IMPROVEMENTS IN BRIDGES
Year: 1869

page_text: LETTERS PATENT to Fleeming Jenkin, of No. 5, Fittis Row, Edinburgh,.
Professor of Civil Engineering in the University of Edinburgh, for
the Invention of "IMPROVEMENTS IN BRIDGES."
Sealed the 12th April 1870, and dated the 21st October 1869..
PROVISIONAL SPECIFICATION left by the said Fleeming Jenkin at the
Office of the Commissioners of Patents, with his Petition, on the
21st October 1869.
I, FLEEMING JENKIN, of No. 5, Fittis Row, Edinburgh, Professor of
Civil Engineering in the University of Edinburgh, do hereby declare the
nature of the said Invention for "IMPROVEMENTS IN BRIDGES," to be
as follows:-
My Invention consists in the better distribution of materials employed
in constructing metal or timber arches or suspension bridges. The
ordinary metal arch consists of a stiff rib either of uniform section or
very slightly tapered towards the crown portion stiffened by extraneous
girders or by iron bracing of various descriptions, and the ordinary
suspension bridge similarly consists of a nearly uniform chain or rope

page_text: stiffened occasionally in a similar manner. I use for the arch a rib
braced to a second member or boom, so that essentially the bridge
consists of three parts, an arch, a boom, and bracing; similarly my
suspension bridge consists of an inverted arch, a boom, and bracing.
The necessary stiffness is given by the bracing between the arch and
boom, but both the arch and boom are compressed in the arch, and both
are in tension in the suspension bridge. I then distribute the material
so that the arch tapers at the crown to a fourth or even less of its cross
section at the haunches, while the boom increases as much or more
from the piers to the centre of the span, the sum of the cross sections
of the boom and arch not being far from constant and about equal to
the cross section of the top member of a bowstring bridge of the full
depth from the boom to the springing. The cross bracing I make nearly
uniform throughout and of about the strength used in the same bow-
spring girder. In the suspension bridge a perfectly analogous dis-
tribution of materials is employed. By this construction a perfectly
rigid suspension bridge or arch is secured with the least possible waste of
materials. As compared with a bowstring girder of the same depth the
bottom boom is saved. As compared with a lattice girder a great part of
the cross bracing is saved, and a much greater depth relative to span
is made possible. As compared with existing arches, all cross strains on
simple or composite members are avoided, and a much better distribution
of materials employed. As compared with existing suspension bridges
perfect rigidity is given by the new bridge without any cross strain on
any simple or composite member, and the distribution of material pro-
posed is entirely novel, as shewn in the accompanying Drawing, where
the best cross section of each part is approximately shewn by the
breadth given to the member in the diagram. The struts must of
course in practice be stiffened to resist bending.
SPECIFICATION in pursuance of the conditions of the Letters Patent, 3
filed by the said Fleeming Jenkin in the Great Seal Patent Office
on the 21st April 1870.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, FLEEMING
JENKIN, of No. 5, Fettes Row, Edinburgh, Professor of Civil Engineering
in the University of Edinburgh, send greeting.
3

page_text: WHEREAS Her most Excellent Majesty Queen Victoria, by Her
Letters Patent, bearing date the Twenty-first day of October, in the
year of our Lord One thousand eight hundred and sixty-nine, in the
thirty-third 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
vend, within the United Kingdom of Great Britain and Ireland, the
Channel Islands, and Isle of Man, an Invention for "IMPROVEMENTS IN
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
performed, to be particularly described and ascertained in and by the
following statement thereof, that is to say :-
My Invention consists in the better distribution of materials employed
in constructing metal or timber arches or suspension bridges. The
ordinary metal arch consists of a stiff rib, either of uniform section or
very slightly tapered towards the crown, further stiffened by extraneous
girders, or by iron bracing of various descriptions, and the ordinary
suspension bridge similarly consists of a nearly uniform chain or rope
stiffened occasionally in a similar manner. I use for the arch a rib
braced to a second member or boom, so that essentially the bridge
consists of three parts, an arch, a boom, and bracing; similarly my
suspension bridge consists of an inverted arch, a boom, and bracing.
The necessary stiffness is given by the bracing between the arch and
boom, but both the arch and boom are compressed in the arch, and both
are in tension in the suspension bridge. I then distribute the material
so that the arch tapers at the crown to a fourth or even less of its cross
section at the haunches, while the boom increases as much or more

page_text: from the piers to the centre of the span, the sum of the cross sections
of the boom and arch not being far from constant and about equal to
the cross section of the top member of a bowstring bridge of the full
depth from the boom to the springing. The cross bracing I make nearly
uniform throughout and of about the strength used in the same bow-
string girder. In the suspension bridge a perfectly analogous dis-
tribution of materials is employed. By this construction a perfectly
rigid suspension bridge or arch is secured with the least possible waste
of materials. As compared with a bowstring girder of the same depth
the bottom boom is saved. As compared with a lattice girder a great part
of the cross bracing is saved, and a much greater depth relative to span
is made possible. As compared with existing arches all cross strains
on simple or composite members are avoided, and a much better dis-
tribution of materials employed. As compared with existing suspension
bridges perfect rigidity is given by the new bridge without any cross
strain on any simple or composite member, and the distribution of
material proposed is entirely novel, as shown in the accompanying
Drawing, where the best cross section of each part is approximately
shown by the breadth given to the member in the diagram, the plates
of which each member is composed being supposed to be of equal
thickness throughout. The struts must of course in practice be
stiffened to resist bending.
In witness whereof, I, the said Fleeming Jenkin, have hereunto set
my hand and seal, this Eighteenth day of April, in the year of
our Lord One thousand eight hundred and seventy.
FLEEMING JENKIN. (L.S.)