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may be termed framed voussoirs, similar to the arehstones of a stone bridge,—a principle that has since been successfully adopted, both in timber and in iron bridges.
One of the best modern methods of construction is that of forming curved ribs for the support of the roadway. This method, introduced by Mr. Price, is thus described by Tredgold. "He proposes the curved rib to rise about one-sixth of the opening, and to divide it into a convenient number of equal parts, according to the span, or to suit the lengths of the timber. For a bridge of 36 feet span, he proposes to make the ribs of oak, in five lengths, and 3 inches in thickness; each rib to consist of two thicknesses, one 12 inches deep, and the other 9 inches deep; the joints crossed, and the thicknesses keyed together with wooden keys. Two of these ribs, with joists framed between, he says, will be sufficient for the roadway." This method of construction, which has been brought to considerable perfection in Germany, America, and other places, will be understood from Fig. 307, in which D E r are three beams of the arch,
consisting of logs of timber of small lengths, such as 10 or 12 feet, or such as can be found of a curvature suited to its place in the arch, without trimming it across the grain. Each beam is double, consisting of two logs, applied to each other side by side, breaking joint, as the workmen term it. They are kept together by wedges and keys, driven through them at short intervals, as at K, L, &c. The manner of joining and strongly binding the two side-pieces of each beam is as follows:—the
mortise aicb and dcio, which is cut into each halfbeam, is considerably larger on the outside than on the in, where the two mortises meet. Two keys, B B c c, are formed, each with a noteh, bed and aio, on its side, which noteh fits one end of the mortise. The inner side of the key is straight, but so formed that when both keys are in their places they have a space between them wider at one end than at the other. A wedge, A A, having the same taper as this space, is put in and driven hard, thus holding the two logs firmly together.
Colonel Douglass, in his work on "Military Bridges," notices the wooden arch, of 250 feet span, across the Portsmouth River, in North America. It was put together with wooden keys, on Price's method of construction, applied to a larger span, except that there is some difference in the form of the keys. The Colonel observes, that the arch is extremely flexible, and that diagonal braces would be an improvement in it. Also that if the three ribs had been placed close above one another, and firmly connected together, the bridge would have been much better adapted to resist any unequal load, because, in such case, they would have formed a solid beam, equal in depth to the sum of their depths. Tredgold remarks that it would have been still better to have made the same quantity of timber into two ribs, with cross ties and diagonal braces between them; that the method of connecting the parts by means of dovetail keys is objectionable, as the timber must be greatly weakened by such large mortises, and a very slight degree of shrinkage renders them useless; that it is still more objectionable as applied to the radial pieces, which would have been much better notehed on in pairs, and bolted through.
There are many excellent wooden bridges in Switzerland. One of the most celebrated was that at Schaffhauscn, constructed in 1757 by John Ulrich Grubcnmann, a village carpenter of Tuffen in Appenzel. It was composed of two arches, one 172 feet, the other 193 feet span, supported by abutments at the ends and by a stone pier in the middle, which remained when the stone bridge was swept away in 1754. In this bridge the oak beams which rested upon the masonry of the abutments and pier not having been properly seasoned, nor raised from the stone-work so as to allow the air to circulate around them, they rotted, and the frames began to settle. Grubenmann being dead, a carpenter of Schaffhauscn named Spengler, undertook in 1783 to supply a remedy. He raised the whole bridge by means of screw-jacks upon scaffolding supported by piles, and replaced the decayed timbers by others of better quality. This was the only repair required by the bridge during the 42 years of its existence. It was burnt by the French army in 1799. The principle of its construction is shown in Fig. 311. Its chief defect was that all the principal supports were so dependent upon one another that a single part could not be removed without first supporting the whole bridge. This bridge in common with others constructed on the same principle bent considerably sideways. Mr. Coxe says, that a man of the slightest weight felt this bridge almost tremble under him; yet waggons heavily laden passed over it without danger. It is frequently stated that the middle pier was not necessary as a support: the bridge however could not have borne its own weight without such assistance.
The general principles upon which timber bridges are constructed are very simple, and belong rather to Carpentry than to bridges. It will, however, be desirable to introduce a short notice of them here. If Ab, Kg. 308, be a solid beam resting upon the supports A and B so as to form a roadway, it will
- . ...v Fig. 309.
less effect in bending the beam than a similar strain in the middle. But if the weight be sufficient it will cause the beam to bend however it may be distributed, in which case the fibres at the upper side d will be compressed, and those on the lower side e extended. A line may however be drawn at the middle of the depth acb, where the Cbres remain in their natural state, being neither exteuded nor compressed. But all the fibres between c and d are compressed, and all those between c and e are stretched; but not equally so, because the nearer a fibre is to the points d or e the more it is strained. Now as the middle part of the beam is very little strained compared with the upper and lower side it is obvious that the same quantity of timber can'be employed more effectually by using a deeper beam and cutting out the middle, as in Kg. 310. On examining the forees exerted by the parts of the beam
very common form. It was adopted by Palladio in a bridge across the Brents. By dividing the span into shorter lengths than is here shown, little or no advantage is gained, because the angles of junction become more obtuse or open, and the strain in the direction of the pieces is much increased. Although such a bridge might bear a constant load, a load moving over it would soon derange it, because the strength of such a system to resist a variable load must depend wholly on the strength of the joinings, which cannot be made very strong. Bridges on this
rib at C,d and E; therefore when the strength of the rib is capable of sustaining the strains at c, D and E, and the curve is a proper curve of equilibrinm to the constant load, this combination is both secure and simple. The use of curved ribs of this kind was known at a very early period, and it has been further improved by bending the pieces that form the ribs. Many considerable structures on this principle have been erected, among which may be mentioned the bridge near Bamberg over the Regnitz, designed by Wiebeking, and built in 1809. Cast-iron bridges are constructed on nearly the same principle, as in Southwark Bridge, desigued by Mr. Rennie, consisting of three arches, the span of the centre arch being 236 feet.
As a bridgo with a curved rib and of considerable span yields at i>, c and E, Fig. 315, when the load is applied at the middle, the strength must be increased by increasing the depth of the rib. This leads to the construction of a framed rib, Fig. 316. But in
such case the two curved ribs must be continuous, and put together so as to resist either extension or compression: for when a load is placed at D, the lower rib will be extended to d and compressed at c and E; while the upper one will be compressed at D and extended at c and e; a weight applied at any other point would produce a similar effect. In bridges of large span framed ribs greatly add to stability. The framed voussoirs by Palladio, Fig. 306, resemble this kind of construction, but they are better adapted for iron bridges, where the parts cannot be very firmly united, in consequence of the expansion and contraction of the material according to variations in temperature; but in timber, where nothing is to be feared from this cause, it is, as Tredgold well remarks, iosing one of the greatest advantages of the material to interrupt the connexion of the parts: besides, many joints should be avoided, on account of the difficulty of making them fit so as to bring every
as is shown in Fig. 317. Where the width of the bridge is considerable a rib may rise in the middle of the width so as to divide the roadway into two parts, or a double rib with a footway between. As cross ties will be required at the top, the middle parts may be covered with a roof to protect them. A continued coping a a a' a' might be put over each truss, which would improve the appearance and protect the framing.
By the application of some of the simple combinations thus noticed, the carpenter will be able to span any opening of moderate dimensions.
Wiebeking states that a rise of 1 in 24 is a convenient ascent for a bridge; that in timber bridges the settlement is generally about 1 part in 72; that is, if a timber bridge of 144 feet span rise one foot in the middle when first framed, it will settle so as to become nearly horizontal; so that when it is intended for the bridge to have an ascent of 1 in 24 when finished, it must be framed so as to have a rise of 1 in 18; for Tv = yi + 7^
Section V.—Suspension Bridges.
A suspension bridge derives its chief value from the fact that it is independent of the bed of the river which it crosses. Hence it can be thrown across an opening where it is impossible, cither from the rapidity of the current or from the altitnde of the banks, to erect centering for a stone bridge. It can also be built with ease and expedition; and as it requires only a small amount of materials, it is economical. There is also an elegant lightness about a suspension bridge; it is however on this account much slighter than stone or iron bridges, and there is probably no suspension bridge that would bear permanently the load that is passing every hour over London Bridge. "A bridge destined to be a great and perpetual thoroughfare, exposed not only to be frequently quite filled with people, and to the passage of troops, but also to the rapid motion of great numbers of heavy vehicles; in short, a bridge in a busy part of a great city, ought not to be on the suspension principle. For if it were made no stronger than our strongest suspension bridges, it would not possess sufficient stability. If on the other hand the strength were increased to a sufficient extent to enable it to bear safely its constant work, tho weight, the difficulty of getting up the chains, and the increase in the masonry part, would so raise the expense, that it is doubtful how far it could be brought under that of a stone or cast-iron bridge. Add to which, a suspension bridge would never equal in stability a common arch bridge, because it is subject to vibrations, the law of which is not sufficiently known to caleulate their precise results in practice, but which certainly are more dangerous in a heavy bridge than in a light one. The object therefore in building a suspension bridge, is either to make it so light that its own vibration shall not hurt it; or if, as in nine cases out of ten, that cannot be done, then to make it so heavy and stiff, in proportion to the load it will have to carry, that the load shall not cause it to vibrate much. This, for a bridge liable to be constantly loaded with as much as it could contain, would be impracticable."' For large openings where the traffic is small, suspension bridges are well adapted, because they can be carried to almost any span and any height, for a comparatively moderate expense. They are also well fitted for military bridges; the chains, or cables, the platform and even timbers being ready prepared to frame suspension piers, might be carried more conveniently than a pontoon bridge, and could be rigged up for use in a short time. They arc also well adapted for crossing chasms in mountainous countries; and on a great military pass, a bridge of this kind would give the inhabitants greater command over it: for by knocking out a few connecting bolts, a whole bridge might be dismantled very rapidly without being destroyed. For piers or jetties on the sea-coast they are well adapted, from the openness of their construction. "If the suspension towers are founded on piles, and themselves made of strong but open framework, and if the chains and platform arc properly combined to get as much stiffness with as little weight as possible, so that they may resist vibration, without being so heavy as to be endangered by the vibration they cannot resist, a suspension pier may be buried in the waves without being hurt."
The bridges of ropes, &c. mentioned at the beginning of this article, arc true suspension bridges in their rndest form. They are very numerous in various parts of the world. In China, where the germ of nearly every thing connected with the Useful Arts is found, suspension bridges are formed of five parallel chains with links one foot in diameter, on which a loose bamboo flooring is laid. Another form is described as consisting of two parallel chains 4 feet apart, suspended over stone piers about 8 feet high on each bank. The ends of the chains pass back from thence, turn obliquely, and are bedded in the rock, each being fastened round a large stone, which is kept down by a mass of smaller stones laid upon it. A plank about 8 inches wide, extending across the river, is suspended from the chains by bands made of roots, of such length that the path is 4 feet below the chains in the middle of the length of the bridge. The suspending bands are renewed every year, and the planks are loose, so that any part can be repaired separately. The length of one of these bridges is described as being 59 feet. It is only used for foot-passengers; but it is a proper
(1) "A Memoir on Suspension Bridges," &c. by Charles Stewart Prewry. London, 1832.
suspension bridge, with a horizontal platform suspended from the main chains.
Bridges of rope or cordage arc described in works on Military Engineering as early as the year 1631, and perhaps earlier, but there does not seem to be any account of the existence of iron suspension bridges in Europe before the middle of the last century. About the year 1741, the first European chain-bridge was built in England across the Tees, two miles above Middleton, chiefly for the use of the miners of that district. The length was 70 feet, the breadth rather more than 2 feet, and the height above the water 60 feet. It appears to have been a very rnde work, not superior to the Chinese chain-bridges. It attracted no attention at the time, nor does the construction of suspension bridges appear to have occupied the attention of English engineers until 1814. In America however iron suspension bridges were erected as early as 1796, by Mr. Finlay, who, in 1801, took out a patent for their construction. In 1811, eight bridges had been built on Friday's plan.
In 1814, a plan was advanced by Mr. Dumbell of Warrington, for making a direct road from Runcorn in Cheshire, across the Mersey to Liverpool. The scheme inclnded a bridge instead of a ferry across Runcorn-gap, and it was suggested to streteh across the river a web of metallic rings. From the nature of the navigation, it was necessary that any bridge to be constructed should consist of not more than three openings, the centre one of 1,000 feet, and the other two of 500 feet each, with a height of at least 70 feet under the bridge above high water. These conditions being submitted to Telford, he proposed an iron suspension bridge to consist of 16 iron cables, each formed of 36 square half-inch iron bars, and of the segments of cylinders proper for forming them into one immense cylindrical iron cable, which was to be nearly half a mile long, inclnding the fixings on shore, and about 4£ inches diameter. The half-inch bars as well as the four segments were to be welded together into so many lines of bars, and laid in a bundle together, secured by bucklings every 5 feet, and wrapped in flannel saturated with a composition of rosin and beeswax, to preserve them from the weather. They were further to be bound together with wire of about -fa th inch diameter. The roadways were to be suspended from 16 of these cables, and to consist of two carriage ways and one central footpath. The main suspension piers of the middle opening were to be about 140 feet high, and the deflection of the cables in the middle jSj-th of the opening or 50 feet.
At the time this bridge was proposed, there was but little experience on the construction of suspension bridges; and in order to obtain data for proportioning the strength of the parts, Telford undertook a course of experiments in 1814 upon the tenacity of malleable iron. "The inquiry was commenced by proving what force would tear asunder lengthways pieces of iron from 1-J- inch to -fa inch in diameter. The experiments upon the first or larger diameters were performed with great accuracy, with an excellent hydrostatic machine, constructed by Mr. Fuller, at Mr. Brunton's patent chain-cable manufactory, Commercial Road, London; those upon the smaller diameters were made by weights attached perpendicularly, and sundry times repeated. Having ascertained their powers when suspended perpendicularly, I next made experiments upon different diameters from ^ to ^ of an inch, drawn horizontally and with different degrees of curvature: all these were done between points 900, 225, 140, 131£ feet apart, and repeated sundry times upon the different distances and diameters. The number of experiments in all amounts to above 200.
"In the experiments made upon -^ inch and under, the wire was drawn over pulleys; sometimes both ends were fixed, and sometimes one end only, the other having weights attached perpendicularly, to show their effects when compared with those loaded upon the curved part of the wire: these last were disposed at the ^, i, and $ divisions of the distance over which it was stretehed. Having completed these experiments, I next proved what force by a blow would break the wire when stretehed nearly horizontal, and at different curvatures, which was done by droppmg weights from a given height. The several wires upon which these experiments were made were weighed, and the weight of 100 feet in length of each noted.
"The results of these experiments were, that a bar of good malleable charcoal iron one inch square, will suspend 27 tons, and that an iron wire -fo inch diameter, (100 feet in length, weighing 3 lbs. 3 oz.) will suspend 700 lbs.; and that the latter with a curvature or versed sine of j^th part of the chord line, will support -foth part of the weight suspended perpendicularly, when disposed equally at $, j, and $ of its length; and with a curvature of ^th of the chord, it will bear -Jd of the aforesaid perpendicular weight, disposed in a similar way. Experiments upon the other diameters correspond sufficiently, and it was found that increasing.the distance only varied the effect by the difference of the weight of metal contained in the wire employed.
"A wire ^ inch in diameter, drawn very tignt between points 32£ feet apart, resisted the impulse of a 20 pound weight falling from a height of 7-J feet. Several other experiments were tried with lesser weights upon this and smaller wires.
"From these experiments I had reason to be satis fled that English iron made with wood charcoal, had sufficient tenacity to bear itself, and a portion to spare equal to the purposes of a bridge across an opening of 1,000 feet, and therefore considered myself justified in proceeding to form designs."
The project for the Runcorn bridge was not carried out on account of the great expense of the undertaking; but it served to direct the attention of engineers to the subject, and may thus be regarded as the origin of the great extension that has since been given to the suspension mode of bridge-building. Between the time here referred to, and the erection of the Menai Suspension Bridge, several small suspension bridges had been erected in Great Britain.
The first was built in 1816, by Mr. Lees at Galashicl, across Gala Water, to establish a communication between different parts of his manufactory. It was made of slender wires: the span was 111 feet, and the cost about 40/. Another bridge of iron wires was built in 1817 across the Tweed at King's Meadows. It is 110 feet long, and 4 feet wide. It was constructed by Messrs. Redpath and Brown, of Edinburgh, and cost about 160/. It is supported by two hollow cast-iron columns 9 feet high, 8 inches diameter, and $ inch thick in the metal, set on the opposite banks of the river 4 feet apart: in each column is placed a wrought-iron bar 10 feet high, 2£ inches square; and the suspending wires are attached each separately to these bars by screw-bolts 1 inch diameter. The lower ends of the cast-iron columns stand on a framed wood foundation. The roadway is formed of wrought-iron frames, to which deal planks 1^ inch thick are bolted. The side railing is of wrought-iron rods: the suspending wires are about -nrths of an inch diameter, and are not arranged in a catenary, but one end of each is fastened to its supporting bar, and the other end is fastened to a part of the roadway, so that the wires are a set of inclined lines forming different angles with the vertical supporting bars, Fig. 318. When the wires are
drawn tight by the screw-bolts the roadway is said to have very little motion. The strength of this bridge was tried shortly after its erection, by filling it entirely with a crowd of people, and it received no injury.
The system of inclined wires was adopted in a bridge built in 1817, at Dryburgh Abbey, across the Tweed, by Messrs. Smith. It was 260 feet span, and 4 feet wide. It was observed that the bridge had a very sensible vibration when crossed by any person, and the motion of the chains appeared to be easily accelerated. In 1818, six months after the completion of the bridge, a violent gale of wind caused so great a vibration of the chains that the longest chains broke, the platform was carried away, and the whole bridge destroyed. The vertical motion of the roadway was said to be, just before breaking, as great as the horizontal motion, and sufficiently violent to have thrown a person off the bridge.
"The defect of this plan of supporting a suspended roadway by inclined chains, fastened at the ends to the platform, is, that the chains, being of different lengths and inclinations, form different curves, and are exposed to very different degrees of tension; the long ones being the most strained, and the shortest the least. In a bridge of large span, and made with chains of any considerable weight, it would be almost impracticable to strain the inclined chains (especially the long ones) quite, or even nearly, tight; for if