Tower Bridge, London

Tower Bridge, London

BHA have been
involved in the engineering aspects of Tower Bridge since designing the
present electro-hydraulic drive system in 1974 . The article below, from
a 1930's partwork, describes the building of the bridge and gives some
details, albeit in the imperial measure of the day. Tons are imperial
long tons, approximately equal to a metric tonne; one foot is 0.3048
metre.
The hydraulic
motors, installed in 1974/75, drive the bridge through gears and the
original final pinions which engage the arcuate racks attached to the
moving bascules. In the horizontal position, some load is taken by
resting blocks beneath the bascules. Work is going on to develop a way
of transferring all the load to these blocks, as the original designers
are believed to have intended.


click
here for a section through the bascule

BUILDING THE TOWER BRIDGE
Description
from an article in the 1930's in Wonders of World Engineering, published by
Fleetway House

The most famous example of the bascule bridge is the Tower Bridge
across the River Thames in the heart of London. Engineers were able to build
this type of bridge without interrupting traffic on the great commercial
waterway.

The problem of building a bridge over a busy river with low banks so
that shipping is not obstructed is one that taxes the resource and
ingenuity of the engineer. He surmounts the difficulty by resorting to the
opening type of bridge, of which the main types are the drawbridge or
bascule bridge, turning about a horizontal axis ; the swing bridge,
turning about a vertical axis ; the rolling lift bridge and the vertical
lift bridge.

One of the most famous examples of the bascule type is the Tower Bridge,
which spans the River Thames just below London Bridge. It is the most
distinctive of London's bridges and its construction was a masterly
engineering achievement. The building of the Tower Bridge came about
because the development of cross-Thames traffic had far outstripped the
capacity of the existing bridges.

By the year 1870 the position had become serious, and between 1874 and
1885 some thirty petitions from various public bodies were brought before
the authorities urging either the widening of London Bridge or the
building of a new bridge.

A two days' census taken during August 1882 showed that the average
traffic for twenty-four hours over London Bridge -which at that time was
only 54 feet wide-was 22,242 vehicles and 110,525 pedestrians. A committee
was appointed to consider the matter and to report upon the different
plans that had been proposed.

These included schemes for low-level bridges with swing openings of
various kinds, and high-level bridges with inclined approaches or with
lifts at either end. There was also a proposal for a railway line to be
built at the bottom of the river and to carry a traveling staging with its
deck projecting above high-water level. Proposals for a subway and for
large paddle wheel ferry boats were also considered. one of these schemes
was approved.

In 1878 Horace Jones, the City architect, put forward a proposal for a
low-level bridge on the bascule principle - that is, a bridge on a level
with the streets with two leaves or arms that could be raised to let ships
pass up and down the river and lowered to let vehicles pass to and from
across the waterway. Successful bridges of this type already existed,
though on a much smaller scale, at Rotterdam and Copenhagen.

"Bascule" is derived from the French word for see-saw,"
and the bascule bridge is a kind of drawbridge which works on a pivot and
has a heavy weight at one end to balance the greater length at the other.
This was the type of bridge finally decided upon, and it has proved a
great success.

The Tower Bridge is, perhaps, the most famous bascule bridge in the
world, and its working from the day it was first opened to the present has
been perfect, far exceeding the hopes even of its most enthusiastic
advocates. An Act of Parliament empowering the Corporation of the City of
London to build the bridge was passed in 1885.

Horace Jones was appointed architect and was knighted, but died the same
year, and Mr. (afterwards Sir) John Wolfe Barry was appointed engineer.
The work was divided among eight different contractors Among them Sir John
Jackson was responsible for the piers and abutments, Sir William Arrol for
the steel superstructure, Sir W. G. Armstrong, Mitchell and Co., Ltd., for
the hydraulic machinery and Perry and Company for the masonry
superstructure.

Work was started on the bridge in April 1886, the foundation stone being
laid, on behalf of Queen Victoria, by the Prince of Wales, afterwards King
Edward VII. The bridge was to have been finished by 1889, but difficulties
arose and Parliament was twice asked to extend the time for the completion
of the work.

It did so, and the bridge was eventually opened on June 30, 1894, having
cost about £1,000,000 sterling to build, a remarkably small sum for
such a bridge in such a position. The total length of the bridge,
including the approaches, is half a mile. The roadway has a width of 35
feet and on either side of it is a footway 12.5 feet wide.

The total height of the towers on the piers, measured from the level of
the foundations, is 293 feet.

140 Feet Headway for Ships

In building the bridge there were used about 235,000 cubic feet of
Cornish granite and Portland stone, 20,000 tons of cement, 70,000 cubic
yards of concrete, 31,000,000 bricks and 14,000 tons of iron and steel.


The bridge is a combination of the suspension and bascule type. The
width of the river between the abutments of the bridge on the north and
south sides is 880 feet. This is crossed by three spans. The two side
spans, each 270 feet long, are of the suspension type. They are carried on
stout chains that pass at their landward ends over abutment towers of
moderate height to anchorages in the shore. At their river ends the chains
pass over lofty towers which are themselves connected at an elevation of
143 feet above high water. Heavy tie bars, at the level of the connecting
girders, unite the two pairs of chains so that one acts as anchorage for
the other at the centre.

The central span has two high-level foot ways side by side, and one
low-level roadway. High-level girders carry the upper footways, which are
reached by hydraulic lifts or staircases in the main towers. The roadway,
or central opening span, is 200 feet long and consists of two bascules or
leaves.

The Tower Bridge Act laid down that when the bridge was open there
should be a clear headway at high tide between the water and the
high-level footways of 135 feet and a headway of 29 feet when the bridge
was closed. These dimensions were exceeded in practice, the open height
being 5 feet and the closed height 6 in. greater than had been prescribed.
This was above high-water level. The greatest extreme between high and low
tide at Tower Bridge is 25 feet.

The Act further stipulated that the piers were to be 185 feet long and
70 feet wide. There was also a clause making it compulsory to maintain at
all times during the building of the bridge a clear waterway 160 feet
wide. This stipulation made it impossible for the two piers to be built at
the same time, because the staging would have occupied far too much of the
river space. As the use of timber cofferdams was prohibited, the builders
had to rely on caissons. The restricted area which they were allowed for
their staging, 130 feet by 335 feet, did not permit the use of one caisson
extending the full length of a pier.

The builders therefore adopted a system of small caissons covering the
area of the pier. By this means it was possible while building one of the
piers to be working also at the shore side of the other. Had both piers
proceeded simultaneously a saving of thirteen or fourteen months might
have been effected.

The piers of the Tower Bridge are much more complicated structures than
the piers of an ordinary bridge. In addition to supporting the towers
carrying the overhead girders for the high-level footways and the
suspension chains of the fixed spans, they also house the counterpoise and
the machinery which operates the bascules.

Triangular Caissons

The caissons used for securing the foundation of the piers consisted of
strong boxes of wrought iron, without either top or bottom. To secure a
good foundation it was found necessary to sink them to a depth of about 21
feet into the bed of the river. There were twelve caissons for each pier.
On the north and south sides of each pier was a row of four caissons, each
28 feet square, joined at either end by a pair of triangular caissons,
formed approximately to the shape of the finished pier. There was a space
of 2.5 feet between all the caissons, this being considered the least
dimension in which men could effectively work. The caissons enclosed a
rectangular space 34 feet by 124.5 feet. The space was not excavated until
the permanent work forming the outside portion of the pier had been built,
in the caissons and between them, up to a height of 4 feet above
high-water mark.

The method adopted in building and sinking the caissons was unusual.
First came the building of the caisson upon wooden supports over the site
where it was to be sunk. The caisson was 19 feet in height and it was
divided horizontally into two lengths. The lower portion was known as the
permanent caisson and the upper portion, which was removable when the pier
was completed, was called the temporary caisson. The object of this upper
portion was simply to keep out water while the pier was being built. When
ready the supports were removed and the permanent caisson lowered to the
river bed (this had previously been levelled by divers) by means of four
powerful screws attached to four lowering rods.

After the caisson had reached the ground various lengths of temporary
caisson were added to the permanent section, till the top of the temporary
portion came above the level of high water. The joint between the
permanent and the temporary caissons was made tight with india-rubber.
Divers working inside the caisson excavated first the gravel and then the
upper part of the clay forming the bed of the river. As they dug away the
soil, which was hauled up by a crane and taken away in barges, the caisson
gradually sank until its bottom edge penetrated some 5 feet to 10 feet
into the solid London clay. London clay is a firm watertight stratum, and
when the desired depth had been reached by the caisson it was safe to pump
out the water, which up to this time had remained in the caisson, rising
and falling with the tide through the sluices in the sides.

The water having been pumped out, navvies were able to get to the bottom
of the caisson and to dig out the clay in the dry. Additional lengths of
temporary caisson were added as the caisson sank, so that at last each
caisson was a box of iron 57 feet high, in which the preparation of the
foundations could be made. The caisson having been controlled from the
first by the lowering rods and screws, its descent any farther than was
desired was easily arrested by the rods when the bottom of the caisson was
20 feet below the bed of the river. The clay was then excavated 7 feet
deeper than the bottom of the caisson, and outwards beyond the cutting
edge for a distance of 5 feet on three of the four sides of the caisson.
In this way not only was the area of the foundations of the pier enlarged
but, as the sideways excavation adjoined similar excavations from the next
caissons, the whole foundation also was made continuous.

All the permanent caissons, with the spaces between them were then
completely filled with concrete, upon which the brickwork and masonry were
begun in the temporary caisson and carried up to 4 feet above high water.
The preparation of the foundations was a long and troublesome task because
of the extent of the river traffic, which made it difficult to berth the
necessary barges. On two occasions "blows" occurred which
hindered the operations. When the cutting edge of one of the caissons had
reached a depth of 16 feet beneath the river bed, water rushed into the
caisson through a rent in the clay. The caisson had to be lowered still
further to seal the opening when the water was pumped out.

The second blow was due to one of the stage piles between the caissons
having been driven in aslant. As the caisson went down its cutting edge
came in contact with the pile and thus loosened the clay in the immediate
neighbourhood. Divers were sent down to ascertain the damage and the pile
was re-driven. The full extent of these handicaps was underestimated and
thus this section of the work occupied much longer than had been expected.
Finally there emerged four feet above high-water mark two gigantic piers
of concrete, granite and bricks able to withstand without settlement a
load of 70,000 tons. From the river bed upwards the piers are faced with
rough picked Cornish granite, in courses between 2 feet and 2.5 feet
thick. The piers called for the excavation of 30,000 cubic yards of mud,
silt and London clay. The material consumed in the piers was 25,220 cubic
yards of cement, 22,400 cubic yards of bricks and 3,340 cubic yards of
Cornish granite. The cost of the piers was £111,122. As soon as the
piers had been finished the building of the towers began.

Stone Over Steel

Because of the fine masonry work of these towers, Tower Bridge is often
mistaken for a stone bridge. It is a steel bridge, however, just as much
as is the Forth Bridge, and it depends entirely for its strength upon the
steel columns and girders of which it is composed. As the authorities
insisted that the design of the bridge should be in keeping with its
surroundings, the steelwork is faced with masonry whose architectural
character is made to harmonize with the general style of the Tower of
London close by.

The masonry is Cornish granite and Portland stone, backed with
brickwork. Each of the steel towers consists of four octagonal columns,
with a diameter of 5 ft. 6 in., connected at a height of 60 feet above the
piers by plate girders, 6 feet deep. Across these are laid smaller girders
which carry the first landing. Twenty-eight feet higher is the second
landing, similarly built, and at an equal distance above that is the third
landing, leading to the high-level footways Each column rests on a massive
granite slab previously covered with three layers of specially prepared
canvas to make the pressure even and the joint watertight. The columns are
keyed to their foundations by great bolts built into the piers.

All four columns in each pier are braced diagonally to resist the wind
pressure, which is calculated at a maximum of 56 lb. to the square inch, a
pressure several times greater than has ever been registered in the
locality. It was important that precautions should be taken to prevent any
adhesion between the masonry and the steelwork of the towers. With this
object the columns were covered with canvas as the masonry was built round
them, and spaces were left in places where any later deformation of the
steel work might bring undue weight upon the adjacent stonework. The
masonry covering forms an excellent protection against extremes of
temperature.

All parts of the metal not accessible for painting purposes after the
bridge was completed were coated thoroughly with Portland cement. Manholes
were provided in the steel columns to make it possible to paint the
interior whenever it became necessary. The abutments of the bridge, which
were built by means of cofferdams in the usual manner and without
difficulty, have similar but shorter towers.

The towers finished, workmen tackled the high-level footways. These are
cantilever structures, each with a suspended span. They were built out
from either tower simultaneously. The footways are cantilevers for a
distance of 55 feet from either tower and suspended girders for the
remaining distance of 120 feet between the cantilever ends. The building
of these cantilevers attracted a great amount of attention on the part of
the public, who watched their gradual approach with keen interest. Every
care was taken to prevent rivets, fragments and tools from falling into
the river below, to the peril of passengers in passing vessels.

Intricate Suspension Chains

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 is composed of eight plates 2 feet deep and 1 inch thick, ending
in large eye-plates to take the pins uniting them to the suspension
chains. The making 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 bracing so as to form a rigid girder. They were
built in the positions they had 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.

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 bored to within 0.5 in. 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 enlarging all the holes to
their full diameter was equivalent to boring a hole with a diameter of 2
ft. 6 in. through 65 feet of solid steel. 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 outstanding feature of the bridge is its opening span, consisting of
two bascules or leaves. Each leaf consists of four parallel girders 13.5
feet apart and about 160 feet long. When lowered the leaf projects
horizontally 100 feet towards the opposite tower, spanning exactly half of
the opening. The point of balance is a solid pivot, with a diameter of 1
ft. 9 in. and a length of 48 feet. It 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 was due to
the condition that compelled the contractors to leave a clear way of 160
feet between the towers. In 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 riverward steelwork and launched the
incomplete girders from the main stage close to the piers into the bascule
chambers. A steel mandrel (cylindrical rod) was inserted to carry their
weight while they were turned into a vertical position. The mandrel was
then withdrawn to make room for the permanent pivot, which weighed 25
tons. The outer ends were added to until a point 53 feet from the pivot
had been reached. Work in this direction then stopped until the raising
and lowering of the leaves for purposes of adjustment had been concluded.


After that the girders were completed vertically. The leaves, each of
which weighs about 1,200 tons, are moved by toothed pinions, engaging with
steel quadrantal racks riveted to their two outside girders. The accurate
attachment of the racks was a somewhat difficult business because 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
counterpoises, 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 1,200-tons 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 lb.
to the square foot.

Opened and Shut in Five Minutes

The source of power is a building on the east side of the southern
approach, where are stationed two large water accumulators with 20-in.
rams loaded to give a pressure of from 700 lb. to 800 lb. per square inch.
The engines are duplicated on either pier to provide against the
possibility of breakdown. The operations of opening and shutting the
bridge are safeguarded by every possible means. When the leaves are
brought together bolts carried on one leaf are locked by hydraulic power
into sockets on the other leaf. In the event of anything going wrong with
the opening and closing mechanism there would be no danger of disaster,
for the leaves would be brought gently to rest in either the vertical or
the horizontal position.

The whole process of opening the bascules, allowing a ship to pass and
bringing them down again for the resumption of road traffic takes only
five minutes. Thus the large hydraulic lifts, which go to the top of the
tower to the overhead footway with eighteen passengers in one minute, are
rarely used. It has been found that the interruption of traffic is so
brief that pedestrians do not take the trouble to go up and over the
footway, but wait for the lowering of the bascules.