Thursday, November 12, 2015

Wind gusts of up to 53 mph are forecast for today.

One of the concerns about not burying the legs is diminished resistance to tipping over during high winds.  I decided to dust off my engineering skills and see if I could ***ESTIMATE*** tipping resistance.

The critical velocity is calculated by setting the "righting moment" equal to the (wind drag TIMES sail height) and solving for V.

The "righting moment" is the narrowest dimension of the base, times the weight AND divided by two. The reason for dividing by two is because the center-of-gravity is probably centered, thus exploiting only half the footprint's capability.

The wind drag is 1/2 * Velocity^2 * air density(slugs/ft^3) * drag coefficient (which is between 1.3 and 2.0 for a flat plate when oblique to the wind direction).  The sail height is the height of the center of the structure.  In this case the floor is 10.5 feet above the ground and the roof will be 18.5 feet above the ground.  Average height will be 14.5 feet.

A little bit a algebra and accumulating of coefficients yields something like:

Vmphtipping = sqrt (142 * weight * narrowest footprint(ft) / sail area (ft^2) / sail height).  Plugging in the numbers for my elevated blind (estimating 500 pound weight) indicates the possibility of tipping over at 21 mph gusts.

NOT. GOOD. ENOUGH.

Clearly, some system of guy-wires or additional support is needed.

In cases like this, Excel is your friend.  A 60 mph gust produces a tipping moment of about 5500 ft-lbs.  A guy-line attached to the top of one of the pilings (approximately 10 feet above the ground) needs to withstand about 350 pounds of tension (in addition to the righting moment of the weight) to counteract that tipping force.

It looks like I will be burying some rebar and concrete and stretching some guy-line before I put any more sheathing on the frame.