Pole Dynamics

I was looking at the some photos from the PDXStrippie awards and the image of (I think) Lark on the pole really struck me. I couldn’t believe how much the pole was deflecting. The pole was really moving. It looked more like a catapult than stage fixture.

Image Copyright AmbeRed (2010)

I went into engineer mode and started to take measurements. I know I’m violating copyright law by modifying the images, but hopefully AmbeRed photography won’t mind too much. It’s a pretty cool image. I took a look at the AmbeRed site, some really talented stuff there.

Anyway, I put in some reference points to make some estimates. I can’t assume the camera was perfectly level when the photo was taken. To compensate for the camera angle I first drew a line perpendicular to the vertical edge of the pole platform. I’m making the assumption that the platform is relatively square. I think counted the number of pixel offset from a second reference line that was straight relative to the edge of the image. Using that pixel offset I drew a reference line from the left edge of the pole in red. This is the perpendicular line to show where the left should be it was no deflected. I then drew a second line in green from the center of pole at the base to the center of the pole at its highest point. I was amazed to see that the pole was both tilting and flexing.

Assuming the pole was 2” in diameter I estimate that the pole was deflecting (both tilt and bending) of seven to eight inches. I think the pole was held in place by weights around the base of the pole. The tilt is probably the result of the edge of the base lifting up against the weights.

The thing to keep in mind is when the pole deflects eight inches, the weight of the dancer suspended shifts eight inches. That is an enormous amount of energy. Have somebody jump off an eight in curb on to your hand for reference. I can only imagine how difficult this pole was to use. Besides the normal stresses the dancer has to deal with, there is the additional force of her weight moving around. That is a lot of force on her hands. Plus if she lets go with one hand the pole will shift. Releasing the top hand will result in the pole returning to a more vertical position. The lower the grip the lower the moment arm applied to the poll. Releasing the lower hand will have the opposite effect. That will increase the moment arm, resulting in the pole deflecting even more. The second she lets go with either hand the pole is going to become a moving target for her to grab.

The flexing is even scarier. If there were any cracks in the pole material the flexing will make them bigger until it breaks. Considering she is roughly three feet in the air, moving, and in a prone position, that would have resulted in a really nasty fall. On top of that the pole will act like a spring trying to catapult the dancer off the stage. I have a lot more respect for the dancers that did pole tricks at the awards.