Thinking clearly · Chapter 5
Why did the Tacoma Narrows Bridge collapse?
The 1940 bridge developed destructive twisting motion driven by interaction between wind and structure. The classroom story of simple forced resonance is misleading. The relevant mechanism is torsional aeroelastic flutter.12
Flutter is not a wind matching a musical note
Billah and Scanlan's engineering paper rejects the elementary forced-resonance account found in many textbooks. The APS history explains why the familiar story is wrong. Wind and a moving bridge can interact so that motion draws more energy into the structure.12
A neat explanation can become popular before people check whether it describes the actual mechanism.
Use a model that fits the failure
Chapter 5 concerns the right tool for the job. I ask what the model includes and what it leaves out. An account of static loading cannot, on its own, settle a problem involving motion and changing airflow.
Identify the conditions under which a model is useful before asking it to certify a result. Familiarity with the model is not evidence that those conditions hold.
A check you can use
- Name the mechanism your model represents.
- List interactions or changing conditions it leaves out.
- Test those omissions instead of repeating a familiar explanation.
Was it resonance?
Not in the elementary sense of a periodic external force matching the structure's natural frequency. The cited paper rejects that oversimplification. Aeroelastic flutter is the more accurate explanation.1
Zafar Alam is a civil engineer and Chartered Construction Manager with about 12 years on major projects. He is the author of What Are We Missing? How to Think More Clearly Before You Decide, planned for 13 February 2027.
Keep thinking
Why did Challenger break apart?Chapter 5 · The Right Tool for the JobHow can you avoid confirmation bias?Chapter 10 · Building Critical Thinking HabitsUse this on a real decision
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