The Sun.
London, England, 1847-11-06 00:00:00

SUSPENSION BRIDGES.

At the last meeting of the Institution of Mechanical
Engineers, held at Birmingham, the following im-
portant paper by Mr. A. E. Cowper, of the London
Works, was read, and deemed highly important and
deserving of further consideration :
" DESCRIPTION OF AN IMPROVED SUSPENSION-BRIDGE
FOR CARRYING A RAILWAY, AND FOR OTHER PUR-

POSES.
"In bringing before the Institution of Mechanical
Engineers a paper on a peculiar form of bridge, I
ought, perhaps, to apologise for introducing matter
which may by some of our friends be thought to be-
long more strictly to our civil brethren ; but possibly
before the conclusion of the paper, I shall have anti-
cipated any objection which might have been made on
that score by showing, in point of fact, that I have
only been explaining a piece of boiler-maker's work,
and which may certainly be considered to be far enough
removed from civil engineering. The object of the
present paper is to call the attention of engineers and
railway directors generally to a mode which I have
invented of constructing suspension-bridges in such
a way that they shall not be thrown out of shape, or
in any way distorted, by the weight of a pass-
ing load, whether it consists of a railway train
or only of the ordinary traffic of a common
road. It is well known that suspension-bridges
are decidedly less costly than any stone bridges, and
we may add than most iron bridges, when the span
is at all above the length of an ordinary girder ; and
although many persons have turned their attention to
them, particularly with regard to their use on rail-
ways, I am not aware that any suspension-bridge has
ever been made, or proposed, that was at all com-
petent to carry the weight of a railway train in
motion, or in other words, that should be safe as a
railway bridge. My attention was particularly called
to suspension-bridges by the proposal of carrying a
railway over the Hungerford-bridge, or over a bridge
placed alongside of it; and it appeared to me that the
weight of a passing train would so move and distort the
chains as to cause the road very soon to get out of
order, if not actually to give way, and I then schemed
the plan of making a chain of such depth as to include
any alteration in the curve of the strain that might
take place. The curve which the chains of an
ordinary suspension-bridge takes is well known to be
a catenary, or rather a curve between a catenary
and a parabola; it would be a true parabola if all
the weight were in the platform, and a true catenary
if all the weight were in the chain. As, however, the
' difference between the catenary and the parabola is very
slight indeed in that portion which would be used
for a bridge, we may assume it to be a catenary for
all practical purposes. Now, on loading an ordinary
suspension-bridge with even a small weight, it at
once assumes a different curve (unless the weight be
equally distributed over the bridge), and if the weight
be large, it will assume a very different curve ; so
much, indeed, will the form be altered as to injure or
strain the material of which the platform or road is
composed. Now, it is evident, that if the road has to
distribute the weight, it must be a very strong and stiff
beam, or in fact, a girder of the full length of the
bridge ; and the strength of this girder would very
nearly be equal to carrying a quarter of the weight
of the load in the centre ; it is, therefore, evident
that the plan of forming a stiff platform or road for a
railway suspension-bridge, although by no means
impossible, must be at least half abandoning the sus-
pension principle, and be the cause of greater out-
lay. The plan of keeping the road in shape, by distri-
buting any weight that might come upon it, by means
of strong diagonal ties, was the first idea that I had ;
but it will be found by calculation that these diagonals
would have to be very strong, and of considerable
height, thereby causing the total depth of the bridge
to •be much greater. But the plan on which I pro-
pose to construct suspension-bridges capable of car-
rying railway trains without being in any way injured
thereby, is simply to construct the chain of such
depth as to include the curve of strain when the weight
is placed on the bridge in the most unfavourable posi-
tions. With this object I construct the chains of boiler
plate of considerable depth—say three or four feet,
or more—and rivet the whole well together without
any moveable joints, or separate links, and at the top
and bottom edges of the chains (I still call them
chains, that I may be clearly understood) I rivet or
otherwise attach bars, either fiat, half-round, or
angle iron, so as to give an accumulation of
metal at those parts, and at the same time
to render the edges of the chains perfectly
secure against any tendency to rip or tear. In the
drawing you will observe that there are two chains.
each four feet deep, which support the ends of cross
wrought iron girders, in the position of sleepers, each
chain being composed of four boiler-plates, riveted
together in pairs, each plate being three-eighths
thick, and at the top and bottom edges there are
securely riveted strong angle irons. The suspension
bars hang between the two pairs of plates forming the
chain, and are supported by a small saddle, which
bears on the top edges of them. The ends of the
cross wrought iron girders are firmly secured to
a light rib of boiler-plate, which runs along each
side of the bridge, as shown in the cross section of
the bridge ; the lower ends of the suspension-bars are
secured to the ends of the girders, with means of
adjustment, so that the road may be trimmed per-
fectly level when the bridge is fixed. There are also
light diagonal ties introduced, for the purpose of
more perfectly staying the road to the chains,
particularly in case of the breaks being applied
whilst the train is passing over the bridge. The
rails, either of the ordinary form placed in chairs, or
of that form commonly called the bridge-rail, are
supported on balks of timber scarphed toge-
ther, which run longitudinally throughout the bridge,
and these are supported by short balks of timber
running from girder to girder, immediately under the
first. There are a series of diagonal ties placed in the
platform, as shown in plan in the drawing. These
act as a means of stiffening the platform, and pre-
venting any vibration or shaking of the parts. The
long red lines indicate stay rods, by which the bridge
is prevented from moving or swinging sideways.
They are attached to the piers, and are very similar
to some used by Mr. Brunel, sen., in a bridge at the
Isle of Bourbon. The drawing before you is of a
bridge 200 feet span, having the cross girders
eight feet from centre to centre, and the chains four
feet deep, which depth has been arrived at by actual
experiment ; the weight of the road for one line of
rails and one chain is one ton per foot run, and the
weight of a train of locomotives I have assumed at
one ton per foot run, and this is allowing some
margin for the continued growth of locomotives ;
and 1 have taken as a proof load, two tons per
foot run ; thus the weight of the load, or disturbing
cause, will be just double the weight of the
bridge. I find the greatest distortion of the curve
strain takes place when the bridge is only half
loaded, i. e., from one end to the centre ; the curve
then approaches the bottom of the chain, very nearly
in the centre of the loaded half, and approaches the
top of the chain in the centre of the unloaded half,
whilst at the piers it approaches the top at the loaded
end, and the bottom at the unloaded end. Again, if
the same load be placed in the centre of the bridge
(covering one-half of the length), the curve of strain
will approach the bottom of the chain in the
centre, and will approach the top of the chain
at very nearly one-fifth from each pier, whilst
at the piers it will be near the centre of the
chain, but rather above it. Take one more case,
and we shall have disposed of all the heavy
disturbing tendencies viz., that of the ends
loaded, and the centre left unloaded : the curve of
strain will then approach the top of the chain in the
centre, and the bottom of the chain at about one•
sixth from each pier, whilst at the piers the strain
will be slightly above the centre. I may add, that
when the bridge is fully loaded throughout, the curve
of strain is in the centre of the chain, throughout its
length. I propose to call bridges made on this plan
inverted-arch bridges.' "
A brief conversational discussion followed the read-
ing of this paper, in the course of which Mr. Stephen-
son and others stated that if the invention could prac-
tically carry out the intentions of Mr. Cowper, it
effected a most important desideratum in railway
engineering.
On a motion for an adjournment of the meeting,
Mr. HUMPHRIES remarked on the good likely to
accrue to railways from the discussions of that so-
ciety, and considered that the authorities of these
companies should pass the members free along the

lines.
Mr. M'CoNNeLr. felt convinced that if the subject
was brought under the notice of the directors, the
suggestion would be acceded to.
Mr. CHARLES GEACH remarked that the whole of
the business that had come before the meeting tended
to show the great advantages which the institution
would confer on railway property, not only directly,
but indirectly ; for while the members were de-
voting their attention to the practical improvements,
their discussion also removed the idea which the public
and the public press had got hold of—namely, that
defects had only to be known to be remedied. The
discussion of that night on railway signals showed the
complication and difficulty of this subject ; and that
fact would go far to exonerate the railway companies
from indifference to the safety of the public. On these
grounds he considered that the members of that so-
ciety had a fair claim to the privilege sought.
Mr. STEPHENSON, and several other gentlemen,
promised to bring the subject before the respective
companies.
In the course of the evening it was intimated
that forty-three members had recently joined the in-
stitute.
A motion for an adjournment to Wednesday, the
24th of November, was carried, and the meeting
separated soon after nine o'clock.

AUSTRALIAN PRODUCE.—The vessel Morayshire
arrived i n the docks, from Launceston, Van Dieman's
Land, has brought the large quantity of 3,849 bags of
wheat, and 541 bags of flour, the production of that
distant.colony.