive study on the part of those interested professionally in these subjects. The former is described in Mr. Drewry's work, together with a number of other bridges, cither proposed or actually erected, up to the date of its publication, in 1832.
Some of the French engmeers nave preferred wire to bar-iron for suspension bridges. The reasons given by them for this preference are, that iron wire is stronger than bar-iron; that cables of wire can be put together more easily and rapidly than chains; that it is more easy to ascertain whether it be sound or not; and, that it is easier to get wire cables up into their places than bar-chains.
The facility of working wire, without heavy machinery, may render its use expedient in small structures; but it has many disadvantages for larger ones. First, as Mr. Drewry states, "although a single wire is stronger per square inch than a bar of iron, it is much to be doubted whether a cable made of wires is stronger than a bar-chain of equivalent dimensions, because of the inequality of tension in the several wires, which throws a greater share of strain on sonio than on others, and therefore reduces the effective length of the cable to that of a cable of less diameter. This inequality it is hardly possible to prevent, even if the wires are drawn, in making up the cables, to the same curvature that they are intended to have when in their places. Secondly, wires, exposing a greater surface than bars of equal section, are more quickly destroyed by oxidation. A coating of varnish, it is true, may somewhat preserve them; but bars may also be preserved by varnish and painting, and are still on that point superior to wire cables. Thirdly, wires are very apt to have kinks and bends in them, which cannot be got out without a very considerable strain; and when that has been done, it is difficult to ascertain whether the wire has not been permanently injured at that part. The long bends, also, that are formed frequently in wire can hardly be got rid of at all. . . . Lastly, although a small cable is very easily got up into its place, it is so, not because it is a cable, but because it is comparatively small and light."
In 1823, a wire bridge was erected over the double ditch of the fortifications at Geneva. The wires were of small size, laid together side by side, and bound up into small cables by wire wound spirally round them. It was erected under the direction of Colonel Dufour, an officer of engineers, at the cost of little more than 640/. The inner ditch is 109 feet wide, the outer one 75g feet wide, and the embankment or countcrguard between the two 82J feet wide. The breadth of each opening is 131 feet clear between the piers, which are of masonry. The main cables pass over the piers in grooves formed in bed stones on the tops of the piers: the edges of the grooves are rounded off where the cables come on them, and are covered with a thin brass plate. The ends of the cables at the tower end are attached to vertical bars, 7-J feet long, and If inch square, which descend close against the back of the pier; their lower ends are linked to horizontal bars laid on edge in the foundations of the piers. The back stays being thus carried down perpendicularly, this pier has to bear all the drag of the bridge to pull it over, and is made larger and stronger than the others in proportion. At the opposite end of the bridge, the cables are fastened to inclined bars. The cables are in several lengths, there being one l«ng cable across each opening, and three short ones over each pier. By varying the different lengths of the short connecting cables that lie on the tops of the piers, the main cables are adjusted. The ends of the long cables are fastened to the short ones by wire links, Fig. 327; loops at the ends of the cables are made like the loop of a cord, by simply turning the wires back upon themselves, and then binding the
washer a is interposed, to prevent the wire being bent too abruptly.
The platform consists of cross joints suspended from the main cables by short vertical cables, each containing 12 wires. The upper ends are looped round the main cables, and tlte ends tied round by a wire, in the way described of the loops at the end of the cables; and the lower ends are simply looped round the ends of the cross joints, and also tied. The parapet is 31 feet high, composed of small iron rods, with a top rail, and two other rails of flat iron. The parapet is inside the suspension wires, to guard them from injury. It is steadied by diagonal ties, about 25 feet from each end.
The load on the larger opening is thus caleulated :— 160 persons .... 23,205 lbs. Weight of bridge, platform, &c 15,912
Chains, &c 1,547
Chance load ... . 221
40,885 = 18£ tons.
The angle of direction of the chains is 19°, and the tension for that angle is about 1.5 times the weight = 27f tons. The ultimate strength of the cables is 1114 tons, and they might be safely loaded with one-third of that, or 37 tons, instead of 27f. The bridge v>brates very much from the great slack ness and lightness of the construction. It is well adapted for its purpose, being intended only for foot passengers; but it would be dangerous to allow it to be completely loaded, or to allow more than 50 men to march over it in step.
The most remarkable wire suspension bridge in Europe, on account of its dimensions and height, is that of Freyburg, in Switzerland, commenced by M. Chaley, of Lyons, in the spring of 1832, and opened to the public on the 23d August, 1834. This bridge has a span, from pier to pier, of 870 feet, and is suspended at the height of 167 feet above the river which flows under it. It is thus 319 feet longer than the Menai Bridge, and 65 feet higher. "It is supported on 4 cables of iron wire, each containing 1,056 wires, the united strength of which is capable of supporting three times the weight which the bridge will ever be Likely to bear, or three times the weight of two rows of waggons extending entirely across it. The cables enter the ground on each side obliquely for a considerable distance, and are then carried down vertical shafts cut in the rock, and filled with masonry, through which they pass, being attached at the extremity to enormous blocks of stone. The materials of which it is composed are almost exclusively Swiss. The iron came from Berne; the limestone masonry from the quarries of the Jura; the woodwork from the forests of Freyburg; the workmen were, with the exception of one man, natives, who had never seen such a bridge before. It was completed at an expense of about 25,000/. sterling, and, in 1834, was subjected to various severe trials, to prove its strength. First, 15 pieces of artillery, drawn by 50 horses, and accompanied by 300 people, passed over it at one time, and were collected in as close a body as possible, first on the centre and then at the two extremities, to try the effect of their concentrated weight. A depression of 39$ inches was thus produced in the part most weighed upon; but no sensible oscillation was occasioned. A few days after, the bridge was opened by the bishop and the authorities of the town, accompanied by about 2,000 persons, who passed over it twice in procession, preceded by a military band, and keeping step. On this occasion a slight horizontal vibration was produced; but it is very improbable that the bridge, in its ordinary service, will ever receive such a multitude at once. The passage of 2 or 3 heavy carriages or carts across it causes only the slightest pereeptible oscillation; and nothing is more extraordinary in this beautiful structure than the combination of stability with such apparent fragility."' We learn from the same authority that another wire bridge, 640 feet long and 317 high, has been suspended across the gorge of Gotteron. It was finished in 1840. The wire cables are attached immediately to the solid rock on each side, and the point of sus
pension is higher on one side than on the other, which gives it the appearance of half a bridge. The object of this mode of construction was economy, the expense of building piers of masonry from the bottom of the valley being thus saved.
But even the Freyburg Suspension Bridge has been excelled by a wire bridge recently constructed over the Niagara River in North America. It is built over the river at a point about If mile below the Falls, and directly over the fearful rapids which commence at this point. The bridge is nearly 800 feet in length, and is suspended 260 feet above the river. Upon the very edge of the precipice which bounds each shore of the river towers 80 feet high have been built, and at a point about 100 feet in their rear the immense wire cables vhich sustain the bridge are firmly secured. These strands pass from their fastenings immediately over the top of the tower upon either cliff; they pass thence across the chasm, and then over the top of the tower upon the opposite shore, in the rear of which the ends are fastened into the rocks. Two of these powerful cables, one on each side, support the bridge. "Stepping upon the bridge, before you walk 20 feet from the shore you find yourself suspended in the air several hundred feet above a mass of jagged and flinty rocks over and among which the waters of Niagara plunge with terrific velocity. To add to the sensations of terror which this fearful scene is caleulated to produce at first glance, you find the bridge oscillating and bending beneath your weight. It requires considerable nerve to cross this aerial structure, and there are few who have firmness enough to look over the side into the awful surf. The bridge is about 10 feet in width, and a temporary railing of wire and slats of wood has been constructed at each side. The flooring is composed of light planks resting upon thin scantlings, to which the wires are fastened." *
Mr. Dredge of Bath has adopted a new and elegant principle in the construction of suspension bridges, which promises to be of great use, both as regards thc quantity of material employed, and the stability and strength which it confers on the structure. In this invention tnc chains are made of sufficient mag
(1) Murray's "Handbook for Travellers ir Switzerland," &c. See also Penny Magazine, Nos. 279, 280
nitude and strength at the points of suspension, to support with safety the greatest permanent and contingent load to which they are ever likely to be exposed; and from thence they taper or diminish gradually to the middle of the bridge, where the strain becomes least. The suspending rods or bars that
(2) "New York Herald," quoted in the ' Athenxum," Sep. 30, 1348.
support the platform, instead of being nung vertically, or at right angles to the plane of the horizon, as is usually done, are inclined to it in angles which vary in magnitude from the abutments to the middle of the bridge, where the obliquity, as well as the stress upon the chains, attains its minimum value. "The principle developed by this obliquity of the suspending rods is singularly beautiful; but much judicious management is necessary on the part of the engineer, to fix upon that degree of obliquity which shall produce the greatest effect. Each bar is considered to perform its part in supporting the load, in proportion to its distance from the abutment, drawn into the sine of the angle of its direction, so that the entire scries of suspending bars transmits the same tension to the points of support as would be transmitted by a single bar reaching from thence to the middle of the bridge."'
Fig. 329 represents a portion of ftic bridge at Balloch Ferry, Loch Lomond, erected on this principle. Several of Mr. Dredge's bridges have also been erected in the Regent's Park, London.
Section VI.—Ox Iron, Girder, And Tubular Bridges.
The extensive employment of iron at the present day, is one of the consequences of the improvements which have taken place in its manufacture, as well as the increased facilities for transit afforded by canal navigation, roads, railroads, and other means of conveyance. It is scareely more than 70 years ago, that the first iron bridge was constructed in England, over the river Severn, near Coalbrook Dale. It consists
arehes is 55 feet. The form of this bridge is well adapted to the high banks of the Severn, at the place where it crosses. The bridge was built by Darby, but the design appears to have originated with Mi. Pritchard.
In 1787, Thomas Paine presented to the Academy of Sciences, at Paris, a model of an iron bridge constructed by him; and in the following year, during his residence at Rotherham in Yorkshire, a bridge chiefly of wrought-iron is said to have been erected, but was afterwards taken down.
In 1790 Mr. Rowland Burdon designed a cast-iron areh for the River Wear at Sunderland, on a peculiar
Fig, 331. IRON BRIDGE AT SUNDERLAND.
plan of construction, for which he obtained a patent. It consisted in " a certain mode or manner of making, uniting and applying cast-iron blocks, to be substituted in lieu of key-stones in the construction of arehes." In this way it was proposed to retain the common form and principles of the old stone areh. The Sunderland bridge consisted of six ribs, 200 feet in span with a rise of 30 feet. The total height from low-water level to the soffit of the areh is nearly 100 feet, and the design is peculiarly elegant and bold. The six ribs which form the areh are parallel to each other, and 6 feet apart. Each rib consists of 105 separate blocks or castings, 5 feet in depth, connected together with bars and collars of malleable iron. The ribs are braced together with cast-iron tubular braces and struts. The spandrils are filled in with cast-iron cireles, Fig. 332, meeting at their peripheries, and supporting the roadway, which is formed upon a strong timber frame, planked over and covered with a mixture of chalk and tar, upon which is placed a layer of marl, limestone and gravel. The bridge is 30 feet wide; the abutments are of stone founded on rock, "and are 24 feet thick, and from 37 to 42 feet wide. The iron work consists of 214 tons of cast, and 46 tons of malleable iron. The bridge was completed within three years, at a total cost of 26,000/. In October 1816, it was disposed of in a lottery for 30,000/. The confined situation of the site rendered it necessary to erect the bridge without interrupting the passage of