IMPROVEMENTS IN THE CONSTRUCTION OF LONG-SPAN BRIDGES
Year: 1880

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LETTERS PATENT to Andrew Barclay, of Kilmarnock, in the County of Ayr,
North Britain, Engineer, for an Invention of IMPROVEMENTS IN THE
CONSTRUCTION OF LONG-SPAN BRIDGES."
PROVISIONAL SPECIFICATION left by the said Andrew Barclay at the Office
of the Commissioners of Patents on the 27th January 1880.
ANDREW BARCLAY, of Kilmarnock, in the County of Ayr, North Britain,
Engineer. "IMPROVEMENTS IN THE CONSTRUCTION OF LONG SPAN BRIDGES."
This Invention, which relates to improvements in the construction of long span
bridges, has for its object to enable the bridge to be self supporting during
construction, to dispense with the use of scaffolding during construction, and to
obviate the necessity of placing any connection across the space to be bridged
during the construction thereof.
The single span of a bridge constructed according to this Invention consists of
two triangular lattice girders, the apex of each triangle being above the centre of
the piers upon which the girders are respectively carried, the base of the triangles
forming the roadway, and meeting together with the hypotenuse at the central
point of the span.
In practice the central part of each girder terminating in the apex of the triangle
consists of a tower constructed of any desired number of vertical tubes or bars
braced together into a rigid structure by ties and struts. The lower part of each
triangle consists of a horizontal strut bearing vertically against the bottom of the
central or lower portion, and the inclined side or hypotenuse consists of ties
connecting the strut at any desired number of points with the tower or vertical
member.
In constructing the bridge, so soon as the tower or central portion has been
raised to a sufficient height to carry inclined service ties, the horizontal or strut
portion is built out at the two opposite sides thereof simultaneously in equal
portions, so that a balance of the structure is at all times maintained; and when
each section of such strut is completed, it is coupled to the vertical central portion
by the permanent tie rods; the building of the vertical towers upwards and the
struts outwards at all points simultaneously enables the work to be carried out in
much less time as compared to other modes of constructing wide span bridges, for
example, suspension bridges, wherein the towers have to be completed to their full
height before the chains can be carried across the span.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Andrew Barclay in the Great Seal Patent Office on the 27th July
1880.
ANDREW BARCLAY, of Kilmarnock, in the County of Ayr, North Britain,
Engineer. "IMPROVEMENTS IN THE CONSTRUCTION OF LONG-SPAN BRIDGES."
My said Invention relates to improvements in the construction of long-span
bridges, and has for its objects to enable the bridges to be self supporting during
construction, to dispense with the use of scaffolding during construction, and to
obviate the necessity of placing any connection across the space to be bridged
during the construction thereof, and is especially applicable to the bridging of deep
channels, such for example, as that of the Firth of Forth, which afford foundations
for piers only at long distances apart.
The single span of a bridge constructed according to my Invention consists of
two triangular lattice girders, the apex of each triangle being above the centre of
the piers upon which the girders are respectively carried, the base of the triangles
forming the roadway and meeting together with the bypotenuse at the central
point of the span.
In practice the central part of each girder terminating in the apex of the triangle
consists of a tower constructed of any desired number of vertical tubes or bars,
braced together into a rigid structure by ties and struts. The lower part of each
triangle consists of a horizontal strut, which may bear or abut against the bottom
of the central or tower portion, or which may be continuous, and the inclined side
or hypotenuse consists of ties connecting the strut at any desired number of points
with the tower or vertical member.
Figure 1 of the accompanying Drawings represents in side elevation a portion of
a bridge shewing a long span constructed according to my Invention; Figure 2 is
a transverse section on the line 1, 2, Figure 1, and Figure 3 is a transverse section
on the line 3, 4, Figure 1. The two triangular lattice girders by which the span is
formed are marked A and B. The girders are carried upon the piers C, as shewn,
the base of the triangles forming the roadway at D. Each of the said girders
consists of the horizontal portion or strut D, or base of the triangle forming the
roadway of the towers E, and of the ties F, which connect the horizontal portion D
with the said towers. Inclined stiffening bars g may be employed to prevent the
sagging of the several ties F, and vertical bars h may likewise be employed to
provent sagging, and thereby impart additional strength to the structure.
In constructing the bridge, so soon as the towers or central portions E have been
raised to a sufficient height to carry inclined service ties, a corresponding portion
of the horizontal roadway or strut portion D is built out at the two opposite sides
of each of the said towers on either side of the channel to be bridged simultaneously
in equal portions, so that a balance of the structure is at all times maintained.
After one portion of the towers and of the roadway portion have been built out
and connected by permanent ties, another portion of the towers is built up, and
then another corresponding portion of the roadway is built out on either side, and
so on until the roadway portions built out from the piers on either side the channel
meet in the centre, and are there connected together in any suitable manner, any
suitable or ordinary means for compensating for contraction and expansion being
provided. As each section of the horizontal portion is completed it is coupled to
the corresponding part of the vertical central portion E by the permanent tie
rods F. The building of the vertical towers E upwards, and the horizontal portion
D outwards at all points simultaneously enables the work to be carried out in
much less time as compared to some other modes of constructing wide spau
bridges.

page_text: In the Drawing I have shewn a central bracing portion or truss i (see Figure 3),
which may be employed to strengthen the bridge at this part.
In Figure 1 the ties F are shewn as being situated at equal distances from
each other along the towers E, but they may be situated at unequal distances apart,
or they may be distributed through only the upper part of the tower, or they may
all be connected to the top portions of the towers, as shown in Figure 5.
To give a diminished height of towers E the centre of the span may be formed
by a lattice or bow-string girder, as at k, Figure 4, which represents a portion of a
span of a bridge constructed according to my Invention. The said girder k is carried
by the ends of the two projecting triangular girders, and it may be attached by
suspending chains to the towers or to the said triangular girders.
In spans with double roadways, the roadways may be situated at any suitable
distance apart to ensure lateral stability, the two roadways and their respective
towers being braced together after the manner illustrated in Figure 2.
Having now described and particularly ascertained the nature of my said
Invention, and the system, mode, or manner, in or under which the same is or
may be used or practically carried into effect, I would observe in conclusion that
what I consider to be novel and original, and therefore claim as the Invention
secured to me by the herein before in part recited Letters Patent is,―
The improvements in the construction of long span bridges consisting in
constructing the long span or spans of such bridges of triangular girders, which are
built out on either sides of the piers, essentially as hereinbefore described.
'
In witness whereof, I, the said Andrew Barclay, have hereunto set my hand
and seal, this Twenty sixth day of July, in the year of our Lord One
thousand eight hundred and eighty.
ANDREW BARCLAY. (L.S.)