Impact loads

[Martin7919]

How do I calculate impact loads due to lowering a load onto a fixed structure? The load is lowered using a crane.

 

[BUGGAR]

To start, you will need to know how much the fixed structure deflects under just the dead load of the weight to compute the elastic energy that will be stored in the structure. Then you need the speed of the weight so you can compute the kinetic energy of the weight as it hits the platform. Equate the energies and solve for the resultant elastic force in the structure. Ignoring other energy losses should be conservative for most situations. If you get plastic losses in your structure, it has failed.

 

[Martin7919]

Hi Buggar, I know the lowering speed (0.2 - 0.5 m/s) but I do not know the deflection. Can I somehow estimate the impact force ignoring the deflection?
I am not a structural engineer and I have only limited information.

 

[BUGGAR]

You need the deflection of the platform because that measures how much force it absorbs. You can calculate the deflection by engineering methods, but absent that, perhaps you could make a realistic guestimate of the likely deflection and go from there (SUBSTANTIATED BY TESTING). If you give us some data on the platform, maybe we can figure a likely deflection.

Here's a reality check: Stopping an object from 0.3 m/s (0.67mph) over a deflection of 1" requires 0.20g's of force. If the deflection is 1/4", then 0.9g's force is required. Add this to your static load and include a safety factor of course.

 

[zeusfaber]

All the above assumes that the load is stiffer than the platform. What are you lowering?

A.

 

[dicer]

Use some load cells and just check it, why guess?