The weaknesses of the early suspension bridges in storms or under repeated rhythmic loads were fatal for most of them. in 1831 the Broughton suspension bridge collapsed because of oscillations set up by a body of troops marching in step. Four other bridges in the U.S. and Britain were destroyed simply by the impact of flocks of sheep or droves of cattle. The Chain Pier Bridge at Brighton, Sussex, and the Union Bridge over the Tweed at Berwick. Northumberland, were both blown down. The first railway suspension bridge, built in I830 to carry the Stockton and Darlington Railway over the Tees, was hammered to destruction in a few years by the weight and impact of the trains. In the U.S.the Fairmount Bridge. supported by a number of small wire cables, over the Schuylkill River, was a success, but a 1.000-foot (300-metre) span ever the Ohio River at Wheeling, West Virginia, survived only five years.
The collapse of the Tacoma Narrows Bridge, at Puget Sound, in 1940, only 4 months after its
completion and after more than 50 years of immunity from suspension-bridge failure of this kind, brought the study of aerodynamic stability sharply to the fore. This bridge was by far the most flexible among its contemporaries, it had a span of 2.800 feet (853-metres) with a width of only 39 feet (12-etres) . and the deck was stiffened throughout its length not by the deep open trusses formerly used but by two plate girders only 8 feet (2. 4-metres) deep. Under quite moderate winds, the deck not only swayed sideways but also was subject to severe torsional (twisting) vibrations, and ultimately, in a wind of only 42 miles (70-kilometres) per hour, the vibrations became so violent that the deck was torn away and crashed into the water. Other U.S. suspension bridges had also shown undesirable aerodynamic action, and further bracing or stiffening was quickly incorporated.
A committee appointed to investigate the Tacoma failure found that the oscillations caused by wind were due to (1) natural turbulence or gusts and (2) the eddies created by the solid cross section and shed from the bridge structure.A dangerous buildup of oscillations, possibly leading to collapse. might result if the frequency of the eddies coincided with any of the natural frequencies of oscillation of the bridge. Bridge designers thereafter reverted to the use of heavy, stiff deck structures to provide torsional rigidity and also left longitudinal openings or slots along the deck, between the dual roadways and the girders, similar to the antistall slots originally used in the wings of aircraft. All these modifications were adopted for the Tacoma Narrows II Bridge, completed in 1950. in which width and rigidity were considerably increased as compared with its predecessor, and also for the Mackinac Bridge, the Forth Road Bridge, and others.