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that were done, the tension to which they would he exposed would so far absorb their strength as to leave them little power of carrying any load beyond their own weight. On the other hand, by leaving them slack enough not to impair their strength, the difference of curvature and tension of the several chains, and their want of connexion and uniformity of action, cause the vibration to which they are exposed, from gusts of wind or the motion of carriages, to act unequally on the several chains; and the vibration is likely to be violent in proportion to the slackness of the chains. These reasons have probably led to the abandonment of this system for large suspension bridges. ... In suspension bridges of large dimensions, and consequently of great weight, the force that the suspended mass will acquire by being put in motion increases rapidly. Hence, it is an object to make it resist motion, and especially to make every part bear its fair share of strain. It is a common doctrine that lightness is the peculiar excellence of a suspension bridge; but that is a principle which must he acted upon with discretion, and not taken generally; for a bridge may be from its size just so heavy that, by being put in motion, it will acquire great momentum, and just so light and slight that it will be unable to resist the effects of its own vibration. Therefore, when it becomes necessary to make the chains of a bridge so heavy that vibration would be dangerous, it is advisable boldly to increase their weight, rather than attempt to diminish it, and to bind and connect the several chains and the roadway firmly together, in order that there may be sufficient mass and stiffness in the bridge to resist motion, rather than yield to it readily."—Drewry.

Mr. Robert Stevenson, in a paper on suspension bridges, inserted in the 5th Vol. of Jameson's Edinburgh New Philosophical Journal, says:—" The effect we have to provide against in bridges of suspension is not merely what is technically termed dead weight. A more powerful agent exists in the sndden impulses or jerking motion of the load, which we have partly noticed in the description of the Dry burgh bridge. The greatest trial, for example, which the timber bridge at Montrose, about 500 feet in extent, has been considered to withstand is the passing of a regiment of foot, marching in regular time. A troop of cavalry, on the contrary, does not produce corresponding effects, owing to the irregular step of the horses. The same observations apply to a crowd of persons walking promiscuously, or a drove of cattle, which counteract the undulating and rocking motion observed on some occasions at the bridge of Montrose, when infantry has been passing along it. Hence, also, the effects of gusts of wind, often and violently repeated, which destroy the equilibrinm of the parts of a bridge of suspension; and the importance of having the whole roadway and side-rails framed in the strongest possible manner."

The destruction of the Dryburgh bridge created a great sensation of regret throughout all parts of the country. Its utility when compared with a troublesome ferry, even in the short experience of six months.

had given it such a decided preference to the boat, that the Earl of Buchau, the owner, ordered it to be restored. This was accordingly done, after a better design, for the additional sum of 220/., (somewhat less than 500/. having been paid for the original structure, which the contractors agreed to uphold against all accidents only during the period of its erection,) and, in less than three months, it was again opened to the public. The bridge was reconstructed on the catenarian principle, the roadway being suspended by perpendicular rods of iron from main or catenarian chains. The chief mechanical alterations upon the former plan consisted in welding both eyes or ends of the links, instead of having one of them simply turned round, and fixed with a collar. The roadway was also strengthened by a strongly trussed wooden rail, which also answers the purpose of a parapet, on each side of the bridge, the good effects of which were particularly exemplified while the bridge was building. A high wind having occurred before the side-rails were erected, one end of the platform was lifted above the level of the roadway, and the undulating motion produced on this occasion is described as resembling a wave of the sea; an effect which pervaded the whole extent of the bridge, and went off with a jerking motion at the further end. But after the side-rails were attached, this vertical motion was checked, and afterwards greatly reduced. There were also added gyes or mooring-chains, consisting of rods of iron fixed to stakes in the opposite banks of the river, and attached to the beams of the roadway. These diagonal moorings are said to have some effect in lessening the motion of the bridge in high winds.

The plan proposed for Runcorn Bridge was, as already stated, to suspend the platform from iron cables, made of small bars, welded together at their ends, and bound up together, so as to form a long flexible bar or cable. That plan has not been adopted, the usual system being to make the chains of straight wrought-iron rods or bars, from 5 to 15 feet long, with either welded eyes or holes drilled out at their ends, by which they are connected together in pairs by short links and bolt-pins. This system was introduced by Captain Samuel Brown, R.N., who, in 1817, took out a patent for his invention. His reasons for preferring this mode of making the main chains to making them of links, like chain cables, or of small wires, were, that link chains are weaker in proportion to their weight than bars, a portion of the strength of the iron being lost in bending it into the form of a link; and small wires are inexpedient for large bridges, on account of the number of parts and complication of joinings required, and also on account of the great surface they expose to damp, and their consequent liability to destruction. The Union Bridge across the Tweed, five miles above Berwick, was the first large bar-chain bridge completed in this country. It was begun in August, 1819, and opened in July, 1820, just after the commencement of the Menai Bridge.

The straits of Menai, which separate the island of Anglesea from Caernarvonshire, had long formed a troublesome obstruction upon the great road from London to Dublin by Holyhead. The connexion between the two shores was maintained by six ferries, the most important of which was that about a mile north of certain rocks in the strait, called the Swellies, named Bangor Ferry, from its proximity to the city of Bangor. The idea had long been entertained of superseding the ferry by a permanent roadway; but no plan had been entertained until after the union of Ireland with England, when the increased intercourse between the two countries led the Government, in 1801, to direct Mr. Rennie to survey the strait, and prepare a plan and estimate for a bridge. Four designs were accordingly prepared for a cast-iron structure; and, while these were under consideration, a violent opposition was made to the erection of a bridge, by interested parties, and it was affirmed that the effect of a bridge would be to destroy the navigation of the strait. A parliamentary committee, however, reported that the bridge was desirable, and in no way likely to prove injurious; and they advised the immediate construction of the proposed bridge, with three cast-iron arches, over the Swellies. Upon this, the Lords of the Treasury directed Mr. Telford to survey the Holyhead Road, and to consider the best mode of passing the Menai Strait. This was done; and in April, 1811, Telford proposed a castiron bridge, of one arch, 500 feet span, 100 feet clear in height, at high spring tides, and 40 feet broad. As it was nearly impossible to construct the centering of the arch from below, on account of the nature of the bottom of the channel, and the great rise and rapidity of the tides, Telford proposed to suspend the centering from above.

This plan was strongly recommended for adoption by the parliamentary committee, in their report, in May, 1811; but the difficulties in which the country had beeu involved by the long protracted war abroad interfered with improvements at home, and nothing was done in the matter until 1818. In the interval, however, public attention had been drawn to the subject of suspension-bridges; and the improvements of the Holyhead Road being in active progress, in pursuance of an Act passed in 1815, under Telford's direction, he was ordered, in 1818, by Government, to give his opinion whether a suspension-bridge was practicable over the Menai Straits, and if so, to prepare a design. Telford accordingly surveyed the localities, and proposed a suspension-bridge at Ynys y Moch. Evidence was taken by the committee, at considerable length, on the plan proposed, and the opinions of engineers being favourable, a report was made, and 20,000/. voted by Parliament to enable the commissioners to commence operations; but, m consequence of the opposition of interested parties, the building of the bridge was delayed until July, 1819, when an Act was passed, giving proper powers for carrying on the erection of the bridge.

lVe come now to describe the finished structure.

F«t. Inches.

The distance apart of the centres of
the summits of the main pyra-
mids is ". 570 10J

The deflection of the chains in the
middle 43 0

The clear height of the roadway

above high water 102 0

Breadth of the platform ... 28 0

It is divided into two carriage-ways, one on each side of the bridge, 12 feet wide, and a central footpath, 4 feet wide.

The piers of suspension are pyramids, built of grey Anglesea marble. The Anglesea main pier stands on the rock Ynys y Moch, which was levelled by blasting to form a foundation, and rises to about the level of high water. On the Caernarvon side the foundation of the pier is sunk 7 feet below the surface of the beach, and is also firm rock. The main piers are 100 feet high, from the level of high water line up to the level of the roadway, and their summits are 53 feet above the roadway. The shape of the piers below the roadway is octagonal; their extreme breadth at the base 70 feet, and at the summit 45 feet. Their thickness at the base is 50 feet, at the level of the roadway 29 feet, and at the summit 11 feet. They are not built solid all the way up; but, in each, four hollow squares, about 91 feet square, were left, commencing above high-water mark, and running up to within about 4 feet of the level of the roadway. Through each main pier two arched openings are formed for the passage of carriages, 9 feet wide, and 15 feet high to the springing of the arches, the wall between them being a little more than 6 feet thick. The masonry above the openings is tied together by iron dowels, 1 inch diameter, and 12 inches long, put through holes drilled through each stone of every course. To prevent the walls of the arches for the carriage-roads being pressed out by the weight of the superincumbent part of the pyramid, 6 wrought-iron tie-bolts, 4 inches wide by 2 inches thick, are laid in the masonry at the springing of the arches, with dovetail heads at each end, let into corresponding sockets in cast-iron plates bedded in the masonry. The main piers are connected with the shores by a series of arches, three on the Caer . narvon side, and four on the Anglesea side. The height of each of the small piers, from high-water line to the springing of the arch, is 65 feet, and the span of each arch 521 fcet. A square chamber is left in each, filled up with rubble.

The main chains are on Capt. Brown's plan of straight bars, A, Fig. 319, united by coupling bolts. The main chains are 16 in number, each containing a series of links composed of 5 wrought-iron bars, 9 feet \\ inch long, 3i inches broad, and 1 inch thick. They are disposed, four chains one under the other, on each side of the central footpath, and four at eacn outside of the platform. So that there are in all 80 bars in the main chains, and their united section is 80 + 3^ = 260 square inches of iron. The bars are united by coupling plates, it, Fig. 319,16 inches long, 71 inches broad, by

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Fig. 31g.

chains are carried through tunnels cut through the solid rock of which the shores are composed, and the extreme ends arc fastened by strong holding bolts in chambers made at the ends of the tunnels. The holding bolts are 9 feet long and 6 inches diameter, and rest in sockets in cast-iron plates 6 inches thick. There are 12 holding bolts at each end of the bridge, viz. 4 for the two middle sets of chains, and 4 for each of the outer ones. The length of each bolt between its bearings against the rock is 18 inches. There are 3 tunnels for the chains, about 5 feet square, at each end of the bridge, viz. one for the two central sets of chains, and one for each of the outer ones. The bars of the chains in the tunnels are much stronger than the other bars, under the idea that in the tunnels they would be more exposed to rust, and less accessible for painting or varnishing them. The back stays are tied down, by vertical rods, to small cast-iron plates laid in the masonry of the arches between the main piers and the shores, to prevent their undulation.

Thus the main chains are fixed firmly at their extremities into the rock, and are also held down between the main pyramids and the shores: but over the pyramids they lie loosely upon cast-iron saddles, laid upon horizontal rollers or trucks, 3 feet 8 inches long and 8 inches diameter, that lie in grooves or channels formed in a cast-iron platform upon the summit of each main pier. The saddles can thus move backwards and forwards a few inches in the direction of the length of the chain, and hence expansion and contraction of the chains produce no other effect than to move the saddles on which they lie, just as much as they lengthen or shorten, and to raise or depress the suspended roadway a little, instead of producing an injurious strain on the materials. The rollers on which the saddles rest, are kept equidistant by their necks or projecting ends being received in brasses held between wrought-iron plates screwed together, so that no one roller can move alone; but there is sufficient space in the grooves in which the rollers are placed, for all of them to move as much as 4£ inches.

The chains are tied together across the breadth of the bridge by transverse ties, which are cast-iron tubes placed between each pair of chains, with wrought-iron

screw bolts put through them, and passing through the chains. The bolts serve to draw the chains together by their nuts, and the cast-iron tubes keep the chains a proper distance apart. There are 8 of these transverse ties in the length of the bridge. The chains are further tied together by a diagonal wrought-iron lacing between each pair of transverse ties. This mode of binding the chains was adopted in consequence of the effect perceived on them from the action of gusts of wind, while the bridge was in progress.

In the length of the chains there are 4 sets of adjusting links to every line of chains, viz. 2 in the opening between the piers, and 1 between each pier and the shore. The object was, to give the means of adjusting the chains to their proper length, in case there should be any inequality in the lengths of the bars. Fig. 320 is a perspective view of one of the adjusting links, and D E, Fig. 319, is a side view of the same. At one end of each of the bars a, a loug

[graphic]

hole is made instead of a round eye, and the corresponding end of the connecting plate b has also a long hole in it. The bar and connecting plate are connected by two semicircular pins, 1 and 2, one bearing against the ends of the bars a, and the other against the ends of the connecting plates b, Fig. 320, and E, Fig. 319. and the space between them is filled up with iron wedges, 3,3. If any one of these iron wedges be taken out, the ends of the bars a can be drawn away a little from the ends of the connecting plates b, and so the chain will be lengthened; or by driving in more wedges, the two semicircular pins, 1, 2, are separated further and the chains shortened. The bars c are connected to the other ends of the connecting plates b, by screwbolts, like the other bars of the chain. The whole distance that each adjusting link can be lengthened is 9j inches: so that as the chains can be lengthened 19 inches between the pyramids, and 9£ more between each pyramid and the shore, allowance is made for 38 inches in all.

The roadway is laid upon wrought-iron cross-bars or roadway bearers, suspended from the ends of the vertical suspending rods. The bearing bars are 3£ inches deep, and j inch thick, placed 1 inch asunder, and the space between them is filled up with a piece of deal. They are trussed with bent ties and struts underneath. The roadway consists of two thicknesses of fir plank, the lower one of 3 inches, the upper one of 2 inches, laid parallel with the length of the bridge, upon the iron cross bearers. The lower course is bolted to the wood that is riveted between the iron roadway bars, and is covered with patent felt saturated with boiled tar; the second course of planking is spiked through to the lower course. In the middle of each carriage-way a third course of planking, 2 inches thick, is spiked through to the second, through an intermediate layer of felting; and an oak guard is bolted on each side of the carriage-way through the two lower courses of planking. Also to stiffen the roadway, an oak plank is bolted to the underside of it between each of the cross bearers.

All the suspended iron-work of the bridge, before being put up, was proved by actual strain in a proving machine, at the rate of 11 tons strain per square inch.1

The chain-work was put up in the following manner. The chains were first put together in the tunnels, working up from the fastenings to the mouths of the tunnels, by bringing down one link of each chain at a time, and bolting it to the one previously brought down. The chains were then put together from the mouths of the tunnels up to the main piers, upon a scaffolding erected on the masonry between the pier and the shore, with the proper inclination for the back stays. The chains were then continued over the Caernarvon pier, and allowed to hang loosely down to the level of high water. This was done by suspending a cradle large enough for two men to sit in, from a crane-arm set up on the top of the pyramid. The cradle was suspended by tackle, so that the men could slack it down or haul it up, to raise or lower themselves at pleasure. The links of the chain that were to be joined on to the ends of the hanging chain, were brought in succession along the road to the . front, or the sea face of the pyramid, through the arched roadway opening. The link was then taken up by tackle from a pair of shears placed on the top of the pyramid, and lifted up to the height of the link it was to be fixed to, where the men in the cradle got hold of it and brought the two links together, and put the coupling bolt through them.

On the Anglesea side of the street the chains were carried just over their saddles on the top of the pier, and their ends were retained by tackle made fast to them, and descending thence to capstans on the shore. The remaining piece or length of

(1) The proving machine was similar to that used by Captain Brown. It consists of two rollers set up in bearings, one at each end of a long iron frame. On the axis of one of the rollers a strong lever projecting out horizontally is fixed; its end is connected, by a short vertical link, to the short end of a counter horizontal lever mounted on a fixed centre-pin; the long end of this lever fs loaded, the proportions of the levers being such, that a very moderate weight on the end of the counter lever, will exert great power to turn the roller round. A short vertical arm projects out below the circumference of the roller, to which a piece of chain is linked. One end of the bar to be proved, is fastened to the end of that chain, and the other end is linked to a chain attached to the opposite roller. This second roller is turned round by 2, 4 or 6 men, according to the size of the bar under proof, by means of a train of 3 wheels and 3 pinions, until the loaded rofler is pulled or turned round enough to raise up its balance weight, which is the measure of the strain the bar is intended to undergo. In the machine used at Capt. Brown's manufactory, the proportions of the two levers were such as to multiply the effect of the balance weight 221 times, and the train of wheelwork was so proportioned, that two men at the handles could exert a force of 30 tons on the bar under proof.

chain which was to unite the two ends of the chains brought up from each shore, was laid on a raft, 400 feet long, and 6 feet wide. One end of that piece of chain was first attached to the end that hung down to the water from the Caernarvon pier, and then the raft was floated across to the Anglesea pier, and the loose end of the chain upon it was fastened to the tackle which hung down from that pier. The tackle was then hauled up by the capstans fixed on the shore, and the chain was gradually raised off the raft until the end of it was brought in contact with the end of chain that hung over the top of the pier. The two loose ends were then bolted together and the operation completed. The first chain was thus raised and fixed on the 26th May, 1825, in 1 hour 35 minutes. The other 15 chains were successively got up in the same way at subsequent periods, the second chain being got up on the 28th April, and the fifteenth on the 28th June.

The hoisting of the first chain is thus described by Mr. Provis, the resident engineer:—" Whilst these operations were going on below, [i.e. securing the two ends of the chain at the Caernarvon pier,] the two capstans were manned by about 150 labourers, and everything above put in readiness. The word "go along" was then given, when a band of fifcrs struck up a lively tune, and the capstans were instantly in motion. As soon as each main fall or leading rope had coiled from the bottom to the top of its respective capstan barrel, the capstans were stopped, the clams screwed hard down upon the main falls, and held there till the ropes were shipped (or fleeted) down, from the tops to the bottoms of the barrels. The clams were then unscrewed, a fresh tune struck up, and the men resumed their task with renewed spirit. At first they had little to do but haul up the slack of the tackles, but when these were tightened, and the weight of the chain, which was gradually rising, began to be felt, the race which they had hitherto run at the capstans was reduced to a steady trot. Before the chain was lifted quite off the raft, the direction of the tide had changed, and the anchors being all on what was the up-stream side of the raft when it was moored, were of no use now in resisting the force of the contrary current. The raft was consequently forced from under the middle of the chain, which swung easily off into the water. Till this time scarcely a word had been heard but the directions to 'hold on' or 'go along,' but as soon as the chain was fairly suspended above the surface of the water, we were greeted with a hearty cheer from the surrounding crowd. As soon as the raft had drifted from under the chain, the anchors were weighed; the ropes which held to the ringbolts at the Caernarvonshire pier were slacked to give the raft sufficient play, and in a few minutes it was floated to its berth on the shore and secured in readiness for another chain." Many of the strangers assembled to witness the operation were eager to relieve the workmen at the capstans, "anxious to have it in their power to say they had helped to put up the first chain of the Menai Bridge." The whole operation was conducted in the most satisfactory manner. "Every man was at Lis 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 lond 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 up and walk over 30 or 40 feet of the middle 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."'

It is related in the appendix to the Autobiography of Telford, (London, 1833,) that after th6 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 radins, 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 I8 inches, but after

(1) "An Historical and Descriptive Account of the Suspension Bridge constructed over the Mcnai Strait," &c. By W. A. Provis, the Resident Engineer. London. 182a

wards it was not found to exceed 6 inches. The chains are scraped and painted once in 3 years. The longitndinal motion on the pyramids by expansion and contraction of the chains has been observed to the extent of 2 inches.

Tons Cwt. qrs. lbs.

The weight of the 16 main chains between the points of support, inclnding connecting plates, screw pins, wedges, &c. is computed at . . . 394 5 0 16

The transverse ties .... 3 16 2 20

The weight of the suspending rods and platform, viz. roadway bars, planking, side-rails, iron-work, &c 245 13 2 27

Making the total suspending weight 643 15 2 7

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

The entire section of the 80 bars is 80 x 3.25 = 260 square inches, which would bear without breaking 260 x 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 x 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 "4* * or about 732 £ tons besides its own weight.

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 magnitnde, 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 withachainmadeof some of the vertical suspending rods 1 inch square, with a chord line of 570 feet, and deflections varying from 35 feet to 4g 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 3,5gi) lbs.; hence the tension was 1.7 times the weight suspended.

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