Floor Vibration 3

[GalileoG]

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.

 

[fcu45]

RC beams may help reducing the vibrations (More stiffness) without the need of thick and deep steel sections, if it is possible to consider it as alternative.

Tiling Floor screed of ~ 2 inch would also reduces the vibrations/Echo, (Happened in a recent project).

 

[frv]

There was a webinar a while back and if I remember correctly, I think the load is actually the Live Load only, not the dead load. The reason is that the live load is what drives the vibration, not the dead load.

 

[MrHershey]

Both live and dead technically drive vibration. But the dead load also typically brings along more stiffness that makes up for the added dead load. So usually, yes, just live load will drive your vibration acceptability. If you've got a large amount of superimposed dead load that causes the planks to deflect without adding any stiffness, then dead could contribute as well.

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.

 

[JoshPlumSE]

Many of the sources of increased mass (full height wall partitions, book shelves, et cetera) also increase damping.

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.

 

[shobroco]

Vibration is a concern on long spans, & I would consider 14m a long span. I had problems at 11m with a composite steel/concrete system & ultimately the only thing that worked was partitions on the floor to deaden it. They originally had portable partitions, essentially furniture, & we built some permanent steel stud & drywall partitions & they made a huge difference. We couldn't put topping on the floor, the place was occupied & it was only one area with a problem. The vibration problem was (surprising to me) not caused so much by occupants but by heavy truck traffic on the street outside. From what I learned though, mass makes a big difference; be conservative.

 

[ThomasH]

Hi

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

 

[PeterDebney]

The usual procedure, at least for European floor vibration guides, is to use 10% of the live load in the modal and response calculations. The thinking behind this is that when there is no live load there is no one to worry about the vibration (stairs and the like are an exception to this) and when there is more live load the additional mass reduces the response.

I wrote a paper on the subject a little while ago, which you can find here

http://www.oasys-software.com/media/press/GSA/Sound&Vibration0911.pdf

 

[Archie264]

The later versions of the Ram Steel software used to have a provision for checking vibration for a steel-concrete composite system, for what it's worth. If you have that software that type of floor system can be checked very quickly. And for clairity, yes, I'm referring to vibrations and not just deflection. It's another potential resource to keep in your toolbox, if it helps.

 

[shobroco]

I was just at a training session for S-Frame software, & it can deal with frequency & vibration. It allows you to change variables almost infinitely and see real responses, including animated visual simulations of them. As with most things like that, it looked simple when the kid showing us did it, but I had trouble even getting to the same screens as he had up.

 

[271828]

For floor vibration analysis, use the absolute best estimate of the actual mass present during the walking event. That'll be the structural weight, accurate estimate of the SDL, and a very small live load. There is no safe side on which to err. A mass overestimate will drive down the natural frequency but will drive up the effective mass. A mass underestimate will do the opposite.

See the following presentation. Dr. Thomas Murray talks about mass somewhere in the first 10-15 slides.

http://www.aisc.org/uploadedcontent/2012NASCCSessions/N4/