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Inclusion of product mass in dynamic analysis

Inclusion of product mass in dynamic analysis

Inclusion of product mass in dynamic analysis

(OP)
I have crusher support steel that I am designing for dynamic loads and to avoid resonance. Do I need to include the mass of the product in crusher and chutes when calculating frequencies and vibrations? My thinking is no because it is not attached to the system.

RE: Inclusion of product mass in dynamic analysis

(OP)
Anyone?

RE: Inclusion of product mass in dynamic analysis

I'd agree with you. I would say no as that mass is either causing the dynamics or it is free to move independent of the structure and therefore i wouldn't rely on it to dampen.

RE: Inclusion of product mass in dynamic analysis

I vote yes. At the least, it's a load case that ought to be considered. When the product is in the bin it is certainly not free to move independently of the structure. And when it is in the chute, it is only free to move independently of the structure once all friction between the product and the chute has been overcome. Lastly, even if the product is able to move independently, you may get something equivalent to the seismic sloshing effect in the bin.

I've dealt with a similar situation in designing libraries. Should the books be part of the seismic weight? Some say no; I say yes. In a long duration, Mexico City style earthquake, I imagine the books falling off the shelves and lying in a big heap that most likely moves along with the floor system as a result of friction.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Inclusion of product mass in dynamic analysis

(OP)
That's my dilemma. With storage bins, I've considered product as part of the mass for dynamics/seismics. But with vibrating screens, I've ignored product mass for dynamics. I'm thinking as a worst case the chute could operate like a bin

RE: Inclusion of product mass in dynamic analysis

canwesteng,

If your structure is supporting your mass at any time, at the very least, you have a case where the product mass matters.

--
JHG

RE: Inclusion of product mass in dynamic analysis

(OP)
My understanding of dynamics is that product mass should only matter for dynamics if it's part of the elastic body that's vibrating. So I would never count the live load of people on a platform as influencing the natural frequencies of the platform, or add any dust load, etc...

RE: Inclusion of product mass in dynamic analysis

canwesteng - Instead of looking for an exact solution, bound the problem. Ignore the product mass in question for dynamics and calculate the result. Then consider the product mass to contribute fully to the mass for dynamics. See what the you get. The "best" answer probably lies between the two results. Chances are that the two results will be very close together anyway. If that is the case, then you can likely design the steel to avoid both cases. If not, at least you have a better idea of the scope of the problem.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Inclusion of product mass in dynamic analysis

(OP)
I'm more interested in understanding the fundamentals of the actual behaviour than coming up with a conservative solution. Since the goal is to high tune the structure, and more mass=lower natural frequencies (at least to my knowledge, and it certainly is the case here), more mass is more conservative.

RE: Inclusion of product mass in dynamic analysis

Understood, and I commend you for gaining understanding of what is happening. In that case the second half of my suggestion applies:

Quote (SlideRuleEra)

If not, at least you have a better idea of the scope of the problem.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Inclusion of product mass in dynamic analysis

It should be included, in fact is like with elevated tanks where you have to take into account the kick that the water produces cause it has inertia of it's own as it's not strictly attached to the tank itself. I'd say this is the same case to a lesser extent but it'd increase inertia forces, damping wouldn't really matter as that's more of a question of stiffness.

RE: Inclusion of product mass in dynamic analysis

I agree with sliderule's approach as there is no exact solution to my knowledge. In my own engineering judgement, as engineeringeric suggested, I also suspect that it would have an overall dampening effect but have no scientific proof or method to calculate it's magnitude....

RE: Inclusion of product mass in dynamic analysis

It's not just damp(en)ing it is also mass loading, which will affect the resonant frequencies. Structure/fluids interaction is a pretty hot topic, I suspect a moving bed of ore will be a similar issue.

Cheers

Greg Locock


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RE: Inclusion of product mass in dynamic analysis

(OP)
Greg

Can you link to any discussions on the matter?

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