Damping to be considered in dynamic analysis 6

[GD_P]

Hello,

For welded steel structures subjected to dynamic load due to machinery, i have been instructed to use damping factor of 2% of critical damp coeff. I dont find any reference in Eurocode OR AISC, some papers recommend it.
But AISC specifies "Damping levels for use in evaluating building motion under wind events are generally taken as approximately 1% of critical damping for steel buildings."
Whereas in case of seismic analysis, the response is calculated for 5% damping (Eurocode 1998-1).
Why they are different?
If response in seismic conditions is calculated on 5%, why I can not use the same damping for dynamic load due to machinery.
Please share you views,
Thanks in advance.



GD_P

 

[canwesteng]

Do a sensitivity analysis and see how much difference it makes. It shouldn't have a large impact if your floor is high tuned away from the natural frequency of the equipment.

 

[WARose]

Damping is dependent on many factors. One of them is the stress level of the material involved. In general, the more stressed steel is, the more damping you will have. For a welded steel structure using no more than about half of it's yield stress, 2% is appropriate. For something stressed at it's yield point, 5% is appropriate.

In a major seismic event, you would expect stresses and strains beyond the yield point (which would get you a 7-10% critical damping value), but perhaps the 5% is being conservative.

(See 'Structural Dynamics: Theory and Application', by: Tedesco, p.423, for a complete list based on stress level.)

 

[BridgeSmith]

I believe the higher damping coefficient for seismic is due to the much larger magnitude of the movements and forces.

 

[JoshPlumSE]

We could get into a long, long discussion about damping. But, I'll just cite a few points:
1) The Raleigh damping we typically assume in our analysis does NOT model how real damping occurs in structures. It is merely a convenient mathematical method to approximate behavior.
2) You have to think about what is producing the damping? For large movements (at seismic load levels) you may have the following:
a) Yielding of members.
b) Damage to non-structural items, cladding, flooring, gyp board, et cetera.
c) Radiation of vibration out of the structure (into surrounding soil, through full height partitions into adjacent floors).
d) misc. minor items like bolt slip, friction / slip of furniture, supported equipment, piping or such.

3) For welded steel structures at very low amplitudes of vibration you will have a lot less damping. Especially industrial steel structures that don't have architectural cladding, or drywall or such. Maybe a, b and c wouldn't apply at all. So, you will have significantly lower levels of damping.

I generally see 2% damping used as a standard. Not many would question it. Sure you could go down to 1% if you feel this structure will have very little damping for that applied motion. Or, you could increase it to 5% if you feel like you will have a lot of damping for the applied motion.

 

[271828]

If there are no nonstructural elements, then I would use 1%. I have measured values below 0.5% for bare steel and concrete floors. There needs to be slipping and friction, like from drywall partitions, to have reliable higher damping.

 

[Denial]

When I was investigating the dynamics of a fully-welded steel pedestrian bridge many years ago I sought advice on this from a couple of experts.[ ] They independently recommended the use of 0.5%, fully consistent with what our numerically-named colleague suggests above.

 

[GD_P]

Thank you all for your response.

Structure we deal with are chemical skids, which are around 3000mm x 3000mm x 12000mm high. and it includes piping & its supports, mesh floors gratings and electric panel & its connections
So I think 2% is okay for our application.

One more point, every one talks about floor and its damping in their posts,
Is dynamic analysis is only performed on floors perpendicular to axis of the floor?
What about the lateral dynamic forces (forces along the plane of floor)?

Is their any guideline for ideal/required frequency for structure?
I know SCI have some guideline about the floor frequency (i suppose it is along vertical direction)?
But what about the lateral direction?

GD_P

 

[WARose]

Is dynamic analysis is only performed on floors perpendicular to axis of the floor?
What about the lateral dynamic forces (forces along the plane of floor)?

It depends on the situation......but (say) in the case of a motor or a fan, you absolutely should include the unbalanced force in the horizontal direction. Your (time-history) FEA model would include (of course) lateral modes of vibration as well.

Is their any guideline for ideal/required frequency for structure?
I know SCI have some guideline about the floor frequency (i suppose it is along vertical direction)?
But what about the lateral direction?

Generally, you try to avoid floors with a natural frequency of 3 Hz or less. AISC Design Guide 11 specifically says this to avoid "rogue jumping". (Due to human excitation.)

IIRC, AASHTO's rules for pedestrian bridges specifies 3 Hz for the first vertical mode (i.e. the natural frequency should be higher than) and 1.5 Hz for the horizontal mode.

But at the end of the day, it depends on what the forcing frequency of your source is.....you are trying to avoid a resonance/near resonance condition.

 

[GD_P]

@WARose
Thanks for the info.

1) "But at the end of the day, it depends on what the forcing frequency of your source is.....you are trying to avoid a resonance/near resonance condition."
How to interpret the near resonance condition, 0.8 to 1.2 times the natural freq.? Any guidelines for this?
I think this will depend more on how accurately we model our masses the in order t get natural freq?




GD_P

 

[ThomasH]

Hi

If you can, look in ASCE 4-98 Seismic analysis for Nuclear Structures, there is a table for damping values. It depends on material, type of structure but also on stress level. Damping is energy loss, and a clean steel beam will lose less energy when it vibrates than a bolted truss.

I have analyzed several pedestrian bridges and the recommended damping values are different for SLS and ULS. I analyzed a steel structure supporting a slab some year ago that had excessive vibrations due to dancing. I talked to one of my professors at the University and he said that in a lab they had measured damping "near zero" for steel. One issue with damping is that you can't calculate it. You need to measure it on the finished structure. So for design you need similair references of have to make a reasonable "guess".

Thomas

 

[WARose]

How to interpret the near resonance condition, 0.8 to 1.2 times the natural freq.? Any guidelines for this?

The guideline is: stay out of that range.

I think this will depend more on how accurately we model our masses the in order to get natural freq?

There are always going to some inaccuracies in this. The model, mass estimate, and so on. One of the biggest is damping. It's the black box of dynamic design because it's hard to say what it actually is. That's why we use safety factors.

 

[271828]

"I talked to one of my professors at the University and he said that in a lab they had measured damping "near zero" for steel."

I can back that up. I measured 0.2% and 0.3% for two specimens composed of just steel and concrete. I've also measured around 0.5% and a little over 1% on similar structures.

One could "get away with" using 1% because that is published in a few places. I wouldn't count on having higher than that unless there are significant nonstructural components.

 

[BridgeSmith]

In a perfectly elastic system, there would be no damping. Damping is produced by plastic deformation and slippage of connections with friction. A strictly steel and concrete system where the connections do not slip and the concrete doesn't crack under the applied loading, has very little damping. The responses would seem to indicate 0.5% or less for a building without significant nonstructural appurtenances that are flexible. If you need more damping, isolating the equipment or machinery shouldn't be difficult.