Monday, November 9, 2009

A strange incident

Between my house and the shop building stands the base of an old radio aerial. The original had a top mast that was cantilevered and I removed that portion when I built the shop. But I left the base, which is quite solid and anchored in concrete. It's steel, tubular, about seven inches in diameter at the base and tapering to about four inches at the top. It is about 24' high.

Yesterday afternoon as I stood at the kitchen sink I noticed that the top of the mast was gyrating. I'd never seen this before and went out to see what was going on. The top of the mast was moving back and forth in a more or less circular movement that could not be missed.

Wind? That's what I thought, but there was no wind in the space between the house and the shop and the trees were completely still. I went out front and looked at the windmill out in the pasture. Not revolving at all. It was a dead calm.

I shall ponder on this, but right now have no explanation for it.


Crotalus said...


Miz Minka said...

Circular motion caused by tectonic plate shifting. :)

Rio Arriba said...

Plate shifting. Could be. The thing looks a little like a sun-dance pole. Ghosts?

Mark B. said...

Small earthquake, perhaps not noticable to the human senses.

That type of structure can do some strange things. I have personally witnessed -- well, okay, personally participated in -- an impromptu experiment in structural resonance phenomena. Did you know that it's perfectly possible for one person to lay over a 25-foot-tall parking lot light pole 5 inches in diameter?

You just need to find the cyclical-motion "sweet spot" -- and some patience.


Anonymous said...

You need to lay off the mushrooms, they're not good for you!

Mark B. said...

Hermit, ever heard of "Galloping Gertie?" The Tacoma Narrows bridge, a suspension structure across the southern end of Puget Sound that was built in 1938-1939 and vibrated itself apart in late 1940.

The cause was aeroelastic flutter, a mechanical resonance phenomenon. Wind speed? 40 miles per hour.

Same thing, different application. In truth, my Composite Materials instructor sorta incited the incident; we were in a parking lot that was being bulldozed anyway to make space for a new campus building. And yeah, it took awhile, but it was fun proving the hypothesis.


Lorimor said...


Anonymous said...

Mark, never heard of that. Seems a logical explanation.