Airborne

A 8ft by 40ft long container full of household goods typically weighs about 20,000 lb. Next assume our container is loaded at about 40,000 lb of after PS building materials and equipment and is sitting on the ground or on a slab at ground level. Now assume wind blows perpendicular to and of equal strength on all parts of one side of this container. The floor is 8 ft by 40 ft long or 320 sq ft. This results in a floor loading of (40000/320) = 125 lb/sq.ft. Typically the floor is about the same size as the sides in surface area. From the graph of wind force on a flat surface we get that at wind speeds roughly greater than 170 miles/hr (274 KM/hr) the unit would begin to slide, assuming a coefficient of friction of 1 or less. At this wind speed there is an equal force on the side to what it weighs. If anchored down it would begin to receive tipping forces trying to pull the near side out of the ground.

At 300 MPH the container will be pushed on the side with more than 3 times it's weight (378/125). With a wind of 400 MPH the container will have a force of 5.3 times it's weight (672/125). Now wind tends to be slower near the earths surface, so in actuality there would be a gradient of wind speed due to height above the earth's surface. Thus the above numbers give only a rough order estimate to give an order of magnitude for the forces involved for average wind speed hitting the side of a container. If hills and other wind barriers surround the container, then this would help increase the maximum wind velocity before the container would move. I doubt the sheet metal sides unprotected would hold up under this kind of air pressure. At this point we assume no blowing rocks, sand or dirt. If the unit ever breaks loose from the ground it would easily tumble or become air born at wind speeds around or greater than 170 Miles/hr (274 KM/hr).

Now one might ask at what wind speed does it take before things in general begin to blow away? Assume coefficient of friction = 1 or force to move it is equal to it's weight.

Assume a 1 ft square rock of density about 3 gm/cc resting on the ground with wind blowing on one side. One ft = approximate 30.5 cm. Thus weight= volume times density = (30.5^3)*3 = 85118gm/(454gm/lb) = 187 lbs. Thus from the wind pressure table if wind only (no sand or dirt) were blowing at greater than 212 miles/hr (341 KM/hr) then this rock would begin to move and begin to take off and rapidly accelerate.

If say the wind speed were 312 MPH then one can expect the maximum projectile speed of this large rock bouncing along the earth surface to be something like about 100 MPH. The best I can recall from a TV show I once watched the trebuchets (siege engines) that were used to break down castle walls could use a round sand stone projectile that could weight up to about 90 lbs and traveled about 60 MPH and were about 19 to 20 inch in size. The many feet thick castle walls were no match for these impacts.

So the bottom line is anything of any weight or size to it becomes a deadly dangerous projectile once it leaves the ground at around 200 MPH.

Assuming a car of about 3,000 lb with a side surface area of about 15*4 = 60 sq ft. This gives about 50 lb/sq ft pressure on the ground. With a drag coefficient of about .5 then a wind that produces about 100 lb/sq ft would be enough to start to move it with the wind. This is equivalent of a wind blowing at 160 MPH (257 KM/hr). Yes, as the wind increases in speed expect cars and vehicles to start tumbling before boulders do.

Summary: In general with just air blowing in the wind (no sand, water, or dirt) one can estimate most objects will begin to move and become airborne at between 160 miles/hr and 220 miles/hr (257 KM/Hr to 354 KM/Hr).

Offered by Mike.