**Updated May 18, 2016:** Use new rotor. See end of post.

Burning Man is a bit like an outdoor museum—a museum that can have highway-speed winds! Part of registering an art piece for display is determining the strength of *hold downs* required to ensure that the art does not tip over or blow away from the wind. The required calculations assume a wind speed of 100 MPH!

*Miles* uses a single earth anchor centered on its base. The geometry of the forces involved is somewhat like those seen when one uses a crowbar to remove a nail from a board: a horizontal force applied to the long end of the crowbar becomes a much larger uplift force which is applied to pull the nail. Since *Miles* is taller than it is wide, the situation is similar.

### Horizontal Force

The force of the wind depends on the speed of the wind and the shape of the object that it blows against. The equations to use are:

*WindPressure* = 0.00256 × *WindSpeed* ^{2}

and

*Force* = *WindPressure* × *Area* × *DragCoefficient*

With these equations *Force* is measured in pounds, *Area* is in square feet, *WindPressure* is in pounds per square foot, and *WindSpeed* is in MPH. The *DragCoefficient* is a unitless value which is selected from a table depending on the shape of the object.

### Uplift Force Calculations

I’ve included the wind-load calculations in this drawing set and this Excel spreadsheet. For simplicity, I’ve performed separate calculations for three different parts of the art piece: Main Enclosure, Rotor, and Mast. The calculations reveal that the total uplift force that an earth anchor must withstand is 1,698 pounds (ignoring the weight of *Miles* itself). Since Burning Man can install 4-ft long screw-in type anchors with an approximate vertical load limit of 3,500 pounds, a single anchor is sufficient.

** Updated May 18, 2016: **I’m going to use 16-inch round-bottom woks for the rotor cups. Also, I’m going to raise the rotor to help discourage participants from trying to slap stationary cups in a calm wind. In the calculation, I used a more accurate estimate of the rotor’s coefficient of drag. The new uplift result is 1,865 pounds, which still is far less than the 3,500-pound capacity of the earth anchor. (In the calculations, “Downfacing Cup” means the cup whose round part faces into the oncoming wind; “Upfacing Cup” means the cup means the cup whose open part faces into the oncoming wind.)