IMPROVEMENTS IN THE CONSTRUCTION OF IRON BRIDGES
Year: 1856

page_text: LETTERS PATENT to John Talbot Pitman, of the American and European
Patent Offices, No. 67, Gracechurch Street, in the City of London, for
the Invention of "IMPROVEMENTS IN THE CONSTRUCTION OF IRON BRIDGES."—
A communication.
Sealed the 6th March 1857, and dated the 11th September 1856.
PROVISIONAL SPECIFICATION left by the said John Talbot Pitman at
the Office of the Commissioners of Patents, with his Petition, on the
11th September 1856.
I, JOHN TALBOT PITMAN, of the American and European Patent Offices,
No. 67, Gracechurch Street, in the City of London, do hereby declare the
nature of the said Invention for "IMPROVEMENTS IN THE CONSTRUCTION OF IRON
BRIDGES,' a communication from a foreigner residing abroad, to be as
follows:-
My Invention consists in certain improvements in bridges, whereby I attain
lightness, strength, durability, and economy.
The arches of my bridge are of a compound character, and are made of
wrought plate iron, so as to give to the whole arch transversely the form and
strength of an arch, and to admit of very long spans without excessive weight,
presenting at once extraordinary strength and lightness.
In transverse section my arch presents an isosceles triangle, the base of

page_text: which is the chord of the arch. The plates composing the arch are so arranged
as to break joints. In the middle of the base plate is a vertical plate bolted
to the base plate and secured at top to the plates composing the sides of the
arch by rivets. In order still further to increase the strength and to prevent
all risk of buckling of the plates, I insert on the sides of the arch loose pieces
of irregular form, which I term saddles. These rest upon the base plate and
also bear upon the side plates, and also support each other by their edges,
which come into contact. They are not secured to either plate, but are
inserted loosely, and their upper edges receive a part of the pressure of the
stirrups, which go over the arch and support the suspension rods to which the
floor of the bridge is hung. The chain of saddles on either side of the com-
pound arch forms an independant arch, and the effect of each saddle is to give
tension to the side plates when under great strain in consequence of the
pressure of the saddle upon the base plate, and thus prevent the buckling of
the plates. The saddle is rendered light by the removal of those parts least
subjected to strain. The suspension rods are radial, or nearly so. The flooring
of the bridge rests upon the chord of the arch, which is secured to, the feet of
the arch and supported upon a suspension plate. The suspension rods pass
between the two plates which compose the chord, and they are bolted to the
suspension plate; the suspension plate is not fastened to the chord, but the
chord is kept in position laterally upon the suspension plate by flanges on this
plate. The feet of the arch rest upon corrugated shoes.
SPECIFICATION in pursuance of the conditions of the Letters Patent, filed
by the said John Talbot Pitman in the Great Seal Patent Office ou
the 11th March 1857.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, JOHN
TALBOT PITMAN, of the American and European Patent Offices, No. 67, Grace-
church Street, in the City of London, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her Letters
Patent, bearing date the Eleventh day of September, in the year of our
Lord One thousand eight hundred and fifty-six, in the
year of Her
reign, did, for Herself, Her heirs and successors, give and grant unto me, the
said John Talbot Pitman, Her special licence that I, the said. John. Talbot
Pitman, and my executors, administrators, and assigns, or such others as I,
the said John Talbot Pitman, or my executors, administrators, and assigns,
should at any time agree with, and no others, from time to time and at

page_text: all times thereafter during the term therein expressed, should and lawfully
might make, use, exercise, and vend, within the United Kingdom of Great
Britain and Ireland, the Channel Islands, and Isle of Man, an Invention for
"IMPROVEMENTS IN THE CONSTRUCTION OF IRON BRIDGES," a communication from a
foreigner residing abroad, upon the condition (amongst others) that I, the said
John Talbot Pitman, or my executors or administrators, by an instrument in
writing under my, or their, or one of their hands and seals, should par-
ticularly describe and ascertain the nature of the said Invention, and in what
manner the same was to be performed, and cause the same to be filed in the
Great Seal Patent Office within six calendar months next and immediately
after the date of the said Letters Patent.
NOW KNOW YE, that I, the said John Talbot Pitman, do hereby declare
the nature of my said Invention, and in what manner the same is to be
performed, to be particularly described and ascertained in and by the following
statement:
My Invention consists in certain improvements in bridges, herein-after de-
scribed, whereby I attain lightness, strength, durability, and economy beyond
any iron structure heretofore used for such purpose.
The same parts are indicated by the same letters in all the Figures of the
accompanying Sheet of Drawings.
The arches A, A, of my bridge are of a compound character, and are built
up of wrought plate iron, in such manner as to give the whole arch transversely
the form and strength of an arch, and to admit of very long spans without
excessive weight, presenting at once the combined features of extraordinary
strength and lightness. Hollow arches for such structures have been essayed
before, but of such form, application, and material as to be objectionable on
the ground of expense, great weight, and derangement from expansion and
contraction by changes of temperature. A transverse section of my compound
arch, as shewn in Fig. 3, exhibits an arch in the form of an isosceles triangle,
the base B of which is the chord of the arch. This form is best adapted for
strength, lightness, and economy of construction, and is, I believe, the only
form with the least weight that can be given to a hollow iron arch for such
purpose, which is not liable to buckle. The plates P, P, P, P, Figures 2 & 6,
composing the arch are so arranged in their construction as to break joints for
5 the purpose of securing greater strength; and to give greater strength to the
triangular arch I insert a vertical plate R, bolted to the base plate B, and
secured to the plates P, P, P, P, by rivets; thus uniting the plates P, R, and
B, in the most advantageous manner for producing a light and rigid structure,
both as regards its own support and that of the bridge below. Under a strain

page_text: in any direction which may come upon this compound arch there is less risk
of buckling of either of the plates than in other structures for such pur-
pose. In order, however, still further to increase the strength and diminish
the liability to buckling of the plates, I insert the loose pieces S, S, which I
term saddles, and which are of the form shewn in the Drawings. These
saddles rest upon the base plates B, and also bear upon the plates P, P, and
also support each other by their edges, which come into contact, as seen at T.
These pieces are not secured to either plate, but are inserted loosely, and their
upper edges receive a part of the pressure of the stirrups E, E, of the sus-
pension rods F, F. The chain of saddles on either side of the compound arch
thus forms an independent arch, and the effect of each individual saddle is to
give tension to the plates P, P, where under great pressure in consequence
of the pressure of the saddle upon the base plate B, and thus prevent the
buckling of the plates; and with this last increment of strength and support, it
is obvious that the arch can give way to extreme pressure only by the rupture of
the metal of plates B and R. The exterior face of the saddle is formed for
lightness and strength, the superfluous metal being removed from those parts
subjected to the least strain. The suspension rods F, F, are radial, or nearly
so, to the curvature of the arch, and therefore all of them inclined to the
versed sine of the arch. The flooring of the bridge rests upon the chord M
of the arch, which is secured to the feet of the arch and supported upon the
suspension plate D. The suspension rods pass between the two plates G, G,
which compose the chord M, and are bolted to the suspension plate D. The
suspension plate is not fastened to the chord M, and the effect in conjunction
with the radial suspension rods is, in case of great weight upon any part of the
bridge, to throw the strain upon the whole span at once. It will be seen that
on no part of the bridge is any weight or pressure under the point of sus-
pension of that part, and that every load draws upon the whole arch in con-
sequence of the sliding movement of the suspension plate under the chord M.
This chord is kept in position laterally upon the plate D, by flanges W, W, on
this plate. The feet of the arch rest upon corrugated shoes K, for the two-
fold purpose of producing friction on the abutments and of working their
way by gradual abrasion into the material of the abutments and securing a
firm hold. The two arches are held together at top by diagonal braces L, L.
In the accompanying Drawings, Figure 1 is a side view of my improved
bridge; Figure 2, a top view of the arch; Figure 3, a transverse vertical
section of the arch, floor, suspension rod, and suspension plate; Figures 4
and 5, top and bottom views of the corrugated shoe K; and Figure 6, section,
in perspective, of the arch, shewing the arrangement of the saddle and stirrup;

page_text: Figure 7 shews the diagonal braces which hold the two arches together at
top.
Having thus described my Invention, what I claim under the above in part
recited Letters Patent is as follows:-
First, the compound arch constructed substantially as herein set forth.
Second, the saddle pieces, in combination with the stirrups and said
compound arch.
Third, the sliding suspension plate in combination with the chord M and
radial suspension rods, as set forth.
Fourth, the corrugated shoes K, constructed and operating as set forth.
In witness whereof, I, the said John Talbot Pitman, have hereunto set
my hand and seal, this Tenth day of March, in the year of our Lord
One thousand eight hundred and fifty-seven.
JOHN TALBOT PITMAN. (L.S.)