Need Info on Dynamic Dampers for Floor Vibration 3

[JAE]

We just completed a report to an owner of a two-story office building where the occupants had been complaining of vibrations in the floors.

We performed some standard vibration analyses and verified that the floor was under-damped and in the unsuitable range of perceptability. We recommended stiffening the steel floor joists and in passing, made mention of the possibility of installing mechanical floor dampers within the structure. We understand these to be quite expensive and have never really used them in practice.

Now the owner is asking for info on the dampers and I'm having difficulty finding anything. Does anyone know of companies that provided dampers for floors? I'm just trying to get approximate pricing to shut the door on this option.

 

[IJR]

Cant let you down JAE, I am also searching for you man.

Good luck

 

[IJR]

JAE

I am back but like a schoolboy excited by a homework assignment.

Do you want to start the day with http:/

http://www.nrc.ca/irc/practice/gen4_E.html

Thanks. IJR

 

[Riz]

Thanks IJR for the useful information.
JAE,
There was a case I had reported on about 4 years back. It was for a RC structure where one could feel vibrations due to heavy trucks passing on the adjacent road. No vibration was felt due to rythmic loads within the building.
Is your structure suffering from vibrations due to loads within the building?
Riz

 

[butelja]

A tuned mass damper can be designed relatively easily to eliminate a single mode of vibration. If the floor vibrates primarily in the 1st mode, where it is acting like a diaphragm, then this mode can be effectively eliminated via a tuned mass damper. HOWEVER, the new system will have 2 resonant peaks, with one slightly lower in frequency than the original, and one slightly higher in frequency. These frequencies are closer together for a small damper and further apart for a larger damper. Small vs. large refers to the mass of the damper compared to the mass of the original system. For this reason, tuned dampers are most effective at eliminating vibration caused by a single frequency excitation. They are less effective if the excitation is random or over a wide frequency range. The % of critical damping on the tuned damper can be optimized to minimize resonant amplification over a given frequency range, but there will still be amplification between the 2 fundamental frequencies. If you are still interested, I can give you a reference for the design details.

If your floor is vibrating at frequencies higher than the 1st mode, then the system will be even more difficult to attempt to control by this method. The system would probably require a detailed FEA modal analysis, multiple dampers, and be very sensitive to changes in floor loading. For higher modes of vibration, the addition of structural damping might be more effective.

 

[JAE]

Thanks IJR for the quick response (and thanks all of you).

I checked the site you posted. I noted that they downplayed the effectiveness of tuned mass dampers. Our first inclination is to add stiffness to the steel floor joists as they contribute the most to the amplitude/frequency.

Riz - the vibrations are definitely due to foot traffic and not outside vehicle traffic. We walked around the floor and personnally experienced the shakes. Noticed that corner bays were the worst, exterior side bays also bad, and interior bays less noticable but still vibrating. The floor is only 3" (76 mm) thick and there are no partitions to speak of in the subject areas. Acoustical ceilings everywhere with cubicle squares and no floor to ceiling partitions.

butelja - your last paragraph tells me what I already suspected, that installing a tuned mass damper is difficult, provides unknown success, and would require a lot of engineering effort.

 

[IJR]

JAE

When you are done, will you please post your final solution here. I am fascinated by this subject

Good luck

 

[butelja]

JAE - When increasing the stiffness of the floor joists, you may want to utilize bolted connections wherever possible rather than welding. The bolted joints have inherently more damping than a welded connection. There is also an interesting article on floor vibrations at the AISC website via the following link:

http://www.aisc.org/documents/sep_91.pdf