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© Tacoma Narrows Bridge Failure (1940) / A Film by Frederick Burt Farquharson / University of Washington / 1988
The construction of the Tacoma Narrows Bridge, its opening to traffic on July 1, 1940, its collapse on November 7, 1940, and footage of testing on a scale model of the bridge (aerodynamic testing in the wind tunnel). The bridge experienced oscillations both while under construction and after opening, and in May of 1940 the Washington Toll Bridge Authority hired Professor Frederick Burt Farquharson (1895-1970) to make wind-tunnel tests and recommend solutions in order to reduce the oscillations of the bridge. The recommendations were delivered days before the collapse, and Prof. Frederick Burt Farquharson was on site at the East Tower taking photographs and motion pictures on the day of the collapse. Historically, the name "Tacoma Narrows Bridge" has applied to the original bridge nicknamed "Galloping Gertie", which opened in July 1940, but collapsed possibly because of aeroelastic flutter four months later, as well as the replacement of the original bridge which opened in 1950 and still stands today as the westbound lanes of the present-day two-bridge complex. Tacoma Narrows Bridge Collapse (Tacoma Narrows Bridge Failure). Resonance and Vortex Lock-in. Early thoughts on the failure mechanism were directed towards resonance from external wind loading (NYT, 1940). Historical failures of the Broughton suspension bridge in 1831 and Angers suspension bridge in 1850 due to marching troops may have contributed to this line of thought. Federal Works Agency (FWA) report’s statement, “Its [Tacoma Narrow’s] failure resulted from excessive oscillations caused by wind action,” was not very clear and inadvertently corroborated the theory. While some attribute the periodicity required for resonance to turbulence in wind (Miller, 1977, McCormick, 1969), others attributed it to the shed Karman vortices. However, the variation in the characteristics of wind loading at the site could not account for the required periodicity for the resonance of the bridge. Modern Consensus. The FWA report concluded: “The vertical oscillations of the Tacoma Narrows Bridge were probably induced by the turbulent character of wind action. Their amplitudes may have been influenced by the aerodynamic characteristics of the suspended structure. There is, however, no convincing evidence that the vertical oscillations were caused by so-called aerodynamic instability. At the higher wind velocities, torsional oscillations, when once induced, had the tendency to increase their amplitudes.” The body of knowledge on aeroelastic phenomenon was limited to Theodorsen’s paper on aerodynamic instability and flutter of airfoil published in 1934, but limited extension to bridges until Scanlan (Scanlan, 1971). The last line of the statement refers to an instability in the torsional mode of oscillation. This instability, dependent upon the aerodynamic characteristics of the bridge, is believed to be a consequence of aeroelastic phenomenon referred to as torsional galloping or stall flutter (stall not due to viscous effects). Aeroelastic phenomena occur in the domain of the intersection of aerodynamic, elastic, and inertial forces. Scanlan demonstrated that the failure mode was “SDOF torsional flutter” of a bluff body. Subsequent publications supported this mechanism. A non-catastrophic 1D flutter in plunge motion translated into a large amplitude 1D torsional flutter observed at the instance of collapse (Blevins, 1977; ASCE, 1987). The reason for the change in the mode of vibration from plunge to torsional is not well understood, with explanations ranging from slip-of-cable-mount during the plunge phase (Ammann, 1941; Malik, 2013) to a theoretically based energy threshold approach (Arioli, 2013; Arioli, 2015). Flutter may be conceptualized as a self-exciting, aerodynamic phenomenon wherein a condition of positive feedback is established on the structure’s vibration by the aerodynamic forces. Aeroelasticity is the branch of physics and engineering studying the interactions between the inertial, elastic, and aerodynamic forces occurring while an elastic body is exposed to a fluid flow. The study of aeroelasticity may be broadly classified into two fields: static aeroelasticity dealing with the static or steady state response of an elastic body to a fluid flow; and dynamic aeroelasticity dealing with the body's dynamic (typically vibrational) response. Aeroelasticity problems can be prevented by adjusting the mass, stiffness or aerodynamics of structures which can be determined and verified through the use of calculations, ground vibration tests and flight flutter trials. Flutter of control surfaces is usually eliminated by the careful placement of mass balances.
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