_THE CLIFTON SUSPENSION BRIDGE._
A.D. 1829--1853. ÆTATIS 24--48.
ORIGIN OF THE UNDERTAKING--THE FIRST COMPETITION, NOVEMBER
1829--DESCRIPTION OF MR. BRUNEL’S PLANS--MR. TELFORD’S DECISION AS
UMPIRE--MR. TELFORD’S DESIGN--THE SECOND COMPETITION--MR. BRUNEL
APPOINTED ENGINEER, MARCH 1831--COMMENCEMENT OF THE WORKS, AUGUST
1836--DESCRIPTION OF THE DESIGN--ABANDONMENT OF THE WORKS,
1853--FORMATION OF A NEW COMPANY AND COMPLETION OF THE BRIDGE,
1864. _NOTE_: THE HUNGERFORD SUSPENSION BRIDGE.
After Mr. Brunel had recovered from his accident in the Thames Tunnel,
he went for a trip to Plymouth, where he examined with great interest
the Breakwater and other engineering works in the neighbourhood. He
notes in his diary that he went to Saltash, and that he thought the
river there ‘much too wide to be worth having a bridge.’ This remark was
no doubt made in consequence of his father having some years before been
consulted as to the construction of a suspension bridge at this place,
which Mr. Brunel himself, eighteen years afterwards, selected for the
crossing of the Tamar by the Cornwall Railway, and built there the
largest and most remarkable of his bridges.
For the remainder of the year 1828, and during the greater part of 1829,
Mr. Brunel kept himself fully employed in scientific researches, and in
intercourse with Mr. Babbage, Mr. Faraday, and other friends; but he was
without any regular occupation, until, in the autumn of 1829, he heard
that designs were required for a suspension bridge over the Avon at
Bristol, and he determined to compete.
This project originated in a bequest made in 1753, by Alderman William
Vick, of the sum of 1,000_l._ to be placed in the hands of the Society
of Merchant Venturers of Bristol, with directions that it should
accumulate at compound interest until it reached 10,000_l._, when it was
to be expended in the erection of a stone bridge over the river Avon,
from Clifton Down to Leigh Down. Alderman Vick stated that he had heard
and believed that the building of such a bridge was practicable, and
might be completed for less than 10,000_l._
* * * * *
The legacy was duly paid to the Society of Merchant Venturers, and
invested by them. The interest accumulated; and in 1829, when the fund
amounted to nearly 8,000_l._, a committee was appointed to consider in
what way it would be possible to carry out Alderman Vick’s intentions.
An estimate for a stone bridge was procured, but as it gave the cost at
90,000_l._, it was evident that this scheme must be abandoned.
* * * * *
The committee then advertised for designs for a suspension bridge. Mr.
Brunel, on hearing through a friend of the proposed competition, went to
Bristol; and, after examining the locality, he selected four different
points within the limits prescribed by the instructions of the
committee, and made a separate design for each of them. His plans were
sent in on the day appointed, Nov. 19, 1829, with a long statement, from
which the following description of them is taken.
The first design was for a bridge of 760 feet span between the points of
suspension, the length of the suspended floor being 720 feet. In order
to obtain a height of 215 feet above high-water mark (which was the
least that the levels allowed of), towers 70 feet high would have had to
be built on the cliffs to carry the chains. The total length of chain,
including the land-ties, was about 1,620 feet. Mr. Brunel did not
approve of this design, as the situation was not favourable to
architectural effect, a point to which the committee attached great
importance; but he suggested it from its being somewhat more economical
in construction than his other plans.
In another design, the situation being some way farther down the river
than that of the design last mentioned, towers would also have been
necessary. The distance between the points of suspension was 1,180 feet,
with a suspended floor of over 900 feet. It is probable that Mr. Brunel
only proposed this plan because the site came within the limits of
deviation, as he does not say anything in favour of it in his report.
The two remaining plans are the most interesting of the series, as there
can be no doubt that, if Mr. Brunel had had his own way, he would have
adopted one of them for execution; and it appears from a little sketch
on the top of one of his earliest letters from Bristol, that his first
idea for the bridge was that which is carried out in these two designs.
The site selected was one where the rocks rise perpendicularly for a
considerable height above the proposed level of the bridge, and
therefore piers and land-ties were dispensed with, the chains being hung
directly from the rock. No masonry was required except for architectural
effect.[25]
[Illustration: CLIFTON SUSPENSION BRIDGE.
Plate I.
Fig. 1.
Elevation of Drawing Nº 3 of Mr. Brunel’s Designs in the first
competition. AD. 1829
Fig. 2.
H. Adlard So.
Elevation of the Bridge according to the Design on which the works were
commenced. AD. 1836.]
The principal difference between these two designs is that in the second
a short tunnel is avoided at one end. The style of architecture selected
for the tunnel-front and the face of the rock, as shown on the drawings
sent in to the committee, is Norman. There are also extant many
beautiful sketches made by Mr. Brunel for different parts of the
design.[26]
In determining upon the mode of construction, which was the same in the
four designs, Mr. Brunel acted upon the principle which guided him in
all his subsequent undertakings, which was, as he states in his report,
‘to make use of all that has been found good in similar works, and to
avail himself of the experience gained in them, and to combine with all
their advantages the precautions which time and experience had pointed
out.’
He dismissed in a few words the plan of breaking the span into two or
three lengths. This was in his opinion unnecessary, and he computed that
the cost of a pier built up from the water’s edge to sufficient height
above the bridge to carry the chains, would be at least 10,000_l._ For
this reason he recommended the adoption of spans, the smallest of which
far exceeded any up to that time constructed.
In designing the chains, he dispensed with the short connecting links,
which had been previously adopted in suspension bridges, introducing
instead the method now universally used, of connecting each set of links
directly with the adjoining one by means of a pin passed through the
holes of both. The number of joints and pins was thus reduced one half,
and a considerable saving of expense, as well as diminution of weight,
effected.
Another improvement, which diminished still further the weight of the
chains, was making the links in lengths of 16 feet, or nearly double
that of the longest links at the Menai bridge. The chief reason for this
alteration was to ensure a near approximation to equality in the strains
on the different links, should all the distances between the holes not
be exactly equal. This improvement was afterwards carried still further
in the Hungerford Suspension Bridge, the links of which were 24 feet
long.[27]
Mr. Brunel also intended to introduce equalising beams in the supports
of the floor, so that each chain should bear an equal share of the load.
By this arrangement, there would have been comparatively few points of
suspension, and ‘the view of the scenery would not be impeded from the
observer being surrounded by a forest of suspension rods.’
The disturbance of the strains on the links arising from the greater
expansion of the metal of the outer links by the direct heat of the sun,
he proposed to obviate by sheet-iron plates placed on each side of the
chains, but separated from them by a small interval, and thus screening
them from the heat. He did not, however, use this protecting covering at
the Hungerford bridge.
All the designs show a camber or rise in the centre of the platform of
the bridge, to the extent of two or three feet; and the main chains are
brought down almost to the level of the platform. To this last
arrangement, as tending to prevent undulation, Mr. Brunel attached some
importance; and he further intended to stiffen the bridge against the
action of high winds by a system of transverse bracing, and by the
addition of inverted chains, similar to those used with success by his
father in the Bourbon bridges.[28]
Such, then, were the main features of the bold and carefully matured
designs placed by Mr. Brunel before the committee. Out of twenty-two
plans submitted, only those of Mr. Brunel and four other competitors
were deemed worthy of consideration. He and his friends were naturally
much gratified at this, and were full of hope for his ultimate victory.
But now, when he seemed to have a fair chance of success in a contest
which he justly deemed would have a most important bearing upon his
future professional career, an obstacle presented itself, which for the
time seemed almost insurmountable; for he met with an unexpected
opponent in Mr. Telford, the foremost engineer of the day, and the
designer of the famous suspension bridge over the Menai Straits.
The committee of the Society of Merchants had, not unnaturally, found
themselves unable to decide upon the merits of designs for a suspension
bridge, and had asked Mr. Telford to act as their adviser in the matter.
Unfortunately for Mr. Brunel, Mr. Telford was of opinion that the
maximum span admissible was that of the Menai bridge, _i.e._ under 600
feet, and that Mr. Brunel’s proposed bridge, though very pretty and
ingenious, would most certainly tumble down in a high wind.
This decision was, of course, fatal to the success of any design which
substituted one large span for two or more smaller ones, and dispensed
with pillars. Mr. Brunel therefore obtained permission to withdraw his
plans from the competition.
Mr. Telford then reported to the committee that none of the remaining
designs were suitable for adoption without the introduction of such
material alterations as would, in fact, constitute a new design.
Whereupon the committee took the only course which, under the
circumstances, was open to them, and requested Mr. Telford to prepare a
design himself.
Mr. Brunel was not a little disappointed at the turn matters had taken;
but, having, as he said,‘smoked away his anger,’ he took leave of his
friends at Bristol, and went for a visit to some of the principal
manufacturing towns in the north.
Meanwhile Mr. Telford prepared his design, and it was exhibited in
Bristol in January 1830. It consisted of a suspension bridge of three
spans (the centre span 360 feet, and the side ones 180 feet each), the
chains being supported at the intermediate points by tall stone piers
rising from the river’s banks at just sufficient distance apart to avoid
interfering with the roadways on either side of the stream. The style of
architecture was a florid Gothic; and, in order to display the peculiar
features of that style, the faces of the piers were covered with
elaborate panelling, and the chains ornamented with fret-work.
This design was received with a flourish of trumpets; numerous
engravings were published, exhibiting the bridge from various points of
view, and ‘thousands of copies were disposed of;’ but, after a time, it
would appear that the captivating effect of the Gothic belfries wore
off, and that the more the citizens of Bristol looked at Mr. Telford’s
plan, the less they were satisfied with it; for, although it was
deposited in the Private Bill Office, on application being made for an
Act of Parliament, the trustees who were appointed under the Act
determined to invite a second competition.
On this occasion, Mr. Telford appeared as a competitor and not as a
referee, that office being filled by Mr. Davies Gilbert, sometime
President of the Royal Society.
The site of the bridge was fixed, being that selected by Mr. Telford;
but the trustees expressly left it to the judgment of the competitors to
decide whether there should be intermediate piers or one unbroken span.
Of the thirteen designs sent in, five, including those submitted by Mr.
Telford and Mr. Brunel, were reserved for further examination. On March
17, 1831, Mr. Davies Gilbert (who had been assisted by Mr. Seward) made
his report. Mr. Telford’s design was put aside, ‘on account of the
inadequacy of the funds requisite for meeting the cost of such high and
massive towers as were essential to the plan which that distinguished
individual had proposed.’
Mr. Brunel’s design was placed second.[29] Although Mr. Gilbert reported
that it presented every desirable strength and security, he saw
objections to many of the details, and therefore did not recommend it
for adoption. However, on the following day, March 18, he stated to the
trustees that he had seen Mr. Brunel, and that it gave him much pleasure
to state that the explanations made by Mr. Brunel had materially altered
his views as to the details of the plans, which he (Mr. Gilbert) was now
satisfied were quite equal to those which he had placed first, and that,
considering the superiority of Mr. Brunel’s design in the essential
particular of strength, he should judge it preferable to any of the
others.
Thereupon the trustees, ‘having considered Mr. Davies Gilbert’s report,
and referred to all the plans, including Mr. Telford’s, unanimously gave
the preference to Mr. Brunel’s,’ and appointed him their engineer.
Subscriptions came in but slowly, and it was not till 1836 that the
works were commenced.
The first stone of the abutment on the Leigh woods or Somersetshire side
of the river was laid on August 27 by the Marquis of Northampton,
President of the British Association, which was then holding its meeting
in Bristol.[30]
The span of the bridge is greater than that of Mr. Brunel’s design for
the second competition, but much less than the spans of the earlier
designs, to which he had given the preference.[31] On this point, as
well as on the question of site, he had to conform to the wishes of the
trustees.[32] The span approved of by them necessitated the building of
a very large abutment on the Leigh woods side, the height of which, from
the surface of the rock to the level of the roadway, is 110 feet. Above
the roadway, the tower to carry the chains is built to a height of 86
feet. On the Clifton side, the base of the tower is formed by one of the
boldest of the range of St. Vincent’s rocks, which here rise almost
perpendicularly to a height of 230 feet above high water, and
consequently a very small abutment was required. The tower on this side
is 3 feet higher than that on the Leigh woods side, and the roadway has
a general inclination of about 1 in 233. Mr. Brunel thought that if the
roadway were level, it would have the appearance of falling towards
Clifton, owing to the ground there being precipitous, while on the
Leigh woods side it is sloping.
He intended, in the construction of the bridge, to have followed out the
ideas embodied in his report of 1829, and would have preferred to have
had only one chain on each side of the bridge, and that much stronger
than was usually adopted; but, in deference to public opinion, he put
two chains, though he doubted if they would expand equally. ‘A rigid
platform would in some degree prevent the unequal distribution of load
thus caused, but he endeavoured to lessen the effect of unequal
expansion by arranging a stirrup at the top of each suspending rod, so
as to hold equally at all times on both chains, and thus to cause each
to sustain its proportion of the load.’
The road platform was to have had beneath it ‘a complete system of
triangular bracing, which would render it very stiff.’
In order to lessen the action of wind on the bridge, he brought down the
main chains in the centre nearly to the level of the platform, and
intended to apply the system of brace chains at a small angle to check
vibration. There were, moreover, to be two curved chains lying
horizontally, and attached underneath the platform, so as to resist the
lateral action of the wind.[33]
He here introduced movable saddles to carry the chains on the top of the
towers, with rollers running on perfectly flat and horizontal roller
beds.[34] By this arrangement no pressure except a vertical one could
come on the towers.
He also devised means, by levers and hydraulic presses, for relieving
the rollers and roller beds from pressure, in the event of their
requiring renewal.
Mr. Brunel ultimately determined to adopt the Egyptian style of
architecture. His brother-in-law, Mr. John Callcott Horsley, R.A., gives
the following account of the proposed designs for the towers:--
‘His conception of the towers or gateways at either end of the bridge
was peculiarly grand and effective, as may be seen from his sketches
still existing. They were to be purely Egyptian; and, in his design, he
had caught the true spirit of the great remains at Philæ and Thebes. He
intended to case the towers with cast iron, and, as in perfect
accordance with the Egyptian character of his design, to decorate them
with a series of figure subjects, illustrating the whole work of
constructing the bridge, with the manufacture of the
materials--beginning with quarrying the iron ore, and making the iron,
and ending with a design representing the last piece of construction
necessary for the bridge itself. The subjects would have been arranged
in tiers (divided by simple lines) from top to bottom of the towers, and
in the exact proportion of those found upon Egyptian buildings. He made
very clever sketches for some of these proposed figure subjects, just to
show what he intended by them. I remember a group of men carrying one of
the links of the chainwork, which was excellent in character. He
proposed that I should design the figure subjects, and he asked me to go
down with him to Merthyr Tydvil, and make sketches of the iron
processes. We accomplished our journey, and all the requisite drawings
for the intended designs were made.’
The works were commenced with the Leigh abutment, which was completed in
1840, great delay having been caused by the failure of the contractors.
This misfortune led to a large excess of expenditure over the original
estimates. In 1843 the whole of the funds raised (amounting to
45,000_l._) were exhausted, and there still remained to be executed the
ornamental additions to the piers (the cost of which was estimated at
about 4,000_l._), half of the iron work, the suspension of the chains
and rods, the construction of the flooring, and the completion of the
approaches, &c., the estimate for the execution of which was 30,000_l._
Unfortunately, all efforts to raise further subscriptions were
unsuccessful; and in July 1853, when the time limited for the completion
of the bridge had expired, the works were closed in, and the undertaking
abandoned.[35]
Several proposals for completing the bridge were made in Mr. Brunel’s
lifetime, and he took every opportunity of furthering this object, which
he had very much at heart. It was not, however, till about a year after
his death that the superstructure of the bridge was actually commenced.
A company was formed in 1860 by some of the principal members of the
Institution of Civil Engineers, ‘who had an interest in the work as
completing a monument to their late friend Brunel, and at the same time
removing a slur from the engineering talent of the country.’[36] Mr.
John Hawkshaw, F.R.S., and Mr. W. H. Barlow, F.R.S., were appointed the
engineers, and Mr. Brunel’s old friend Captain Christopher Claxton,
R.N., the secretary. The works were carried on with vigour; and the
bridge was opened with much ceremony on December 8, 1864.
The chains were brought from the Hungerford Suspension Bridge, then in
process of demolition. A description of the Hungerford bridge will be
found in the note to this chapter.[37]
Although the Clifton bridge was not completed by Mr. Brunel, his
connection with it forms a very important passage in the history of his
life. Doubtless, if he had never heard of the proposed competition in
1829, or if he had been one of the disappointed competitors, he would
have found some other opportunity of making a name in his profession;
but, as a matter of fact, the Clifton bridge competition did give him
the opportunity he desired, and all his subsequent success was traced by
him to this victory, which he fought hard for, and gained only by
persevering struggles. He never forgot the debt he owed to Bristol, and
to the friends who helped him there; and he would have greatly rejoiced
to see the completion of his earliest and favourite work.
[Illustration: HUNGERFORD SUSPENSION BRIDGE
H. Adiard Sc.]
NOTE (p. 58).
_The Hungerford Suspension Bridge._
The suspension bridge which spanned the Thames at Charing Cross, on the
site of the present railway bridge, was designed and constructed by Mr.
Brunel between the years 1841 and 1845. It consisted of a centre span of
676 feet, and two side spans of 343 feet each. Being intended for foot
passengers only, its width was 14 feet. The versed sine, or deflection
of the middle of the catenary, was 50 feet. The two river piers, which
still exist up to the level of the railway, and form piers of the
present bridge, were of brickwork, with large footings at the bottom, so
as to distribute the pressure over a considerable area. The whole
structure was made hollow and as light as possible. From the level of
the footway the piers were carried up as ornamental campanile towers,
the weight of the chains being taken by four solid pillars of brickwork,
7 feet 3 inches square, forming the angles. Mr. Brunel introduced here
many of the arrangements he had designed for the Clifton bridge. In
order that the pressure from the chains might be always vertical on the
piers, the saddles rested on rollers working in oil, on the level
surface of a large cast-iron bed-plate. By this arrangement it was
rendered possible for the chains of the land spans to leave the tower at
a greater inclination than those of the middle span, so that the chains
were made shorter, and as they were at a lower level where they met the
abutment, there was less change in their direction at that point, and
consequently less thrust on the brickwork. Freedom of horizontal motion
was also secured, so that, in the case of unequal loading of the spans,
the chains might accommodate themselves to the strains, and move
horizontally until equilibrium was restored. At each of the land
abutments the chains passed down over a fixed saddle, at an inclination,
to anchorages placed at the bottom of the abutment. The brickwork under
the fixed saddle was so disposed as to resist directly the thrust
resulting from the change of direction between the main chains and the
anchor chains. To resist any movement of the abutments, the piles on
which they rested were driven obliquely, with their heads inclined from
the river. These piles were very numerous, the abutments spreading out
so as to cover a large area at the foundations. Nearly all the spaces
between the longitudinal, cross, and outside walls were filled with
concrete, in order that the abutments might be as massive as possible.
The details of the brickwork in the piers and abutments showed Mr.
Brunel’s skill in the economical employment of this material. The chains
were constructed so that the sectional area was proportional to the
strain; the total area at the centre was 296 square inches, while near
the piers it was 312 square inches. There were four chains, two on each
side of the bridge, placed one above the other, and consisting each
alternately of ten and eleven links. The links were 24 feet long and 7
inches in depth, the thickness varying so as to give the requisite
sectional area.
The relative diameter of pin, and proper form of the ends of the link,
were subjects of much consideration, and many experiments were made in
order to determine these points. The fact that two specimens of iron,
apparently identical in every respect, sometimes exhibit considerable
difference in their breaking weights, shows that an average of a great
number of experiments is required in order to test satisfactorily any
proposed refinements of construction. Mr. Brunel, however, convinced
himself by experiment that he had practically arrived at such a form of
link and diameter of pin that the chain would have no tendency to break
at one point rather than another. The links were forged with shoulders
near the eyes, in order that by means of clamps the pin could be taken
out and the links disengaged, if necessary.
The efficient action of the rollers was demonstrated shortly after the
completion of the bridge. On the occasion of the opening of the Corn
Exchange by Prince Albert, one of the land spans was crowded with
people, while the centre span was nearly empty. In consequence of this
the land chains became depressed considerably below their normal
position; and the saddles on the top of the tower nearest to the loaded
span moved horizontally on the rollers to the extent of 3 inches; and,
when the crowd had dispersed, they returned to their original position.
Many years after the completion of the bridge a proposal was made to
widen it for carriage traffic; but this was not carried out, and
eventually the superstructure was removed, to make way for the bridge of
the Charing Cross Railway. As the Hungerford Suspension Bridge has
ceased to exist, an engraving has been given of it (Plate II. p. 59), in
order that some record of its appearance may remain.