Relation between mass, stiffness and Damping

[gopi9]

Hi,

Will there be any relationship between Mass, Damping and Stiffness. I want to use a 5 node example and I don't know what values should I use for mass, stiffness and damping.

 

[btrueblood]

Well, yes, they are related by how the mechanical system you are discussing is configured. This should have been covered in your freshman physics course at college.

The generic mechanical oscillator system is a mass-spring-damper, and is considered equivalent to a sparky's RLC circuit. Consider a mass suspended from a fixed point by a spring, and a damper (think shock absorber) in parallel with the spring, then the mass m, damping constant c, and spring stiffness k are related by the differential equation

m d2x/dt2 + c dx/dt + kx = f(t), where f(t) is a force vs. time function.

 

[Tmoose]

I'd say I could safely draw no conclusions about two of those properties knowing just the third lone property of a nameless material.

Wood is "lighter" than water but way stiffer in bending.

 

[msquared48]

Personally, since water cannot be bent, I would say that it is much stiffer.

Please. Demonstrate the bent shape of water...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[msquared48]

OK... Wave theory, but I thought I'd give it a shot anyway...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[elogesh]

Every component has three inherent (Mass, stiffness, damping) in them.

If you take a steel wire, it has more axial stiffness and low mass. If you attach steel ball at the end of the wire. The steel ball has more mass and rigid(High stiffness). Compared to the steel ball, the mass of the wire negligible.

Instead of steel wire, if you use rubber cord, it may have still lower stiffness and better damping.

Logesh

 

[gopi9]

Thanks everyone.
I want to know how the parameters vary for a bridge. Thanks elogesh

 

[gopi9]

I have seen in many examples that mass is low, damping is close to mass values but slightly higher than mass values and stiffness is very high when compared with both mass and damping. Can any one justify this?
One more thing I found in many examples is that they are using all the stiffness and damping values as the same for all DOF.

 

[amanuensis]

Mass and stiffness are related together through the resonance frequency
f0=(1/2pi)*sqrt(Stiffness/Mass).

Damping effect is negligible/unimportant compared to both mass or stiffness, except when the SDOF is near from the resonance frequency. Why ? Because the mass effect is balanced/canceled by the stiffness effect.