On the Theory of Suspension Bridges, with some account of their
early history. By Mr G. F. FORDHAM. Head at the Scientific
Society, March 12, 1840.
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. Humboldt
informs us that in South America there are numerous bridges of this kind
formed of ropes made from the fibrous parts of the roots of the American
agavey (_Agave Americana_). These ropes, which are three or four inches
in diameter, are attached on each bank to a clumsy framework composed
of the trunk of the _Schinus molle_; where, however, the banks are flat
and low, this framework raises the bridge so much above the ground as to
prevent it from being accessible. To remedy this inconvenience, steps
or ladders are in these cases placed at each extremity of the bridge,
by ascending which all who wish to pass over readily reach the roadway.
The roadway is formed by covering the ropes transversely with small
cylindrical pieces of bamboo. The bridge of Penipé erected over the
Chamboo is described as being 120 feet long and 8 feet broad, but there
are others which have much larger dimensions. A bridge of this kind will
generally remain in good condition 20 or 25 years, though some of the
ropes require renewing every 8 or 10 years. It is worthy of remark, as
evincing the high antiquity of these structures, that they are known to
have existed in South America long prior to the arrival of Europeans.
The utility of these bridges in mountainous countries is placed in a
striking point of view by the fact mentioned by Humboldt, of a permanent
communication having been established between Quito and Lima, by means of
a rope bridge of extraordinary length, after 40,000_l._ had been expended
in a fruitless attempt to build a stone bridge over a torrent which
rushes from the Cordilleras or the Andes. 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 most
remarkable bridge of this kind of which we have any knowledge in Thibet,
is the bridge of Chuka-cha-zum, stretched over the Tehintchieu river,
and situated about 18 miles from Murichom. Turner, in his Embassy to the
Court of Thibet, says, “Only one horse is admitted to go over it at a
time: it swings as you tread upon it, re-acting at the same time with a
force that impels you every step you take to quicken your pace. It may be
necessary to say, in explanation of its construction, that on the five
chains which support the platform, are placed several layers of strong
coarse mats of bamboo, loosely put down, so as to play with the swing of
the bridge; and that a fence on each side contributes to the security of
the passenger.” The date of the erection of this bridge is unknown to
the inhabitants of the country, and they even ascribe to it a fabulous
origin. The length of this bridge appears to be about 150 feet.
Turner describes in the following terms a bridge for foot passengers of
an extraordinary construction. “It was composed of two chains stretched
parallel to each other across the river, distant four feet from each
other, and on either side resting upon a pile of stones, raised upon each
bank about eight feet high; they were carried down with an easy slope
and buried in the rock, where, being fastened round a large stone, they
were confined by a quantity of broken rock heaped upon them. A plank
about eight inches broad hung longitudinally suspended across the river
by means of roots and creepers wound over the chains with a slackness
sufficient to allow the centre to sink to the depth of four feet below
the chains. This bridge, called Selo-cha-zum, measured, from one side of
the water to the other, 70 feet. The creepers are changed annually, and
the planks are all loose; so that if the creepers give way in any part,
they can be removed, and the particular part repaired without disturbing
the whole.”
Numerous suspension bridges formed of iron chains exist also in China;
and though the accounts which travellers have transmitted respecting
them are less detailed and explicit than would have been desirable,
descriptions of two of them have been furnished, which are sufficiently
minute and intelligible to excite considerable interest. The first to
which I refer is contained in Kircher’s China Illustrata. The following
is a translation of the author’s words: “In the province of Junnan, over
a valley of great depth, and through which a torrent of water runs with
great force and rapidity, a bridge is to be seen, said to have been
built by the Emperor Mingus, of the family of the Hamæ, in the year of
Christ 65, not constructed of brickwork, or of blocks of stone cemented
together, but of chains of beaten iron and hooks, so secured to rings
from both sides of the chasm, that it forms a bridge by planks placed
upon them. 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.
of the “Histoire Generale des Voyages.” The following is a translation:
“The famous _Iron_ Bridge (such is the name given to it) at Quay-Cheu, on
the road to Yun-Nan (Junnan?) is the work of an ancient Chinese general.
On the banks of the Pan-Ho, a torrent of inconsiderable breadth, but of
great depth, a large gateway has been formed between two massive pillars,
6 or 7 feet broad, and from 17 to 18 feet in height. From the two pillars
of the east depend four chains attached to large rings, which extend
to the two pillars of the west, and which being connected together
by smaller chains, assume in some measure the appearance of a net. On
this bridge of chains a number of very thick planks have been placed,
some means of connecting which, have been adopted in order to obtain a
continuous platform; but as a vacant space still remains between this
platform and the gateways and pillars, on account of the curve assumed
by the chains, especially when loaded, this defect has been remedied by
the aid of planking supported on trusses or consoles. On each side of
this planking small pilasters of wood have been erected, which support
a roof of the same material, the two extremities of which rest on the
pillars that stand on the banks of the river.” The writer proceeds to
remark, that “the Chinese have made several other bridges in imitation of
this. One, on the river Kin-cha-Hyang, in the ancient canton of Lo-Lo,
which belongs to the province of Yun-Nan, is particularly known. In the
province of Se-Chuen there are one or two others, which are sustained
only by ropes; but though of an inconsiderable size, they are so
unsteady and so little to be trusted that they cannot be crossed without
sensations of fear.”
While our attention is directed to early accounts and to the origin of
suspension bridges, it may be proper to remark, that although, as we have
seen, the inhabitants of the mountainous districts of South America,
or the wild and barbarous regions of Thibet, appear to have been well
acquainted with the purposes for which these structures are best adapted,
and to have practised their construction from the most remote ages,
neither the Greeks, the Romans, nor the Egyptians, according to all we
know of those nations, had any knowledge of their uses or properties,
or ever employed them as a means for crossing a river, or other natural
impediment. It is not, therefore, from these celebrated nations of
antiquity that the engineer has derived his first hints for the
construction of suspension bridges, but from those rude and unpolished
people, the results of whose ingenuity have just been described.
But it will now be interesting to inquire how far we can trace back the
antiquity of suspension bridges in more civilized countries--on the
Continent, in the British Isles, and in the United States of America.
Scamozzi speaks of suspension bridges existing in Europe in the beginning
of the seventeenth century, but it is very questionable if he employed
that term to designate the same structure to which it is now applied;
and this is rendered the more improbable, as no such bridges are now
in existence, and other writers are totally silent upon the subject.
It does not appear, then, that suspension bridges of other than recent
erection have existed on the Continent, and in England the oldest of
which we have any account has not been constructed more than a century.
The first suspension bridge in the United States was erected in the
year 1796. In England the oldest bridge of the kind is believed to have
been the Winch Chain Bridge, suspended over the Tees, and thus forming
a communication between the counties of Durham and York. Mr Stevenson
(Edinburgh Philosophical Journal) expresses his regret at not having
been able to learn the precise date of the erection of this bridge; from
good authority, however, he concludes it to be about the year 1741. It
may also be mentioned here, that at Carric-a-rede, near Ballintoy, in
Ireland, there is a rope bridge, which in 1800 was reported to have been
in use longer than the present generation could remember.
In the years 1816 and 1817, some _wire_ suspension bridges were executed
in Scotland, and, though not of great extent, are the first example of
this species of bridge architecture in Great Britain. As, however, full
descriptions of these bridges are to be met with elsewhere, it will not
be necessary to notice them further.
In 1818, Mr Telford was consulted by government as to the practicability
of erecting a suspension bridge over the Menai Strait, and was
commissioned to prepare a design, if upon an examination of the
localities he found the project feasible. Having accordingly surveyed the
spot, he was led to propose the construction of a suspension bridge near
Bangor Ferry, and in 1819 an act was obtained, authorising the erection
of the bridge, a sum of money having been previously voted by Parliament
for that purpose. This structure, which will always be regarded as a
monument of the engineering abilities of Telford, was commenced in August
1819, and opened to the public on the 30th January 1826, having occupied
six and a half years in its erection. The Union Bridge across the Tweed
was designed and executed by Captain Brown, and was the first bar chain
bridge of considerable size that was completed in this country. It was
commenced in August 1819, and finished in the month of July 1820. After
the completion of the Menai Bridge, bridges on the suspension principle
began to be universally adopted throughout Europe; but it was not till
_iron wires_ had been proved to be more _firm_ than bars _of a greater
thickness_ that these bridges received their most extensive applications.
Since 1821, Messrs Sequin have constructed more than fifty wire bridges
in France with the most complete success. The wire suspension bridge at
Freyburg, in Switzerland, the largest in the world, was erected by Mons.
Challey, and depends across the valley of the Sarine. It was commenced in
1831, and thrown open to the public in 1834. A suspension bridge has also
been erected at Montrose, the size of which is scarcely inferior to that
of the Menai Bridge. At Clifton a very large suspension bridge is now in
progress of erection by Mr Brunel, and a suspension bridge of 1600 feet
in length is about to be erected over the Danube, between Pest and Offen,
the design for which is the production of Mr W. Tierney Clark, and under
whose able superintendence its construction will be effected.--_Civil
Engineer and Architect’s Journal._