The Tower Bridge
From:
The Romance of Modern
Engineering (1908)
by Archibald Williams
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Frontispiece:
This interesting view of the Tower Bridge shows the "bascules" of the
central span raised to permit the passage of shipping.
A notable feature of the structure is that the
ironwork is quite independent of the masonry, which is out of contact
with the real supports, concealed inside.
The foundations of the two towers are capable of supporting without settlement, a weight of 70,000 tons.
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LESS
imposing as a structure than the giant conqueror of the Forth is the
new bridge that spans the Thames, a short distance east of the Tower of
London, from which it derives its name.
The Tower Bridge is, however,
of such importance and interest, both on account of the problems that
it has solved, and from the manner in which it has solved them, that
this great framework of metal and masonry, so familiar to the Londoner,
deserves inclusion among the chief engineering feats of modem times.
The general outlines of the
Bridge, being so well known, need little detailed description.
Technically, it is a three-span bridge, the two outside spans of the
suspension type carried on stout chains that pass at their landward
ends over abutment towers of moderate height to anchorages in the
shore, and at their river ends over very lofty towers, themselves
connected at an elevation of 143 feet above high-water level. Extremely
powerful ties, borne on the connecting girders, unite the two pairs of
chains, making the suspension spans to support one another in a
horizontal direction.
The central span has two
footways and one road. way. The high-level girders bear the upper
footway, reached by two hydraulic lifts situated in each of the main
towers.
The most notable feature of
the Bridge, unless we except the unique combination of steel and
masonry work in the towers, is the method of enabling traffic,
pedestrian and vehicular, to cross the 200-foot space between the
towers, at the level of the roadway of the two outer spans.
History repeats itself in engineering as elsewhere, and, as an example,
we see here a reversion to the idea of the drawbridge that shut off the
mediaeval fortress or town from the hostility of the outside world.
Principle apart, however, it is a far cry from the wooden platform,
heaved laboriously aloft by creaking chains, to the massive 1200-ton
steel leaf raised noiselessly by the unseen energy of hydraulic engines.
Before entering into details
of construction, it will be interesting to glance for a moment at the
antecedents of this latest-born of Thames bridges—the reason
for its
erection, and the considerations that cast it into its present form.
Let the reader take a map of
London and fix his eye on Blackfriars Bridge. A line drawn due north
and south through the bridge would approximately bisect the metropolis.
A steamboat travelling westwards from this point passes in succession
under Waterloo, Westminster, Lambeth, Vauxhall, Chelsea, Albert,
Battersea, Wandsworth, and Putney Bridges—nine in
all—open to
vehicular traffic. On an eastward journey of equal length it would,
however, have to lower its funnel for but two—the Southwark
and
London—assuming the Tower Bridge to be still in the future.
Yet both
banks are thronged by some of the most densely-populated districts of
London, so near each other and yet so far for want of means of
communication.
A further reference to the map
shows us why things should be so. This is a region of docks and
wharves, the latter reaching up to London Bridge, from which we have
often watched the unloading of cargoes.
The engineer, called in to
effect a compromise between the crying needs of road traffic on the one
hand and the equally important interests of river traffic on the other,
is able to suggest several methods of cutting the Gordian knot:
1. A
low-level bridge, with an opening for vessels through it.
2. A high-level bridge, with inclined road approaches.
3. A high-level bridge, with hydraulic lifts at each end.
4. A tunnel under the river, with inclined approaches.
5. A tunnel with hydraulic lifts at each end.
6. A ferry.
Of these the first would be
most convenient for the landsman, but most inconvenient for the sailor.
The second and fourth necessitate very costly approaches, the third and
fifth continual blocks in the traffic; and as regards ferries, they are
at best but very poor substitutes for a bridge.
Among the many plans submitted
since 1867 for a bridge, one is particularly noticeable for its
originality—that of Mr. C. Barclay Bruce. He
proposed a
rolling bridge, to consist of a platform 300 feet long and 100 wide,
which should be propelled from shore to shore over rollers placed at
the top of a series of piers 100 feet apart. The platform would have a
bearing at two points at least, and, according to the designer's
calculations, make the journey in three minutes, with a freight of 100
vehicles and 1400 passengers. Another engineer, Mr. F. T. Palmer,
proposed a bridge which widened out into a circular form near each
shore, enclosing a space into which a vessel might pass by the removal
of one side on rollers while traffic continued on the other side. As
soon as the vessel had entered the enclosure the sliding platform would
be closed again, and that on the other side be opened in turn.
In 1878 Sir Joseph Bazalgette,
engineer to the Metropolitan Board of Works, recommended the
construction of a bridge that should give a clear head-way of 65 feet
above Trinity high-water level, but a Bill brought into Parliament for
power to build it was thrown out on the ground that the headway would
be insufficient, and on account of the awkward special approaches.
To avoid wearying the reader with a list of projects we will pass
straight on to that of Mr. Horace Jones, the late City architect, who
in 1878 was asked to report upon the various projects of Sir Joseph
Bazalgette and make suggestions on his own account. He maintained that,
as a high-level bridge would not give satisfaction, a structure of the
same level as London Bridge, opening at the centre by means of hinged
platforms, or bascules, might be advantageously employed. From his
design has sprung that of the Tower Bridge—the joint work of
him and
Sir J. Wolfe Barry—which provides a central opening of 200
feet clear
and a headway of 135 feet. An Act for its construction having been
passed in the autumn of 1885, contracts were let for the foundations of
the piers and abutment towers up to the level of 4 feet above
high-water mark. On June 21, 1886, the (then) Prince of Wales laid the
foundation stone.
The masonry piers on which the
main towers stand are remarkable for their size—100 a feet
wide by
205 long—which exceeds that of any in the world, with the
exception
of those of the Brooklyn Bridge. The piers being but 200 feet apart,
the engineers, who were under agreement to leave a clear way of 160
feet between them, could not build both simultaneously as a whole,
since the scaffoldings would have narrowed the opening beyond legal
limits. They therefore adopted a system of small caissons, which should
be sunk so as to form a broad wall round the area of the pier, and
enclose a space of 34 by 124½ feet, to be dealt with as soon
as the exterior caissons were in position.
On the north and south sides
of each pier four caissons were sunk, 28 feet square and 21 feet apart,
each end of the rows being joined by a triangular caisson. While one
pier was in course of construction, the shoreward row of caissons for
the other pier was also sunk, thus saving time without obstructing the
river.
Reference has been made in the previous
chapter to the sinking of caissons; so it need here only be stated that
at the Tower Bridge no pneumatic caissons were employed, but only the
open variety. Divers cleared away the gravel and mud until a caisson
had descended such a distance into the stiff London clay at which it
was thought safe to pump out the water at low tide, and then navvies
were turned in with pick and shovel. At a depth of 19 feet the caissons
were undercut, i.e. the workers burrowed beneath their lower edges into
the clay for a distance of 5 feet horizontally, and 7 feet vertically.
The undercutting proceeded in sections—filled with concrete
in
succession—so that the caisson should not be left
unsupported. When
all the ten external caissons had been sunk and filled in, the narrow
spaces between them were also filled, and the interior enclosure pumped
dry and excavated. Finally, there emerged from the water a couple of
gigantic piers of concrete, granite, and bricks, able to withstand
without settlement a load of 70,000 tons. Their cost was £
111,122.
The contract for the steelwork
in the superstructure was let to Sir William Arrol & Co., of
Glasgow, who, as the reader will remember [from a previous chapter],
had already taken an important part in the construction of the Bridge.
Before any metal-work could be
placed in position, it was necessary to erect stagings from the shore
abutments to the centre piers. This work occupied some months, and when
it was completed operations at once commenced on the main towers.
Each tower consists of four
octagonal columns, connected at a height of 60 feet above the piers by
plate girders, 6 feet deep, across which are laid smaller girders to
carry the first landing. Twenty-eight feet higher is the second
landing, similarly constructed, and above that, at an equal distance,
the third landing leading to the high-level footway. The columns each
rested on massive granite slabs previously covered with three layers of
specially prepared canvas to make the pressure even and the joint
water-tight. They were keyed to the bed-stones by great bolts built
into the piers.
The first length of column plates
having being riveted in position by
hydraulic riveters, the second length was added by means of a crane
placed on the piers, and when the crane had been raised aloft on
special trestles the third length followed. The first landing served as
a platform from which to build upwards in like manner to the second,
which in turn became the base of operations. All four columns in each
tower were braced diagonally to resist the wind
pressure—calculated
at a maximum of 56 lbs. to the square inch, or several times greater
than has ever been registered in that locality.
The columns finished, and the
top landing girders in position, the workmen attacked the high-level
footway. This was built out from both towers simultaneously on the
overhang principle. First, the portions of the cantilevers immediately
over the towers were erected and anchored to the shoreward columns.
Then cranes were placed on the completed portions and moved forward to
add fresh plates until the cantilevers had reached the point where the
central suspended girder began. As at the Forth Bridge, this was built
on to the cantilever ends, to which it was attached by temporary ties,
and when the centre plates had been made secure, the ties were cut,
allowing it to ride free at each extremity. Throughout the construction
of the upper footway the greatest care had to be observed to prevent
rivets, fragments, and tools falling into the river below to the peril
of passengers on passing vessels.
Along the upper boom of the
footway run the great ties connecting the suspension chains at their
river ends. Each of the two ties is 301 feet long, and composed of
eight plates 2 feet deep and 1 inch thick, terminating in large
eye-plates to take the pins uniting them to the suspension chains. The
construction of these chains was one of the most interesting and at the
same time most delicate parts of the whole undertaking. Each chain is
composed of two parts, or links; the shorter dipping from the "top of
the abutment tower to the roadway, the longer rising from the roadway
to the summit of the main tower. The links have each a lower and upper
boom, connected by diagonal bracings so as to form a rigid girder. They
were built in the positions they had finally to occupy, supported on
trestles, and were not freed until they had been joined by huge steel
pins to the ties crossing the central span and to those on the abutment
towers. In order that the reader may have a clear conception of the
action of the ties and chains, we will personally conduct him from end
to end of the series. At the north end of the bridge is a huge mass of
concrete surrounding an anchorage girder 40 feet long, 4 feet wide, and
4 feet deep, to which is attached a land tie springing up to the shore
edge of the abutment top. At the anchorage end the tie is joined by a
pin, 2 feet in diameter, to the girder, and at its upper end to the
horizontal links crossing the abutment tower. The tie is built up of
twelve plates 21 inches wide and nearly an inch thick. The link plates
are 5 to 5½ feet wide and 7/8 inch thick and 22 feet long.
At each end they rest on roller bearings moving over 3-inch steel
plates very carefully levelled. Then comes the short link of the chain,
attached by eye-plates and a steel pin, 2½ feet in diameter,
to the tie and also to the lower end of the long link, at which point
both are joined to the girders of the roadway. Passing up the long link
we reach the top of the towers and note the great pins and roller
bearings at each end of the 301-foot ties. Then down the south long
link to the roadway, up the short link, and over more roller bearings
to the last section of the series—twelve plates 35 inches
wide
secured by rivets to the south anchorage girder, which is of larger
dimensions than its northern fellow. This arrangement of chains, links,
and ties permits a slight amount of horizontal motion to compensate the
stresses of unequal loading on the two suspension spans, and the
alterations in the length of the metal connections in varying
temperatures. Roller joints are also made in the flooring of the side
spans at each end and at the junction of the links to allow for
longitudinal expansion and contraction.
The boring of the pin holes
was a matter of great delicacy and considerable difficulty. The holes
in the eye-plates of ties and chains had been cleared to within
half-an-inch of their final diameter before leaving the contractor's
works at Glasgow, and the finishing touches were added when the plates
were in position. The labour of expanding out the holes to their full
diameter was equivalent to boring a hole 2 feet 6 inches in diameter
through 65 feet of
solid steel; and most of this boring had to be done
in somewhat awkward positions at the top of the main towers and
abutments, whither it was necessary to transport engines, boilers, and
boring tools. The fixing of these generally occupied as long a time as
the actual boring, since the greatest accuracy had to be observed
throughout the process.
The roadway of the suspension spans is carried on cross girders, 61
feet long, weighing 22 tons. At each end they are connected by 6-inch
pins to the suspension rods hanging vertically from the chain links.
The rods are from 5½ to 6 inches in diameter, and furnished
with a screw-coupling at their centres to enable the accurate
adjustment of the girders to the true level of the roadway. Before
leaving the works each rod had been subjected to a tension of 200 tons,
so that of their sufficiency there can be no doubt. Longitudinal
girders of smaller section were then laid on the transverse girders,
and on these again corrugated floor plates, afterwards filled up with
concrete to form a slightly convex surface, over which wood paving
blocks were placed.
We may now turn our attention
to the central span of the roadway, which forms, perhaps, the most
interesting part of the whole structure.
Each bascule, or leaf, of the
drawbridge consists of four parallel girders, 131 feet apart, and about
160 feet long. When lowered it projects horizontally 100 feet towards
the opposite tower, spanning exactly half of the opening. The point of
balance is a solid pivot, 1 foot 9 inches in diameter and 48 feet long,
that passes through the girders 50 feet from their shore ends. The
pivot is keyed to the girders, and rotates on roller bearings carried
by eight girders crossing the piers horizontally from north to south,
themselves borne on girders under their ends.
The chief difficulty attending
the erection of the bascules resulted from the condition compelling the
contractors to leave a clear way of 160 feet between the towers. Under
other circumstances the girders might have been completed before being
brought into line and connected together. As it was, the engineers
first built the portions on the shore side of the pivot, added a short
section of the river side steelwork, and launched the incomplete
girders from the main stage close to the piers into the bascule
chambers. A temporary steel mandrel was inserted to carry their weight
while they were turned into a vertical position, and then withdrawn to
make room for the permanent pivot, weighing 25 tons. The outer ends
were added to until a point 53 feet from the pivot had been reached,
and work in this direction then stopped until the raising and lowering
of the leaves for purposes of adjustment had been concluded; after
which the girders were completed vertically.
The leaves are moved by means
of pinions (or cog-wheels) engaging with racks fixed to the edge of two
steel quadrants riveted to their two outside girders. The accurate
attachment of the racks was a some-what difficult business on account
of the confined space in which the men had to work.
To preserve the balance of the
bascule it was necessary to load the shorter, or inner, arm with
counter-poises, consisting of 290 tons of lead and 60 tons of iron
enclosed in ballast boxes at the extreme ends of the girders. The
function of the raising gear is merely to overcome the inertia of the
1200 tons of the leaf, and the friction caused by wind pressure on the
exposed surface. In designing the hydraulic machinery allowance was
made for a wind pressure of 56 lbs. to the square foot, which would
produce a force of 140 tons acting with a leverage of 56 feet.
The source of power is a
building on the east side of the southern approach, where are stationed
two large accumulators with 20-inch rams loaded to give a pressure of
from 700 to 800 lbs. per square inch. An accumulator is the hydraulic
counterpart of the reservoir bellows in an organ. It ensures a steady
pressure, as its capacity is greater than that of the engines it
operates; and since the pumping engines can be constantly at work
filling it, there is always a plentiful supply of energy stored against
the periodical opening and shutting of the bascules. The water is led
through two 6-inch pipes, provided with flexible joints at points of
movement, to the two sets of engines on the south pier; and to those on
the north pier through continuation pipes passing up the south tower,
across the footway, and down the north tower. After use, the water is
returned through a 7-inch pipe to the pumping engines placed in two of
the arches forming the southern approach to the bridge.
The engines are duplicated on
each pier to avoid the inconvenience that would result from the
breakdown of a single installation. The power of the engines is
transmitted to the racks through a series of cog-wheels, which increase
the effective pressure of the pistons almost sevenfold. Hydraulic
energy is also used to work the two hydraulic lifts in each main tower,
and to shoot home and withdraw the four locking bolts at the outer
extremity of the southern leaf.
In this connection the
following extract from Mr. J. E. Tuit's fine book on the bridge will be
of interest. "Every precaution has been taken so that the operation of
opening and shutting the bridge shall be rendered as safe as possible.
By an automatic arrangement attached to the hydraulic engines on the
piers they are caused to close the valves which admit the high-pressure
water just at the end of the operation of raising or lowering the
leaves, so that even if the man in charge were to make a mistake
through an error of judgment, or be prevented from attending to his
duties, the leaves would gradually bring themselves to rest either in a
vertical or horizontal position without the least chance of any
catastrophe. As a still further precaution, however, hydraulic buffers
are fixed in such positions that if the men in charge lost control of
the bridge, and at the same time the apparatus above alluded to for
bringing up the motion of the leaves were to fail, their impact would
be taken by these buffers, which would bring them to rest in the same
manner as that in which the hydraulic cylinders that are attached to
heavy guns take up the recoil."
In cabins at the east and west
ends of each pier are indicators to tell the men in charge whether the
accumulators are full before starting the engines, and whether the
locking bolts are in their proper position. Further provision is made
to prevent the raising of the bascules before they are cleared of
traffic. The policemen in charge have to stretch a chain across the
entrance to each pier. As soon as the chain is fixed, the man carrying
it will be able to turn on the water to a small cylinder that draws it
tight and at the same time releases the locking arrangement of the
levers in the cabin. So that until the chain has opposed a barrier to
the traffic, it is impossible to draw the locking bolts at the centre
of the span.
The masonry of the towers is
independent of the steelwork that it encloses. In fact, great care has
been taken that there shall be no adhesion between the two substances.
This part of the structure, carried out by Messrs. Perry & Co.,
calls for no special attention here, though it impresses itself
favourably on the eye of the spectator. Objections have been raised to
the external masonry on the ground that it is a "hollow sham," but we
fancy that were the covering suddenly stripped away, so as to expose
the steel skeleton beneath, many objectors would be silenced. The
general opinion is that with so many metal structures exposing the
nakedness of their outlines the London Corporation is to be
congratulated on having thus boldly made a concession to the aesthetic
tastes of the community which does not detract from the value of the
bridge as a utilitarian erection. The cost of construction was
enhanced, but the result is one of which Londoners will be proud in
years to come.
The Tower Bridge, typical of
modern engineering skill, has an interesting connection with the old
London Bridge—itself a mechanical triumph considering the
science of
the time—built towards the end of the twelfth century. That
bridge,
which stood the wear and tear of nearly 700 years, was endowed with
certain lands which, with the growth of London, became extremely
valuable, and are now known as the Bridge House Estates. The revenue
from them has enabled the Corporation of London to rebuild the London
Bridge, throw another across the Thames at Blackfriars, and also to
construct the subject of this chapter.
We may conclude the account by
a few figures. The bridge is exactly half a mile long, including the
approaches, the side spans each occupying 270 feet clear. Its extreme
height, measured from the bottom of the foundations to the summit of
the main tower ridge-tiles, is 293 feet. The roadway of the side spans
is 35 feet wide, flanked on each side by a 121-foot paved footway. In
the central span the widths are reduced by 3 and 4 feet respectively.
Its construction, which occupied eight years, consumed 235,000 cubic
feet of granite and stone, 20,000 tons of cement, 70,000 cubic yards of
concrete, 31 million bricks, and 14,000 tons of iron and steel. The
columns on the main piers and abutments required five miles of steel
plates.
The total cost was estimated
at three-quarters of a million pounds, of which the bridge itself
represents rather more than half a million.
Sir J. Wolfe Barry, the
engineer responsible for the construction, includes among his other
important works the great Barry Dock near Cardiff, and the completion
of the Inner Circle Railway between the Mansion House and Aldgate
stations.
From:
The Romance of Modern
Engineering Containing Interesting Descriptions
in Non-technical Language
of the Nile Dam, the Panama Canal, the Tower Bridge, the Brooklyn
Bridge, the Trans-Siberian Railway, the Niagara Falls Power Co, Bermuda
Floating Dock, Etc., by Archibald Williams, Seely
& Co., London (1908), Chapter
5, pages
110-126.
