Floor Vibration
Floor Vibration
(OP)
I am in the preliminary design phase of a 4-storey office building and I am trying to select a suitable floor system. I am currently leaning towards hollow cores which would span approximately 14 metres and would be bearing on steel beams. I do however have some concerns with floor vibrations.
I am going through the methods outlined in the CPCI manual (or PCI for the yanks) in regards to assessing the vibrations.
The first exercise is to determine the frequency of the floor system, CPCI provides a formula fn = 18 / SQRT(deflection). The manual is not clear as to whether the calculated deflection is dead load deflection only or full dead + live load deflection. The latter would yield a low frequency value which makes it difficult to meet the 3Hz or greater criterion.
However, on another section of the manual, "Many vibration problems are more critical when the mass (or weight) is low." - This intuitively makes sense, but adding mass reduces the frequency (as per the above equation.) Can someone elaborate on this point? I think I'm missing a link somewhere.
I am going through the methods outlined in the CPCI manual (or PCI for the yanks) in regards to assessing the vibrations.
The first exercise is to determine the frequency of the floor system, CPCI provides a formula fn = 18 / SQRT(deflection). The manual is not clear as to whether the calculated deflection is dead load deflection only or full dead + live load deflection. The latter would yield a low frequency value which makes it difficult to meet the 3Hz or greater criterion.
However, on another section of the manual, "Many vibration problems are more critical when the mass (or weight) is low." - This intuitively makes sense, but adding mass reduces the frequency (as per the above equation.) Can someone elaborate on this point? I think I'm missing a link somewhere.






RE: Floor Vibration
Tiling Floor screed of ~ 2 inch would also reduces the vibrations/Echo, (Happened in a recent project).
RE: Floor Vibration
RE: Floor Vibration
If you look at AISC Design Guide 11 (focuses on steel, but the basic principles are the same regardless of material), all of the examples use dead+live load for the deflection when calculating natural frequency.
RE: Floor Vibration
It's another application, but in idustrial work we will semi-arbitrarily increase the mass of foundations which supported dynamic equipment. When it is a relatively small piece of equipment,we just make sure that the block footing weighs substantially more (3 or 4 times) than the weight of the equipment. The idea there was that it takes more energy to move the mass. Therefore, adding mass tends to reduce the response. Of course, this only works when the magnitude of the forcing function is relatively light... Hence, the limitation on this only applying to light weight equipment.
RE: Floor Vibration
RE: Floor Vibration
The first frequency for a simply supported beam is f = pi /2 *( E I / (m L^4))
E module of elasticity
I moment of inertia
m mass / length
L span
Mass is usually dead load only and if you increase the mass you will reduce the frequency. But you will also reduce the response (acceleration) since the dynamic force will work against a larger mass.
There are several methods to analyze a floor for vibration. Some are crude "rules of thumb", others try to describe peoples movement as frequency dependent forces. If you calculate the frequency and get a high frequency with the assumption of high mass then you are probably okay. The value for "high" depends on activity but also on material. It's easier to get a high frequency value for steel than concrete since the mass is lower but it is also easier to start vibrations in steel since the mass is lower.
And it is actually only a part of the live load that drives the vibration. But you don't need that to calculate the frequency. What you do need is understanding of the method you use. But I think you know that since you ask.
Good Luck
/Thomas
RE: Floor Vibration
I wrote a paper on the subject a little while ago, which you can find here http://www.oasys-software.com/media/press/GSA/Soun...
RE: Floor Vibration
RE: Floor Vibration
RE: Floor Vibration
See the following presentation. Dr. Thomas Murray talks about mass somewhere in the first 10-15 slides.
http://www.aisc.org/uploadedcontent/2012NASCCSessi...