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.
[Illustration: THE LATEST TYPE OF BRIDGE
The Transporter Bridge at Bizerta, Tunis. It has a span of 500 feet,
and the suspension girder is 120 feet above high water, so that the
largest vessels may pass under it from the Mediterranean to the inland
lakes. The car is seen near the bottom of the right-hand tower.]
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.
The second "transbordeur" is that of Rouen, already referred to. Its
span is rather less--467 feet--but the suspension girder lies higher
by 14 feet. The car is 42 feet long by 36 broad, and weighs, with a
full load, 60 tons. A passage, which occupies 55 seconds, costs one
penny first class, one halfpenny second class; while a vehicle and
horses pay 2-1/2d. to 4d., according to weight. The car is propelled by
electricity, under the control of a man in the conning-tower perched on
the roof.
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 2-1/4 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.