GIRDERS AND SUSPENSION CHAINS COMBINED. 239 
becomes in large spans, with its supports, by far the largest portion of the 
structure, and destroys the bridge by its own weight, the weight of metal 
being fully doubled to produce equal strength, and quadrupled to produce 
equal stiffness, if loaded equally all over. 
The great difference in weight produced by this and other causes will be 
seen by comparing suspension girder bridges with ordinary girder bridges ; 
and I will take as an example the case of the two largest railway openings 
yet constructed, the Niagara suspension girder bridge, and compare its weight 
of metal with that of the Britannia Tube. 
The quantity of material in the Niagara Bridge, having a roadway and a 
single railway of three gauges in a span of 820 feet, is in round numbers 
1000 tons, and the weight in the Britannia Tube of 460 feet span 3000 tons 
for a double line. 
If the Britannia Tube had been made on the same principle as the Niagara 
Bridge, the quantity of material to give the same strength and rigidity would 
not have exceeded ith part of what has actually been employed. 
So great a difference in the weight renders it obvious that the principle of 
an ordinary girder involves great extra material, and it became an interesting 
and important inquiry to ascertain the cause of this difference. 
The view that has hitherto been generally adopted on this subject, is that 
advanced by Mr. E. Clark in his work on the Britannia Tube, in which he 
states, speaking of the proposal to use the Menai suspension bridge for 
railway purposes,—‘‘ With respect to the use of the present suspension bridge 
for the proposed traffic, it was found difficult to devise any means of suffi- 
ciently strengthening it that did not involve an almost entire reconstruction, 
and great difficulty was similarly found in attempting to render any suspension 
bridge sufficiently rigid for railway traffic, by means of ordinary trussing. 
“ When the passing load is small compared to the weight of the chains and 
of the structure itself, there is indeed no difficulty ; but the construction of a 
platform 450 feet long, sufficiently rigid for railway traffic, almost amounts 
_ to the construction of the tube itself.” ’ 
: Although unsupported by fact or experiment, this theory has been received 
_ and acted upon, not only by a large portion of the public, whose impressions 
_ of suspension bridges are derived from what had hitherto been constructed 
_ of insufficient strength, and without being combined with a girder, but it has 
been received and acted upon by engineers of eminence in this country. 
3 _ These experiments, however, distinctly prove that a suspended girder, as 
_ designed for the Londonderry Bridge, is rendered equally rigid with less than 
; ith of the metal required in the girder alone, so that the most important 
- economy arises from the combination of a girder with a chain. 
_ Experiments on Suspension Girders—I have had the model accurately 
‘made, which is now submitted to the meeting, on a scale of ard part of the 
_actual span, the length being 13! 6" between the bearings,—a length exceed- 
ing that of the average of the models used by the Iron Commissioners in their 
experiments, and is amply sufficient, due allowance being made for the scale 
to determine with accuracy the deflections on the actual girder, although the 
rE eflections of the chains will be somewhat more on the model than on the 
girder, from the weight not being sufficient to bring the surfaces into perfect 
contact. 
# _ The principal object of the experiments was to ascertain the deflection of 
the wave of a girder attached to a chain, as compared with the deflection of 
_ the same girder detached. 
_ This being obtained, it was perfectly easy to arrive at the deflection of the 
_ wave of the Londonderry Bridge, because we have sufficient experiments on 
girders to enable a calculation to be made of what the Londonderry girder 
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