Vibration analysis of Concrete Joists

[cnasta]

I have a project with one way wide module concrete joists that is going to be used for a laboratory and it has strict vibration criteria. The only design guides that I can find for vibration are for steel. Does anyone have a method for determining the vibration of a concrete floor system?

 

[prsconsultant]

We are currently doing a project exactly similar to yours. Once we have the concrete members designed, a vibration consultant is involved to determine if the design meets the vibration criteria.

 

[271828]

You could look in the British SCI Vibe DG. They have a simplified method similar to the one in the AISC DG11. They also have a more elaborate method that can be used in conjunction with a finite element program. The FE program needs to be able to calculate natural frequencies and mode shapes, but does not need to be able to perform dynamic analyses beyond that.

With judgment, you can use the AISC DG11 or SCI simplified procedures, but more elaborate analyses might be called for here.

 

[271828]

Another thing. Note that the main difference between concrete and steel systems is often that the concrete systems have much higher natural frequencies. These will often be outside the range of walking, even at the 3rd or 4th harmonic of the step frequency. In this case, the vibration problem is fundamentally different from a low frequency steel framed floor. It's impossible to cause resonance, so the response is more like the response due to a series of unrelated impulse loads.

The SCI elaborate procedure accounts for this. They have one way for low frequency floors and one way for high frequency floors.

 

[Lion06]

If I remember correctly, the acceptable frequency for steel is somewhere around the 6-8 hz range, depending on application.
If following the AISC DG procedure, be careful to use appropriate, acceptable frequencies.
I had some literature for wood that recommended the natural frequencies be in the 22-25 hz range, which differs significantly from the steel recommendations.
I can imagine concrete would have different recommendations.

 

[271828]

StrlEIT, there's no magic 6-8 Hz range for steel. A good floor can have a frequency from 4 Hz to whatever, depending on mass, damping, application, etc.

There are ranges of walking frequencies. Perhaps that's what you're thinking about. People only usually walk between about 1.6 Hz (96 bpm) and 2.2 Hz (132 bpm).

Wood is a completely different ballgame. If anybody needs them at some point, I can provide refs to several good papers.

 

[cnasta]

We were instructed to design the floor to meet the Class A criteria for sensitive equipment in AISC DG11. This limits the vibrational velocity to 2000 uin/sec.

I did find a new guide developed by Arup that was published in England in January 2007, " A design Guide for footfall Induced Vibration of Structures". I am still going through this guide, but their method for a high frequency structure using FE analysis requires using the modal mass in conjuction with the walking frequency and the structure's frequency. Their FE program gives them the modal mass for each mode, but I am using Risa 3D and I would have to manually calculate the modal mass from the mode shapes which would take some time.

 

[Lion06]

271828-
I would love to see the papers you have for vibrations (wood or whatever else you have).
I am sure you are right about the DG11, but I swear I remember some natural frequency limits in the 6-8 hz range depending on application (dancing, dining, office floor, etc..), in addition to acceleration limits.

 

[271828]

StrlEIT, there have been some blanket statements made over the years. Another whopper was the one in the Vulcraft catalog about how 28' is death for a joist design, LOL.

 

[271828]

cnasta, the Arup guide should have the more elaborate procedure from the new SCI. I'm 99% sure that Willford & Pals came up with it.

 

[cnasta]

271828, Close, the authors are Willford and Young.

 

[271828]

LOL, that's what I meant. I couldn't remember the guy(s) other than Willford.