Thursday, March 14, 2019

Niagara Falls Suspension and Arch Bridges

(Bridge Hunter3D Satellite, 425+ photos)

Joe Dockrill posted
bridge built outside a bridge, Niagara Falls, from suspension to steel span.................clever
Unknown - Buck, Richard (December 1898). "The Niagara Railway Arch". Transactions of the American Society of Civil Engineers 40 (836): Plate VIII. New York: American Society of Civil Engineers. [Wikipedia]
The suspension bridge being replaced is the second one built at this site. John A. Roebling built the first one between 1852 and 1855. I recognize the name because he was the first to manufacture wire rope in North America; and he built several suspensions bridges, including the Brooklyn Bridge, to expand the market for his wire. This was the first suspension bridge that carried a railroad. Many didn't thing a suspension bridge could carry a train because they were too flexible to carry the focused weight of a crossing train. The key was that John built a stiff, 18' deep double-deck truss. The upper level carried the tracks and the lower level was for carriage and pedestrian traffic. There were "four main cables 10 inches in diameter with 3,640 no. 9 gauge wires each." [StructureMag]
Historic Photo from Frank Griggs Jr.
The four masonry towers were 60.5' tall. What neither of these photos show is the "56 under deck cables (river stays) anchored to the bedrock on the banks of the river." They were added to brace the bridge against the winds that would howl through the gorge. In the truss, the diagonals (tension members) were wrought iron rods 1" in diameter and the verticals (compression members) were wood timbers. [StructureMag] It cost $400,000. [NiagaraFallsTourism] Unfortunately, I don't know how to convert 1855 dollars into today's dollars. But I do know that a penny could buy more than just a little piece of candy back then.

From the Niagara Falls Public Library [Public domain], via Wikimedia Commons
Some sources call the steel bridge below the second suspension bridge, others consider it just maintenance of the first bridge. I consider it the second suspension bridge because between 1877 and 1886, Leffert L. Buck completely rebuilt the bridge. Just because he was the engineer responsible for the maintenance of the bridge and because he did the rebuild with little interruption to rail traffic doesn't nullify the fact that it was a new bridge when he was done. Buck used steel, which would have been a bleeding edge material in 1877, to replace the deck truss and towers. [StructureMag] "This new structure was capable of carrying an increased load from 300 tons to 350 tons which was much more than any train and load was capable of weighing. All that remained of the second bridge was the anchorage's and cables but they too had been overhauled and improved." [NiagaraFrontier]

See page for author [Public domain], via Wikimedia Commons
Even though the steel suspension bridge seemed more than adequate when it was completed, just a decade later, the growth of train sizes was making it obsolete. So Buck built the arch bridge we see today under the suspension bridge as illustrated by the photo at the top of these notes. By this time, using a steel arch for a railroad bridge was considered better than a suspension bridge. The deck was "reused" so no trains were delayed and the highway floor was closed only two hours per day. "The bridge was completed on August 27th 1897. Extensive tests proved that a steel arch bridge possessed much greater strength than ever anticipated. This bridge continues in use to this day. With only minor changes from the original, it is capable of handling the heaviest of loads and bridge engineers do not indicate how many years the future life span of this bridge might have." The original name was Lower Arch Bridge, but it is now known as the Whirlpool Rapids Bridge. [NiagaraFrontier]

1900 from Widipedia






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