static equivalent to a dynamic load

[JD80]

I need some help converting a load. Right now I have a dynamic load. I have mass=76.5 kg, acceleration=6g's (58.8 m/s^2) and time=0.667 s. The load is out on a cantilever and is being subjected to an impulse (there is no impact).

The setup is similar to someone on a diving board, quickly going from standing to crouching. There is deflection at the end of the diving board, but there is no impact. I do not think energy method can be applied here because, height is difficult to determine.

I know the current setup holds up under the above conditions. I want to test the setup until failure. I would like to use a pull station to isolate this part. I should be able to do this if I can turn the result of the static test into an equivalent impulse result.

I cannot remember how to convert a static load into an equivalent impulse. Can anyone point me in the right direction?

 

[JoshPlumSE]

For a quick "back of envelope" calculation:

Dyn Amplification factor = 1 / (1-fa/Fn)

Where fa is the frequency of the equipment and Fn is the natural frequency of the cantilever beam.

 

[rb1957]

from the data provided, why not F=ma ? but i guess you'll say the acceleration isn't constant, maybe a sin (or cos ?) with 1/4 wavelength of 0.667s ??

if it is similar to a person quickly crouching ... isn't the person giving up potential energy (lowering their CG) ? where does that energy go ? strain energy in the diving board ??

 

[JD80]

You are correct that the center of gravity lowers, but measuring that distance is not possible.

The acceleration is not constant, but I do have the acceleration profile over time. This is a one shot test. The setup has to survive once to "pass"...

Does the accelerometer data (6g's) take in to account the fact that this is an impulse?

 

[rb1957]

accelerometers measure acceleration ...

but you can derive impulse from acceleration ...

 

[TXStructural]

Suddenly applied loads (not impact from a moving object with sizeable mass) are simulated by doubling the load. I am away from my office so I don't have my original source (Terzaghi.) This was originally based on the observation that a suddenly applied load would result in a doubling of the elastic elongation of a steel rod as compared to the same load applied under static conditions. While not accurate for relatively high energy transfer, it works for gravity-magnitude transactions for typical members.