Slab vibration 2

[SemiPE]

Hi All,

In concrete design, does the allowable span depth ratio ensures that vibrations due to live load will not affect occupants? say for example I have 12' (3.75m)span continuous slab with a residential live load of 42psf (2kPa), if I use of 6" (150mm) thk slab would i feel significant vibration if people at the next room are running, jumping or doing some sort of activity?

I know that the span depth ratio is there to control excessive deflection. I dont know if applying the span depth ratio would also take care of the vibration problems, and the code has no guideliness on how to control vibrations.

 

[hetgen]

Hi SemiPE,

The allowable span to depth ratio does not ensure acceptable vibration response. But I don't think you will have vibration issued for the span you dealing with.

 

[clarke1973]

Vibration is generally only a concern with steel or lightweight floors having spans greater than 8-9m. it is not usually a problem with concrete floors having a L/D ratio less than 25.

if you are ever worried about vibration however there is a very simple hand check you can do to determine the natural frequency of the floor structure, which is a function of the dead load deflection - see the attached extract with the formula. You basically work out the natural frequency and if it is greater than 4.5 Hz vibration should not be an issue.

 

[steellion]

If it is residential, are there a lot of partition walls along this span? The partition walls will do a lot to dampen the vibration.

 

[CELinOttawa]

Steel I on: Agreed, with the pointed exception of the newer engineered joists when they are being pushed... Seeing more and more agregious SLS failures in those!

 

[CELinOttawa]

Lol... Egregious. *smiles*

 

[hetgen]

.... You basically work out the natural frequency and if it is greater than 4.5 Hz vibration should not be an issue.

4.5Hz is very close to the second harmonic walking frequency and the structure will be susceptible to both impulsive and resonance response. The vibration could be unacceptable if the peak acceleration is >0.75%g. See attached figure from AISC.

 

[SemiPE]

Hi All thank you for the valuable comments. In my experience if I use thin concrete slabs (150mm>, for 3.5m span) and RC frames people would feel significant vibrations/shaking of the floor when a person relatively close to them would impose forceful impacts on the slab such as when a person stomps or jump. This makes them feel that the structure is not safe or it will easily collapse, even though the slab is compliant to the span depth ratio and was designed properly. As a personal precaution I often avoid utilizing the span/depth ratio near to its limits, however thicker slabs means $$$ for the over-all structure.

Thank you for the reference vibration calculation.

 

[MikeHalloran]

I think I worked out once that a static deflection, i.e., from self weight alone, of L/360, gave a natural frequency of 4-ish Hz, regardless of material or span.

It came to my attention again one day when I was messing with an assembly driving by a small stepping motor (nothing nominally unbalanced, just commercial components), and decided to slowly sweep it up from 1Hz to 1000Hz to test the electronics.
... on a heavy table in my second floor lab.
As the machine swept through 3 to 4 Hz, the entire population of the machine shop downstairs came running up to see how the hell I was shaking their ceiling so badly.
I had to run the sweep again in their presence, after they had searched the lab and not found some kind of seismic death ray machine, to show them how little energy it takes to excite a resonance.

After that I programmed the machine to just start above 20Hz.



Mike Halloran
Pembroke Pines, FL, USA