TRANSPORTER BRIDGES

The Romance of Modern Mechanism by Archibald Williams is part of the HackerNoon Books Series. You can jump to any chapter in this book here. CHAPTER XIX

TRANSPORTER BRIDGES

When the writer was in Rouen, in 1898, two lofty iron towers were being constructed by the Seine: the one on the Quai du Havre, the other on the Quai Capelier, which borders the river on the side of the suburb St. Sever.

The towers rose so far towards the sky that one had to throw one's head very far back to watch the workmen perched on the summit of the framework. What were the towers for? They seemed much too slender for the piers of an ordinary suspension bridge fit to carry heavy traffic. An inquiry produced the information that they were the first instalment of a "transbordeur," or transporter bridge. What is a bridge of this kind?

Well, it may best be described as a very lofty suspension bridge, the girder of which is far above the water to allow the passage of masted ships. The suspended girder serves only as the run-way for a truck from which a travelling car hangs by stout steel ropes, the bottom of the car being but a few feet above the water. The truck is carried across from tower to tower, either by electric motors or by cables operated by steam-power.

The transporter bridge in a primitive form has existed for some centuries, but its present design is of very modern growth. With the increase of population has come an increased need for uninterrupted communication. Where rivers intervene they must be bridged, and we see a steady growth in the number of bridges in London, Paris, New York, and other large towns.

Unfortunately a bridge, while joining land to land, separates water from water, and the dislocation of river traffic might not be compensated by the conveniences given to land traffic. The Forth, Brooklyn, Saltash, and other bridges have, therefore, been built of such a height as to leave sufficient head-room under the girders for the masts of the tallest ships.

But what money they have cost! And even the Tower Bridge, with its hinged bascules, or leaves, and bridges with centres revolving horizontally, devour large sums.

Wanted, therefore, an efficient means of transport across a river which, though not costly to install, shall offer a good service and not impede river traffic.

Thirty years ago Mr. Charles Smith, a Hartlepool engineer, designed a bridge of the transporter type for crossing the Tees at Middlesbrough. The bridge was not built, because people feared that the towers would not stand the buffets of the north-easterly gales.

The idea promulgated by an Englishman was taken up by foreign engineers, who have erected bridges in Spain, Tunis, and France. So successful has this type of ferry-bridge proved, that it is now receiving recognition in the land of its birth, and at the present time transporter bridges are nearing completion in Wales and on the Mersey.

The first "transbordeur" built was that spanning the Nervion, a river flowing into the Bay of Biscay near [Bilbao, a Spanish town famous for the great deposits of iron ore close by. A pair of towers rises on each bank to a height of 240 feet, and carry a suspended trussed girder 530 feet long at a level of 150 feet above high-water mark. The car, giving accommodation for 200 passengers (it does not handle vehicles), hangs on the end of cables 130 feet long, and is propelled by a steam-engine situated in one of the towers. Motion is controlled by the car-conductor, who is connected electrically with the engine-room. The lofty towers are supported on the landward side by stout steel ropes firmly anchored in the ground. These ropes are carried over the girder in the familiar curve of the suspension bridge, and attached to it at regular intervals by vertical steel braces. The cost of the bridge—£32,000—compares favourably with that of any alternative non-traffic-blocking scheme, and the graceful, airy lines of the erection are by no means a blot on the landscape.

At Bizerta we find the third flying-ferry, which connects that town with Tunis, over a narrow channel between the Mediterranean Sea and two inland lakes. It replaced a steam-ferry which had done duty for about ten years.

The lakes being an anchorage for war vessels, it was imperative that any bridge over the straits should not interrupt free ingress and egress. This bridge has a span of 500 feet, and like that at Bilbao is worked by steam. Light as the structure appears, it has withstood a cyclone which did great damage in the neighbourhood. It is reported that the French Government has decided to remove the bridge to some other port, because its prominence would make it serve as a range-finder for an enemy's cannon in time of war. Its place would be taken either by a floating-bridge or by a submarine tunnel.

The Nantes "transporter" over the Loire differs from its fellows in one respect, viz. that it is built on the cantilever or balance principle. Instead of a single girder spanning the space between the towers, it has three girders, the two end ones being balanced on the towers and anchored at their landward extremities by vertical cables. The gap between them is bridged by a third girder of bow shape, which is stiff enough in itself to need no central support. The motive power is electricity.

All these structures will soon be eclipsed by two English bridges: the one over the Usk at Newport, Monmouthshire; the other over the Mersey and Manchester Ship Canal at Runcorn "Gap," where the river narrows to 1,200 feet.

The first of these has towers 250 feet high and 685 feet apart. The girders will give 170 feet head-room above high-water mark. Five hundred passengers will be able to travel at one time on the car, besides a number of road vehicles, and as the passage is calculated to take only one minute, the average velocity will exceed eight miles an hour. The cost has been set down at £65,000, or about one-thirtieth that of a suspension bridge, and one-third that of a bascule bridge. The bridge is being built by the French engineers responsible for the Rouen transbordeur.

Coming to the much more imposing Runcorn bridge we find even these figures exceeded. This span is 1,000 feet in length. The designer, Mr. John J. Webster, has already made a name with the Great Wheel which, at Earl's Court, London, has given many thousands of pleasure-seekers an aerial trip above the roofs of the metropolis. The following account by Mr. W. G. Archer in the Magazine of Commerce describes this mammoth of its kind in some detail:—

"The two main towers carrying the cables and the stiffening girders are built, one on the south side of the Ship Canal, and the other on the foreshore on the north bank of the river; and the approaches consist of new roadways, nearly flat, built between stone and concrete retaining walls as far as the water's edge, and a corrugated steel flooring, upon which are laid the timber blocks on concrete, resting on steel elliptical girders and cast-iron columns. The roadway in front of the towers is widened out to 70 feet, for marshalling the traffic, and for providing space for waiting-rooms, etc. The towers are constructed wholly of steel, rise 190 feet above high-water level, and are bolted firmly to the cast-iron cylinders below. Each tower consists of four legs, spaced 30 feet apart at the base, and each pair of towers are 70 feet apart, and are braced together with strong horizontal and diagonal frames. Each of the two main cables consists of 19 steel ropes bound together, each rope being built up of 127 wires 0·16 inches in diameter. The ends of the cable backstays are anchored into the solid rock on each side of the river, about 30 feet from the rock surface. The weight of the main cables is about 243 tons, and from them are suspended two longitudinal stiffening girders, 18 feet deep, and placed 35 feet apart horizontally, the underside of the girders being 82 feet above the level of high water.... Upon the lower flange of the stiffening girders are fixed the rails upon which runs the traveller, from which is suspended the car. The traveller is 77 feet long, and is carried by sixteen wheels on each rail. It is propelled by two electric motors of about 35 horse-power each.... The car will be capable of holding at one time four large wagons and 300 passengers, the latter being protected from the weather by a glazed shelter.... The time occupied by the car in crossing will be 214 minutes, so, allowing for the time spent in loading and unloading, it will be capable of making nine or ten trips per hour. This bridge, when completed, will have the largest span of any bridge in the United Kingdom designed for carrying road traffic, the clear space over the Mersey and Ship Canal being 1,000 feet.... The total cost of the structure, including Parliamentary expenses, will be about £150,000."

Mr. Archer adds that, in spite of prophecies of disastrous collisions between transporter cars and passing ships, there has up to date been no accident of any kind. To those in search of a new sensation the experience of skimming swiftly a few feet above the water may be recommended.

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