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During the period comprised in the previous
chapter Mr. Fairbairn was engaged, for four years, on a work of such
importance and novelty as to merit special description. This was the great
series of experimental investigations necessary to determine the details and
proportions of the colossal wrought-iron tubular bridges erected on the
Chester and Holyhead Railway.
After the close of his connection with this
work, in 1849, Mr. Fairbairn published a book, the title of which is as
follows :—
'An Account of the Construction of the Britannia
arid Conway Tubular Bridges; with a complete history of their progress, from
the conception of the original idea to the conclusion of the elaborate
experiments which determined the exact form and mode of construction
ultimately adopted.' By William Fairbairn, C.E., Memb. Inst. Civil
Engineers, Vice-President of the Literary and Philosophical Society,
Manchester, &c. London: Weale; Longman & Co. 1849.
As this work expressed Mr. Fairbairn's matured
views on this subject, it will naturally form the most appropriate basis for
the brief notice to be given in this chapter, Reference may be made to the
work itself for further details.
The following extracts give an account of the
origin and early history of the proceedings :—
In the construction of the Chester and Holyhead
Railway two formidable obstacles had to be overcome. The deep and rapid
tidal streams at the Conway and Menai Straits had to he crossed by bridges
which must necessarily be of extraordinary span, and of great strength. No
centerings or other substructures, such as are usually resorted to for
putting such massive structures together, could be erected.
Under such circumstances the most obvious
resource of the engineer was a suspension bridge, but the failure of more
than one attempt had proved the impossibility of running railway trains over
bridges of that class with safety. Some new expedient of engineering was
therefore required, and an engineer bold and skilful enough to conceive such
an expedient and to apply it. That engineer was found in Mr. Robert
Stephenson, and that expedient is the one, the history of which it is the
object of the following pages to relate.
Having to encounter extraordinary difficulties
of execution, and being compelled by the Admiralty [who opposed the erection
of any structure which should offer a hindrance to the free passage of
vessels under it] to abandon the ordinary resources of the engineer, Mr.
Stephenson conceived the original idea of a huge tubular bridge, to be
constructed of riveted plates and supported by chains, and of such
dimensions as to allow of the passage of locomotive engines and railway
trains through the interior of it.
It was with reference to this expedient, after
all others had been found inapplicable, that I was consulted by him, and
that my opinion was requested, first as to the practicability of the scheme,
and secondly as to the means necessary for carrying it out. This
consultation took place early in April 1845, and, as far as could be
gathered from Mr. Stephenson at the time, his idea then was that the tube
should be either of a circular or an egg-shaped sectional form.
At th^S .period there were no drawings
illustrative of the original idea of the bridge, nor had any calculations
been made as to the strength, form, or proportions of the tube. It was
ultimately arranged that the subject should be investigated experimentally,
to determine, not only the value of Mr. Stephenson's original conception,
but that of any other tubular form of bridge which might present itself in
the prosecution of my researches. The matter was placed unreservedly in my
hands; the entire conduct of the investigation was entrusted to me; and, as
an experimenter, I was to be left free to exercise my own discretion in the
investigation of whatever forms or conditions of the structure might appear
to me best calculated to secure a safe passage across the Straits. This
freedom of action was obviously necessary to the success of my experiments.
I cannot but feel myself to have been honoured by that confidence in my
judgment which it implied.
The whole series of experiments (detailed in the
Appendix) was conducted at my works, Jlillwall, Poplar.
By July 21 a considerable number of experiments
had been made; nearly the whole of the cylindrical tubes had been tested,
and preparations were then in progress for the rectangular and elliptical
forms. The difficulties experienced in retaining the cylindrical tubes in
shape, when submitted severe strains, naturally suggested the rectangular
form. Many new models of this kind were prepared and experimented on before
the end of July, and others, with different thicknesses of the top and
bottom plates, or flanches, before August 6.
On this (lay he wrote a letter to Mr.
Stephenson, which clearly pointed to the principle thenceforward adopted in
regard to the benni—namely, that of treating it as a hollow girder. The
letter says:—
From these investigations we derive several
important facts, one of which I may mention, namely, the difficulty of
bringing the upper, as well as the lower, side of the bridge into the
tensile strain. For this object several changes were effected, and attempts
made to distribute the forces equally, or in certain proportions throughout
the parts, but without effect, the results being in every experiment that of
a hollow beam, or girder, resisting, in the usual way, by the compression of
the upper and extension of the lower sides. In almost every instance we have
found the resistance opposed to compression the weakest; the upper side
generally giving way from the severity of the strain in that direction.
These facts are important so far as they have
given rise to a new series of experiments calculated to stiffen or render
more rigid the upper part of the tube, as well as to equalise the strain,
which in our present construction is evidently too weak for the resisting
forces of compression.
Mr. Fairbairn continues his narrative :—
It w ill be seen by this letter that the
weakness of the tube had been recognised in its upper surface, which yielded
to compression before the under side was upon the point of yielding to
extension; and that the course which the experiments henceforth took, of so
strengthening the upper surface that it should not be on the point if
yielding to compression until the under surface was about to yield by
extension, had been already shaped out ... I had ordered the top of the
tube to be thickened. It now occurred to me that the top might be
strengthened more effectually by other means than by thickening it, and I
directed two additional tubes to be constructed, the one rectangular and the
other elliptical, with hollow triangular cells or fins to prevent crushing.
These experiments led to the trial of the
rectangular form of tube with a corrugated top, the superior strength of
which decided me to adopt that cellular structure of the top of the tube
which ultimately merged in a single row of rectangular cells. It is this
cellular structure which gives to the bridges now standing across the Conway
Straits their principal element of strength.
In a letter to Mr. Stephenson, dated September
20, 1845, Mr. Fairbairn, after describing the experiments with the tubes,
adds:—
It is more than probable that the bridge, in its
full siae, may take something of the following sectional shape.
The parts a a being two longitudinal plates,
elivided by vertical plates so as to form squares, calculated to resist the
crushing strain in the first instance; and the lower parts b b, also
longitudinal plates, well connected with riveted joints, and of considerable
thickness to resist the tensile strain in the second.
Mr. Fairbairn remarks on this :—
The reader will not fail to observe how much
this sketch resembles the tubes actually constructed for the Conway and
Britannia Bridges.
Mr. Faibrairn's first sketch for the tube of the
Britannia bridge.
Towards the end of August Mr. Fairbairn
considered that the experiments had assumed a shape which seemed to require
the assistance of a mathematician, in order to deduce, from the trials on a
small scale, formula; and modes of calculation applicable to a larger size.
For this purpose he invited the assistance of Mr. Eaton Hodgkinson, who, it
will be recollected, had already been associated with Mr. Fairbairn in
investigations on the strength of iron. Mr. Stephenson concurred in the
proposition, and Mr. Hodgkinson first visited Millwall on September 19.
The square cell tube, although so clearly
indicated in the above letter, was not, however, at once tried; for Mr.
Fairbairn preferred to experiment on another modification of the same
principle—namely, a rect angular tube having a corrugated top, resembling in
section the eyes of a pair of spectacles. This was tried on October 14, and
Mr. Fairbairn, writing the next day to Mr. Stephenson, says:—
Our experiments of yesterday were the best and
most satisfactory we have yet made; and, agreeable to expectation, the form,
as per annexed sketch, gave not only the greatest strength, but what was of
equal importance, there was a near approximation to an equality of the
forces on the top and bottom sides. .... It is evident we are approaching
the strongest form. . . I think we have sufficient data to guide you as to
the security of such a structure.
Mr. Fairbairn adds:—
It is from this period that I date the
disappearance of almost every difficulty respecting the construction and
ultimate formation of the Britannia and Conway tubes. The powerful
resistance offered to compression by the cellular form of the top, as
exhibited in the last experiment, at once decided in my mind the form to be
adopted in those for the large tubes; and from that time forward I had no
doubts as to the practicability and complete success of the undertaking.
Towards the end of the year it became necessary
for Mr. Stephenson to make some report to the directors of the Chester and
Holyhead Railway. They had up to this time shown a great deal of patience,
and had watched with much interest the progress of the experiments at
Millwall; but as the next general meeting of the shareholders was
approaching, the directors naturally desired to have some definite
statements to produce.
It was accordingly arranged that Mr.
Stephenson's own report to the directors should be accompanied by two
separate ones, by Mr. Fairbairn and Mr. Hodgkinson respectively, each giving
his own views relative to the experiments, as well as to the chance3 of
ultimate success in the construction of the bridges.
Mr. Stephenson's Report was dated February 9,
1846, and the three documents are given entire iu Mr. Fairbairn's book. A
few extracts will serve to illustrate Mr. Fairbairn's position in the
matter. Mr. Stephenson says:—
I will lay before you the results of the
experimental investigation, which, with your sanction, I commenced some
months ago in reference to the construction of the bridge over the Menai
Straits.
In conducting this experimental investigation I
saw the importance of avoiding the influence of any preconceived views of my
own, or at least to check them, by calling in the aid of other parties
thoroughly conversant with such researches. For this purpose I have availed
myself of the assistance of Mr. Fairbairn and Mr. Hodgkinson; the former so
well known for his thorough practical knowledge in such matters; and the
latter distinguished as the first scientific authority on the strength of
iron beams.
lie then gives a resume of the experiments made
to that time, which had, he said, served to determine finally two essential
points—namely, the form of the tube, which should be rectangular, and the
distribution of the material, which should be such as to throw the greatest
thickness into the upper side. The important question remaining to be
determined was the absolute ultimate strength of a tube of my given
dimensions, which required further experimental elucidation.
There bad been an idea, in the first instance,
of using, for the erection of the tubes, large suspension chains on each
side, and Mr. Stephenson had contemplated retaining these permanently m
their position as an auxiliary support for the tubes. Mr. Fairbairn had
expressed the opinion that these were unnecessary, and Mr. Stephenson
remarks on the subject as follows:—
You will observe in Mr. Fairbairn's remarks that
he contemplates the feasibility of stripping the tube entirely of all the
chains that may be required in the erection of the bridge ; whereas, on the
other hand, Mr. Hodgkinson thinks the chains will lie an essential, or at
all events a useful auxiliary, to give the tube the requisite strength and
rigidity. This, however, will be determined by the proposed additional
experiments, and does not interfere with the construction of the masonry,
which is designed so as to admit of the tube, with or without the chains.
The application of chains as an auxiliary has
occupied much of my attention, and I am satisfied that the ordinary mode of
applying them to suspension bridges is wholly inadmissible in the present
instance; if therefore it be hereafter found necessary or desirable to
employ them in conjunction with the tube, another mode of applying them must
be devised, as it is absolutely essential to attach them in such manner as
to preclude the possibility of the smallest oscillation. In the
accomplishment of this I see no difficulty whatever, and the designs have
been arranged accordingly, in order to avoid any further delay.
It will be noticed that Mr. Fairbairn was the
only one of the three reporters who gave a positive and decided opinion
against the use of chains; Mr. Hodgkinson decidedly recommending them, and
Mr. Stephenson appearing, by his expressions, rather favourable to them than
otherwise. Now, as ultimately the chains were abandoned, not only for
permanent, but even for temporary use, the event testified strongly to Mr.
Fairbairn's sagacity and soundness of judgment in a matter so confessedly
novel and obscure.
Mr. Fairbairn's Report gave a succinct account
of the experiments which had been conducted—namely, 9 on cylindrical tubes,
5 on elliptical, and 10 on rectangular tubes. These tubes varied from about
17 to 30 feet long, and from 7 to 24 inches in transverse dimensions, and
the trials clearly proved the superiority of the rectangular form and the
cellular top. Mr. Fairbairn expressed great confidence as to the ultimate
success of the undertaking and the self-supporting power of the tube.
After the presentation of these reports, the
experiments were continued, with the view of determining more accurately the
dimensions and strength of the structure ; but before much more was done Mr.
Hodgkinson, in March 1846, requested that his share of the work should be
performed separately and under his own control; and as Mr. Stephenson
acceded to this, Mr. Hodgkinson had no further connection with Mr.
Fairbairn's proceedings.
In April Mr. Fairbairn communicated to Mr.
Stephenson an account of further experiments, which had enabled a rough
preliminary estimate to be made out of the dimensions of the real tube. Mr.
Fairbairn also began to give some attention to the details of construction,
proposing certain modes of connecting the plates by riveting, which he
considered would be advantageous.
It was further determined to construct a large
model tube, in every respect accurately proportioned to one-sixth of the
dimensions of the real structure; this, Mr. Fairbairn remarked to Mr.
Stephenson, would complete everything necessary for their practical
guidance.
About April 184(1, the design of the bridges was
commenced in earnest, the drawings were put in hand, and measures were
considered and discussed for obtaining the material and arranging the
manufacture. The distribution of the metal, the sizes of the plates, and the
methods to be pursued for putting them together, became matters of
considerable importance, and much time and thought were devoted to them Mr.
Fairbairn's duties now became more onerous. It was no longer the making and
testing of small models that he had to do. He was required to render
efficient aid iu the designs and manufacture of the largest and most
important iron constructions that had ever been known, thousands of tons in
weight, and involving great novelty, both in principle and detail. Hence it
became desirable that his position and occupation in regard to the work
should be acknowledged and clearly defined; and, with Mr. Stephenson's
concurrence, this was done at a board meeting of the directors of the
Chester and Holyhead Railway on May 13, 1816. The following is an extract
from the official minutes:—
Resolved:—1. That Mr. Fairbairn be appointed to
superintend the construction and erection of the Conway and Britannia
Bridges, in conjunction with Mr. Stephenson.
2. That Mr. Fairbairn have, with Mr. Stephenson,
the appointment of such persons as are necessary, subject to the powers of
their dismissal by the directors.
3. That Mr. Fairbairn furnish a list of the
persons he requires, with the salaries that he proposes for all foremen or
others above the class of workmen.
4. That advances of money be made on Mr.
Fairbairn's requisition and certificates, which, with the accounts or
vouchers, are to be furnished monthly.
The works connected with the first bridge it was
intended to erect, that at Conway, may be said then to have fairly
commenced, and we find Mr. Fairbairn hard at work iu regard to various
practical matters connected with the construction—visiting ironworks,
arranging workshops and tools, preparing for letting the contracts, and so
on. The large model tube was pushed on, and was completed, ready for
experiment, in June. It was 75 feet long, 4 feet 0 inches high, and 2 feet 8
inches wide. It was tested to destruction, by hanging weights on it till it
gave way, the object being to find out the weak places, and to ascertain how
it would fail. After each trial the injured and defective parts were cut out
and the tube was restored to its original form, with plates of altered
strength, as indicated by the nature and appearance of the fracture, and as
circumstances might require. This was done seven different times, until
proportions were arrived at which appeared to be satisfactory, as giving all
the strength of which such a tube was capable. By the middle of July a
decision had been come to as to the proportions and distribution of material
to be adopted in the real tubes.
About this time we find Mr. Fairbairn
considering and proposing plans for the erection and fixing of the bridges,
and earnestly urging on Mr. Stephenson the abandonment of the proposed
suspension chains. In August he was at the Menai Straits attending to the
arrangements there.
Mr. Stephenson was away on the Continent till
the end of September, and on his return the contract drawings and
specifications, which had been prepared by Mr. Fairbairn in conjunction with
Mr. Edwin Clark (Mr. Stephenson's chief assistant on the bridge), were
ready.
The contracts took some time to settle, but they
were not of such a nature as to shut out alterations and improvements in the
forms or proportions of the tubes, as new information was obtained. The
experiments and investigations still went on, and the forms of the cells and
other points of detail underwent careful discussion.
At the end of the year 1846 Mr. Fairbairn, after
visiting several manufactories, reported progress in the preparations for
the construction of the tubes.
When the contracts were first considered, it was
proposed that Mr. Fairbairn's firm should take an important share in the
manufacture. Mr. Stephenson, writing to Mr. Fairbairn on October 25, said:—
I am sincerely glad that your son and Ditchlmrn
[another maker] have succeeded in arranging with the Company. We must put
the whole of the Britannia into their hands, as I am sure the others are
unequal to the thing. We must visit both their establishments when I come
down to Manchester.
In reference to this, Mr. Fairbairn says
afterwards:—
A joint contract, which had been entered into by
my son, as representative of the tirm of Messrs. Fairbairn and Sons,
Millwall, with Messrs. Ditchburn and Mare, of Blackwall, for constructing
the greater part of the tubes for the Britannia Bridge, was looked upon w
ith suspicion by the board. Although interested as a partner, I had not
personally interfered in the matter, and I was even unacquainted with the
terms of agreement between the two firms; but when the feelings which were
entertained by the directors were made known to me, and as it appeared
difficult for me, in consequence of the partnership, to maintain a perfectly
independent position, I urged a transfer of the whole contract into the
hands of Messrs. Ditchburn and Mare. This transfer was afterwards
satisfactorily arranged by my son and Mr. J tare, and approved of by the
Company.
The detailed dimensions of some parts of the
tubes continued to be under consideration, as more light was thrown on the
nature of the forces and resistances, until about the spring of 1847, when
the whole may be said to have been finally arranged.
All this time Mr. Fairbaim was occupied in
various matters connected with the work, and, among others, with the mode of
erecting it. On March 24, 1817, he wrote to Mr. Stephenson :—
I have now completed, or nearly completed, the
whole of the drawings fur the framework, girders, &e., for lifting the
tubes. The arrangement of the hydraulic apparatus, chains, &e., is also
complete; and as soon as we. have copied the drawings &c„ the whole shall be
laid before you. 1 am now well satisfied as to the security of the ends of
the tubes, where the chains [for lifting them] are to be attached, as also
the large girders, and all the roller platforms, which are now secure and in
a most satisfactory position.
The actual manufacture of the tubes also engaged
his attention, although the superintendence of this was not strictly within
his province. On June 8 Mr. Stephenson wrote to him :—
I am much gratified at your resolution to devote
a considerable portion of your time to looking the tube builders up, and
getting a good job made of the whole affair.....What would be most valuable
is a regular periodical visit, so that the progress may be narrowly watched,
and advantage taken of every new continuation [contrivance] as it occurs. Of
these there will be many, which must suggest improvements in our
arrangements.
Mr. Fairbairn answered, June 9 :—
I have made up my mind to devote my best
energies to the construction and due completion of the tubes, and I will
watch narrowly and regularly the progress of each construction, that the
work be well done, and free from blemish in every respect.
As the time approached for making arrangements
for the erection, Mr. Fairbairn wrote, August 1(1, to Mr. Stephenson:—
Will you write me whether it is your wish that I
should take charge of the floating and raising of the tubes ? I have no
objection to do it, and to take the management of the whole thing, subject
to your approval, and to be responsible for the result.
Mr. Stephenson answered, August 23 :—
I was surprised at your letter this morning,
asking if I wished you to take charge of the floating and lifting. I
consider you as acting with me in every department of the proceedings, and I
shall regret if anything has been done which has conveyed to you the idea
that I was not desirous of having the full benefit of your assistance in
every particular.
On January 7, 1818, Mr. Stephenson wrote .—
I am glad to hear from your note, received this
morning, that all is progressing satisfactorily, though not with that
despatch which could be desired. Your presence will do much, and I hope you
will give as large a portion of your time as you can possibly spare.
It hail been decided that, in order to ascertain
the strength of the structure by actual trial, the first tube completed,
that at Conway, should, before putting it in its place, be tested by
supporting it on its ends and loading it with a considerable weight. This
test was made at the end of January, and on February 2 Mr. Fairbairn wrote
to the effect that the anticipations derived from the experiments on the
model had been fully borne out by the trials of the real tube. A few weeks
later the tube was hoisted into its place, and the trains passed through it
in April 1848, Mr. Fairbairn continuing to give his aid in the matter until
the solution of the great problem was practically completed.
Shortly after this time, some misunderstandings
having unfortunately arisen as to the precise nominal position Mr. Fairbairn
occupied (there could be none as to the value of the services he had
rendered) in regard lo the bridges, he did not feel it consistent with his
self-respect that he should continue his connection with them, and on May
22, 1848, he wrote to the directors resigning the appointment he had
formally received from them two years before.
He then put in hand the book mentioned at the
beginning of the present chapter, with the object of giving an authentic
record of the proceedings he had been a party to, in reference to these
bridges, and thereby establishing his claims to what he considered an
important share in the merit of their construction. In the preparation of
this work (the largest literary effort that had yet proceeded from his pen)
he had the assistance of many friends, among others the late Eev. IT.
Moseley, Canon of Bristol, and Mr. Tate, of Battersea, the latter gentleman
furnishing the many mathematical calculations which the book contained.
Many other men eminent in science also actively
interested themselves in Mr. Fairbairn's work on these bridges, among whom
may be mentioned Sir David Brewster, Mr. George Rennie, Mr. James Nasmyth,
Dr. Andrew Tire, Mr. C. Babbage, and Professor James D. Forbes.
Mr. Babbage wrote thus in answer to an
invitation from Mr. Fairbairn:—
My dear Sir,—I very much regret the
impossibility of my accepting your very agreeable invitation for next week.
I have been compelled to leave London on account of my health, and am
endeavouring, by the aid of sea air and quietness, to recruit it. This will
detain me in the West of England as long as circumstances permit. It is now
several years since I have visited your part of England, and I know how
rapidly it advances. I am, therefore, very anxious to take the earliest
opportunity of renewing my acquaintance with it, and of studying those great
mechanical advances in which you have taken so large a part.
I am, my dear Sir, yours faithfully,
C. Bibbage.
Ashley Combe, Purtlick, Somersetshire, September
3, 1818.
Another letter, from one of the cleverest
practical mechanics of the age, contains also some interesting passages:—
Patricoft, December 15, 1819.
My dear Sir,—Feeling such a lively interest as I
shall always do in all that relates to your well-earned fame, and having,
from the first, through your great kindness, noted the development of this
masterpiece of your genius, I did not fail to purchase a copy of your work
when it first came out, and have perused it again and again with the deepest
interest. I assure you I feel most proud in being thought worthy of
receiving a copy of your work direct from the author, and shall store it up
along with a few other much valued treasures, and so let my locomotive copy
free to run ahout telling the truth in many a quarter where the truth ought
to be known, and where it can be justly appreciated.
Tile Earl of Ellesmere has taken a most lively
interest in this affair, and, after carefully perusing your work, I think it
would have done your heart good to have heard the way in which he gave forth
his verdict, one afternoon, before some rather influential folks. Long may
you live to enjoy the fame (and,I trust, the profit) which shall attend your
triumphant introduction of a new era in engineering, which is destined to do
mankind most mighty service!
With kindest regards to Mrs. Fairbairn, in which
Mrs. Kasmyth desires to unite,
Believe me, I am yours most faithfully,
James Nasmyth.
During the course of Mr. Fairbairn's experiments
it seems to have occurred to him that the principle which was being
developed might be made of extended application for bridges generally,
particularly on railways; and as its application involved points of novelty,
he, with Mr. Stephenson's concurrence, took out a {latent for the
improvement. It is dated October 8, 1816, and bears the official number
11,401. The title is for 'Improvements in the construction of iron beams for
the erection of bridges and other structures.' It states the nature of the
improvement to consist—
In the novel application and use of plates of
metal, united by means of rivets and angle iron, for such or similar
purposes, and forming by such combination a hollow iron beam or girder.
The drawings show several varieties of
wrought-iron girders, all embodying the hollow or ' box' construction with a
cellular top, combining peculiar stiffness and lightness with great facility
of construction.
Mr. Fairbairn states in regard to this patent:—
The patent for wrought-iron girder bridges was a
joint affair between Mr. B. Stephenson and myself. It was in my name as the
inventor, but he paid half the expense, and was entitled to one half the
profits, but it ultimately became a dead letter, and was abandoned by Mr.
Stephenson.
Under the circumstances the question was, shall
I continue to build the bridges. I chose to do so, and I believe I did
right, as the principle was quite new, and no one understood the
construction so well. I therefore gave designs, and received orders for more
than one hundred bridges in the course of a very few years. Up to the
present time, 1870, I have built and designed, with the assistance of the
Fairbairn Engineering Company, nearly one thousand bridges, some of them of
large spans varying from 40 to 300 feet. |
