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# Effect of Dropped Loads From Structure

## Effect of Dropped Loads From Structure

(OP)
Assume a 1k at rest load instantaneously dropped from a cantilever structure.  The arguement in the office is how to model this instant detachment of the load and determining a relevant response spectra for dynamic analysis.

Does the instant release of the 1k load at the end of the cantilever have the same effect on the structure as merely exciting the same load while not being released?

If the period of the guyed steel structure is .7 is it prudent to use a 5% damping ratio?

Is it valid to excite this 1k load using UBC response spectra on the vertical axis as a means for excitation?  If so, any recommendations on scaling the spectra for this condition?

### RE: Effect of Dropped Loads From Structure

In a linear structure this will provoke the same dynamic response in magnitude as applying a 1 kN impact with the same time history. the direction will be reversed, obviously.

In the modal analysis world this is called relaxation excitation and is used for things like bridges and buildings.

"Does the instant release of the 1k load at the end of the cantilever have the same effect on the structure as merely exciting the same load while not being released?"

No, obviously. Equating statics to dynamics is usually wrong.

"If the period of the guyed steel structure is .7 is it prudent to use a 5% damping ratio?"

I've seen steel strucures with damping ratios of <1%. So it isn't very prudent, unless you have measured similar structures.

"Is it valid to excite this 1k load using UBC response spectra on the vertical axis as a means for excitation?  If so, any recommendations on scaling the spectra for this condition?"

Dunno, but I'd be asking on the vibration forum, and/or the FE guys.

The problem is that the shape of the impulse (and so the spectrum) will be strongly affected by the elasticity of the trigger/restraint system, and that of the cantilever.

Having said that this does not look very difficult to model with LS Dyna or something like that.

Cheers

Greg Locock

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