across the Menai Straits, is perhaps the most celebrated example of an
iron bridge on the girder principle. It was designed by Stephenson, but
the late Sir W. Fairbairn contributed largely by his knowledge of iron
to the success of the undertaking, if he did not, in fact, propose the
actual form of the tubes. Stephenson fixed upon a site about a mile
south of Telford’s great suspension bridge, because there occurred at
this point a rock in the centre of the stream, well adapted for the
foundation of a tower. This rock, which rises 10 ft. above the low-water
level, is covered at high water to about the same depth. On this is
built the central tower of the bridge, 460 ft. from the shore on either
side, where rises another tower, and at a distance from each of these of
230 ft. is a continuous embankment of stone, 176 ft. long. The towers
and abutments are built with slightly sloping sides, the base of the
central or Britannia tower being 62 ft. by 52 ft., the width at the
level where the tubes pass through it, a height of 102 ft., being
reduced by the tapering form to 55 ft. The total height of the central
tower is 230 ft. from its rock foundation. The parapet walls of the
abutments are terminated with pedestals, the summits of which are
decorated by huge lions, looking landwards. As each line of rails has a
separate tube, there are four tubes 460 ft. long for the central spans,
and four 230 ft. long for the shorter spans at each end of the bridge.
Each line of rails, in fact, traverses a continuous tube 1,513 ft. in
length, supported at intervals by the towers and abutments. The four
longer tubes were built up on the shore, and were floated on pontoons to
their positions between the towers, and raised to the required elevation
by powerful hydraulic machinery. The external height of each tube at the
central tower is 30 ft., but the bottom line forms a parabolic curve,
and the other extremities of the tubes are reduced to a height of 22¾
ft. The width outside is 14 ft. 8 in. Fig. 144 shows the construction of
the tube, and it will be observed that the top and bottom are cellular,
each of the top cells, or tubes, being 1 ft. 9 in. wide, and each of the
bottom ones 2 ft. 4 in. The vertical framing of the tube consists
essentially of bars of ⟙-iron, which are bent at the top and bottom, and
run along the top and bottom cells for about 2 ft. The covering of the
tubes is formed of plates of wrought iron, rivetted to ⟙- and ∟-shaped
ribs. The thickness of the plates is varied in different parts from ½
in. to ¾ in. The plates vary also in their length and width in the
different parts of the tubes, some being 6 ft. by 1¾ ft., and others 12
ft. by 2 ft. 4 in. The joints are not made by overlapping the plates,
but are all what are termed _butt_ joints, that is, the plates meet edge
to edge, and along the juncture a bar of ⟙-iron is rivetted on each
side, thus: ╬. The cells are also formed of iron plates, bolted together
by ∟-shaped iron bars at the angles. The rails rest on longitudinal
timber sleepers, which are well secured by angle-iron to the ⟙-ribs of
the framing forming the lower cells. More than two millions of rivets
were used in the work, and all the holes for them, of which there are
seven millions, were punched by special machinery. The rivets being
inserted while red hot, and hammered up, the contraction which took
place as they cooled drew all the plates and ribs very firmly together.
In the construction of the tubes no less than 83 miles of angle-iron
were employed, and the number of separate bars and plates is said to be
about 186,000. The expansion and contraction which take place in all
materials by change of temperature had also to be provided for in the
mode of supporting the tubes themselves. This was accomplished by
causing the tubes, where they pass through the towers, to rest upon a
series of rollers, 6 in. in diameter, and these were arranged in sets of
twenty-two, one set being required for each side of each tube, so that
in all thirty-two sets were needed. There are other ingenious
arrangements for the same purpose at the ends of the tubes resting on
the abutments, which are supported on balls of gun-metal, 6 in. in
diameter, so that they may be free to move in any manner which the
contractions and expansions of the huge tubes may require. Each of the
tubes, from end to end of the bridge, contains 5,250 tons of iron. The
mode in which these ponderous masses were raised into their elevated
position is described in the article on “Hydraulic Power,” as it
furnishes a very striking illustration of the utility and convenience of
that contrivance. The foundation-stone of the central tower was laid in
May, 1846, and the bridge was opened in October, 1850. The tubes have
some very curious acoustic properties: for example, the sound of a
pistol-shot is repeated about half a dozen times by the echoes, and the
tubular cells, which extend from one end of the bridge to the other,
were used by the workmen engaged in the erection as speaking-tubes. It
is said that a conversation may thus be carried on with a person at the
other end of the bridge, a distance of a quarter of a mile. The rigidity
of the great tubes is truly wonderful. A very heavy train, or the
strongest gale, produces deflections in the centre, vertical and
horizontal respectively, of less than one inch. But when ten or a dozen
men are placed so that they can press against the sides of the tube,
they are able, by timing their efforts so as to agree with the period of
oscillation proper to the tube, to cause it to swing through a distance
of 1¼ in.—an illustration of facts of great importance in mechanics,
showing that even the most strongly built iron structure has its own
proper period of oscillation as much as the most slender stretched wire,
and that comparatively small impulses can, by being isochronous with the
period of oscillation, accumulate, as it were, and produce powerful
effects. Bridges are often tried by causing soldiers to march over them,
and such regulated movements form the severest test of the freedom of
the structures from dangerous oscillation. The main tubes of the
Britannia Bridge make sixty-seven vibrations per minute. The expansion
and contraction occurring each day show a range of from ½ in. to 3 in.
The total cost of the structure was £601,865.
A stupendous tubular bridge has also been built over the St. Lawrence at
Montreal, and the special difficulties which attended its construction
render it perhaps unsurpassed as a specimen of engineering skill. The
magnitude of the undertaking may be judged of from the following
dimensions: Total length of the Victoria Bridge, Montreal, 9,144 ft., or
1¾ miles; length of tubes, 6,592 ft., or 1¼ miles: weight of iron in the
tubes, 9,044 tons; area of the surface of the ironwork, 32 acres; number
of piers, 24, with 25 spans between the piers, each from 242 ft. to 247
ft. wide.
[Illustration:
FIG. 145.—_Albert Bridge, Saltash._
]
Another singular modification of the girder principle occurs in the
bridge built by Brunel across a tidal river at Saltash, Fig. 145. Here
only a single line of rails is carried over the stream, which is,
however, 900 ft. wide, and is crossed by two spans of about 434 ft.
wide. A pier is erected in the very centre of the stream, in spite of
the obstacles presented by the depth of the water, here 70 ft., and by
the fact that below this lay a stratum of mud 20 ft. in depth before a
sound foundation could be reached. This work was accomplished by sinking
a huge wrought iron cylinder, 37 ft. in diameter and 100 ft. in height,
over the spot where the foundation was to be laid. The cylinder
descended by its own weight through the mud, and when the water had been
pumped out from its interior, the workmen proceeded to clear away the
mud and gravel, till the rock beneath was reached. On this was then
built, within the cylinder, a solid pillar of granite up to the
high-water level, and on it were placed four columns of iron 100 ft.
high, each weighing 150 tons. The two wide spans are crossed by girders
of the kind known as “bow-string” girders, each having a curved
elliptical tube, the ends of which are connected by a series of iron
rods, forming a catenary curve like that of a suspension bridge. To
these chains, and also to the curved tubes, the platform bearing the
rails is suspended by vertical suspension bars, and the whole is
connected by struts and ties so nicely adjusted as to distribute the
strains produced by the load with the most beautiful precision. When the
bridge was tested, a train formed wholly of locomotives, placed upon the
entire length of the span, produced a deflection in the centre of 7 in.
only. This bridge has sometimes been called a suspension bridge because
of the flexible chords which connect the ends of the bows; but this
circumstance does not in reality bring the bridge as a whole under the
suspension principle. The section of the bow-shaped tube is an ellipse,
of which the horizontal diameter is 16 ft. 10 in. and the vertical
diameter 12 ft., and the rise in the centre about 30 ft. Beside the two
fine spans which overleap the river, the bridge is prolonged on each
side by a number of piers, on which rest ordinary girders, making its
total length 2,240 ft., or nearly half a mile; 2,700 tons of iron were
used in the construction. As in the case of the Britannia Bridge, the
tubes were floated to the piers, and then raised by hydraulic pressure
to their position 150 ft. above the level of the water. The bridge was
opened by the late Prince Consort in 1860, and has received the name of
the Albert Bridge.
_SUSPENSION BRIDGES._
The general principle of the suspension bridge is exemplified in a chain
hanging between two fixed points on the same level. If two chains were
placed parallel to each other, a roadway for a bridge might be formed by
laying planks across the chains, but there would necessarily be a steep
descent to the centre and a steep ascent on the other side. And it would
be quite impossible by any amount of force to stretch the chains into a
straight line, for their weight would always produce a considerable
deflection. Indeed, even a short piece of thin cord cannot be stretched
horizontally into a perfectly straight line. It was, therefore, a happy
thought which occurred to some one, to hang a roadway from the chains,
so that it might be quite level, although they preserved the necessary
curve. In designing such bridges, the engineer considers the platform or
roadway as itself constituting part of the chain, and adjusts the loads
in such a manner that the whole shall be in equilibrium, so that if the
platform were cut into sections, the level of the road would not be
impaired.
Public attention was first strongly drawn to suspension bridges by the
engineer Telford, who, in 1818, undertook to throw such a bridge across
the Menai Straits, and the work was actually commenced in the following
year. The Menai Straits Suspension Bridge has been so often described,
that it will be unnecessary to enter here into a lengthy account of it,
especially as space must be reserved for some description of other
bridges of greater spans. The total length of this bridge is 1,710 ft.
The piers are built of grey Anglesea marble, and rise 153 ft. above the
high-water line. The distance between their centres is 579 ft. 10½ in.,
and the centres of the main chains which depend from them are 43 ft.
below the line joining the points of suspension. The roadway is 102 ft.
above the high-water level, and it has a breadth of 28 ft., divided into
two carriage-ways separated by a foot-track. The chains are formed of
flat wrought iron bars, 9 ft. long, 3¼ in. broad, and 1 in. thick. In
the main chains, of which there are sixteen, no fewer than eighty such
bars are found at any point of the cross section, for each link is
formed of five bars. These bars are joined by cross-bolts 3 in. in
diameter. The main chains are connected by eight transverse stays formed
of cast iron tubes, through which pass wrought iron bolts, and there are
also diagonal ties joining the ends of the transverse stays. The time
occupied in the construction was 6½ years, and the cost was £120,000.
This bridge has always been regarded with interest for being the first
example of a bridge on the suspension principle carried out on the large
scale, and also for its great utility to the public, who, instead of the
hazardous passage over an often stormy strait, have now the advantage of
a safe and level roadway.
[Illustration:
FIG. 146.—_Clifton Suspension Bridge, near Bristol._
]
The Clifton Suspension Bridge over the Avon, near Bristol, is noted for
having a wider span than any other bridge in Great Britain, and it is
remarkable also for the great height of its roadway. The distance
between the centres of the piers—that is, the distance of the points
between which the chains are suspended—is more than 702 ft. Part of the
ironwork for this bridge was supplied from the materials of a suspension
bridge which formerly crossed the Thames at London, and was removed to
make room for the structure which now carries the railway over the river
to the Charing Cross terminus. Five hundred additional tons of ironwork
were used in the construction of the Clifton Bridge, which is not only
much longer than the old Hungerford Bridge, but has its platform of more
than double the width, viz., 31 ft. wide, instead of 14 ft. A view of
this bridge is given in Fig. 146, where its platform is seen stretching
from one precipitous bank of the rocky Avon to the other, and the river
placidly flowing more than 200 ft. below the roadway. The picturesque
surroundings of this elegant structure greatly enhance its appearance,
and the view looking south from the centre of the bridge itself is
greatly admired, although the position may be at first a little trying
to a spectator with weak nerves. The work is also of great public
convenience, as it affords the inhabitants of the elevated grounds about
Clifton a direct communication between Gloucestershire and
Somersetshire, thus avoiding the circuitous route through Bristol, which
was required before the completion of the bridge.
[Illustration:
FIG. 147.
]
The use of iron wire instead of wrought bars has enabled engineers to
far exceed the spans of the bridges already described. The table on page
199 shows that iron wire has a tenacity nearly one-third greater than
that of iron bars, and this property has been taken advantage of in the
suspension bridge which M. Chaley has thrown over the valley at
Fribourg, in Switzerland. This bridge has a span of no less than 880
ft., and is constructed entirely of iron wires scarcely more than ⅒ in.
in diameter. The main suspension cables, of which there are two on each
side, are formed of 1,056 threads of wire, and have a circular section
of 5½ in. diameter. The length of each cable is 1,228 ft., and at
intervals of 2 ft. the wires are firmly bound together, so as to
preserve its circular form. But as the cable approaches the piers, the
wires are separated, and the two cables on each side unite by the
spreading out of the wires into one flat band of parallel wire, which
passes over the rollers at the top of the piers, and is again divided
into eight smaller cables, which are securely moored to the ground. Each
of the mooring cables is 4 in. in diameter, and is composed of 528
wires. In order to obtain a secure attachment for the mooring cables,
shafts were sunk in the solid rock 52 ft. deep, and the ingenious mode
in which, by means of inverted arches, an anchorage in the solid rock is
formed for the cables, will be understood by a reference to Fig. 147.
The cables pass downwards through an opening made in each of the middle
stones, and are secured at the bottom by stirrup-irons and keys. The
suspension piers are built of blocks of stone, very carefully shaped and
put together with cramps and ties, so as to constitute most substantial
structures. These piers are embellished with columns and entablatures,
forming Doric porticoes, enclosing the entrances to the bridge, which
are archways 43 ft. high and 19 ft. wide. The roadway is 21 ft. wide,
and is supported on transverse beams, 5 ft. apart, upon which is laid
longitudinal planking covered by transverse planking. The roadway beams
are suspended to the main cables by vertical wire cables, 1 in. in
diameter. The length of these suspension cables of course varies
according to their position, the shortest being ½ ft. and the longest 54
ft. in length. Each suspension cable is secured by the doubling back of
the wires over a kind of stirrup, through which passes a plate of iron,
supported by the two suspension cables, the latter being close together,
and, indeed, only separated by the thickness of the suspension cables,
which hang between them. The roadway has a slight rise towards the
centre, its middle point being from 20 to 40 in. above the level of the
ends, according to the temperature.
To test the stability of the bridge, fifteen heavy pieces of artillery,
accompanied by fifty horses and 300 people, were made to traverse it at
various speeds, and the results were entirely satisfactory. Indeed, a
few years afterwards the people of Fribourg had another wire bridge
thrown over the gorge of Gotteron, at about a mile from the former.
This, though not so long (640 ft.), spans the chasm at a great height,
and in this respect is probably not surpassed by any bridge in the
world—certainly not by any the length of which can compare with its own.
The height of the roadway above the valley is 317 ft., or about the same
as that of the golden gallery of St. Paul’s Cathedral above the street.
The structure is very light, and the sensation experienced when, looking
_vertically_ downwards through the spaces between the flooring boards,
you see the people below diminished to the apparent size of flies, and
actually feel yourself suspended in mid-air, is very peculiar, as the
writer can testify.
The Americans have, however, outspanned all the rest of the world in
their wire suspension bridges. They have thrown a suspension bridge of
800 ft. span over the Niagara at a height of 260 ft. above the water, to
carry not only a roadway for ordinary traffic, but a railway. Suspension
bridges are not well adapted for the latter purpose, but there seemed no
other solution of the problem possible under the circumstances. The
bridge, however, combines to a certain extent the girder with the
suspension principle. The girder which _hangs_ from the main cables (for
they are made of wire), carries the railway, and below this is the
suspended roadway for passengers and ordinary carriages. The engineer of
this work was Roebling, who also designed many other suspension bridges
in America.
The spans of any European bridges are far exceeded by that of the wire
suspension bridge which crosses the Ohio River at Cincinnati, with a
stride of more than 1,000 ft.; and this is, in its turn, surpassed by
another bridge which has been thrown over the Niagara. This bridge,
which must not be confounded with the one mentioned above, or with the
Clifton Bridge in England already described, merits a detailed
description from the audacity of its span, which is nearly a quarter of
a mile, and entitles it to the distinction of being the longest bridge
in the world of one span.
[Illustration:
FIG. 147_a_.—_Clifton Suspension Bridge, Niagara._
]
The new suspension bridge at the Niagara Falls, called the Clifton
Bridge, of which a view is given in Fig. 147_a_, is intended for the use
of passengers and carriages visiting the Falls, and it is also the means
of more direct communication between several small towns near the banks
of the river. The bridge is situated a short distance below the Falls,
crossing the river at right angles to its course at a point where the
rocks which form the banks are about 1,200 ft. apart. The distance
between the centres of the towers is 1,268 ft. 4 in., and the bridge has
by far the longest single span of any bridge in the world, the distance
between the points of suspension being more than twice that of the Menai
Bridge, and more than six times the span of the widest stone bridge in
England. This remarkable suspension bridge was constructed by Mr. Samuel
Keefer, and was opened for traffic on the 1st of January, 1869, the
actual time employed in the work having been only twelve months. The
cables and suspenders are made of wire, which was drawn in England at
Warrington and Manchester, and the wires for the main cables were made
of such a length, that each wire passed from end to end of the cable
without weld or splice. The length of each of the two main cables is
1,888 ft., and of this length 1,286 ft. usually hangs between the
suspending towers, the centre being about 90 ft. below the level of the
points of suspension. This last distance, however, varies considerably
with the temperature, for in winter the contraction produced by the cold
brings up the centre to 89 ft. below the level line, while in summer it
maybe 3 ft. lower. The centre of the bridge is about 190 ft. above the
water in summer, and 193 ft. in winter. The cables are each formed of
seven wire ropes, and each rope consists of seven strands, each strand
containing nineteen No. 9 Birmingham gauge wires of the diameter of
0·155 in. The cables of this bridge do not hang in vertical planes,
since in the centre they are only 12 ft. apart; while at the towers,
where they pass over the suspension rollers, they are 42 ft. apart. The
end of the platform which rests on the right bank is 5 ft. higher than
the other, and if a straight line were drawn from one end to the other,
the centre of the roadway would be in winter 7 ft. above it, and in
summer 4 ft. From each point of suspension twelve wire ropes, called
“stays,” pass directly to certain points of the platform. The stays are
not attached to the cables, but pass over rollers on the tops of the
towers, and are anchored in the rock, independently of the cables. The
longest stays are tangential to the curve formed by the main cables, and
they are fixed to the platform at a point about half-way to the centre.
Other stays proceed from the platform at intervals of 25 ft., between
the longest and the end of the bridge. The thickness of the stays is
varied according to the strain they have to bear, and they form not only
a great additional support to the platform, but they also serve to
stiffen the bridge and lessen the horizontal oscillations to which the
platform would be liable from the shifting loads it has to bear. There
are also stays which transversely connect the two cables. The wire ropes
by which the platform is suspended to the main cables are ⅝ths of an
inch in diameter, and have such a strength that the material would only
yield to a strain of 10 tons. These suspenders are placed 5 ft. apart
and are 480 in number, the lengths, of course, being different according
to the position. To each pair of suspenders is attached a transverse
beam, 13½ ft. long, 10 in. deep, and 2½ in. wide. Upon these beams—which
are, of course, 5 ft. apart from centre to centre—rests the flooring,
formed of two layers of pine planking 1½ in. thick; and the roadway thus
formed constitutes a single track 10 ft. in width. Along each side of
the platform is a truss the whole length of the bridge, formed of an
upper and a lower beam, 6½ ft. apart, united by ties and diagonal
pieces. The lower chord of the truss is 2 ft. below the road, and on it
rolled iron bars are bolted continuously from one end of the bridge to
the other. The last arrangement contributes greatly to stiffen the
platform, vertically and horizontally. In the central part of the bridge
the flooring-boards are bolted up to the cables, and there are studs
formed of 2 in. iron tubes, so that the platform cannot be lifted
vertically without raising the cables also; and as thus 81 tons of the
weight of the cables vertically rest upon the platform, great steadiness
is secured, inasmuch as the central part of the cables must partake of
any movement of the platform, and their weight greatly increases the
inertia to be overcome. In order still further to prevent oscillations
as much as possible, a number of “guys” are attached to the bridge.
These are wire ropes of the same thickness as the suspenders, and they
connect the platform with various points of the bank—some going
horizontally to the summit of the cliffs, others vertically, but the
majority obliquely. There are twenty-eight guys on the side of the
bridge next the falls, and twenty-six on the other side. The thickness
of the wire rope of which they are made being little more than ½ in.,
they are scarcely visible, or rather appear like spider lines. About 400
ft. of the length of the bridge in the centre is without either guys or
stays except two small steel ropes, which, tightly strained from cliff
to cliff, cross each other nearly at right angles at the centre of the
bridge. The suspension towers are pyramidal in form and are built of
white pine, the timbers being a foot square in section and very solidly
put together, so that they are capable of bearing forty times the load
which can ever be put upon them. The towers are surmounted by strong
frames of cast iron, to which are fixed the rollers carrying the cables
and stays to their anchorage. The weight of the bridge itself, together
with the greatest load it can be required to bear, amounts to 363 tons.
Its cost was £22,000, and it was constructed without a single accident
of any kind.
The foam of the great falls is carried by the stream beneath the bridge,
and in sunshine the spectator who places himself on the centre of its
platform sees in the spray driven by the wind, not a mere fragment of a
rainbow, or a semicircular arc, but the complete circle, half of which
appears beneath his feet. The gorge of the Niagara is very liable to
furious blasts of winds, for by its conformation it seems to gather the
aërial currents into a focus, so that a gentle breeze passing over the
surrounding country is here converted into a strong gale, sweeping down
with great force between the precipitous banks of the river. Indeed, one
would suppose that the cavern from which Æolus allows the winds to rush
out, must be situated near Niagara Falls. The bridge is not disturbed by
ordinary winds, although during its construction, before the stays and
guys were fixed, it was subject to considerable displacement from this
cause. The peculiar arrangement of the cables, by which they hang, not
vertically, but widening out from the centre of the bridge, giving what
has been termed the “cradle” form, has proved of the highest advantage,
so that, with the aid of the guys and stays, and the plan of attaching
the central part of the roadway to the cables, the bridge is believed to
be capable of withstanding without damage a gale having the force of 30
lbs. per square foot, although its total pressure on the structure might
then amount to more than 100 tons. The stability of the structure was
severely tested soon after its erection by a furious gale from the
south-west, by which the guys were severely strained; in fact, many of
them gave way. In one case an enormous block of stone, 32 tons in
weight, to which one of the guys was moored, was dragged up and moved 10
ft. nearer the bridge. This and some lateral distortion of the platform,
which was easily remedied, was all the damage sustained by the bridge.
By an increase of the strength of the guys, &c., and the addition of the
two diagonal steel wire ropes mentioned above, the bridge was soon made
stronger than before. Some years ago, when the Menai suspension bridge
was exposed to a storm of like severity, that structure suffered great
damage, the platform having been broken and some of it swept away. In
the great gale which swept down upon the Niagara bridge, although the
force of the wind was so great that passengers and carriages could not
make headway, the vertical oscillations of the bridge never exceeded 18
in., an amount which must be considered extremely satisfactory in a
bridge of the kind, having a span of nearly a quarter of a mile.[4]
Footnote 4:
Notwithstanding the skill displayed in its construction, this bridge
has, since the above account was written, been destroyed by a
tremendous hurricane.
[Illustration:
FIG. 147_b_.—_Living Model of the Cantilever Principle._
]
_CANTILEVER BRIDGES._