IMPROVEMENTS IN THE CON- STRUCTION OF BRIDGES
Year: 1861

page_text: LETTERS PATENT to Fleeming Jenkin, of Stowting, in the County of
Kent, Civil Engineer, for the Invention of "IMPROVEMENTS IN THE CON-
STRUCTION OF BRIDGES."
Sealed the 12th September 1861, and dated the 16th March 1861,
PROVISIONAL SPECIFICATION left by the said Fleeming Jenkin at the
Office of the Commissioners of Patents, with his Petition, on the 16th
March 1861.
I, FLEEMING JENKIN, of Stowting, in the County of Kent, Civil Engineer,
do hereby declare the nature of the Invention for "IMPROVEMENTS IN THE
CONSTRUCTION OF BRIDGES," to be as follows:-
This Invention has for its object improvements in the construction of bridges.
Heretofore, in constructing arched bridges, where rigid arched ribs are used, it,
has been found necessary to limit the compression of the arched rib (considered
as an equilibrated arch) to a strain much below what the materials would bear
safely in order to provide strength to meet extra strains caused by flexure when,
the arch is not equilibrated, and it is very difficult to calculate, these extra,
strains.
Now, according to my Invention a flexible arched rib is used, which is braced.
or connected by, struts and ties to an upper rib placed at a sufficient distance,
above it to form with it an inflexible frame wherein the lower arched rib will.
be in a state of compression, thrusting against abutments at points which

page_text: support the weight of the bridge. For this purpose it is preferred to employ
a polygonal flexible pin-jointed arched rib in place of the rigid arched rib
heretofore used. The upper rib is connected to the pin-jointed angles of the
polygonal arched rib by means of struts and ties or bars, which act alternately
as struts and ties. The upper rib may be continuous, or it may be in parts,
and pin-jointed. The struts are pin-jointed at both ends, and such is usually
the case in regard to the ties; but in some constructions their flexibility will
render such jointing unnecessary. By this construction a jointed frame is
produced, each member of which is subject to a strain in the direction of its
length only, consequently the sectional area of each member or part of the
structure can be readily and exactly calculated and proportioned to the
maximum strain which it will have to bear under any distribution of the load.
The upper rib strengthened to meet a transverse strain may also be used to
bear the roadway and load between each strut. Although I prefer that the
lower arched rib should be a pin-jointed polygon, yet it may be a continuous
rib, which must then be comparatively very flexible in its whole length, but
the lengths into which it is divided by the connection of the struts and ties
thereto must each be so stiff and rigid as to act as if each were.a separate
member of a polygonal-arched rib, such as is above described; the pin joints
throughout the bridge may then be omitted. In the frame thus constructed,
each member, besides the chief strain in the direction of its length, is subject
to a transverse strain, which is comparatively very small.
SPECIFICATION in pursuance of the conditions of the Letters Patent, filed
by the said Fleeming Jenkin in the Great Seal Patent Office on the
16th September 1861.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, FLEEMING
JENKIN; of Stowting, in the County of Kent, Civil Engineer, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria by Her Letters
Patent, bearing date the Sixteenth day of March, in the year of our Lord
One thousand eight hundred and sixty-one, in the twenty-fourth year of Her
reign, did, for Herself, Her heirs and successors, give and grant unto me, the
said Fleeming Jenkin, Her special licence that I, the said Fleeming Jenkin,
my executors, administrators, and assigns, or such others as I, the said Fleeming
Jenkin, my executors; administrators, and assigns; should at any time agree
with, and no others, from time to time and at all times thereafter during the
term therein expressed, should and lawfully might make, use, exercise, and

page_text: vend, within the United Kingdom of Great Britain and Ireland, the Channel
Islands, and Isle of Man, an Invention for "IMPROVEMENTS IN THE CONSTRUC-
TION OF BRIDGES," upon the condition (amongst others) that I, the said
Fleeming Jenkin, my executors or administrators, by an instrument in writing
under my, or their, or one of their hands and seals, should particularly 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 Fleeming Jenkin, do hereby declare
the nature of the said Invention, and in what manner the same is to be per-
formed, to be particularly described and ascertained in and by the following
'statement thereof, that is to say :-
This Invention has for its object improvements in the construction of bridges.
Heretofore, in constructing arched bridges where rigid arched ribs are used, it
has been found necessary to limit the compression of the arched rib (considered
as an equilibrated arch) to a strain much below what the materials would
bear safely in order to provide strength to meet extra strains caused by flexure
when the arch is not equilibrated, and it is very difficult to calculate these.
extra strains.
:
Now, according to my Invention, a flexible arched rib is used, which is braced
or connected by struts and ties to an upper rib placed at a sufficient distance
above it to form with it an inflexible frame wherein the lower arched rib will
be in a state of compression, thrusting against abutments at points which
support the weight of the bridge. For this purpose, it is preferred to employ
a polygonal flexible pin-jointed arched rib in place of the rigid arched rib
heretofore used. The upper rib is connected to the pin-jointed angles of the
polygonal arched rib by means of struts and ties or bars, which act alternately
as struts and ties. The upper rib may be continuous, or it may be in parts
and pin-jointed. The struts are pin-jointed at both ends, and such is usually
the case in regard-to the ties; but in some constructions their flexibility will
render such jointing unnecessary. By this construction a jointed frame is
produced, each member of which is subject to a strain in the direction of its
length only, consequently the sectional area of each member or part of the
structure can be. readily and exactly calculated and proportioned to the
maximum strain which it will have to bear under any distribution of the load.
The
upper rib, strengthened to meet a transverse strain, may also be used to
bear the roadway and load between each strut: Although prefer that the
lower or arched rib should be a pin-jointed polygon, yet it, may be a continuous

page_text: rib, which must then be comparatively very flexible in its whole length, but
the lengths into which it is divided by the connection of the struts and ties
thereto must each be so stiff and rigid as to act. as if each were a separate
⚫ member of a polygonal arched rib, such as is above described; the pin joints
throughout the bridge may then be omitted. In the frame thus constructed,
. each member besides the chief strain in the direction of its length is subject
to a transverse strain, which is comparatively very small.
Figure 1 illustrates one form of my Invention,. showing the general elevation
for a bridge of 300 feet span. C, C, C, is a polygonal flexible arched rib, pin-
jointed at P, P, P; T, T, is the upper rib connected to the pin-jointed angles
.P, P, P, by the bars B, B, B, which act alternately as, struts and ties; these
bars are pin-jointed at P and at Q..
Figures 2 to 6 show the details of construction for this example, and will be
.readily understood by those conversant with mechanical Drawings. Each
member of the arched rib consists of a hollow cast-iron tube, as C, (Figure 2
'to 6) cottered or bolted to a single and double eye of cast iron at each end
respectively. A strong wrought-iron pin H connects the single eye of one
member with the double eye of the next, and also supports the double bar B, B,
(Figure 3) which is braced so as to resist compression, without buckling; this
double bar is connected with the upper rib T by the strong wrought-iron pin G.
Each member of the upper rib T is formed of four links or bars of wrought
iron placed side by side, as shown in Figure 5, and the several members, are
connected by the pins G at Q, Q, Q. Figures 3 and 4 show the usual bracing
required to give lateral stiffness. The links T carry the roadway, which is not
shown; the form shown is only an example.. The details, materials, and pro-
portions may be varied according to circumstances; the pins may, in some
cases, as before said, be replaced by rigid connections. Thus, if the lower rib
were made of wrought-iron plates, the bars B, B, might be rivetted to those
-plates, provided always the plates were so disposed as to make the arched rib
very flexible until framed to the upper rib, and were at the same time stiff
enough to resist compression between the points P, P, without buckling ; thús
plates lying flat might be used stiffened with angle iron between the points P, P;
when pins are used each member is strained as a strut or tie only, but when
the pins are omitted, a transverse strain is thrown on each member of the
arched rib by a passing load; but this transverse strain is so small, if the
arched rib is flexible, as to be practically of no importance, so that the strength
of each member can be calculated in the same manner as if pins were used:
The thrust on the abutments being known, the strain on every member is
easily determined by the principle of moments, or by composition of forces!

page_text: The strain on each member of the arched rib is nearly constant when the load
is uniformly distributed over the whole arch; the strains on the upper rib and
bars or diagonals are in that case comparatively small. To resist a passing
load, the members of both upper and lower ribs should be strengthened towards
the centre of the bridge. The bars or diagonals must also be stronger than
would be necessary to support the same load uniformly distributed. The tri-
angular trussing shown in Figure 1 may be replaced by diagonals and uprights,
as in Figure 7. The upper rib may be curved, as in Figure 8. The roadway
may be hung to the structure at any convenient level, as in Figure 9; and
many similar modifications might be used. I do not therefore limit my claim
to the particular forms shown, but claim generally a flexible arched rib thrusting
against abutments and framed to an upper rib, as described.
In witness whereof, I, the said Fleeming Jenkin, have hereunto set my
hand and seal, this Sixteenth day of September, in the year of our
Lord One thousand eight hundred and sixty-one.
FLEEMING JENKIN. (L.S.)