Expert: Paul Soderman - 7/9/2006

Question
Hi;

I'm planning on building a structure on my property in order to provide a shaded area for barbequing and such.  

I'm planning on using 30 to 50 foot diameter circular steel ring with a circular tarp firmly attached in the center.  This will form the roof. There will not be any walls.  The plan is to have it supported by 4 cantilevered steel arcs that will allow the entire roof to move.  Basically, it's an idea I saw on Discovery Channel called the Kinetic Pavillion.

My concern is given that the roof will act as a large sail/wing and that the angle of the roof will vary in relation to the wind, it could create an immense amount of lift in windy conditions.  So, how would I calculate the lift/force generated by such a structure for a number of different angles of attack?  Any help and or pointers in the right direction will be greatly appreciate.

Thanks
Brett

Answer
HI Brett

I understand you already bought the steel ring.  OK - all is not lost.  First off, my assumptions in the previous answer were extreme.  I don't know where you are - maybe you will never get a 100 mph wind.  The load varies as velocity squared.  And the angle of attack may be high.

In any case, you might also find a way to protect the system.  What if decorative panels were cut in the roof that were sewn shut to be waterproof, but were able to open in high wind before the structure failed.  Or what if the tie downs were allowed to fail before the structure did.  I don't know enough about your system to design the best solution, but there is probably a way to make this work.

Paul

p.s. I had a typo in my definition of air density; it should be
0.002378 lb sec^2/ft^4