First, let’s choose some simple geometry, say a box 3 m tall, 2 m wide, and 2 m deep.  (Scientists like to work in meters, not feet!  I will convert back into English units in the end.  The weight may be thought of as acting at the geometrical center of the volume (if the weight is more or less uniformly distributed and the force from the wind may be thought of as acting at the center of the face being blown on (as long as the wind has the the same speed everywhere.  What I can say for sure is that the force of the wind will be proportional to v2 where v is the speed of the wind.  The proportionality constant is where the geometry of the object comes in.  There is a handy formula which is approximately true for the force on a sphere of diameter D which is in a wind of speed v: .  So, suppose we say that the force is about the same order as that on a sphere of radius 3 m, then  approximately.  Now, the wind is trying to tip the tower about the far edge (where the little “donut” is) and the weight is trying to keep it from tipping.  The torques must be the same and then if the wind is any faster it will tip over.  The torque is force times moment arm and the moment arms are 1 m for the weight and 1.5 m for the wind force.  The weight (500 lb) is about 2200 N (newtons).  Therefore, .  Solving,  m/s which is about 60 mi/hr.