Impact loading due to wind forces - stiffness and more

[Sjqlund]

Dear engineers

I am working on a concept and cannot give too many details.
Basically its for lifting something up in a windturbine. See left side of IMAGE

During the lift, wind forces may push the lifted object into the tower.
I wish to find the impact force to see if the tower takes damage or not.

I will do this, by finding the kinetic energy of the lifted object and equating it to the strain energy of the impacted tower. For this, i need the stiffness coefficient k of the tower. My idea is to make an FE model, apply a unit displacement delta, find the reaction load F, and then take k=F/delta.

My first idea was to do this on the full model, as seen in the crossed part of the image. But intuitively this seems wrong! A quick impact of a (relative to the tower) small object will not cause the full tower to bend. It seems more correct to isolate part of the tower, and use this for finding the stiffness, as seen on the right side of the image. Do you agree with me on this?
The way i've reasoned about this, have been to imagine a bullet hitting the Eiffel tower; here the only stiffness that would matter would be of that local piece of plate which the bullet hit.

-sjqlund

 

[TLHS]

Unless there's some reason you can't avoid lifting in reasonable winds, put a windspeed limit on your lift. You shouldn't be making a lift that you can't properly control anyway.

If there's going to be light bumps, put some foam or something along the edge of the item being lifted. Once you add something that can actually deflect you get a much nicer result when you check the math.