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.