the first bridge on the cantilever and central girder principle that has
been erected in Great Britain, and it has also the distinction of being
by far the widest spanned bridge in all the world. We are told by the
engineers of the bridge that the cantilever and girder principle is by
no means new, for it has been adopted hundreds of years ago by
comparatively rude tribes in the construction of timber bridges, to
which it readily lends itself. Such bridges are described as having been
erected by the natives of Hindoostan, Canada, Thibet, etc., even at
remote periods. The principle of the cantilever and girder construction
was well illustrated by Mr. Baker, one of the engineers of the bridge,
at a lecture given by him at the Royal Institution, by means of what he
termed “a living model,” of which (Fig. 147_b_) shows the general
arrangement. Two men, seated on chairs, extend their arms and hold in
their hands sticks, of which the other ends butt against the chairs. The
central girder is represented by a shorter stick, suspended at _a_ and
_b_. We have here the representation of two double cantilevers, the
ropes at _c_ and _d_, connected with the weights, representing the
anchorages of the landward arms of the cantilevers. When a weight is
placed on _a b_, which was done in the “living model,” by a third man
seating himself thereon, a tensile strain comes into action in the ropes
and in the men’s arms, while the sticks abutting on the chairs have to
resist a compressing force, and the weight of the whole is borne by the
legs of the chairs, also under compression. Now let the reader imagine
the men’s heads to be 360 feet above the ground, and about a third of a
mile apart, while the distance between _a_ and _b_ is 350 feet, and he
will have a rough but sufficiently clear idea, not only of the principle
upon which the Forth Bridge is constructed, but also of the magnitude of
one of its spans. To complete the comparison, Mr. Baker further invited
his hearers to suppose that the pull upon each arm of the men is equal
to 10,000 tons, and that the legs of each chair press on the ground with
the weight of more than 100,000 tons.
The Forth Bridge spans the estuary at Queensferry nine miles north-west
from Edinburgh, and its purpose is to afford uninterrupted railway
communication along the eastern side of Scotland. It will, in effect,
shorten the railway journey between Edinburgh and Perth, or Aberdeen, by
nearly two hours. Queensferry had long been established as a usual place
for crossing the Forth, and readers of Scott’s “Antiquary” will remember
that the first chapter describes how Monkbarns and Lovel, by some
accidental delays to the coach, lost the tide, and had to wait, to sail
“with the tide of ebb and the evening breeze,” finding themselves, in
the meanwhile, pretty comfortable over a good dinner at the “Hawes Inn.”
This inn still stands, its situation being close to the southern end of
the great bridge. A design for the erection of a light suspension bridge
at the same spot was published at the beginning of the present century,
but although the spans were to be equal to those of the present bridge
(17,000 feet), the different scale of the projects may be inferred from
the total weight of iron to be used being estimated at 200 tons, while
50,000 tons will be required for the structure now approaching
completion.
In 1873, an Act of Parliament was obtained authorizing the construction
of a suspension bridge at Queensferry, to carry the railway over the
estuary. The design comprised practically two bridges, each carrying a
single line of rails, the bridges being braced together at intervals.
The central towers were to have been 600 feet high, or about 100 feet
loftier than any other erection then existing in the world. The designer
was the late Sir Thomas Bouch, and preparations were made for carrying
out the plans by the erection of workshops and the manufacture of bricks
for the piers. But the project was knocked on the head by the terrible
disaster at the Tay Bridge, in December, 1879, when several of the
central piers were overturned by the force of the wind, with swift
destruction to a passing train, which was precipitated into the water,
and every one of about ninety persons in the train perished. Sir Thomas
Bouch having been the designer of the Tay Bridge, public confidence in
his plan was shaken to such an extent, that the four railway companies
who were promoting the construction of the suspension bridge abandoned
the project in favour of a design on the cantilever and central girder
system, which was then brought forward by Mr. (now Sir John) Fowler and
Mr. Baker. When the Bessemer process had made steel attainable at a
cheap rate, these engineers recognized the advantages which cantilever
bridges, made of that material, presented for the wide spans required
for carrying railways across navigable rivers, and in 1865 they had
designed such a bridge, with 1,000 feet spans for a viaduct, across the
Severn, near the position of the present tunnel. It was not, however,
until 1881 that the designs for the Forth Bridge were published in
English and American engineering journals. These designs at once
attracted attention, and scarcely a year had elapsed before a railway
bridge was built for the Canadian and Pacific Railway, on the same
principle, and this has been followed by others since. It is, however,
absurd to allege that the engineers took their ideas from America,
merely because these smaller undertakings have been completed before the
great work that dwarfs them all was open for traffic. The construction
of the Forth Bridge on its present design was commenced in January,
1883. Its site at Queensferry is at a point where the estuary narrows,
and where, in the very middle of the channel, there is a small rocky
island, called Inchgarvie, that furnishes a solid foundation for the
great central pier. On each side of this island the channels are about
one-third of a mile wide, and more than 200 feet deep, and through them
the tide rushes with great velocity. The impossibility of building up
any intermediate piers, under such circumstances, is sufficiently
obvious—the currents must be crossed at one span, if a railway bridge
had to be made. The formation of the piers for such a work presented
many novel problems, and much of the work had to be commenced in deep
water; that is, the ground of rock or hard clay had to be prepared, in
some parts, as far as 90 feet below high water. Each pier stands on four
caissons, which are great tubes or drums of iron and steel, filled up
with concrete. Each weighed, when empty, about 400 tons, but when filled
up with concrete, the weight would be about 3,000 tons. The diameter of
each is 70 feet, and the deepest one is sunk 89 feet below the water,
and it was with no little labour that some of them were put in their
places. Each caisson has an outer and an inner tube, is 70 feet in
diameter at the base, and 60 feet at the top. Seven feet from the
bottom, an air-tight partition formed a chamber in the lower part of the
caisson, about 70 feet in diameter, by 7 feet high, and shafts
sufficiently large to admit the passage of men and tools led from the
top. Air was forced into this chamber, when the caisson had been sunk,
expelling the water, and then men descended through the shafts and
locks, in which a high pressure of air was also maintained, and
excavated the material at the bottom, until the caisson had, by its own
weight, sunk to the depth required. The work in this air chamber was
carried on by means of electric lights, and ten or twelve weeks were
occupied in sinking each caisson. The pressure of the air in the working
chamber was sometimes as high as 35 pounds per square inch, or
sufficient to maintain the mercurial column in a barometer 72 inches
high, instead of the ordinary 29 or 30 inches. It was found that the
labour in the compressed air chamber could not be done by our home
workmen, as they were quite unaccustomed to the high air pressures
required to keep out the water; but arrangements were made for the
assistance of a staff of French workmen, inured to the conditions by
long working under water in the construction of the docks at Antwerp.
[Illustration:
PLATE XIII.
THE FORTH BRIDGE.
]
The stores, offices and workshops, situated on a slight eminence near
the south end of the bridge, are very extensive, occupying, it is said,
an area of 50 acres. Here are great furnaces, cranes and machinery for
shaping and fitting the steel plates and bars ready for taking their
appointed places in the vast structure. An hydraulic crane may, for
instance, be seen lifting a ton weight flat steel plate that has been
heated to redness in a regenerative gas furnace, and transferring it to
an hydraulic press, where it is quickly and quietly bent to the required
shape. The plate is then cooled, and, when the edges have been planed,
it is placed in position with the adjoining plates, and the rivet holes
are drilled by an ingenious machine, specially designed by Mr. Arrol,
the contractor, for that purpose. It works upon 8–feet lengths of the
tubes, and simultaneously cuts ten rivet holes at different points in
the circumference. All the different parts of the structure are
temporarily fitted together to ascertain that every piece is properly
adjusted. They are then marked according to the position they are to
take, and are laid aside until they are wanted. Thus the work at the
bridge has proceeded without any awkward hitches arising from ill
adjusted sections being brought together. At times, 1,800 tons of
finished steel-work has been turned out of these shops in a month, and
this material, which was supplied by the Steel Company of Scotland, has
been found thoroughly trustworthy in every respect. Its strength is
one-half greater than that of the best wrought-iron, and the plates have
thrice the ductility of iron plates. The steel plates for the great
tubes are supplied in lengths of 16 feet, and of different thicknesses,
between ⅜ths of an inch and 1¼ inch.
[Illustration:
FIG. 147_c_.—_Principal Dimensions of the Forth Bridge._
]
The sketch, Fig. 147 _c_, shows the general dimensions of the bridge
proper, or that part of the viaduct which will actually span the
estuary. Of the three great piers that support the cantilevers, it will
be observed that the central one, which rests on Inchgarvie, is wider
than the other two. Each consists mainly of four tubes, 12 feet in
diameter, made of plates of steel 1¼ inch in thickness, and these rise
to the highest part of the bridge, which is 361 feet above the water, so
that the structure is as lofty as St. Paul’s Cathedral. These great
tubes are not placed vertically, but incline inwards towards the top, so
that while the “straddle legs” of each pair are 120 feet apart at the
base, they are only 33 feet apart at the top. These lofty columns are
also braced together diagonally by other steel tubes—that is, a tube
passes from the foot of every column to each of the other three. At the
base of each column, the lowest spanning member springs also (which
appears like an arch, but is not so), as a tube of 12 feet diameter.
Thus abutting or resting on enormously thick plates of steel that cap
the masonry of each pier, are five tubular steel limbs, three of which
are 12 feet in diameter, and two are 8 feet, and, besides these five,
girder members diverge from nearly the same centre. One of the large
tubular members is the first strut that rises obliquely to support the
upper structure. From the point where this strut meets the upper member,
a stay passes downwards with an opposite inclination to the lower
member, from its point of junction with which another strut rises, and
so on. All the struts, as being subject to compressing force, are made
of steel tubes; the straight upper members and the stays are lattice
braced girders of rectangular section. The apparent curve of the lower
member—for it is really made up of sections of straight tubes—may
suggest the notion of an arch; but the reader must remember that the
principle of this bridge has no relation to that of the arch. The
cantilevers do not unite the long arms they stretch, but each is an
independent structure with its own perfect stability, and it will not be
clutched on or locked up to its neighbours by the central girders. The
weight of one of these 1,700 feet spans is about 16,000 tons, and the
heaviest train loads might be two coal trains, weighing together, say
800 tons, or only one-twentieth of the dead weight of the structure.
But, what would not generally be supposed, the pressure of the wind is
an element of much more importance in considering the stability of the
bridge than the weight of the rolling load. It is to resist the wind
pressure that the lofty columns that are only 33 feet apart at the top
across the bridge, plant their bases 120 feet asunder. The estimated
lateral pressure of the wind on one of the cantilevers, assuming it as
equal to 56 lbs. per square foot, would amount to 2,000 tons. These
strains are so fully provided for that the engineers are confident that
a hurricane of such a force as would desolate the country would leave
the Forth Bridge intact, even if the wind blew in opposite directions on
the two arms of the cantilever. To rend asunder the top ties, a pull
equivalent to the weight of 45,000 tons would be required, whilst the
utmost strain that passing trains could possibly bring upon these ties
would be less than 2,000 tons. A striking illustration of the strength
of these huge brackets was lately given by Mr. Baker himself, when in a
public lecture he assured his audience that half a dozen of our
ponderous modern ironclads might be hung from the cantilevers. Everyone
knows that a bracket requires to be strongest nearest the base, and the
lower steel arms that stretch out 680 feet each diminish in diameter
until at the end it has decreased to five feet, and the pairs approach
each until, from being 120 feet apart at the base, they are only 33 feet
apart at the ends. The central girders will each weigh about 1,000 tons,
and only one end of each will be attached to a cantilever, the other
ends will simply rest on what are called “rocking columns,” so that
there may be freedom of motion to allow play for the changes of position
that will be induced by changes of temperature expanding or contracting
the huge masses of metal.
The reader can hardly have failed to observe that the chief element in
the stability of the structure depends upon balancing a great mass of
metal on the one side of a pier by an equal mass on the other side. But
while each end of the central cantilever bears half the weight of a
central girder, the two shoreward cantilevers have this load at their
inner ends only. How is their balance maintained? In this way: the
shoreward arms are made about 10 feet longer than those that stretch
over the water and their extremities are also loaded with about 1,000
tons of iron, built up within the shore piers.
The lofty columns of the piers were erected without any external
staging, from a temporary platform surrounding the piers and supporting
the necessary machinery. The weight of this platform with the machinery
on it was about 400 tons, and as the work proceeded it was raised as
required by hydraulic machines placed within the vertical columns. As
the height of these increased, the men and materials had to be conveyed
to the platform by cages moving between guide ropes and worked by steam
engines. From this platform were constructed not only the main columns,
but the great diagonal tubes, the bracing girders, and the viaduct
girder. The cantilevers were also put together without scaffolding. When
the first few feet of the lower member had been built out from the base,
a movable platform was hung round it, and on this platform were the
cranes for putting the plates into position, the furnace for heating the
rivets, and the hydraulic riveter of specially designed construction,
without noise or hammering, the riveting being completed by the
application of a pressure equal to 3 tons per square inch. The building
up of the cantilever arms on either side of each pier always proceeded
at the same rate, so that the balance was constantly maintained. This
building out from each side of the pier, without the necessity of
relying upon any temporary scaffolding from below, is one great
advantage of the cantilever system, as it is both easier and safer than
a system which relies upon the temporary scaffolding raised from below.
The Forth is for the time the longest spanned bridge in the world; but
it may not retain that honour long, for the legislature of the United
States has already authorized the construction of a cantilever bridge,
the spans of which are to be 2,480 feet. Still more gigantic is the
project lately put forward by some competent French engineers of
bridging the English Channel from Folkestone to Cape Grisnez in 70 spans
on the cantilever system. The designs have been completed and the
calculations made, and no one doubts of the engineering practicability
of the scheme. But the cost is estimated at about 34 million pounds
sterling, or nearly six times as much as that required for constructing
the proposed Channel Tunnel; so that the scale could be turned in favour
of the bridge only if the political reasons that were opposed to the
tunnel were held not to be applicable to the bridge. But it is difficult
to conceive that the existing traffic could ever be developed to such an
extent as to make an undertaking of this magnitude a commercial success.
Since the above account was written, the Forth Bridge was formally
opened on the 4th March, 1890, by the Prince of Wales, in the presence
of a great gathering of railway directors, eminent engineers, and other
distinguished persons from all parts. A very strong gale was blowing at
the time, and at this very hour the bridge was therefore subjected to
another severe but undesigned test of its stability. The perfect
steadiness and security of the structure impressed all who were present
on that occasion, and the train crossed the bridge, exposed to a wind
pressure, registered by the gauge, of 25 lbs. per square foot. At the
luncheon following the opening ceremony, the Prince announced that
baronetcies had been conferred upon Mr M. W. Thompson (the chairman of
the Bridge Company) and upon Sir John Fowler, and that Mr. Baker and Mr.
Arrol, the contractor for the works, were to be knighted. Sir John
Fowler, the engineer-in-chief, was born in 1817, and has been engaged in
many other important works of railway construction in Yorkshire, in that
of the London and Brighton Railway, in the Sheffield Waterworks, &c. The
Metropolitan Railway in London, which also was carried out by Sir John
Fowler, would alone suffice to make him famous as an engineer. Sir
Benjamin Baker is a much younger man, who has had a large and varied
practice in railway engineering in various parts of the world. He is in
much request on the American continent, and is now engaged in carrying
out a ship railway in Canada and a tunnel under the Hudson at New York.
Sir William Arrol began life at nine years of age as a “piecer” in a
cotton mill, but was afterwards apprenticed as an engineer. Subsequently
he was employed as a foreman by engineering firms in Glasgow. In 1866,
he began business on his own account at Dalmarnock, and obtained
contracts at first for smaller then for larger works connected with
bridge and viaduct building. He is distinguished for the energy and
inventive resources he displays in carrying out his undertakings.
_THE TOWER BRIDGE, LONDON._