Mike's Spacetime

Building a 15ft x 15ft cubic object with an essentially wind impermeable surface clearly creates a significant force even in low winds.
A quick look round the internet (e.g. Wikipedia on Drag Forces and Drag Coefficients ) gave a simple formula:

Fd is the drag force (Newtons)
u is windspeed (meters/second)
p is the density of Air (1.2 kg/m³ at sea level, so a little less at playa height)
A is the cross-sectional area, normal to the wind.

and CD is the drag coefficient of the object in question. c accounts for the shape of the object and varies over at least 2 orders of magnitude from a sail shape to a fighter airplane or a fish.

Drag coefficients

It encapsulates empirically all the non ideal properties of the object in question, it’s shape, the amount of surface drag, turbulence etc. etc. etc. The whole formula accounts for both the pushing force the wind exerts on the frontal surface of the object as well as the suction effect that occurs on the backside of the object due to the low pressure region created. The latter can make substantial difference as can be seen by the different coefficients for say a flat disk (1.15) compared to a long cylinder (0.82).

A (smooth) cube has various coefficients (and cross sectional areas) depending on the angle of attack. Face on its close to 1.2 while on edge its only 0.8! Sadly I could not find any data what a cube head-on-vertex is, presumably a little less then the cube on edge, but not necessarily.
Note that this all assumes smooth cubes! Our cube isn’t exactly smooth. But its also not super rough either, since the black spandex fabric was to keep all the holes closed and hugged the scaffold in soft outlines.
And even if we knew the coeffcient for the cube, the coefficient itself depends on the wind speed and air density itself too (or rather on the Raynolds number), as object create considerably more drag force when the fluid flow is turbulent compared to when it’s laminar.
Another scary factor is that, should one of the panels on the upward windside fail and rip, it would suddenly dramatically increase the wind coefficient, since the whole thing would act as a box kite. On the other hand, if this were to happen, most likely this would cause a cascade event ripping out further panels on the downwind side ultimately causing total destruction of the cube guts but in effect removing most of the wind load on the structure itself (it would be a giant 3D clothing line)

Summa summarum, the true coefficient for the cube in a 60mph windstorm could conceivably be anywhere between 0.8-1.2. I deceded to assume that it would be around .9 or so and engineer in generous margins into the guying design.
I calculated the maximal cross sectional area as around 390 sqft, being exactly head-onto a vertex. On-edge the crossectional areas is less and exactly face on (impossible however in the orientation we intended for the cube) its 225sqft.

Note: A week or so before departure i checked the wind load calculations again for the millionth time and noticed i had made a mistake when calculating the maximal surface area: When sitting exactly on one vertex, none of the sideways pointing vertices point *exactly* at 90degrees to the ground, in fact they are about 19.5deg  off, thus the actual maximal cross-sectional area is smaller then I initially worked with. I reanalyzed the cross-sectional aras again and in fact, depending on wind direction the cross-sectional areas would vary between 318 sq ft and 367 sq ft.

With all these parameters, the wind load in a 70mph wind then came out at around 4000 lbs of force, about as much as the cube turned out to weigh! I checked the figure against various sites and figures for sailing boat sails of the appropriate size (but with larger coefficients obviously, since there you *want* to catch the wind).

DISCLAIMER: I do not recommend using any of the calculations on this page for your own wind load calculations and neither I nor anyone ont he Groovik’s Team take any responsibility for its accuracy or suitablity for any purpose stated or not stated.