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Full text of "The tower bridge; a lecture"



: 



TT-'iO-T 



THE TOWER BRIDGE. 



A LECTURE. 



BY 

JOHN WOLFE BARRY, 

M.INST.C.E. 



XonDon : 

BOOT, SON AND CARPENTER, 
24, OLD BAILEY, B.C. 

1894. 



ERRATA. 

Page 43, line 11, for 12 inches" read " 27 inches; 
Page 45, line 3, for "their" read "these;" 
Page 51, line 16, for "down" read "into." 



PREFACE. 



Lecture was composed in 1893 for an audience 
consisting in part of persons well acquainted with 
engineering matters, but more largely of those who took an 
interest in the Tower Bridge as members of the general 
public. It is not to be regarded as being such a professional 
description of the Bridge as would be suitable to a body of 
experts ; but it may be of interest to a considerable number 
of persons who have seen the Bridge in its various stages of 
progress, or who wish to form a general conception of its 
design and of the various considerations which led up to 
its inception and determined its mode of execution. 

J. W. B. 



THE TOWER BRIDGE. 

A LECTURE. 

BY 

JOHN WOLFE BARRY, M.lNST.C.E. 



Introductory. 

T)EFORE entering upon a description of the structure of 
the Tower Bridge, it may be interesting if I touch 
first upon the history of the principal bridges of London, and 
then upon the necessity of means being afforded for crossing 
the river below London Bridge, with a glance at the various 
suggestions which have been made to meet the want. 

A bridge appears to have existed on the site, or a few 
yards lower down the river than the site, of the present 
London Bridge, from remote antiquity. We read of Canute 
attacking and being repulsed from London Bridge, and some 
writers have held that a bridge at this spot dates from the 
Roman occupation of London, which was then known to the 
world as the town Augusta, and must have been of con- 
siderable importance. Stow says that in the year 994 the 
Danes were repulsed in an attack which they made on 
London, because they took no heed of the bridge ; but 
probably there is some misconception here, for it is recorded 
that only in the previous year Anlaf, the Dane, sailed up 



2 A LECTURE ON 

the Thames as far as Staines with ninety-three ships and 
ravaged the country. Discarding the tradition of a Roman 
bridge, it seems clear that the first London Bridge of which 
any record exists was erected between the years 993 and 
1016, when Canute attacked London a second time, and made 
a canal on the south side of the river so as to bring his ships 
past the bridge. The first bridge was probably of wood, and 




FIG 1. OLD PUTNEY BRIDGE. 

somewhat of the form which many of us remember in the 
cases of old Putney and old Battersea bridges, the piers being 
formed of piles driven into the bed of the river, and the 
openings spanned by timber beams. (The view (Fig. 1) is 
taken from a photograph of old Putney Bridge, and iUustrates 
that mode of construction.) This wooden bridge had great 
vicissitudes. It was washed away in a flood in 1 091, was rebuilt 
m 1097, and burnt in 1136. It was again rebuilt, but was 



THE TOWER BRIDGE. 3 

in so bad a state in 1163 that a new bridge was resolved on, 
and this time in more durable materials. 

A certain monk, Peter Curate of St. Mary, Colechurch 
being known for his skill in bridge building, was employed to 
erect the new bridge, which was begun in 1176 and finished in 
1209. Peter of Colechurch did not live to see the completion 
of his great work, but died in 1202. 

The bridge though of stone, gave cause for anxiety early 
in its history, for the structure was in bad condition in 1280, 
and five of its arches were borne down by the ice and floods 
after the great frost of 1282. When the design of the 
bridge, with its 20 small arches and thick piers, is considered, 
it will be seen what an obstruction it must have caused 
to the flow of the water, and how serious the wash must 
have been through it. In fact, we know that there was, 
even without any exceptional flood, a fall of five feet on the 
surface of the water through even the widest of the arches. 
Apart from the danger to the structure from such a rush of 
water, the impediment to river traffic must have been very 
great, and shooting London Bridge in a boat must have been a 
hazardous enterprise at most times of the tide. 

The piers were built on a forest of piles, the tops of which 
were about 9 feet above the bed of the river ; and either 
originally, or as the necessity for protection to the piles was 
recognised, starlings or projections all round the piers were 
added. The starlings consisted of pilework, carried 3 feet above 
the tops of the main piles of the bridge, the intervening 
space being filled in with stone or chalk. 



4 A LECTURE ON 

The bridge at first was like modern bridges in being 
unencumbered with houses, except that St. Thomas's Chapel 
was built on its east side on the ninth pier from the north end. 
The chap-dl was 65 feet long, 20 feet wide, and 14 feet high, and 
it must have projected beyond the roadway on to the pier. 

There was a drawbridge near the southern end, partly for 
defence, but probably also for the passage of ships. To this 
extent, it was prophetical of the Tower Bridge. 




FIG. 2. OLD LONDON BRIDGE. 

In 1426 a tower was built on the north side of the draw- 
bridge to resist an enemy, and in 1471 a commencement was 
made in building houses on the bridge. These were added to 
from time to time, and amongst them was a famous structure 
called Nonsuch House, which was a very beautiful building in 
the architecture of its period, and was greatly admired. I 
suppose that the houses were chiefly of wood ; for it is recorded 



THE TOWER BRIDGE. 5 

that, in 1632, forty-two houses were burnt, which were subse- 
quently rebuilt ; and, again, in the fire of London in 1666, almost 
all the houses were burnt, great lamentations being made over 
their destruction. One would have thought that the bridge wa,s 
rather improved by the catastrophe : but Londoners did not 
think so in those days, for it appears to have been an important 
business street, and the sites were let out to tenants at large 
rents. In ten years' time the houses were all rebuilt, and the 
bridge appeared as in the view (Fig. 2) which dates from 1725. 

At that time the bridge was described as 9 1 5 feet and 1 inch 
long. Observe the nicety of one inch in such a measurement. 
Its height above the water was 43 feet 7 inches. The whole width 
was 73 feet, of which only 20 feet was devoted to the roadway, 
and 53 feet to the buildings on both sides of it. The approaches 
to the bridge must have been very steep and inconvenient. 

Such was old London Bridge, and so it existed till 1758. 
It might, I think, be described as inconvenient for road traffic, 
dangerous for river traffic, and more like an indifferent weir 
than a bridge over a navigable river. Indeed, its functions 
as a weir were utilised in 1582 ; for water wheels were erected 
in several of the arches at the north end, by which water was 
pumped for the supply of London. 

Whatever we may think now of the ancient bridge, 
people in the 18th century were proud enough of it, and 
occasionally burst into poetry about it. Here is a translation, 
made in 1725, of a Latin ode of the same date in its honour : 

" When Neptune from his billows London spyed, 
Brought proudly thither by a high Spring-tide, 



(5 A LECTURE ON 

As through a floating wood he steered along, 
And dancing castles clustered in a throng ; 
When he beheld a mighty bridge give law 
Unto his surges and their fury awe ; 
When such a shelf of cataracts did roar, 
As if the Thames with Nile had changed her shore ; 
When he such massy walls, such towers, did eye, 
Such posts, such irons, upon his back to lye ; 
When such vast arches he observed, that might 
Nineteen Rialtos make for depth and height : 
When the Cerulean God these things surveyed, 
He shook his trident, and astonished said 
Let the whole earth now all the wonders count, 
This bridge of wonders is the paramount ! " 

There is a little poetical license in more ways than one in 
the ode ; for the Rial to has a span of 90 feet as compared with 
the narrow openings of old London Bridge, varying from 10 to 
30 feet in width and obstructed by the starlings round the 
piers. You will also see that the local poet was even proud of 
the cataracts, which endangered the lives of travellers in boats 
when they shot the bridge with the current, and made it impos- 
sible for any one to pass the bridge against the stream. 

The houses on London Bridge or sufficient of them to 
aiford a wide roadway and two good 'footpaths were ordered 
to be finally pulled down in 1746. 

Various designs were made from time to time for improving 
London Bridge, and especially in respect of the waterway. 
The views (Fig. 3) show two proposals, the upper one by 
Labeleye, the other by Sir Christopher Wren. Neither design 
was executed ; but that of Wren is a good example of 



THE TOWER BRIDGE. 



the grasp by that distinguished architect of the problem to be 
dealt with, and would, if it had been carried out, have been an 
immense improvement of the navigation. 

In 1758-9 an arch 70 feet wide was made (by removing the 
centre pier of the bridge) for the accommodation of the river 
traffic : but before long it was felt that the old bridge had 
outlived its day, and it was replaced by the present beautiful 




FIG 3. LABELEYE'S & WREN'S DESIGNS FOR IMPROVING OLD LONDON BRIDGE. 

bridge, by Rennie, which was begun in 1824 and opened in 
1831, at a cost, including the approaches, of about 1,500,000. 
The gradients leading to Rennie's bridge from the south are 
unfortunately steep, being 1 in 27 from the Borough and 1 in 
21 from Tooley Street, and are very distressing to the horses 
of heavily laden wagons and omnibuses. 

It is remarkable how long London existed and flourished 
with only one bridge across the Thames ; for it was not till 1729 
that any other bridge was built. This was Putney Bridge, 



A LECTURE ON 



already alluded to, which could, indeed, from its distance from 
the town, scarcely be called a bridge to serve the needs of 
London. Such, however, was the jealousy of the Corporation 
of London that they espied in the project of building a bridge 
at Putney an attack on the interests of London, petitioned 
Parliament urgently against it, and prophesied that, if it were 
authorised, the trade of London would wane and decay. 

Putney Bridge cost the modest sum of 23,750, and lasted 
till 1885, when the handsome stone bridge now existing was 
built out of the rates, costing, with its approaches, about 
600,000. For some years before the bridge was pulled down 
it had a central opening, which did not exist in the original 
structure, but was made by removing some of the old piers and 
spanning the opening by iron girders, supported on iron cylinders. 




;. 4. OLD WESTMINSTER BRIDGE. 

Old Westminster Bridge (Fig. 4) followed soon after Putney 



THE TOWER BRIDGE. 9 

Bridge in order of date, an Act of Parliament having been 
granted for it in 1735, and the bridge having been opened in 
1750. It was 44 feet wide and 1,223 feet long. The centre arch 
was 76 feet wide, and the whole structure was greatly admired. 
The cost of it appears to have been about 390,000, which 
sum perhaps includes the cost of the approaches. The engineer 
was Charles Labeleye. It fell into decay from failure of the 
foundations, which rested on timber caissons, and was replaced 
by the present bridge about 1861. The view is taken from an 
old print, and gives some idea of the old bridge in its palmy 
days, before settlements and decay had set in. 

Old Blackfriars Bridge (Fig. 5) was authorised in 1750, and 
opened in 1769. It was not unlike Westminster Bridge and was 




FIG. 5. OLD BLACKFRIARS BRIDGE. 



built by Robert Mylne. As in the case of Westminster Bridge, 
its foundation failed, and from the same causes. It was replaced 



10 A LECTURE ON 

in 1869 by the present fine bridge, constructed of iron and 
stone, from the designs of Joseph Cubitt. 

It is usual to hear, in these days of iron and steel 
construction, that the old stone bridges had an element of 
permanence in them greater than can be hoped for in the case of 
those constructed of iron or steel. But it is a curious com- 
mentary on this view that within our memory Westminster 
and Blackfriars bridges had to be pulled down within 100 
years of their construction, that the piers of Waterloo Bridge 
have required to be strengthened, and that some of the founda- 
tions of new London Bridge have given rise to anxiety. 

The main reason of these failures or partial failures is, no 
doubt, to be found in the increased scour of the river, due to 
the removal of old London Bridge, which, as has been said, 
acted very much as a weir, and prevented the free flow of the 
tide. But it is doubtful whether this was the only cause. 
Apart from the question of scour, there can be no doubt 
that stone bridges bring a heavy weight on the foundations, 
which therefore require great care. They also involve an 
arched form of construction. The Eastern proverb says truly 
that the arch never sleeps, and thus defective foundations in 
an arched bridge involve serious results. 

Another bridge of the 18th century was old Battersea 
Bridge, which was similar to old Putney Bridge. 

Vauxhall Bridge was opened in 1816, at a cost of 300,000. 
It is a poor example of bridge building, and is unworthy of the 
age which produced Southwark Bridge. It is already found 
inadequate, and is to be replaced by a better structure. 



THE TOWER BRIDGE. 



1 1 



Waterloo Bridge was designed by Rennie, and was opened 
in 1817 at a cost of 800,000. It is a very fine work, but 
is narrower than foresight should have predicted. 

Southwark Bridge (Fig. 6), designed by the same engineer 
as New London and Waterloo Bridges, was, at the time of 
its erection, and still remains, an excellent example of iron 
and stone construction. It was opened in 1819 at a cost of 




FIG. 6. SOUTHWARK BRIDGE 



800,000, but it also is narrow and the approaches are unduly 
steep. In the view of Southwark Bridge old London Bridge 
appears with the central arch as widened in 1759. 



Proposals for Crossing the Thames beloiv London Bridge. 

We see, from the above imperfect history of bridge building 
in London during this period, that, though London Bridge alone 



12 A LECTURE ON 

served our forefathers for upwards of seven centuries, no less 
than eight bridges were built in a single century, from 1730 to 
1830. To this number five more bridges, exclusive of railway 
bridges, were added in the next half century. 

All this activity must have resulted from a great develop- 
ment of South London, and no doubt aided immensely in that 
development. But London Bridge still remained the bridge 
lowest down the river, and but little was heard of any other 
means of crossing the river below this spot till 1824, when Sir 
Marc Isambard Brunei conceived the project of the Thames 
Tunnel between Wapping and Rotherhithe. 

The difficulties of that work are known to everybody. 
There were frequent irruptions of water into the works, and it 
was not till 1843 that the Tunnel was opened for foot traffic 
only, the original idea of accommodating vehicular traffic having 
been abandoned, owing to the expense which had already been 
incurred, and which had amounted to about 468,000. The 
Thames Tunnel was purchased about twenty-five years ago by 
the East London Railway Company, whose trains now run 
through it. 

South London, probably, did not begin to extend much 
below London Bridge till after Lambeth and Southwark had 
first been developed. I came upon an old print lately (repro- 
duced in Fig. 7) which indicates what Bermondsey was like only 
90 years ago. It shows the ruins of the old and famous Abbey 
of Bermondsey, which stood near the present Tooley Street, and 
you will see that fields and hedges at that time extended in all 
directions where now an enormous population lives and labours. 



THE TOWER BRIDGE. 13 

This is merely an instance of what is true of all South 
London for miles down the river. 

It has been stated, I believe correctly, that 39 per cent, of 
the population of London now live east of London Bridge, and, 
indeed, a town as large as Manchester and Birmingham put 
together has rapidly grown up south of the Thames below 
London Bridge, as is evidenced by any map of London of the 
present day, which compares strikingly with the print of the 




7. BERMOXDSEY ABDEY. 

ruins of Bermondsey Abbey. The question of further com- 
munication thus became more and more the subject of discussion, 
and various attempts (apart from the Thames Tunnel already 
referred to) were made to deal with the subject, commencing 
with the Tower Subway. 

The Tower Subway, which consists of an iron tube about 



14 A LECTURE ON 

7 feet in diameter, extending from Great Tower Hill (on the west 
side of the Tower of London) to near Pickle Herring Stairs 
on the Surrey side of the river, was constructed by a joint- 
stock company, and was opened in 1871, Mr. Peter Barlow 
being the engineer. It was originally intended to work the 
traffic through the subway by means of a car drawn by 
ropes, but, this plan having been found inconvenient, the 
subway has been since used merely as a footway. In spite of 
its small size and the inconvenience of its being approached by 
long nights of stairs, nearly a million foot passengers have 
passed through it per annum, at a charge of ^d. each. 




FIG. 8. METROPOLITAN BOARD OF WORKS PROPOSED TOWEK BRIDGE. 

In 1879 the Metropolitan Board of Works applied to 
Parliament for powers to construct a high level, or rather medium 
level bridge, on the site of the present Tower Bridge. This 
bridge, designed by the late Sir Joseph Bazalgette, was 
proposed to consist of a single arch of 850 feet span, supporting 
a horizontal roadway at a height of 65 feet above high water, 
and is shown in Fig. 8. It was intended to be approached on 
the north side by a straight raised approach, rising from 
opposite the Mint, and on the south side by a long spiral 
approach, each with a gradient of 1 in 40. 



THE TOWER BRIDGE. 15 

The estimated cost of the bridge was 600,000 for works, 
800,000 for property, total 1,400,000, and this expenditure 
was to have been met out of the rates. 

The proposed bridge was strongly opposed by shipowners 
and wharfingers as being too low, and the long gradients (no 
less than 5,700 feet in length of 1 in 40) being also thought very 
objectionable, the scheme was rejected after a long enquiry. 

In the Session of 1883 a proposal was made by a private 
company for a large subway for vehicular traffic, from Great Tower 
Hill to the Surrey side of the river. Access to this subway would 
have been afforded exclusively by large and numerous hydraulic 
lifts. The Bill was opposed by the Metropolitan Board of 
Works, and was rejected. The subway would not have been 
toll free. 

In 1884 the Metropolitan Board of Works promoted their 
Thames Crossings Bill, by which they sought to construct a 
subway for vehicular and foot traffic under the Thames at 
Nightingale Lane, about a mile east of the Tower. This subway 
would have been approached by gradients 5,800 feet long of 1 in 
40, or somewhat steeper. In the same Session an independent 
company promoted a " Duplex " bridge at the Tower, on the 
site of the present Tower Bridge. This bridge, as its name 
implies, was a double one at the central portion, and its action 
resembled that of a lock. The down river bridge being opened 
would allow a vessel to pass up into the space between the two 
bridges, and when this was done the down river bridge would be 
closed and the up river bridge opened to allow the vessel to 
proceed. Of course for vessels descending the river the operation 



16 A LECTURE ON 

would be reversed. The " Duplex" bridge would have been a 
great obstruction in the river, and it would have been difficult 
to manage vessels passing through it with a tide running as 
it does in the Thames. 

The two Bills were referred to a hybrid committee of the 
House of Commons, who reported that neither Bill ought to be 
proceeded with, and that a low level opening bridge at the east 
side of the Tower was the best way of meeting the various 
opposing views, and they invited the Corporation of London to 
undertake the work. 

Prior to 1884, there had been several proposals for low 
level bridges at or near the site of the present Tower Bridge 
one for example by Colonel Hay wood for a fixed bridge without 
an opening span, others by Sir Douglas Galton and Sir John 
Hawkshaw for an opening bridge of the ordinary type, with 
horizontal girders revolving on a central pier. There were also 
proposals by Sir G. Bruce for a bridge which would roll 
transversely across the river on isolated piers, and various 
modifications of Sir Joseph Bazalgette's proposal. Early among 
these various suggestions was a proposal made by Sir Horace 
Jones (the City Architect) for a bascule bridge. 

I may mention, in passing, that the French word " bascule " 
means a see-saw, and, as applied to a bridge, means a balanced 
bridge, which can move up and down. The view (Fig. 9) shows 
one of the old bascule bridges across a Dutch canal. It will 
be seen that there are two beams balanced on two upright 
posts, the inner ends of the beams being attached by chains 
to a hinged platform across the canal, and the outer ends having 



THE TOWER BRIDGE. 17 

counter-balance weights on them. As the outer or landward 
ends of the beams are lowered, the inner ends are raised and 
pull up the hinged platform and thus open the bridge for the 
passage of craft. Numerous examples of similar bridges exist 
in this and other countries. 

The Bridge House Estates Committee of the Corporation 
of London, who, as representing that body, had given for many 




Fie. 9. BASCULE BRIDGE ACROSS A DCTCH CANAL. 

years much attention to the subject, appointed in 1884 
a deputation to visit the Continent, Newcastle-upon-Tyne, 
and other places where there were opening bridges of various 
descriptions, and finally they came to the conclusion to recommend 
the Corporation to accept the invitation of the hybrid committee 
of 1884 and promote a Bill for the erection at the Tower site 
of a " bascule " bridge as the best means of meeting the case. 



18 



A LECTURE 



After the original sketches made by Sir Horace Jones, 
^hich are reproduced in the two views (Figs. 10 and 11), 




FIG. 10. SIR H. JONES'S DESIGN* OF A B.\scn,E BKIDGK (.shut against i- 




FIG. 11. SIR H. JONES'S DESIGN OF A BASCULE BRIDGE (open for ships.) 

it was seen that any arched form of construction across 
the central opening would be very objectionable; as the 
masts of ships would be in danger of striking the arch 
unless they were kept exactly in the centre of the span. 



THE TOWER BRIDGE. 19 

Accordingly, various other sketches were made, and this was 
the condition of affairs when in 1884 I became connected 
with the undertaking as engineer, Sir Horace Jones being 
at the same time appointed architect to the work, and we 
decided that any girders over the central span, when open 
for the passage of ships, must be horizontal and not arched. 
The result of these considerations was an amended sketch 




FIG. 12. DESIGN SUBMITTED TO PARLIAMENT. 

shown in Fig. 12. In 1885 the Corporation, in accordance 
with these ideas, promoted the Act of Parliament, of which the 
present structure, though greatly altered from the original 
sketch, is the result. 

The problem to be solved is one of no small difficulty, for it 
is necessary to reconcile the requirements of the land traffic with 
the very important interests of the trade of the Upper Pool. 
This part of the river is always crowded with craft of various 
kinds, and this fact made the "bascule" system unusually 
desirable. Any opening bridge revolving horizontally would 
have occupied so large an area of the river as to be 
undesirable from many important points of view, whereas 

a bridge revolving in a vertical plane, not only occupies the 

c 



20 A LECTURE ON 

minimum of space in the river, but also at an early stage of the 
process of opening affords a clear passage for ships in the central 
part of the waterway, increasing in width rapidly as the operation 
of opening is continued. 

The mode in which the traffic of the Pool is conducted pre- 
scribed the general arrangement of the spans of the bridge. 
Sea-going vessels of all kinds are moored head and stern in two 
parallel lines in the Upper Pool, on each side of the centre line 
of the river, leaving a central channel from 200 to 250 feet wide 
free for the passage of vessels up and down the river, and this 
space is frequently contracted, by barges and small craft lying 
alongside the larger vessels, to a width of from 160 to 180 feet. 
The spaces in the river occupied by the large vessels on 
each side of the free central channel are called tiers, and as 
vessels lie in the tiers two or sometimes three abreast, with 
barges alongside them, it will be seen that if the piers of a 
bridge were made alignable with the tiers there would be no 
obstruction to navigation and little to the flow of water by two 
piers of a width not greater than that of the tiers. On each 
landward side of the tiers channels are preserved for the passage 
of vessels to and from the wharves, and it was of course necessary 
that these side channels should not be obstructed by any pier of 
the bridge. Thus the mode, in which the river traffic has for 
many years adjusted itself, made it evident that a bridge with a 
clear central opening of from 160 to 200 feet, and two side 
openings of about 280 or 300 feet, would meet all requirements, 
and that there could be no objection to piers wide enough to 
accommodate a counter-balance, seeing that the width of two 



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 clauses 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. 



General Description of the Bridge. 

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, 
with 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 in length 
and 70 feet in width. 



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 



The doffed //nes she**" 
re/sresonr oufs'de ///7 o 
above toed of fttver . 




(first fty' of Works) 

.. ,__ t-le*- Hf*t*r_W*j[_ 
(Second stag* of Works) 



#?/, 2. 3 . S.6.78 /J J4. /S /6. /9. Sf. 

22 $ 23 *re Square Cussons 
f/.' 9, /O. // /2, /7. IS. 2O. j Z4- JLT* 

Tr/asiffuJa-r 




FIG. 13. PLAN TO SHOW 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, 



THE TOWEtl BfclDGE. 23 

as the opening 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 give 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 be 
occupied by a portion of the works. If this concession had not 
been made, the cost of the land for the undertaking would have 
been almost prohibitory. It 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 



24 



A LECTUBE 



illustrations (Figs. 14, 15 and 16.) Theirfonn in plan (Fig. 15) 
may be described as a square of 70 feet elongated by cutwaters 




__ _. _ 

LONGITUDINAL SECTION OF PIER. 

FIG. 14. 



QUARTER PLAN AT A. 8. 




THE TOWER BRIDGE. 



25 



at each end, bringing the total length to 185 feet 4 inches. 
Their total depth from the roadway level to the London clay, on 
which they rest, is 102 feet. 




TRANSVKRSK SECTION Of PIER 

x- 
FHJ. 10. 

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 (l) 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 



26 A LECTURE ON 

actuating the opening span. The old bascule bridges 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 opening 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 20 feet 4 inches, and are 50 feet in height, extending 
from below the floor of the machinery chamber to within 
26 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 



THK TOWER BRIDGE. 27 

high level (Fig. 30, page 49). These ties are carried by two 
narrow bridges 10 feet in width, which are available as foot 
bridges when the bascule span is open for the passage of vessels. 
The foot bridges are 140 feet above Trinity high water, and, as 
their supports stand back 1 5 feet from the face of the piers, their 
clear span is 230 feet. Access is given to them 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 264 feet from the bottom of the foundations. 

Having now given the leading dimensions of the structure, 
I will proceed to describe (l) 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. 

Tlie Mode of Constructing the Substructure of 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 



28 



A LECTUHE ON 



bottom, and with the bottom edges made sharp and strong (Fig. 
18,) so as to easily penetrate the ground. 




There are twelve caissons for each pier, as will be 



THE TOWER BRIDGE. 

understood from Fig. 13. 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. 17 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 
having been made with piles, the next 
operation was to erect the caissons upon 
the staging. 




ROLLED STEEL CUTTING FOGE. 
Fio. 18. 




HALF CROSS SECTION OP PIER. ' HALF CROSS SECTION OF PIER, 

SHOWING CAISSON BEFORl LOWERING SHOWING CAISSON WHEN SUNK 

TO THE BED OF THE RIVER. 6 BEING FILLED WITH CONCRETE. 

FIG. 19. 

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 





30 A LECTURE ON 

the water during the process of building the pier, and it could 
be removed when the brickwork and masonry were finished. 
This part was thus called the temporary caisson. 

The permanent caisson was 19 
feet in height, divided horizontally 
into two portions. It 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, 14 inches square. It was then 
lifted slightly by four powerful screws 
attached to four rods, from which was 
slung the weight of the caisson and the 
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 10 feet into 
the solid London clay. 



PETAI L OF 
INDIA-RUBBER JOINT. 

FlG. 20. 



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. 
It 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 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 



32 



A LECTURE ON 



28 FEET CAISSONS . 
HALF INSIDE ELEVATION. HALF OUTSIDE ELEVATION. 

1 1 




FIG. 21. COMPLETE CAISSON WITH TIMBERING AND SUSPENSION RODS. 



THE TOWER BRIDGE. 



33 



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 
between them were then completely 
filled with concrete, upon which the 

BETWEEN W TWO ADjo", C N U G TT cA ( lssoNi. brickwork And masonry were com- 

FIG. 22. , . . , 

menced in the temporary caisson, and 

carried up to 4 feet above Trinity high water, as shown 
in Fig. 23. 





Fiu. 23. CROSS SECTION OF PIER, SHOWING OUTSIDE WALL COMPLETED. 

It 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 



34 



A LECTURE OX 



permit of continuous building. For this purpose the corners of 
the caissons were united by timber piles, which were driven 
in a groove on each caisson (Fig. 24), and when these 
had been driven and made water-tight (as to which no difficulty 



Xolca Cor- 1 " 
ZK-ct afxirV 
' uu/ IftujtJiJt cf Gtlascfn'. 

LI.3-X 3fi 




DETAIL OF VERTICAL dOlNTS 
AND PILE GROOVES 



FIG. 24. ENLARGED PLAN OF THE ANGLE OF A CAISSON. 



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, 



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 
arid the pier completed. 



N CAISSONS BEFORE DRIVING THE P.LES,^ 
BETWHN THC CAISSONS / 




FIG. 25. HALF PLAN OK PIER AT DIFFERENT 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. 



36 A LECTURE ON 

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. The substructure thus occupied a considerably longer 
time than was anticipated. 




FIG. 26. VIEW OF THE FOUNDATIONS IN PROGRESS. 



The view Fig. 2G gives an idea of the appearance of the 
works during the construction of the piers. 



The Opening 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 



THE TOWER BRIDGE. 37 

construction to 140 feet) from Trinity high water mark. I may 
mention in passing that I think these dimensions constitute the 
largest opening span in the world. The next largest opening 
is, I believe, at the Newcastle bridge, where there are two 
separate spans of 100 feet each. 

The opening span of the Tower Bridge consists, as already 
explained, of two leaves, each turning on a horizontal pivot of 
solid forged steel. This pivot, which rests on live rollers, is 
1 foot 9 inches in diameter, and weighs 25 tons. Each leaf is 
formed with four main longitudinal girders, 13 feet 6 inches apart 
from centre to centre, which together provide for a clear width of 
bridge between the parapets of 50 feet. This width will be 
divided into 32 feet for a roadway (which is sufficient for four 
lines of road traffic) and for two footpaths each 9 feet wide. The 
spaces between the longitudinal girders are filled with cross 
girders and roadway plates, on which will be laid the wood 
pavement of the roadway and footpaths. 

The total length of the moving girders resting on each pier is 
162 feet 3 inches (Fig. 27). They turn on the main pivot, the 
position of which is 12 feet 9 inches back from the face of the 
pier. Thus from the centre of the main pivot riverwards to the 
end of the girder is 112 feet 9 inches, and to the end of the 
girders landwards is 49 feet 6 inches. The centre of gravity of 
the part over the river is 48 feet from the centre of the main 
pivot, and the estimated weight of this portion of the semi -span 
is 424 tons. A counterbalance box is attached to the landward 
end of the girders, and is intended to be filled with 422 tons of 
iron and lead. The centre of gravity of this portion of the 



gg A LECTURE ON 

girders and the counterbalance weights will be 32 feet 9 inches 
from the centre of the main pivot, and the total weight land- 
ward of the main pivot will be 621 tons. Thus the total 
weight of each leaf of the opening span, resting on the main 
pivot when the bridge is being moved, 'or on the blocks 
when the movement is completed, will be the sum of the two 
weights given above, or 1,045 tons. The pier has, therefore, 
to carry on its riverward face a heavy load, which is distributed 
on the masonry by a longitudinal and several cross girders.* 
>. 



i - 
r---fe-49 6 *-- - ur H2-9 



LUX* 




FIG. 27. DIAGRAM OF WEIGHTS OF BASCULE. 

The two outside girders carry at their counterbalanced ends 
a quadrant (Fig. 28), on which are fixed teeth, and these teeth 
engage with similar teeth on revolving pinions, which are 
actuated by the hydraulic machinery. It will be seen that, when 
the pinions are turned round, the whole of each leaf of the 

* The various weights above given are matters of estimate. They may be slightly 
modified in execution. 



THE TOWER BRIDGE. 



39 



opening span is made to rise or fall at will. It may be 
mentioned that there are two distinct pinions to each quadrant, 
making four pinions to each leaf, and that any one of the four 
pinions is strong enough to actuate and control the whole leaf. 



|_ 100.0 




FIG. 28. DIAGRAM OF BASCULE TO SHOW QUADRANT OR SECTOR. 

It will be observed that the landward ends of the 
main girders are bent downwards, and extend backwards into 
the bascule chamber in the pier, so as to carry the counterbalance 



40 A LECTURE ON 

box ; but it is, of course, necessary that the road and footpaths 
should extend continuously up to the opening span. This is 
effected by eight fixed longitudinal roadway girders, which carry 
the roadway over the bascule chamber, and between which the 
longitudinal moving girders can rise and fall. Where the 
moving roadway over the central span adjoins the fixed roadway 
over the bascule chamber, there must be, of course, a cross 
slit or joint. This space is covered by a hinged flap, which 
rises and falls with the movement of the moving girders. 

Such are the main features of the opening span, and 
before leaving the subject, I will give a short account of the 
actuating machinery in its main features. It will be, of course, 
impossible to describe it within the time now available in any 
minute detail. 

Some of my hearers may not be acquainted with the 
leading principles of hydraulic machinery in general, and so I 
may be excused if to others my remarks appear somewhat 
elementary. The principle of the application of water to 
hydraulic engines is shortly as follows. First of all, a pump is 
required, which is powerful enough to pump water under great 
pressure in the present instance under a pressure of about 
850lbs. on every square inch. This pressure is nothing unusual, 
but its magnitude will be appreciated when we remember that in 
the boiler of a locomotive engine the steam pressure is usually 
not more than about one-fifth of the above amount. 

The water pressure, being so high per square inch, enables 
the pistons and pipes of hydraulic machinery to be comparatively 
small, and, as the water can be conveyed anywhere in pipes of 



THE TOWER BRIDGE. 41 

suitable strength, the power produced by the pump can be 
applied at any desired place. In the present case the pumps 
are on the Surrey side of the bridge, and are actuated by two 
steam pumping engines of 360 horse power. The high-pressure 
water is brought thence by pipes along the Surrey fixed span, 
up the Surrey tower, across the high level foot bridge, and down 
the Middlesex tower. Return pipes convey the water when it 
has exerted its pressure at the hydraulic engines, which are 
fixed below the footway on each pier back again to the pumps 
at the steam engines, where it is again subjected to pressure 
and made ready for work again. 

There is, however, one important feature of hydraulic 
machinery to be explained, and that is the accumulator. The 
object of the accumulator is, as its name suggests, to accumu- 
late power, which is effected as follows. We have seen 
that the origin of the power is the steam engine actuating 
a pump or pumps, and, if the machinery were always working 
and always requiring the same amount of power, an accumulator 
would be of comparatively little use ; for the pumps could pump 
the water direct to the hydraulic engines, to be used there in a 
continuous effort. But such is not the requirement. The 
hydraulic machinery at the Tower Bridge, and in almost every 
other application of hydraulic power, is not called upon for a 
continuous effort, but, as it were, for spasmodic efforts lasting 
over a certain number of minutes, and the accumulators are 
employed to store up the power provided by the pump, in order to 
give it out at a greater speed than the pump, though working 
continuously, would provide. 



42 



A LECTURE ON 



In accordance with these principles, the accumulator (Fig. 
29) is a large cylinder, in which fits a long plunger, on the top 
of which are placed weights, which bring a pressure of 850lbs. 
on every square inch of the area of the cylinder. The steam 
pumps pump water into the accumulator, with that pressure, and 



STEAM EJVG/fJS 




Fio. 29. DIAGRAM OF APPLICATION OF ^HYDRAULIC POWER. 

as it is a principle of hydraulics that any pressure applied to 
water in a closed vessel is communicated to the whole of the 
water in that vessel, the pumps, though much smaller than the 
large plunger of the accumulator, raise it with its superincum- 
bent weight, but proportionately slowly. 

The pipes to work the hydraulic engines are in communi- 
cation with the cylinder of the accumulator, and this, being of 
great internal capacity, can supply high pressure water with 
great rapidity to work the hydraulic engines for what I have 
called their spasmodic efforts. In the meantime, the pumps are 
working away to re-supply the accumulator. 

At the Tower Bridge there are six accumulators, viz., two 
near the pumps on the Surrey side of the river and two on each 



THE TOWER BRIDGE. 43 

pier. Their capacity is ample for the most liberal demands of 
the hydraulic engines. 

It will not be possible to describe the hydraulic engines 
themselves in any detail. It will be sufficient to say that there 
are two sets of engines on each pier, and that their leading 
principle is that of reciprocating cylinder engines. Rams in 
cylinders are worked to and fro by the high pressure water, and 
by cranks communicate rotatory motion, through a series of geared 
wheels, to the toothed pinions, to which allusion has already been 
made, and which actuate the quadrants of the openmg span. 
Each engine has three rams with a stroke of tt inches ; 
those on the smaller engines are 7^ inches in diameter, and 
those on the larger engines 8|- inches in diameter. 

When the Tower Bridge was being discussed in Parliament, 
the disaster to the Tay Bridge was fresh in the minds of many, 
and some alarm was expressed lest the machinery might not be 
strong enough to control the opening span in heavy winds. 
The Board of Trade had reported with regard to the Tay 
Bridge, that provisions should be made in all future structures 
for a wind pressure of 56lbs. per square foot, and, though this is, 
I think, an excessive estimate, even for the wind in such an 
exposed place as the Firth of Tay, and is much more excessive 
in the comparatively protected position of the Tower Bridge, it 
was considered right not only to provide for the extreme 
pressure of 56lbs. per square foot, but also to provide the 
machinery of this strength in duplicate on each pier. Thus we 
have machinery equal to twice the requirements of the Board of 
Trade, An ordinarily strong wind, however, will riot give a 



44 A LECTURE ON 

pressure exceeding about 17lbs. per square foot, and therefore 
the hydraulic engines are arranged in pairs, one engine of each 
pair exerting a power equal to a 17lbs. wind, and the other 
equal to a 39lbs. wind, the two together being equal to the 
extravagant pressure of a wind of 56lbs. On each pier the 
duplicate pair of engines follow up the work of the first pair 
and provide against breakdowns. 

I should mention that when the two leaves of the opening 
span are brought together, there will be long wedge-shaped 
bolts, actuated by hydraulic machinery, fixed on one leaf and 
shooting into the other leaf, to complete the union of the two. 

All the machinery of the opening span will be worked from 
cabins on the piers, in which there will be levers like those in a 
railway signal box, so interlocked one with the other that all 
the proper movements must follow in the arranged order. 

The time required for the actual movement of the opening 
span from a position of rest horizontally to a position of rest 
vertically is estimated at about 1% minutes. To this must be 
added the time necessary for stopping the road traffic and clearing 
the bridge, and withdrawing the bolts. This may take, perhaps, 
some 1 J minutes more, and we then have to add the time for 
the passage of a ship and the lowering of the bridge. The time 
of Ij- minutes for opening or shutting the bridge gives a 
mean circumferential speed at the extremity of each leaf of 2 feet 
per second, which is a moderate speed for an opening bridge. 

Signals will be provided by semaphores by day and signal 
lamps by night, to show ships whether the bridge is open or 
shut. By night when the bridge is open for ships, four green 



THE TOWER BRIDGE. 45 

lights will be shown in both directions, and when it is shut 
against ships four red lights will be similarly exhibited, and 
then- lights will be interlocked with the machinery, so that 
wrong signals cannot be shown. By day similar intimation will 
be afforded by semaphore arms on the same posts as those which 
carry the signal lamps. During foggy weather, a gong will be 
used in specified ways. 

One other part of the machinery remains to be mentioned. 
This is that of the passenger lifts between the roadway level and 
the high level foot bridge. 

There are two lifts in each tower, consisting of a cage, 13 
feet by 6 feet, and 9 feet high, raised and lowered by an ordinary 
hydraulic ram with chain gearing, and capable of lifting 20 to 25 
passengers in about lj minutes, including the delays of opening 
and shutting the doors. As the lift will have to descend carrying 
a cargo of passengers before it can take a second load of ascend- 
ing passengers, we may assume three minutes from one start to 
the next ; or, as there are two lifts on each tower, 1^ minutes. 

In addition to the lifts, there are ample flights of stairs 
in the towers. 

The Fixed Superstructure. 

The fixed parts of the superstructure of the Tower Bridge 
consist, as has been said, of two shore spans, each of 270 feet, 
and of a central high level span of 230 feet. The fixed bridge 
is of the suspension form of construction, and the chains are 
carried on lofty towers on each pier and on lower towers on 
each abutment. 

I will first describe the towers. When an opening bridge 



46 A LECTURE ON 

was first proposed there was some outcry by aesthetical 
people on the score of its ruining the picturesqueness of the 
Tower of London by hideous girder erections, and it seemed to 
be the universal wish that this bridge should be in harmony 
architecturally with the Tower. 

You will have seen the various architectural ideas evolved 
out of these considerations, and observed that it was originally 
intended that the towers should be of brickwork in a feudal 
style of architecture, and the bridge somewhat like the draw- 
bridge of a Crusader's castle. Subsequently, Sir Horace Jones 
suggested a combination of brick and stone, and towers of a 
form similar to that of the view Fig. 12 on page 19. 

The ideas were in this condition when I was appointed 
engineer to the scheme, with Sir Horace Jones as architect, and 
the Corporation went to Parliament for powers to make the 
bridge. Sir Horace Jones unfortunately died in 1887, when 
the foundations had not made much progress, and up to 
that time none of the architectural designs had proceeded further 
than such sketches and studies as were barely sufficient to 
enable an approximate estimate to be made of the cost. Since 
the death of my coadjutor I have preserved the general 
architectural features of the Parliamentary sketch designs, 
but it will be seen that the structure as erected differs 
largely therefrom, both in treatment and material. 

The width, and consequently the weight, of the bridge was 
increased by the requirements of Parliament, and the span of 
the central opening was enlarged from 160 feet, as originally 
intended, to 200 feet. At the same time, the provision of 



THE TOWER BRIDGE. 47 

lifts and stairs, to accommodate foot passengers when the 
bridge was open, was felt to be a necessity. 

In this way it became apparent that it would not be 
possible to support the weight of the bridge on towers wholly 
of masonry, as in the first designs, unless they were made of 
great size and unnecessary weight. It was, consequently, 
necessary that the main supports should be of iron or steel, 
which could, however, be surrounded by masonry, so as to 
retain the architectural character of the whole structure. 

It was clear that in any event a large part of the steelwork 
of the towers must be enclosed in some material, for the 
moving quadrants project upwards some 40 feet from the 
level of the roadway, while the stairs and lifts also required 
protection from the weather. It thus became a question of 
surrounding the towers either with cast iron panelling or with 
stone, and eventually a granite facing with Portland stone 
dressings was adopted. 

^Esthetically speaking, stone seems better than cast iron, 
which would equally hide the constructive features, and 
practically speaking I think it also better, for I know of no mode 
so satisfactory for preserving iron or steel from corrosion as 
embedding it in brickwork, concrete or masonry. Careful pro- 
vision has been made in all parts for expansion and contraction 
of the two materials, and, though we have had great extremes 
of heat and cold since the masonry has been built, no effects 
resulting from any difference of temperature have been observed. 

The Fig. 30, which is a half elevation of the steelwork of 
the bridge, shows the general arrangement of the towers, 



48 A LECTURE ON 

which have been since enclosed with masonry and brickwork, 
and may be described as being steel skeletons clothed with 
stone. 

I am afraid some purists will say that the lamp of truth 
has been sadly neglected in all this, and that the old architects 
would not have sanctioned such an arrangement as a complex 
structure of steel surrounded by stone. 

One reason may be that the old architects did not know 
much about iron or steel. Perhaps, if they had been acquainted 
with their capabilities, they would have been as ready to 
employ them as they were to back up stone-faced walls with 
brick, and to hide the constructive features of their buildings as 
Sir Christopher Wren did when he used a brick cone to support 
the internal and external domes of St. Paul's. 

However all this may be, " needs must when Parliament 
drives," and, if the appearance of the Tower Bridge is approved, 
I hope that we may forget that the towers have skeletons as much 
concealed as that of the human body, of which we do not think 
when we contemplate examples of manly or feminine beauty. 

I return, however, to the details of construction. The 
skeleton of each tower consists of four wrought steel pillars, 
octagonal in plan, built up of rivetted plates. The pillars 
start from wide spreading bases, and extend upwards to the 
suspension chains, which they support. They are united by 
horizontal girders and many diagonal bracings, to which it is not 
necessary here to refer in detail. The chains are carried on the 
abutments by similar but lower pillars. All these and other 
particulars appear in Fig. 30. 



THE TOWER BRIDGE. 



49 




50 A LECTURE ON 

Between the pillars are spaces for the public stairs and the 
passenger lifts, and for the quadrants of the opening span when 
in their upward position. When all these necessary things are 
accommodated, it will be seen that there is very little room left 
in the towers for the first 40 feet of their height, that is to say, 
up to the level of the archway over the road. The horizontal 
girders in the towers above the archway carry various floors to 
provide for landings from the public stairs and possibly for 
rooms for the police and staff. 

On the tops of the octagonal pillars rest a series of rollers, 
which will allow the chains so to move as to accommodate 
themselves to changes of temperature and to unequal distribu- 
tion of the road traffic. 

The arrangements of the rollers are peculiar. The amount 
of space available for their reception on the tops of the steel 
octagonal pillars is limited, while the weight which they have 
to support is very large, being estimated when the bridge was 
fully loaded at 1,000 tons. It was thus necessary that the 
weight should be equally distributed over the whole series of 
rollers, and that there should be no possibility of a concentration 
of weight on any one or two rollers. This is effected by an 
arrangement by which the weight brought on the top of the 
system is first carried by two blocks, thus ensuring equal division 
between them, then the weight so divided is sub-divided 
between two plates, and the weight on each plate is again sub- 
divided between two rollers. 

The main chains, which are 60 feet 6 inches apart from 
centre to centre, extend from the rollers on the piers to other 



THE TOWER BRIDGE. 51 

rollers on each abutment, and support the platform of the bridge 
by suspension rods, extending from the bottom of the chains to 
the cross girders of the platform. These arrangements appear 
in the view, and do not call for any further detailed description. 

It may be asked why are these structures, which look like 
girders, called chains? They are, in fact, chains, stiffened to 
prevent deflection, and the object of the form is to distribute the 
local loads due to passing traffic, which, in the case of an ordinary 
suspension chain, distort the chain, continually depressing each 
part as the load passes, and consequently distorting the 
platform of the bridge. By making the chain, as it were, double, 
and bracing it with iron triangulations, these local deflections of 
the chain are avoided. 

The ends of the chains on the abutments and on the towers 
are united by large pins to the ties. The ties on the abutments 
are carried (4S5S? the ground below the approaches, and are there 
united to anchorage girders, which rest against very heavy 
blocks of concrete, and are abundantly adequate to resist the pull 
of the chains. The ties between the towers are for the purpose 
of uniting the ends of the two chains, and by their means the 
stress on the chains is conveyed from anchorage to anchorage. 

The platform of the bridge is formed of cross girders, 
which extend transversely from side to side, their ends being 
immediately under the chains, to which they are suspended by 
solid steel rods. Between the cross girders are short longitudinal 
girders, and on these rest corrugated steel plates, which carry 
the paving and footways. The total width between the parapets 
on the fixed spans of the bridge and on the approaches is 60 feet, 



52 



A LECTUBE ON 




Fu:. 31. PROGRESS OF TOWER ON PIER ix RIVER. 



THE TOWER BRIDGE. 53 

which is divided into 36 feet for the vehicular traffic and into 
two pathways each 1 2 feet wide. I may mention in passing that 
London Bridge is 54 feet wide between the parapets. 

The total weight of steel and iron in the Tower Bridge will 
amount to nearly 12,000 tons. 

The Erection of the Superstructure. 

The erection of the superstructure of the bridge has been 
effected without any considerable difficulty. Temporary stagings 
of pile work were erected in the river forming piers, upon which 
wrought iron horizontal girders were erected which supported 
timber beams and planking thus forming continuous tem- 
porary bridges communicating from the shores to each pier. 
Less difficulty .was experienced than was expected in 
driving the pile work either round the permanent piers or to 
form the supports of the temporary bridge, and but little 
damage was ever caused to it during the time of the erection 
of the bridge by collisions with it of river craft, which from 
time to time occurred. Access being thus given from the 
shores, the next thing to be done was to erect the pillars, 
girders and bracing, forming the steel framework of the 
abutment and river towers. All the steel work had been put 
together at the works of Messrs. Sir W. Arrol & Co., at 
Glasgow, and was brought thence by sea in small pieces to be 
rivetted together at the bridge. The view (Fig 31) shows this 
work in progress at one of the river towers. 

The next work taken in hand was the erection of the high 



54 



A LECTURE ON* 



level footway bridges between the towers. These bridges when 
finished are cantilevers for a distance from eacli tower of 55 feet, 
and are girders for the remaining space of 120 feet between the 
ends of the cantilevers. They were erected, however, by 




FIG. 32. PROGRESS OF HIGH LEVEL_BRIDGES. 

temporary expedients, as cantilevers, piece by piece from the tops 
of the towers and without scaffolding from below, for the whole of 
the semi-span of 1 1 5 feet till they met over the centre of the river. 
The progress of the girders from week to week as they advanced 
towards each other, was watched with much interest by the 
public from London Bridge. A view (Fig. 32) shows the girders 
a short time before they met. 



THE TOWER BRIDGE. 55 



The chains were erected in their position by means of 
scaffolds and trestles resting on the temporary bridge. The 
succeeding views (Figs. 33, 34 and 35) show this work in 
progress. Cranes on stages, travelling along the temporary 




FIG. 33. PROGRESS OF ERECTION OF MAIN CHAINS. SHORT SEGMENT.] 

wooden bridge, served to place the various parts of the chain on 
the trestles, where they were rivetted together in their 
permanent positions. In the meantime the land ties from the 
anchorages had been brought up to the tops of the abutment 
towers, and the long horizontal ties, which are carried by the high 
level bridges, had been erected and rivetted in their places. The 
holes for the connecting pins at the ends of the ties and at the 



56 



A LECTURE ON 



junctions of the short and long chains were then bored finally 
and the pins inserted, thus forming a through connection from the 




i'lo. 34. FEOGRESS OF J^KECTION 01- MAIN LUA 

anchorage on the north side of the river to the corresponding 
anchorage on the southern side. 



THE TOWER BRIDGE. 57 

Another part of the work was the erection of the fixed 
roadway girders and the moving girders on the river towers. 
The fixed girders were first erected on the temporary bridge, 
and moved from thence into their position over the bascule 
chamber. The portions of the moving girders which would 
eventually be landward of the main pivot and extend about 10 feet 
riverwards therefrom were similarly erected on the temporary 




FIG. 35. PROGRESS OF ERECTION OF MAIX CHAINS. GENERAL VIEW. 

bridge, launched forward, and placed in a vertical position. The 
main pivot could now be threaded through the moving girders 
and the bearings on it adjusted. A sufficient length of the moving 
girders to reach about 40 feet over the central span of the 
bridge was then erected vertically, with the accompanying cross 
girders and bracing, after which the quadrants to carry the 



58 



A LECTURE ON 



teeth by which the moving span was to be actuated, were erected 
in their places. The teeth of the quadrants were bolted on 
to them, and, to ensure an accurate fit, with the pinions, which 
engage with the teeth on the quadrants, the moving girders 
were revolved by temporary means, so that the teeth on the 
quadrants could be fitted carefully to those of the pinions. The 
view (Fig. 36) shows this work in progress. The regulations of the 




FIG. 36. MOVING GIRDERS IN PROGRESS. 

Act of Parliament rendered it necessary to confine operations in 
the first instance to a length of 40 feet of the moving girders, so 
as not to reduce the free passage for vessels through the central 
opening during the process of adjustment to less than 160 feet. 
When the adjustment was completed the erection of the 
remaining portions of the moving girders was taken in hand 
and the work completed to a height of about 100 feet above the 



THE TOWER BRIDGE. 



59 



roadway where the ends of the girders are when in their 
vertical position. 

The succeeding view (Fig. 37) shows the general appaarance 
which the bridge will present when opened for the passage of 
sea-going vessels. It will be observed that the tops of the 
towers and the ends of the moving girders are incomplete, and 




FIG. 37. VIEW OF THE BRIDGE IN THE AUTCMX OF 1893. 

that part of the temporary staging in the river, on which the 
steelwork of the side spans was erected, is still standing.* 

I am glad to say that the loss of human life during the 
construction of the bridge has, considering the magnitude and 
nature of the works, been small. In all, six men have met with 
fatal accidents, and at least one of these was the result of 
sudden illness, or of a fit. 



A more recent view of the Bridge, taken early in 1894, i? given as a frontispiece. 



GO A LECTURE ON 

The Approaches. 

The northern approach to the bridge is constructed partly 

on a viaduct of brick piers and arches, faced with stone, and 

partly by means of retaining walls, and extends from the northern 

abutment to Tower Hill, opposite the Royal Mint, a distance of 

330 yards. The ground on which these works stand was 

acquired from Government, and was the glacis and part of the 

eastern ditch of the Tower. An entrance to the Tower property 

from the east is afforded by a wide archway beneath the 

approach at its southern end. Some of the arches of the viaduct 

adjoining this entrance are used for a guard room, and others 

for stores for the fortress and for the bridge. Most of the 

northern approach is level, and there is a gradient of 1 in 60 

on the remainder, extending across the north shore span to the 

northern pier of the bridge. The southern approach stands 

wholly on property acquired from private owners, and extends 

from the southern abutment to Tooley Street, a distance of 280 

yards. It is partly constructed on a viaduct near the bridge, and 

under the arches of the viaduct are placed the engine and 

boiler houses with the coal stores for the hydraulic pumping 

engines. The rest of the southern approach is upheld by 

retaining walls, built go as to form cellarage for houses to be 

built on each side of the approach. The gradient of the 

southern approach is, throughout its length, 1 in 40, arid this 

gradient extends across the south shore span to the southern 

pier of the bridge. From the piers the inclines are continued to 

the centre of the river by gradients of 1 in 75, on both leaves of 

the opening span. 



THE TOWER BRIDGE. 61 



Communication betiveen existing thoroughfares and the 
Tower Bridge. 

The approach to the bridge on the north side of the river 
connects directly with the southern end of the Minories, 
where that important street, which runs north and south, joins 
wide thoroughfares extending eastward and westward, but the 
access from the southern bank of the river is not at present so 
satisfactory. The approach constructed by the Corporation 
extends from the bridge to Tooley Street, which is an important 
street running east and west, but there is no good thoroughfare 
in a southerly direction from Tooley Street. The London 
County Council undertook to make a wide southern approach, 
extending from Tooley Street to the Old Kent Road and 
Bricklayers' Arms Station ; but up to the present time nothing- 
has been done, except to bring into Parliament bills which have 
been abortive, from the fact that the Council had included 
in them the principle of what is called betterment. It is much 
to be hoped that this greatly needed southern approach will be 
made expeditiously, as it cannot be doubted that, though the 
heavy traffic of Tooley Street will be served by the present 
arrangement, much of the utility of the Tower Bridge will 
remain unrealised, until a direct north and south thoroughfare 
for traffic is opened up. The Corporation have performed their 
part of the enterprise, and it is to be regretted that the London 
County Council are wholly in arrear with their share of the 
undertaking. 



62 .A LECTURE ON 

Conclusion. 

The accommodation of the interests of the road and river 
traffic at the site of the Tower Bridge has presented many 
difficulties, hut I venture to hope that the bridge which is now 
being erected to a great extent solves the problem. Being a low 
level bridge the total rise of the road traffic is not great, and the 
gradients of the approaches are short and easy. The river traffic 
has ample width and height for passing through the bridge, 
and the machinery for opening the bridge for the passage of 
ships will be rapid in its action. Lastly, there are arrangements 
for the continual accommodation of foot traffic. 

The seagoing ships which pass the site of the Tower Bridge, 
and for which the central span would have to be opened, number 
on the average, about 17 daily. They pass by chiefly at or near 
the time of high water, and it may well be arranged that several 
may pass one behind the other. The number of seagoing ships 
proceeding above the site of the bridge does not show any 
tendency to growth, but, on the contrary, the volume of such 
traffic will rather, I think, gravitate to the docks down stream 
as time goes on. 

I am afraid that some disappointment will occasionally be felt 
when vehicular traffic is stopped by the opening of the bridge, 
but it may be hoped that no serious delays will occur either to 
seagoing ships or to vehicular traffic, as the periods during 
which the opening span will be raised, though sufficient for the 
accommodation of the river traffic, will not be of frequent 
occurrence or of long duration. The Tower Bridge will, 



THE TOWER BRIDGE. 63 

it is thought, fairly meet all the difficulties of the case, but if 
the road traffic becomes of greater importance, and the 
sea-going river traffic grows less, I suppose the fate of the bridge 
will be to become a fixed bridge. How soon this may happen no 
one can tell. It is able to fulfil its duties either as an opening 
or as a fixed bridge. 

The cost of the bridge, with its approaches and including the 
cost of the property purchased, will be about a million sterling, 
and the whole of the expense will be defrayed out of the funds 
carefully husbanded and administered by the Bridge House 
Estates Committee. Londoners will thus be presented, without 
the charge of one penny on the rates, with a free bridge. The 
expense of working the bridge, which will be very considerable 
from the quantity of machinery comprised within it, will also 
be paid by the Corporation. 

As this is the first public authorised notice of the Tower 
Bridge, I am very glad to take the opportunity of saying how 
much the works generally, and myself in particular, are indebted 
to many gentlemen who have assisted in the undertaking. First 
and most important of all, my acknowledgments are due to my 
partner, Mr. H. M. Brunei, who has supervised the whole of the 
complicated calculations and details of the structure, and has 
taken a very active share in the carrying out of the work from 
first to last. Afterwards follow the resident engineer, Mr. 
Cruttwell, who has been in control of the works from their 
commencement ; Mr. Fyson, who has had the duty of the prepa- 
ration of most of the detailed working drawings and calculations 
of engineering matters, and Mr. Stevenson, who has acted as my 



64 THE TOWER BRIDGE. 

architectural assistant. In connection with this subject, I 
cannot but express my great regret that the work was so soon 
after its commencement deprived of the architectural knowledge 
and experience of Sir Horace Jones, and that he has not lived 
to see the mode in which his conception of a large bascule 
bridge across the Thames has been realised. In another branch of 
duty I have to express my thanks to the various contractors 
who have been employed to Mr. Jackson and Mr. Webster, 
who made the foundations and approaches ; to Sir William 
Arrol to whom the erection of the steelwork of the super- 
structure is entrusted ; to Messrs. Perry, who are carrying out 
the masonry ; lastly, and in a very important degree, to the 
firm of Sir W. G. Armstrong, Mitchell and Co., to whom is 
entrusted the hydraulic machinery, which, I believe, is without 
rival in size and power. 

The time of construction, some 7 years to the present time, 
has seemed long, but it may be some comfort to those who are 
impatient, to remember that old London Bridge was 33 years 
in building, old Westminster Bridge llf years, and new London 
Bridge 7J years, and I think my hearers will have seen that 
the Tower Bridge is no ordinary bridge, and in no ordinary 
position. The structure and its machinery are full of the most 
elaborate and complicated work of all kinds. 

In drawing this description of the works to a conclusion, I 
may be allowed to express a hope that the Tower Bridge, when 
finished, will be considered to be not unworthy of the 
Corporation of the greatest city of ancient or modern times. 



BOOT, SON AND CARPENTER, PRINTERS, 24, OLD BAILEY, E.C, 



TG 



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