Dear Colleagues

You have probably heard of the age-old instruction to break step to a squad of soldiers marching across a bridge so as to prevent a collapse. This is because the rhythm of marching can match the natural frequency of the structure – thus causing the bridge to collapse.

Natural frequency is a key part of all our lives and is thus worth briefly considering.

Natural Frequency
The natural (or resonant) frequency of a structure is the time taken to complete one full cycle when disturbed. Hence, when a force acts on this structure at the same interval as the natural frequency, it will tend to reinforce the movement thus possibly eventually damaging it (or indeed, destroying it).

Effects can range from humming to strong oscillations to total collapse. I remember being somewhat alarmed working on a new iron ore processing plant when the shaking of the mechanical jigs (used to separate out the ore) caused the structure of the entire plant to vibrate – both significantly and loudly. Although the structural consultants indicated there was nothing to fear; it was of concern to everyone (in line with the wariness of experienced plant engineering staff to external consultants). However a simple (?) stratagem of changing the weight distribution of the plant, fixed the problem, resulting in minimal vibration and noise.

The Uncontrollable Shaking of the London Millennium Bridge
You may recall that the London Millennium Bridge had to shut down shortly after opening in 2000 when the rhythm caused by the thousands of pedestrians walking across matched the natural frequency of the structure.

But Galloping Gertie Was Not Due to Natural Frequency
However, as I always remark - The devil is in the detail. The recently built (1940) Tacoma Narrows suspension bridge in Washington State (Galloping Gertie) began to oscillate wildly when a driver crossed resulting in the bridge collapsing (resulting in the loss of car, dog and bridge – the driver managed to bolt in time).

Apparently this was not due to the rhythmic wind gusts matching the natural frequency of the bridge but aerolastic flutter (response to air movement seen on aircraft wings) leading to torsional flutter (repetitive twisting). Yes, indeed.

The replacement bridge (Sturdy Gertie) used far wider open web stiffening trusses and has been problem-free.

Nikola Tesla makes a good comment: If you want to find the secrets of the universe, think in terms of energy, frequency and vibration.

Thanks to 101 Things I learned in Engineering School by John Kuprenas with Matthew Frederick.

Yours in engineering learning,


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