DLF for Harmonic Analysis

[Crusader911]

I have recently done a super-rushed analysis of shaking pipe caused by slug flow. I used a harmonic analysis, calculating the forces at the elbows from the product of density, slug velocity squared and cross-sectional area. In a spectral analysis, one would derive a dynamic loading factor to apply to this force.

For a harmonic analysis, should I multiply the force calculated by a dynamic loading factor before entering it in the force field on the Harmonic Forces tab? If so, do I just use a DLF of 2, since that is the maximum for a force applied without a 'free drop,' or does a slug that hits the elbow with a non-zero velocity actually have a free drop, which would allow the DLF to be higher?

 

[richay]

That DLF of 2 is only appropriate for impulse loads. You said it's harmonic, implying that there is some residual response from the previous hit when the current impulse occurs.

If you're activating a mode of vibration, the applied static load will be valid (only?) if the shape from the applied load is similar to the excited mode shape.

There is a lot of gray area here.

If the forcing freq. equals a nat. freq. and damping is 1%, the associated DLF is 50 not 2.


Richard Ay
COADE, Inc.