233. Undoubtedly the finest specimen of a bridge of large span upon the suspension principle, or indeed upon any principle, is that built by John A. Roebling, across the Niagara River, a short distance below the falls. The dimensions below of this admirable structure are from the final report of the above-named engineer.
| Length of bridge from centre to centre of tower | 821′ | 4″ |
| Length of floor between towers | 800 | ft. |
| Number of wire cables | 4 | |
| Diameter of each | 10″ | |
| Solid wire section of each cable | 60.40 | sq. in. |
| Total section of four cables | 241.60 | sq. in. |
| Whole section of lower links of anchor irons | 276 | sq. in. |
| Whole section of upper links of anchor irons | 372 | sq. in. |
| Ultimate strength of chains | 11,904 | tons. |
| Whole number of wires in cables | 14,560 | |
| Average strength of a wire | 1,648 | lbs. |
| Ultimate strength of four cables | 12,000 | tons. |
| Permanent weight supported by cables | 1,000 | tons. |
| Resulting tension | 1,810 | tons. |
| Length of anchor chains | 66 | ft. |
| Length of upper cables | 1,261 | ft. |
| Length of lower cables | 1,193 | ft. |
| Deflection of upper cables (mean temperature) | 54 | ft. |
| Deflection of lower cables (mean temperature) | 64 | ft. |
| Number of suspenders | 624 | |
| Aggregate strength of suspenders | 18,720 | tons. |
| Number of over-floor stays | 64 | |
| Aggregate strength | 1,920 | tons. |
| Number of river stays | 56 | |
| Aggregate strength | 1,680 | tons. |
| Elevation of grade above mean water | 245 | ft. |
| Depth of river | 200 | ft. |
| Cost of the bridge | $400,000. |
231. The following items are extracted from the report above referred to:—
“The trains of the New York Central, and Canada Great Western Railroads have crossed regularly at the rate of thirty trips per day for five months. (At present over two years.)
“A load of forty-seven tons caused a depression at the centre of five and a half inches.
“An engine of twenty-three tons weight, with four driving wheels, depressed the bridge at the centre 0.3 feet. The depression immediately under the engine was one inch; the effect of which extended one hundred feet.
“The depression caused by an engine and train of cars is so much diffused as scarcely to be noticed.
“A load of three hundred and twenty-six tons produced a deflection of 0.82 feet only. The Conway tubular bridge deflects 0.25 feet under three hundred tons; the span being only one half that of the Niagara bridge.
“The specified test for the wire was, that a strand stretched over two posts four hundred feet apart should not break at a greater deflection than nine inches; also, that it should withstand bending square and rebending over a pair of pliers without rupture. This test corresponds to a tensile strain of 90,000 lbs. per square inch, or I,300 lbs. per wire of twenty feet per pound.”
The wire is preserved from oxidation by coating with linseed oil and paint. Upon the durability of wire cables employed for suspension bridges the following fact came to light: Upon taking down the cables of the footbridge, put up in 1848, by Mr. Ellet, the wire was found so little impaired that Mr. Roebling did not hesitate to work it into the new cables; also, the original oil was found to be still soft and in good condition, having been up six years.