RISA Floor, Vibrations and AISC Design Guide 11

[marinaman]

I have a project that is to be steel framed (composite floor system). Typically, the floor receives 2" metal deck with a total floor thickness of 5 1/4" lightweight concrete as the composite floor deck.

I've modeled the floor in RISA Floor.

Part of the floor is to receive some relatively sensitive equipment that doesn't like too much vibration. I am looking at ways of reducing vibration in certain parts of the floor, such as reducing spans, using a thicker and heavier concrete deck, etc etc.

My question is: In the AISC design guide 11 for vibration, in chapter 6 for sensitive equipment, the design guide talks about VCA, VCB, VCB floor designations, depending upon the vibration demand of the equipment on the floor in the area of the floor that the equipment is placed. For example, for an area designated VCB, the tolerance limit is 1,000 mips (velocity tolerance unit in micro-in/second).

Well, when working thru RISA Floor and looking into the RISA Floor vibration results, I am seeing a result for natural frequency....and a result for vibration acceleration as %g.

The confusion I have is, AISCs mips limit is really the one third octave velocity. While the RISA results are natural frequency and acceleration in %g.

I'm guessing one has to enter the spreadsheet vibration results data in RISA Floor and still has to hand calculate the V1/3 by hand in order to compare it to the allowable mips for the floor designation?

Is this what others have done?

I do not understand why RISA didn't go ahead and calc the one third octave velocity, because it seems that the graphic result of natural frequency and accelerations are not of much use, if the AISC design guide is comparing and working via mips.

I'd appreciate anybody's input here.

Thank You Guys!

 

[271828]

Pull the natural frequency from RISA's output and manually compute V per Chapter 6 equations.

 

[dik]

Do you need a vibration consultant? Depends on the permissible vibrations; maybe only achieved by extreme stiffness. Had one project with 3'x3' columns on 18' grid (single storey), 3'x4' beams interconnecting the columns and 1'x4' beams at 3' o/c between. Super stiff for a microprocessor lab. Can the equipment go on vibration isolators?

Dik

 

[JoshPlumSE]

RISAFloor vibration results are geared towards floor vibration for walking excitation. So, it's based mostly on chapter 4 rather than chapter 6.

Though the detail reports for the members should give you much of the variables used to calculate those results. Which should help you handle some of the vibration calculations per the other chapters.