156 REPORT—1857. 
case of the great suspension bridge over the Menai Straits, the platform has 
been repeatedly damaged by storms of wind, which twisted it as if made of 
pasteboard. ‘The late Mr. Rendel was the first engineer who perceived the 
mistake which had been hitherto committed in this respect. When the 
suspension bridge at Montrose had been destroyed about twelve or fourteen 
years ago, he reconstructed the platform and _ stiffened it by bracings 
so effectually that it has since remained uninjured. This principle of 
strengthening the suspended platform was carried out to a greater extent by 
the writer of these observations at the bridge over the Dnieper at Kieff, in 
Russia, and the successful resistance of this well-braced platform to the 
effect of hurricane winds, and to vibration, oscillation, and undulation, has 
been very remarkable. , 
The desideratum is, that the platform of a suspension bridge intended to 
sustain a railway train should be made as stiff as possible; and the first 
natural consideration is, how is this stiffness or rigidity to be best obtained ? 
The mode in which this has been effected in the great Niagara suspension 
bridge, is on the system of a deep trellis frame,—in fact, a timber tube, the 
sides of which are of lattice-work, the railway passing on the top. 
It is generally understood, and a print published at the time seems to 
confirm this, that the oiiginal intention of Mr. Stephenson was to have 
added suspension chains for supporting the tubular platform of the Britannia 
Bridge, although that intention was subsequently abandoned, and the tubes 
made sufficiently stiff not to require their assistance. 
Another great point in this discussion seems to relate to the adapting of 
suspension bridges for passing railway trains in localities and under cireum- 
stances where fixed bridges could not be erected except at an unjustifiable 
expense, or not at all, from the onerous conditions naturally or judicially 
imposed. 
According to the locality, timber or iron may be best suited for con- 
structing the platform, the platform being made as deep and as stiff as 
possible, and thus becoming a girder held up by suspension chains ; and the 
stiffness being augmented by the increased depth of framing, it will be 
advisable that the rails should be attached thereto as high up as practicable. 
But the weight of the platform must be kept within reasonable limits, to 
avoid too great an increase in the sectional area and weight of the chains, 
which would otherwise become necessary ; and further precautions have to 
be taken as regards the distribution of the load on the platform, and to guard 
against oscillation and undulation, for all which due consideration must be 
given as to the proper breadth of the platform. 
The weight of the platform of an ordinary suspension bridge was formerly 
scarcely more than 36 lbs. to the square foot of the area of the whole platform ; 
the present weight of the Menai Bridge platform, after having been strengtk- 
ened, is about 383 Ibs. to the square foot; the weight of the platform of the 
Montrose Bridge, as reconstructed by Mr. Rendel, is 413 lbs. to the square 
foot; and the weight of the platform of the Kieff Bridge is 493 lbs. to the 
square foot, including the two footpaths which are corbelled out from the 
main part of the framing; but the weight of that part of the platform 
between the chains, and which sustains the roadway, is about 60 lbs. to the 
square foot. ‘The ordinary test-load for a suspension bridge was about 
62 lbs. to the square foot; the proof-load put upon the Kieff Bridge was 
really about 84 lbs. to the square foot. 
Now a railway-load passing over a suspension bridge being taken at one 
ton per foot forward, the weight per square foot upon the platform will 
vary as the breadth of the bridge: if the bridge be 20 feet, the passing load 
= ee 
