Torsional Oscillation
Definition
Twisting motion of a bridge deck about its longitudinal axis, the mode responsible for the catastrophic final phase of the Tacoma Narrows failure.
- Motion
- Twisting about the bridge deck's longitudinal axis
- Notable case
- Tacoma Narrows Bridge failure
- Role
- Responsible for the catastrophic final failure phase
- Field
- Structural/bridge collapse forensic engineering
Common questions
How did torsional oscillation cause the Tacoma Narrows collapse?+
The deck first developed vertical bending oscillation in moderate wind, but as amplitude grew the motion shifted into a twisting mode where the two edges of the deck rose and fell out of phase with each other. This torsional motion placed far greater stress on the structure than the earlier vertical motion and led to structural failure.
Why is torsional oscillation more dangerous than simple vertical bending in a bridge deck?+
Twisting motion stresses the deck and its connections in ways vertical bending does not, and it can couple with aerodynamic forces to grow in amplitude rather than damp out, a self-reinforcing effect that vertical bending alone typically does not produce as readily.
What design changes came out of studying this failure mode?+
Post-collapse investigation of Tacoma Narrows drove wind-tunnel testing of bridge deck sections and led to stiffer deck designs and truss configurations specifically intended to resist the onset of torsional flutter in subsequent suspension bridge construction.
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