صور الصفحة
PDF
النشر الإلكتروني

chains are scraped and painted once in 3 years. The
longitudinal motion on the pyramids by expansion
and contraction of the chains has been observed to
the extent of 2 inches.

The weight of the 16 main
chains between the points of
support, including connect-
ing plates, screw pins, wedges,
&c. is computed at
The transverse ties
The weight of the suspending
rods and platform, viz. road-
way bars, planking, side-rails,
iron-work, &c.

Tons Cwt. qrs. Ibs.

394 5 0 16 3 16 2 20

operation was conducted in the most satisfactory | wards it was not found to exceed 6 inches. The manner. "Every man was at his post and anxious to do all he could to ensure success. Not the slightest accident, nor even a single blunder, occurred from the time of casting off the raft till the chain was secured in its place. Mr. Telford now ascended the pyramid to satisfy himself that all was right; and there, surrounded by his assistants, the contractors and as many as could find a place to stand on, received the congratulations of many a friend to the Bridge. The hats on the pyramid were soon off, the signal was understood by all around, and three cheers loud and long closed the labour of the day. The ceremony was scarcely over when one of the workmen got astride the chain, and passed himself over it to the opposite side of the Strait. Another followed soon after and actually had the temerity to raise himself and walk over 30 or 40 feet of the middle up of the chain, though the slightest slip must have sent him to destruction; the chain being only 9 inches wide, and its height at that time above the surface of the water not less than 120 feet." 1

It is related in the appendix to the Autobiography of Telford, (London, 1838,) that after the second chain was got up, a labourer, after finishing his day's work, sat himself down quietly on the centre of the curved part of the upper suspension chain, with his feet resting on the one below it, and in that position actually went through the regular operation of making a pair of small shoes in the short space of 2 hours: he afterwards sold the shoes for a, sovereign, and was led to suppose that they were purchased for public exhibition at the British Museum.

The hoisting tackle consisted of two ropes, each leading from a capstan fixed on the shore between 400 and 500 feet back from the pyramid, and there passed over two pulleys 14 inches diameter, fixed in a strong frame placed on the top of the pyramid. Hence the ropes passed through a block of four sheaves made fast to the end of the piece of chain on the raft. Hence the rope led up again through a three-sheaved block at the top of the pyramid, lashed to one of the chains of the back-stays, and lastly, through a single-sheaved block made fast to the end link of the chain hanging over the summit of the pyramid, to which the end brought up from the raft was to be attached. The barrels of the capstans were 1 foot 8 inches diameter; the axles about 4 inches diameter, worked by 8 spanners of 10 feet radius, and manned by about 150 men for the two.

The bridge was opened on the 30th January 1826, or 6 years after its commencement; several improvements and alterations were made during the summer months of 1826, in order to stiffen the chains and roadway. There is no perceptible transverse vibration of the roadway: the vertical undulation before the introduction of the transverse ties and diagonal lacing was about 18 inches, but after

(1) "An Historical and Descriptive Account of the Suspension Bridge constructed over the Ménai Strait," &c. By W. A. Provis, the Resident Engineer. London. 1828.

[ocr errors]

245 13 2 27

643 15 2 7

Making the total suspending weight Whence the tension of the iron at each point of suspension is by the weight of the bridge itself, according to experiment,' 1.7 times 643,, 15 „, 2 „, 7 = 1,094.42 tons.

The entire section of the 80 bars is 80 × 3.25 = 260 square inches, which would bear without breaking 260 × 27 = 7,020 tons.

=

According to Telford's experiments, about half the breaking strain, or 3,513 tons, would produce permanent elongation; and if we take the standard of 9 tons per square inch, the chains will bear constantly without any risk 9 × 260 2,340 tons or 2,340 1,094.42 = 1,245.5 tons of strain produced by the weight of the bridge itself. Therefore it may be perpetually loaded without any injury with 1245-5 or about 732 tons besides its own weight.

[ocr errors]

Such are the details of the Menai Suspension Bridge. Few works connected with the profession of a Civil Engineer have excited more interest, "for," as Mr. Provis remarks, "though the principle of its construction is old as the spider's web, the application on a scale of such magnitude, the durability of the materials of which the bridge is composed, and the scientific combination of its various parts, render it one of the noblest examples of British skill. As a public convenience too, it is of the highest importance; for instead of an uncertain ferry over an often tempestuous strait, at all times crossed with trouble and delay, and frequently at the risk of life, a commodious roadway has now been established between its shores, that can be passed at all times with safety and comfort."

The Conway Suspension Bridge was built to replace a ferry across the estuary between the town of Conway and the opposite shore that leads to Chester, and to connect by a safe and permanent road across

(2) Before suspending the main chains experiments were made by Mr. Rhodes to find the tension required to draw them up to the required curvature. They were tried with a chain made of some of the vertical suspending rods 1 inch square, with a chord line of 570 feet, and deflections varying from 35 feet to 19 feet. He found the tension of the chain at 43 feet deflection, viz. the intended deflection of the chains 6,134 lbs. The weight of the chain was 3,329 lbs.; hence the tension was 1.7 times the weight suspended.

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 are carried through two tunnels in the cliff and properly

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 longitudinal side bearer made of bars of flat iron, bolted together and extending from tower to tower. On the side bearers are 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 Osler'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 113. There was a double motion in the pier, for both chains and roadway oscillated laterally, and undulated longitudinally; but the latter movement increased greatly, whilst the former diminished, just before the fracture took place.

[graphic]

THE 29TH NOV. 1836.

secured. The back stays at the pier head are fastened Fig. 323. THE BRIGHTON CHAIN-PIER, AFTER THE STORM OF 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

[blocks in formation]

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 sketch 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, who 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 against hurricane action. The bridge at

Montrose had 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 balcony. 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. Rendell 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. 324 is a longitudinal section of the trussed rail, which,

Fig. 324.

it will be seen, passes below the bridge as well as above it; and Fig. 325 is a transverse section, showing cross-bracing at every 35 feet, below the roadway.

The facility of working wire, without heavy ma chinery, 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 some 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 6407. The inner ditch is 109 feet wide, the outer one 75 feet wide, and the embankment or counterguard between the two 82 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 Our limited space will not admit of any details are covered with a thin brass plate. The ends of the respecting the many beautiful suspension bridges cables at the tower end are attached to vertical bars, which have been erected in this country and elsewhere 7 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, either proposed or actually erected, up to the date of its publication, in 1832.

Fig. 325.

Some of the French engineers have 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.

feet long, and 13 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 beg one long 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 ci 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

[graphic]

Fig. 326. WIRE SUSPENSION BRIDGE OVER THE NIAGARA RIVER. loose ends of the cable together by a wire wound tight round them at a a. The cables I and L are

Fig. 327.

united by bolt-pins, made hollow, that they may be of large diameter without being heavy. Where the cables are looped round the ends of the iron bars for the back stays, b, Fig. 328, a small semicircular brass

Fig. 328.

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 the 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 34 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 tics, about 25 feet from each end.

The load on the larger opening is thus calculated:160 persons.

Weight of bridge, plat

form, &c. Chains, &c.

Chance load.

23,205 lbs.

[ocr errors]
[ocr errors]
[ocr errors]

15,912

1,547

[ocr errors]

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 =27 tons. The ultimate strength of the cables is 111 tons, and they might be safely loaded with one-third of that, or 37 tons, instead of 27%. The bridge vibrates very much from the great slackness 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

[ocr errors]

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 1 mile below the Falls, and directly over the fearful rapids which commence at this point. The bridge is nearly 800

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 ex-feet in length, and is suspended 260 feet above the pense of about 25,000l. sterling, and, in 1834, was river. Upon the very edge of the precipice which subjected to various severe trials, to prove its strength. bounds each shore of the river towers 80 feet high First, 15 pieces of artillery, drawn by 50 horses, and have been built, and at a point about 100 feet in accompanied by 300 people, passed over it at one their rear the immense wire cables which sustain the time, and were collected in as close a body as possible, bridge are firmly secured. These strands pass from first on the centre and then at the two extremities, their fastenings immediately over the top of the tower to try the effect of their concentrated weight. A upon either cliff; they pass thence across the depression of 39 inches was thus produced in the chasm, and then over the top of the tower upon part most weighed upon; but no sensible oscillation the opposite shore, in the rear of which the ends was occasioned. A few days after, the bridge was are fastened into the rocks. Two of these powerful opened by the bishop and the authorities of the town, cables, one on each side, support the bridge. "Stepaccompanied by about 2,000 persons, who passed over ping upon the bridge, before you walk 20 feet from it twice in procession, preceded by a military band, the shore you find yourself suspended in the air several and keeping step. On this occasion a slight horizontal hundred feet above a mass of jagged and flinty rocks vibration was produced; but it is very improbable over and among which the waters of Niagara plunge that the bridge, in its ordinary service, will ever with terrific velocity. To add to the sensations of receive such a multitude at once. The passage of 2 terror which this fearful scene is calculated to produce or 3 heavy carriages or carts across it causes only the at first glance, you find the bridge oscillating and slightest perceptible oscillation; and nothing is more bending beneath your weight. It requires considerextraordinary in this beautiful structure than the able nerve to cross this aerial structure, and there are combination of stability with such apparent fragility."1 few who have firmness enough to look over the side We learn from the same authority that another into the awful surf. The bridge is about 10 feet in wire bridge, 640 feet long and 317 high, has been sus- width, and a temporary railing of wire and slats of pended across the gorge of Gotteron. It was finished wood has been constructed at each side. The flooring in 1840. The wire cables are attached immediately is composed of light planks resting upon thin scantto the solid rock on each side, and the point of sus-lings, to which the wires are fastened." "

[graphic]

Fig. 329.

ISOMETRICAL VIEW OF ONE HALF OF A SUSPENSION BRIDGE ON MR. DREDGE S PRINCIPLE.

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 the quantity of material employed, and the stability and strength which it confers on the structure. In this invention the 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 Athenæum," Sep. 30,

1848.

« السابقةمتابعة »