Ordnance gazetteer of Scotland

FOETH BRIDGE
for upwards of thirty years. In 1861 the same engineer
proposed to extend the 'floating railway' idea to Queens-
ferry in connection with a projected railway from Edin-
burgh to Perth. This plan not commending itself, three
years later he proposed his first design for bridging the
firth. The bridge was to be 3 miles long, crossing the
broader but shallower part of the river a mile above
Charleston, with a height of 125 feet above the river,
and five spans of 500 feet each in the fairway. But in
1873, after the Tay Bridge had been begun, the bolder
design of crossing at Queensferry, using the island of
Inchgarvie as the central support for two spans of 1600
feet each, was put forward by him. The plan involved
a double bridge, one for each set of rails. The two were
to be braced together by lateral diagonal stays. This
scheme was eagerly taken up, despite the fact that it
was to be partly on the suspension principle, and re-
quired piers of 600 feet high to bear the chains, this
elevation being about 100 feet above the highest ex-
isting structures. When the Tay Bridge fell, however,
the feeling against the Forth Suspension Bridge became
so pronounced that the idea was given up. A conference
of engineers was held on the subject, and, after exhaus-
tive consideration, it was resolved that a steel cantilever
bridge, with central connecting girders, was the best, if
not the only possible solution of the problem. Fortified
by this unanimous and unqualified decision on the part of
the best engineering authorities, the Forth Bridge Kail-
way Company took the necessary steps to have the new
project carried into effect, and in 1882 obtained powers to
proceed with the plans of their chief engineers, Sir John
Fowler and Mr Baker. The Midland and East Coast
Railway Companies, along with the North British, in-
terposed their credit for the necessary financial obliga-
tions; the North British being responsible for one-half
of the four per cent, payable on the capital expenditure.
The cantilever principle is as old as the science of
engineering, but never before has it been applied on so
magnificent a scale. It is that of projecting brackets,
gradually extended, till they come near enough to be
connected by a central girder. The central or Inch-
garvie cantilever is balanced by having a girder to sup-
port at both of its extremities; whereas the south and
north cantilevers have at their shoreward ends about
1000 tons each of cast-iron ballast to counterbalance
the half weight of the connecting girder each has to
support at its other end. From each tower of tubes
the great brackets had to be extended at an exactly
equal rate, that the poise might be preserved. Each
cantilever is in effect composed of two brackets — an
ordinary and an inverted bracket, the former resting
more directly on the pier foundation, and the latter
suspended from the great steel tower.
To carry the tension parts of the cantilevers, it was
necessary that the great steel towers should be 360 feet
in height. Each of the three great towers includes four
steel columns, 12 feet in diameter, and each of these
columns rests on its own foundation of solid masonry,
built from the rock or boulder clay, 70 feet in diameter
at the bottom and tapering to 49 feet at the top. The
foundations of the north and south cantilevers are 91
feet below high-water level, so that the total height of
the structure from its base is fully 450 feet. The foun-
dations of the central cantilever, at Inchgarvie, were cut
out of the hard trap rock to 72 feet below the surface of
the water. Two of the piers for the Fife cantilever were
constructed practically on shore, and other four were
erected without the aid of caissons ; but the remaining six
had to be laid in deep water by means of caissons, 70 feet in
width and about 60 feet in height. These caissons were
made on shore, launched, towed to the spot where they
were wanted, and there ballasted till they sank to the
bottom. The floor of each caisson was 7 feet above
its lower or cutting edge, and below this floor the water
was expelled by means of compressed air, leaving a
working chamber 70 feet in diameter and 7 feet in
height, in which the work of excavation was carried on.
This working chamber communicated with the surface
by three shafts, closed with air-tight double doors or
612
FORTH BRIDGE
air-locks, on the principle of a canal lock. Two of these
shafts were used to bring up the excavated material,
and the third was for the use of the workmen and
officials. The working chamber was lighted by elec-
tricity; and when the caisson was at the bottom of the
foundations the pressure of air had to be maintained at
35 lbs. to the square inch. Most of the men employed
at this part of the work were Italians who had acquired
full experience of similar employment while construct-
ing the foundations of the great new quays at Antwerp.
They used dynamite for blasting, and took refuge some
distance up the shaft when a shot had to be fired. To
work in the boulder clay, which proved too tough for
.ordinary digging implements, Mr Arrol, the contractor,
invented for them diggers with hydraulic rams in their
hollow stems. When these diggers were placed against
the roof of the working chamber, the men had but to
turn on the hydraulic power, when the cutting part of
the implement went down into the clay with a force of
which human muscle is incapable. As the work went
on round the cutting edge of the caisson, it gradually
sank to the required depth; and when the foundation
was found to be satisfactory the whole of the interior
of the caisson was built full of solid masonry, for which
the caisson itself is left as a temporary covering. One
of the deep piers contains 20,000 tons of masonry. Into
the upper part of the piers are built strong steel ties, 24
feet deep, and fixed to secure anchors in the masonry.
By these ties the bed-plates are held down on the top
of the masonry. These plates bear the enormously strong
skew-backs, in which are combined the bases of all the
limbs of the cantilevers — -perpendicular, horizontal, and
diagonal, amounting to 50,000 tons of steeL It took
three years to lay the foundations, and, considering the
nature of the task, the time was considered short by
those competent to judge. They contain 120,000 cubic
yards of concrete and 400,000 cubic feet of granite.
The piers that serve as the bases of the great vertebral
steel columns are in pairs, 120 feet apart from east to
west. The east columns of each tower are therefore that
distance apart from the west at their base, but they
approach to within 33 feet of each other at the top.
This arrangement greatly enhances the stability of the
bridge and its power to resist wind pressure. The piers
of the Fife and of the Queensferry cantilevers are 150
feet apart from north to south; but those of the Inch-
garvie cantilever have been placed 270 feet apart,
because, the arms of the central cantilever being free,
a greater thrust-resisting base has been deemed necessary
for its support. Lengthwise the skew-backs, or boxes
that receive the bases of all the great columns, are joined
together by cylinders 12 feet in diameter; crosswise
they are bound together by lattice girders. On these
skew-backs all the thrusts, vertical and lateral, con-
nected with the weight of the bridge meet and counter-
balance each other. The construction of the great tube
columns was first gone about on shore. The steel plates
of which they are composed had to be heated to a dull
red and bent by hydraulic pressure into the exact curve
needed. The edges of every plate had to be carefully
planed, and they were fitted round a frame which had
been prepared of the exact shape and dimensions of the
great tube. The rivet holes were then all carefully
drilled, and, after the tube had thus been completed,
every plate was numbered and the whole taken to pieces
for erection on its permanent site. The construction
of the cantilevers began in the early part of 1886.
Large temporary platforms for the workmen were used
in the first instance, and gradually raised, as necessary,
by hydraulic rams. When the work advanced, it
served as the basis of its own scaffolding. Cranes
res'ted on the rising vertical and extending horizontal
members of the cantilever, making additions which, in
their turn, became new supports for the cranes and
starting points for further extension. Plates and other
material, brought out in barges, were raised to the level
of the viaduct by a crane stationed there. Goliath
cranes lifted the plates into position, and held them
there till they had Deen securely riveted. The columns