Suspension bridge

“ Over this bridge of ropes, which is erected near Santa, travellers with loaded mules can pass in safety.
But suspension bridges composed of stronger and more durable materials than twisted fibres and tendrils of plants, are found to exist in these remote and semi-barbarous regions; in Thibet, as well as in China, many iron suspension bridges have been discovered, and it is no improbable conjecture that in countries so little known and visited by Europeans, others may exist, of which we have as yet received no accounts. ”

“ The defect of the suspension bridge is its flexibility. It can be stiffened by girders and bracing and is then of mixed type, when it loses much of its advantage in economy. Nevertheless, the stiffened suspension bridge will probably be the type adopted in future for very great spans. A bridge on this system has been projected at New York of 3200 ft. span.
The immense extension of railways since 1830 has involved the construction of an enormous number of bridges, and most of these are girder bridges, in which about half the superstructure is in tension and half in compression. ”

“ If we take the design of an arch bridge and turn it upside down we have a contour such as that of the Williamsburg Suspension Bridge, opened in 1903 between Brooklyn and Manhattan, depicted on page 33. For the utmost length this is the only available span; it brings into play the tensile strength of wire, the strongest form that steel can take. A steel cable of suitable diameter, if it had to support only itself, might safely be three miles long. A suspension bridge has another advantage in employing an anchorage to bear strains which would break down a simple truss resting on piers. ”

“ Suspension Bridges appear to be of very ancient origin: travellers have discovered their existence in South America, in China, in Thibet, and in the Indian peninsula. They are most frequently met with in mountainous regions, and being suspended across a deep ravine, or an impetuous torrent, permit the passage of the traveller where the construction of any other kind of bridge would be entirely impracticable. ”

“ The popular misconception as to suspension bridges is due to the many insufficiently stiffened structures of this kind. No other bridge system can be built so imperfectly stiffened, and yet be safe, as the suspension bridge. An erect arch bridge built in the same manner would fall of its own weight. ”

“ The idea of a suspension bridge was dismissed as inapplicable; a degree of rigidity and strength, greater than could be secured by any bridge constructed on the principle of suspension, being considered an indispensable condition of the proposed structure. ”

“ The Suspension Bridge, by being raised above the possible action of the water, is liable to none of the casualties that operate against the old bridge, whose piers and arches, though formed of solid masonry, are not proof against the powerful battering-rams formed by huge blocks of ice and heavy logs of wood, aided by the violent opposing force of the current. ”

“ The present Suspension Bridge which connects the South Stack rock with the Head, was formed in 1827. It is over a chasm 110 feet in width, and built on the same principle as the Menai Bridge, two chain cables passing across, firmly fixed in the rocks on either side, and carried over two massive stone pillars erected for the purpose. The chain supports the platform of timber five feet wide, and 70 feet above high-water mark. The crossing of the bridge to some will be a little trying to the nerves. ”

“ It is a vast evolution from these natural things to the art of Ulric Grubenmann, the forerunner of metal suspension bridges. [52] Unfortunately it is also an evolution which we cannot follow through many consecutive phases, artists and historians having failed to record its growth. We cannot suppose that the [Pg 145] ancients neglected suspension bridges; from the spider alone they must have learnt that pendent ropes made a good bridge; but we know not what they achieved in this airy handicraft. ”

“ There are 20 chains, each of which is 20 perches or 300 palms in length. When many persons pass over together, the bridge vibrates to and fro, affecting them with horror and giddiness, lest whilst passing it should be struck with ruin. It is impossible to admire sufficiently the dexterity of the architect Sinensius, who had the hardihood to attempt a work so arduous, and so conducive to the convenience of travelling.” Another suspension bridge in this country is described in the 6th vol. ”

“ When the Marquis of Worcester first proposed, by the boiling of water, to produce an effective force, no one could have conceived the incalculable advantages which have since followed its improvement by our illustrious countryman, Watt.”
A prophetic announcement, which has had its full realisation in the Suspension Railway Bridge of 821 feet span at Niagara Falls, and in the still bolder design now in execution for connecting New York and Brooklyn by a steel wire suspension bridge, having a clear opening between the piers of no less than 1600 feet. ”

“ Suspension bridges of large span have been generally considered as entirely unfit for railroad purposes; but John A. Roebling has proved the contrary by erecting a wire suspension railroad bridge of eight hundred feet clear span across Niagara River; which with heavy loads and violent gales has shown itself to be both stiff and strong to any desired amount. The construction of a bridge upon any other plan would have been hardly possible at the site of Mr. Roebling’s Niagara bridge, there being no opportunity for scaffolding or for piers, pontoons or hydraulic presses. ”

“ Roebling’s system of trussed suspension bridges. The cost of material alone in one span of the Britannia bridge, of 460 feet, exceeds the entire cost of the Niagara bridge of 800 feet span; add to this that we are sure of the strength of wire cables, but not of tubes, and that the 800 feet span of the Niagara bridge weighs only 1,000 tons in itself against 1,400 in a 460 feet span of tube, and it will not be difficult to prove the superiority of the suspension over the tubular system; thus,
A suspension bridge of 800 feet span costs $400,000. ”

“ After entering Yünnan we came to a substantial200 suspension-bridge, made of eight parallel iron chains, and with chain hand-rails; but the planks on it were strewn about in a desultory way, calculated to upset unseasoned travellers. At either end of the bridge was a solid block of masonry, surmounted by a temple through which the chains passed and were buried deep into the ground. These bridges are often a considerable length, and do not hang at a natural swing, but are drawn up as tightly as possible with hawsers; which, of course, makes a heavy strain upon the chains. ”

“ Railways have created an entirely new species of bridge, to enable a train to intersect a road, to cross canals in slanting directions, to turn amid jagged precipices, and to cross arms of the sea at a sufficient elevation not to interfere with the passage of ships,—objects not to be accomplished by suspension-bridges because of their oscillation, nor girder for lack of support, the desiderata being extensive span with rigid strength, so triumphantly realized in the tubular bridge. ”

“ They are in principle similar to those required for highways, but many of these great works would never have been undertaken for such traffic as is carried by a highway. The only railway suspension bridge ever built was the Niagara bridge, opened in 1855, and replaced by a steel arch in 1898. The development of bridges and traffic may be judged from the fact that the Victoria single-track tubular bridge over the St. Lawrence, at Montreal, which was opened in 1859, was replaced in 1897–98 by a double-track railway and roadway truss bridge on the same piers. ”

“ The last-named bridge is a remarkably elegant structure, thrown over the Spey at a point where the river, rushing obliquely against the lofty rock of Craig-Ellachie,* [11] has formed for itself a deep channel not exceeding fifty yards in breadth. Only a few years before, there had not been any provision for crossing this river at its lower parts except the very dangerous ferry at Fochabers. The Duke of Gordon had, however, erected a suspension bridge at that town, and the inconvenience was in a great measure removed. ”

“ It is for this reason that soldiers crossing a suspension bridge are often made to break step, lest the steady tramp of armed men should happen to set up a perilous state of vibration in the bridge. It is not untruthful to say that a boy with a pea-shooter could in time break down Charing Cross Railway Bridge over the Thames. If we suppose a pea shot against one of the sections of this iron bridge, there is no doubt that it would produce an infinitesimal displacement of the bridge. ”

“ The intermediate piers should also have considerable stability, so as to counterbalance the thrust arising when one arch is loaded while the other is free from load.
For suspension bridges the abutment forming the anchorage must be so designed as to be thoroughly stable under the greatest pull which the chains can exert. The piers require to be carried above the platform, and their design must be modified according to the type of suspension bridge adopted. When the resultant pressure is not vertical on the piers these must be constructed to meet the inclined pressure. ”

“ The draw would, therefore, require to be made at least three hundred feet long, from the face of the western abutment, to allow the passage of steamboats of the dimensions stated, in the several stages of water, from six to thirty feet in depth.
It is, in my opinion, impracticable to construct so large a draw in a suspension-bridge, because from its flexible character, and the constant change of position of its cables, which would be caused by the movement of a mass of so great weight as the draw, it would not admit of the adaptation of machinery for its movement. ”

“ In April 1858 he took out a patent (No. 771) for an improvement in suspension bridges, in which the roadway consisted of a rigid girder suspended at several points by inclined straight cliains, which carried the whole of the load and the weight of the bridge. ”

“ If the western channel of the river shall be made to afford an equally safe and unobstructed passage for boats, as the eastern channel, before the structure of the suspension-bridge, excepting the mere passage of the draw, and the increased distance, no appreciable injury is done to commerce. ”

“ In trail and road construction it is very often necessary to build bridges. Sometimes a very simple log bridge meets the need, but in bridging many large mountain torrents, which become very high and dangerous in the spring, large bridges are necessary. Cable suspension bridges and queen and king truss bridges are built where occasion arises for them, but only after being planned in detail and after the District Forester has approved their design and method of construction. ”

“ No bridge involves more simple calculations, and in none can we proceed with more absolute safety, than in the wire suspension. European suspension bridges are generally formed of cables made by linking bars of wrought iron together. This method is more expensive and more liable to failure than the American plan of forming cables of iron wire. ”

“ A suspension bridge cannot be built so high but that the chimneys of the boats will grow up till they cannot pass. The steamboat men will take pains to make them grow. The cars of a railroad cannot without immense expense rise high enough to get even with a suspension bridge or go low enough to get through a tunnel ”

“ Between the piers the weight of the structure, including the chains, amounts to nearly a thousand tons.
In all these suspension bridges, however large, the principles are much the same. The platform, or roadway, is hung from chains or cables, which pass over piers and are anchored fast at the ends. Some are stiffened with girders and bracing to prevent undue undulation. The chains take a graceful and definite curve, that of the Menai Bridge dipping fifty-seven feet. The strain is the greatest at the lower part, and is increased, should the chain be drawn flatter over the same space. ”

“ To introduce such an amount of material into the suspension bridge as would supply increased p. 361rigidity, would only be approximating to the original beam, and neutralizing any advantages in point of cheapness which might be derivable from this form of structure, without securing the essential stiffness and strength. Iron arches were also considered inapplicable, because of the large headway required for the passage of the ice in winter, and the necessity which existed for keeping the springing of the arches clear of the water-line. ”

“ Notches were cut in the staves to hold
Fig. 98. The Suspension Bridge. the ropes from slipping off. The flexible flooring thus constructed 100 was stretched across the river and secured to stakes driven firmly in the ground. A pair of parallel ropes were extended across the stream about three feet above the flooring, with which they were connected at intervals of five feet. The bridge was 25 feet long, and while rather shaky, owing to the fact that there were no braces to prevent it from swaying sidewise, still it was very strong and did excellent service. ”

“ Fig. 101.
Every part of a bridge not resisting some force is worse than useless, as it adds to the weight. Lightness not only increases the economy directly, but indirectly by removing a part of the permanent load.
222. Foremost in class number two stands Wendel Bollman’s Iron Suspension and trussed bridge. For simplicity of construction and directness of action, this bridge is unsurpassed. The weight at each post is transferred at once to the abutment or pier. The upper chord is of cast iron, hollow, octagonal without, and circular within. ”