Tower Bridge 4

8/19/2019 Tower Bridge 4

1/17

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


2/17

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.


3/17

22

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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T r t ~ . / u / i U e ~ J s S I I ~

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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


4/17

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


5/17

24

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


6/17

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 .

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~ _ ~ . . 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


7/17

26

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


8/17

THE TOWER

BRIDGE

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


9/17

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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10/17

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


11/17

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.


12/17

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


13/17

32

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


14/17

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


15/17

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,


16/17

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.


17/17

36

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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