“Introduction:

Most engineers and technicians are aware of the Tacoma Narrows bridge disaster. This suspension bridge failed catastrophically in November 1940, five months after completion, due to high amplitude oscillations caused by the wind! Though most believe this disaster was due to resonant excitation, the root cause was a mechanical instability.

Mechanical instabilities are a class of self-excited vibration phenomena*. Self-excited vibration problems begin to vibrate of their own accord spontaneously. The distinguishing feature of mechanical instabilities is the presence of a feedback mechanism between the structural vibration and the oscillation of some key component of the process or structure. When this “feedback loop” becomes unstable, the vibration amplitude increases with time, sometimes to destructive levels.

For the Tacoma Narrows bridge, an unstable feedback loop existed between the bridge lateral (i.e., across wind) vibration and the aerodynamic forces exerted on the bridge by the wind. That is, the aerodynamic forces varied as a function of the bridge vibration and vice versa. As the vibration velocity grew in amplitude, so did the aerodynamic forces. The feed-back loop was unstable in that the aerodynamic forces and the bridge oscillation reinforced one another over time. Finally, the amplitude of oscillation became so large that the center section of the bridge disintegrated.

Instabilities, while relatively rare compared to resonance, must be understood by the practicing vibration specialist. These problems give rise to potentially destructive levels of vibration. The purposes of this paper are the following: (1) to present the characteristics of mechanical instabilities so that they can be identified in the field; (2) to explain the nature and causes of instabilities; and (3) to present practical methods for solving instabilities. The paper concludes with two examples.

“How to identify an Instability:

Mechanical instabilities are identified through careful analysis of the operating vibration characteristics of a structure in conjunction with knowledge of the structural dynamics. Mechanical instabilities generate the following unusual vibration characteristics:

“Intermittent high vibration:

Instabilities generate episodes of very high vibration relative to “normal” levels. In the unstable regime, operating vibration can be hundreds of times higher than in the stable regime. It often appears as if the vibration is either “on” or “off’.

“Absence of a periodic excitation source

Since instabilities are self-generated, they often develop in the absence of a periodic vibration source. The alternating forces which sustain the instability are generated by the instability itself. External forces are not required. The absence of a periodic excitation source is an indicator that the problem may be an instability.

*Other classes of self-excited vibration problems include aeromechanical (flutter), aerodynamics (stall, separation, musical instruments), aerothermodynamics (flame instability), and feedback networks (electromechanical, hydraulic, pneumatic).”