Impact factor suggestions needed

[CTW]

I'm working on a precast concrete wall that will protect bldg columns and equipment foundations from the impact produced by a crane magnet. The magnet weighs 33 kips. I started out by calculating a dynamic load factor and came up with 244 kips (assuming the max falling distance, or swinging distance in this case, was 5 feet). This force seems excessive. Does anyone have any suggestions for a more reasonable impact factor?

 

[jheidt2543]

You might check the AISC Manual for their discussion of impact forces. I seem to remember that, for crane runways, the recommended minimum impact load was 25% of the lifted weight.

 

[CTW]

Correct, but that's the impact on the runway produced by lifting the load vertically. The impact in this situation will be more severe as the magnet can potentially hit the wall producing a horizontal force. Keep posting suggestions. In the mean time, I'm going to brush up on my dynamics skills (or lack there of) and see what force I come up with using the mass and the acceleration of the trolley. This may be a more feasible answer than using a dynamic load factor since the DLF is for falling objects, and this object is not falling.

 

[cyak]

Let Dst be the static deflection of your wall upon application of the weight W = 33kips. Dst = W/k, where k is some appropriate stiffness of the wall....

Then the dynamic load Pdyn you can use is:

Pdyn = W [1 + (1 + 2h/Dst)^(1/2)], where h is the falling

distance. The deflection will then be:

Dmax = Dst [1 + (1 + 2h/Dst)^(1/2)].

See Engineering Mecahnics of Solids by Popov, 1st ed, page 96. Note that no damping is assumed here. In reality there will be some damping and energy dissipation so the impact factor would be less.

If this simpe calcultation does not satisfy your needs, you can model the impact as an impulsive force assuming a square or triangular impulse and solve the equation of motion of the wall (with or without damping) subjected to this impulsive excitation. The solution will give you everything you need.

To develop and solve the equation, you can consult a book on engineering vibrations...

 

[coco1953]

I suggest a simple approach by doing the following:
Find the energy carried by the magnet at the moment of hitting the wall. It will be kinetik but is equivalent to mass by vertical distance traveled before hitting. If you can estimate the hitting speed would be better, because in addition to vert travel, there could be some addinional energy imparted by the crane while turning.
Apply an unitary horizontal force to the wall at the point where it will be hit, and find the displacement of the wall at that point. If I'm not wrong, the energy absorbed by the wall will be the unitary force by one half the displacement, like in the elementary spring problem. Then multiply the unitary force by the quotient of both energy numbers and you have a static force to design the wall. I would say that dynamics consideration would surely be result in lower forces, so you would be on the safe side. Also remember material has additional capacity for dynamic loads. One more aspect is regarding the capacity design of the precast wall: If you want the wall to remain within service levels when hit or to break/yield and being replaced. Sorry for the deficient english, hope being understood and helpful.