Niagara Falls Suspension Bridge - Engineering

Engineering

Roebling's bridge was supported by two limestone towers on each side of the gorge. These Egyptian-style towers stood 88 feet (27 m) tall on the American shore and 78 feet (24 m) tall on the Canadian shore. With their foundations 28 feet (8.5 m) in the earth, the limestone structures could support up to 12 million pounds (5.4 million kg). Four 10.5-inch (27 cm) thick main cables held up the bridge; two cables ran through iron saddles at the top of each tower. Each cable comprised 3,059 wires that were spun with Roebling's patented technique used in his Allegheny Suspension Aqueduct. The ends of each cable were secured to 6-square-foot (0.56 m2) cast-iron plates sunk 20–30 feet (6.1–9.1 m) deep in the bedrock. Support lines hung down from iron clamps that encircled the main cables, and held up the decks. Deep trusses—never before seen on a large suspension bridge—lined the sides of the bridge, and joined the two decks so that the structure looked like a cage. The trussed sides and the upper and lower decks, which spanned 825 feet (251 m), formed a "hollow straight beam", reinforcing the rigidity of the bridge.

The Suspension Bridge was further stiffened by guy-wires which ran from its upper deck to the top of its towers. Criticism of suspension bridges was growing after the Wheeling Suspension Bridge collapsed under strong winds in 1854. To address these concerns, Roebling added more guy-wires to secure the lower deck to the shores below. Roebling's efforts ensured that his Suspension Bridge remained standing while other suspension bridges across the Niagara River collapsed because of strong winds. Although he was not the first engineer to appreciate the need for a suspension bridge to be sufficiently rigid or to implement the methods to do so, Roebling was the first to understand the principles behind the methods and combine them in the building of a suspension bridge. Roebling proved that despite popular opinion, properly built suspension bridges can safely support the passage of heavy railway traffic. The engineer's combination of stiffening methods created the first modern suspension bridge. Such was the rigidity of the Suspension Bridge that it withstood the shockwave caused by the nearby fall of a 5,000-short-ton (4,500 t) mass of rock in 1863; the force of the impact manifested itself as a wave, rippling through the decks of the bridge from the American side to the Canadian side and back.

From the United States, the New York and Erie Rail Road's Canandaigua and Niagara Falls Railroad and New York Central Railroad's Buffalo and Niagara Falls Railroad crossed over the bridge and reached into Ontario. Similarly, the Great Western Railway in Canada extended its network from Canada into New York. At the time of the bridge's opening, the three railroads were of different gauges: 4 ft 8 1⁄₂ in (1,435 mm) Standard gauge on the New York Central, 5 ft 6 in (1,676 mm) on the Great Western, and 6 ft (1,829 mm) on the Erie. Instead of accommodating three railways side-by-side on a single wide deck, the bridge saved space by overlapping the tracks over each other. This method used only four rails; one pair formed the track for one railway, and the other pair formed another. One rail from each pair would then form the final track. In the first year of the bridge's operation, an average of 30 trains trundled across it each day. Five years later, 45 trains passed over the structure daily.

Roebling mandated that the trains be limited to a maximum speed of 5 miles per hour (8.0 km/h) to ensure absolute safety. He was confident the bridge could handle faster train traffic, but he preferred a safe operation. In his tests the bridge supported a 326-short-ton (296 t) train, bending 10.5 inches (27 cm) under the weight. This was within the maximum load of 450 short tons (410 t) specified in the design of the bridge. The figure was a conservative estimate. The cables and guy-wires could support 7,300 short tons (6,600 t), and travel journalist Alfred J. Pairpoint commented that it was normal to see 1,200-short-ton (1,100 t) trains pass over the bridge without danger. The bridge shook whenever a train trundled over it, although this had no effect on its integrity. When the frequency of passing trains was high, the trembling was noticeable to travelers on the lower deck and proved uncomfortable to some; writer Mark Twain noted, "You drive over to Suspension Bridge and divide your misery between the chances of smashing down two hundred feet into the river below, and the chances of having a railway-train overhead smashing down onto you. Either possibility is discomforting taken by itself, but, mixed together, they amount in the aggregate to positive unhappiness." Despite such commentaries, thousands of people crossed over the bridge safely every day.

American engineers regard the Suspension Bridge as a major achievement of efficiency. In a fledgling country where resources—material and financial—were limited, they had to make do with whatever was available. This goal was espoused by the American Society of Civil Engineers, which opined, "That is the best engineering, not which makes the most splendid, or even the most perfect work, but that which makes a work that answers the purpose well, at the least cost." Roebling had built a bridge that rivaled grander bridges of leading European nations at a much lower cost. His Suspension Bridge used only one-sixth the material of Stephenson's Brittania Bridge, but was twice as long and had a capacity that exceeded the tubular bridge. Moreover, the expenditure on Roebling's Suspension Bridge was $400,000, whereas a tubular bridge of equivalent length and load-bearing capability would have cost $4 million. Roebling's success established him as the master of suspension bridges. The inclined guy-wires that stretched from the top of towers to the roadway of the Suspension Bridge became the signature of his future works.

Although the Suspension Bridge proved that the suspension system could be safely used to carry railroads, no more suspension railway bridges were built. The outbreak of the American Civil War diverted attention from such civil engineering ventures, and by the time attention was paid to building bridges again, cantilever bridges were in vogue for railway bridges. Regardless, the Suspension Bridge's success made it a model for suspension railway bridges. When the city of Quebec called for a structure to span the St. Lawrence River in 1850, it looked to the Suspension Bridge for inspiration. Seventeen years later, the British journal Engineering called for a suspension railway to bridge the Straits of Messina and also referred to Roebling's bridge. Lastly, Stuart opened his 1871 work on the history of American engineering, Lives and Works of Civil and Military Engineering in America, with an illustration of the bridge.