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THE TOWER RIDGE
LECTURE
BY
JOHN
WOLFE
B RRY
M INST C E
J L O ~ O l :
BOOT ON A
m
CARPBNTEH
4 1 D BALL};\ E C
I 94
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THE TOWER
BRIDGE.
21
vessels lying in
the
tiers would be more than the width necessary
for such an extension of the moving girders into
the
piers, as
would provide for a sufficient counterpoise.
The proposals of
the
Corporation, however, encountered
a strong opposition in Parliament from the wharfingers, who
carried on business with sea-going vessels above the site of the
bridge, and
the
Bill was also opposed by
the
Board of
the
Thames Conservancy. The Bill was referred to strong Com
mittees of both Houses of Parliament, and eventually was
passed very much in the condition in which
it
was brought in,
but with several stringent l u s e ~ for the protection of the
interests of
the
river traffic.
With
these few words on the principles
that
governed
the
main features of the Tower Bridge, we will proceed to consider
the
details of the structure generally.
ener al Description the ridge
The Act of Parliament defined the leading dimensions of
the
Tower Bridge to be as follows:
1
A central opening span of 200 feet clear width,
v ith a height of 135 feet above Trinity high water when
open, and a height of 29 feet when closed against vessels
with high masts. It may be mentioned in passing that
the height of the centre arch of London Bridge
is
29
feet above Trinity high water.)
2) The size of the piers to be 185 feet 111 length
and 70 feet in width.
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A
LECTURE
ON
3
The length of each of the two side spans to be
270 feet in
the
clear
The
Act
also defined the utmost permissible SIze of
the
temporary stagings in the river
The Conservators of
the
Thames who very properly con-
sidered chiefly the importance of
the
river traffic procured
the
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13. PLAN TO
Snow
LIMIT OF TEMPORARY WORKS
insertion in the Act of Parliament of a clause obliging the
Corporation to maintain
at
all times during
the
construction
of the
bridge a clear waterway of 160 feet in width and this necessity
occasioned much delay in the construction of
the
permanent piers
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TH
TOWER BRIDGE.
as the openmg defined was too wide to permit of both piers being
constructed simultaneously. The plan Fig. 13 shows
the
limits
of
the
temporary works as defined by Parliament. The outer lines
round each pier are the limits of the necessary stagings, and
it
will be seen
that
in order to gi
ve
a navigable width
at
all times
of
160 feet, there could be only one staging at a time of the
full width required for building the piers.
The formal ceremony of commencing the works of the
Tower Bridge was performed by
the
Prince of Wales on 21st
June, 1886.
The Government authorities gave every facility for
the
execution of the works, and, to enable the north approach to the
bridge to be made without interfering with very important
wharf property, allowed a small part of the Tower Ditch to lle
cccupied by a portion of
the
works.
this concession had not
been made, the cost of the land for the undertaking would have
been almost prohibitory.
t
was stipulated in
return that
the
design of the bridge should be made to accord with
the
archi-
tecture of
the
Tower, and at one time it was intended that the
new works should be made suitable for the mounting of guns and
for military occupation. The
latter
idea was afterwards to a
great extent
discarded.
The piers of
the
Tower Bridge are essentially different from
the piers of
an
ordinary bridge, inasmuch as they have to contain
the counterpoise and machinery of the opening span, as well as
to support
the
towers which carry
the
suspension chains of
the
fixed spans and the overhead girders above
the
opening span.
They are
thus
very complex structures, as will be seen by the
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A LECTURE ON
illustrations Figs. 14, 15 an d 1G Their form
plan Fig.
5
may be described as a square of 70 feet elongated by cutwaters
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L
L N G I TU
D IN
A
l e T ION F P IE R •
FIG 4
P N T
Q U R T E R P N T
C O
. J l 2 _ ~ . J
H L PL N T F
FIG
15
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TH E TOWER BRIDGE.
25
at
each end, bringing
the
total length to
185 fe
t
4
inches.
Their total depth from the roadway level to the London clay, on
which they rest,
is
102
feet.
7 --- - --
. : T £ 1 .
. 2 . C C ~
~ _ ~ . . 1 ' ~ ~ ~ . ~ ~ . ~ ~ ~ ~ . - ~ ~ ~ f ( ~ ~ ~ ~ t . ~ ~ ~ . f : . ~ ~ . ~ . I 0 0 : ' ; D . ~ : . ~ . ~ . ~ ~ . : ~ , - . : .. t
T R AN S VE R SI : S E qT IO N o ~ P IER
FIG.
16.
We will first consider the form of the piers up to the level
of the roadway, which is 32 feet above Trinity high water.
Each contains 1 a large cavity to receive the landward end and
counterbalance weight of one leaf of the opening
span
(2) two
large chambers for the hydraulic accumulators; 3 two chambers
for the machinery which actuates the opening
span
and
(4)
two long tunnels, one for receiving the main pivot shaft on
which
the leaf
of
the
opening span revolves, and
the
other for
the pinion shaft
by
which power is transmitted to the opening
span from
the
machinery.
A diagram Fig. 28, page 39, will explain
the
method of
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A
LECTURE
ON
:;l.ctuating the opening sp3 n The old bascule
r i g ~ s
of Holland
Fig. 9 had their counterbalance above the roadway level
mounted on posts at the abutments
and attached
to the bridge
by chains or ropes. The dimensions of the Tower Bridge forbade
such an arrangement of an overhead counterweight and
the
counterbalance is there applied as shown in Fig. 28 directly to
a prolongation of
the
girders of the opening span. These girders
turn on
the
main pivot behind which a space or cavity
has been provided to permit of the movement up and down of
the
landward ends of
the
girders and
the
counterweight.
This space which is called the bascule chamber or opening is
in the form of a quadrant and its dimensions are 50 feet from
north to south
44
feet from east to west;
it
is 50 feet in
height
next the central or ening span diminishing to nothing next
the
landward or fixed span of. the bridge. The two machinery
chambers are each
35
feet
by
30 feet
and
10 feet high
and
the
two chambers for accommodating the accumulators are each 30
feet
by
feet
4
inches
and
are 50 feet in height extending
from below the floor of the machinery chamber to within
feet
of the
bottom of the foundations.
Before describing the mode in which the
substructure
of
the
pier was constructed it will be best shortly to describe
the
general arrangement of
the
remainder of
the
fixed portion
of the bridge.
The mode adopted for spanning
the
landward openings is
by suspension chains which in this case are stiffened. The
chains are anchored in
the
ground
at
each end of
the
bridge
and united by horizontal ties across
the
central opening
at
a
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THE TOWER
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27
high level Fig. 30, page
40 .
These ties are carried
by
two
narrow bridges 10 feet in width, which are available as foot
bridges when
the
bascule span is open
or
the passage of vessels.
The foot bridges are 140 feet above Trinity high water, and, as
their
supports
stand
back
15
feet from
the
face of
the
piers, their
clear span is 230 feet. ·Access is given to th II I by hydraulic
lifts and by commodious staircases in the towers.
Above the landings, at the tops of the stairs and on which
the foot passengers land from the lifts, come the roofs of the
towers, the tops of which are 162 feet above the roadway level,
or 64 feet from the bottom of the foundations.
Having
now given the leading dimensions of the structure,
I will proceed to describe 1 the mode in which the piers were
constructed up to the roadway level;
2)
the details of the
opening span
and
machinery; 3)
the
details of
the
fixed super-
structure, namely, the towers,
the
suspension chains, and the
overhead footways; 4) the mode of erection of the super-
structure.
The Mode Constructing the Substructur e the Piers.
Iron caissons
strutted
with strong timbers were used in
excavating the bed of the river
and
building the foundations
of
the
piers. During these operations the external pressure of
the
water and
earth surrounding the caissons was very great,
as there is a depth of 32 feet of
water
at high tide at this
part
of the river, and the caissons had to be carried about 21 feet into
the
bed of
the
river to secure a good foundation. The caissons
employed were boxes of wrought iron, without either top or
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A LECTURE
ON
There are twelve caissons for each pier,
bottom, l:md
with the
bottom edges made sharp and strong Fig.
18,) so as to easily penetrate the ground.
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THE
TOWER
BRIDGE.
29
FIG 8
understood from Fig
3
Those about
the
central parts
of
the
pier are
28
feet square in plan and those near
the
cutwaters are triangular in plan
the
dimensions being
35
feet by
33
feet 8 inches Fig 7 shows a square caisson and a
triangular caisson in plan with
their
timbering and other details
I will describe the mode of sinking one
caisson and
the
description will apply more
or less to all though of course
the
circum-
stances attending the different caissons
required some differences of treatment
The temporary staging for the pier
haying been made with piles
the next
ROLLED S T £ £ L ; ; ; ; ~ ~ G FOGE. operation was to erect
the
caissons upon
the
staging
r lV
ALP CROSS St CTION OF P I ER,
SHOWINO C IISON
E ORI
LOWIRIHG
TO
,THE
BED THE RIVER
n
HALF CROSS SECTION OF
PlEA,
SHOWINQ CAIS SQ WHIN
UHt
• El Mo FlUID
WITtt
eONCRITI
FIG 9
The bottom part of
the
caisson having to be sunk deep into
the
bed of
the
river could not be removed on
the
completion
of
the
pier and was thus named
the
permanent caisson The
purpose of the upper part of the caisson was merely to exclude
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30
A
LECTURE
ON
FIG.
2
the
water during the process of building
the
pier,
and
it
could
be removed when
the
brickwork
and
masollI y were finished.
This
part
was
thus
called
the
temporary caisson.
The permanent cai son was 19
feet in h ight, divided horizontally
into hvo portions. was erected
on timber supports, which were
slightly
above low
water
mark Fig.
19),
where
it
was
rivetted
together
and
firmly strutted inside
with
strong
timbers, inches square. was then
lifted slightly
by
four powerful screws
attached to four rods, from which was
slung the weight of the cai son and the I H i l : _ E : ~ ~ ~ E : ~ O I T .
timbering in it. The timber supports
were removed,
and
the caisson was lowered by the screws on to
the bed of the river, which had previously been levelled by divers.
After
the
permanent caisson reached the ground various
lengths of temporary caisson were added to it till the top of the
temporary caisson came above
the
level of high water. The
junction between
the
permanent
and
temporary Caissons was
made
with
india rubber, as shown in Fig 20.
Divers working inside the caisson, excavated first the
gravel
and
then the upper part of the clay forming the bed of
the river,
and
as they dug away the soil, which was hauled up
by a crane and taken away in barges, the caisson gradually sank,
until at
length its bottom edge penetrated some 5 to 1 feet into
the
solid London clay.
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THE
TOWER
BRIDGE
31
London clay is a firm, water tight stratification, and, when
the
above-mentioned depth was reached,
it
was safe to pump out
the
water, which up to this time remained in
the
caisson, rising
and falling with
the
tide through sluices in the sides. The
water
having been pumped out, navvies proceeded to the bottom
of the caisson
and dug
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 and of
the
dimensions above stated, in which
the
preparation of
the
foundations could be commenced. A detailed
view of one of
the
completed caissons is given in Fig. 21
I may mention in passing how important it is in order to
ensure success in sinking caissons or cylinders,
that
they should
be controlled from above and be prevented from sinking unevenly.
is very easy to prevent a caisson from going wrong like many
animate subjects as well as inanimate by timely control, but
it
is a very different thing to put the matter right when a wrong
course has been pronouncedly taken.
London clay being peculiarly hard and uniform in texture,
advantage was taken of this circumstance to increase
the area
of
the
foundations by digging out sideways or undercutting
below the edge of the caisson, as shown at the bottom of Fig. 21
The caisson having been controlled from
the
first by
the
suspending rods to which allusion has been made, its descent
any
further than was desired was easily arrested
by
the rods,
when
the
bottom of
the
caisson was
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
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A L TUR ON
H LF
T
I N S D £ £L£VATION
C A I S S O N S
H LF OUTSI £
~ L £ V T I O N
FIG 21
COMPJ ETE CAISSON
WITH
TIMfiJ;RING D
SUSPENSION RODS
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THE TOWER BRIDGE.
33
SHOWINO UNDERCUTTING
lTWIItf W AD JOIHING CAISSONS
carried up to
4
feet
.
F °3
m Ig
.
.6 .
FIG.
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 excavations adjoined similar
excavations from the next caissons
Fig. 22 , the whole foundation was
made continuous. The whole of the
permanent caissons with the spaces
_ I
between them were then completely
filled with concrete, upon which
the
brickwork and masonry were com-
m ~ n c e d in the temporary caisson,
and
above
Trinity
high water, as shown
J
•
FIll.
3
CROSS S E T I O ~ OF PlER SHOWING
OUT I E
WALL
O ~ I P L E r E D
t was
not
desirable to build isolated portions of the brick-
work
and
masonry, even
if they
were joined together afterwards.
Accordingly the temporary caissons were so designed as.to admit
of their
sides being removed Fig.
15
and
of
the
whole area
enclosed by their front and back plates being thrown together to
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4
A LECTUHE ON
permit of continuous building.
For
this purpose the corners of
the
caissons were united by timber piles, which were driven
in a groove
each caisson Fig.
24 ,
and when these
had been driven and made water-tight as to which no difficulty
JIol e4 /0.,.
B0lt 1
KL etP.JP fl.ooWU'.-«n J
-tVnbcr fhx ..c,.
L I 3 3 - 1 :
DETAIL OF V lR TIC L O IN TS
AND PILE
GRVDV\:.lI·
FIG ENLARGED PLAN OF THE ANGLE OF A CAIS ON.
occurred , the sides of the temporary caIssons were removed.
In
this
way
the
outside portions of
the
piers were built,
and
eventually formed a continuous ring of a strong masonry wall,
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THE TOWER BRIDGE.
35
water- tight and
able to resist the external pressure of the
water. Fig.
25).
The foundations of the central portion of
the
pier, enclosed
by the
outside walls, were then excavated
and
the pier completed.
S ~ O W R S T R U · T I ~ G
IlE TWEEN
C A I S S O N S M A S O N R
FIG.
5
HALl PLAN OF PIER AT D U F E I l E r r PERIODS OF CONSTRUCTION.
The abutments of the bridge were built within ordinary
coffer dams, and, though formidable in size and depth, presented
no new features of construction such as have been explained
with regard to
the
piers.
When the piers
and
abutments had reached a
height
of
4
feet above high water,
the
first contract was finished, and new
contracts for the superstructure were let.
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A
LECTURE
0
The work of the foundations was troublesome and tedious,
owing to the isolation of the piers, and still more to the
great amount of river traffic, rendering the berthing of barges
difficult. substl uctUl e
thus
occupi d a considerably longer
time
than
was anticipated.
---- ,---_o,J '
, - - - - - ~ -
=
.
26. VIEW OF TilE 0
~ D ~ T I O : S
IN PROf:RESS
The view Fig. 26 gives an idea of the appearance of the
works duriug
the
construction of the piers.
h pening Span
The stipulated dimensions of the opening span have been
already given, as providing, when the bridge is open for ships, a
clear waterway of 200 feet in width, with a clear height
throughout
the
200 feet of 135 feet which has been increased in
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