Vibration Analysis 1

[NateTBA]

I am trying to analyze a floor slab for vibration. I have referred to the Steel Design Guide 11 (Floor Vibrations Due To Human Activity) however cannot find any solid information concerning a conventional reinforced two way slab.

Does anyone know of any places that I can refer to to find any information on how to analyze the situation.

Thanks.

 

[SperlingPE]

I don't have any direct answers, but suggest to start looking at PCA or ACI.

 

[WillisV]

NateTBA - you can use a finite element package (such as SAP2000) to determine the fundamental mode shapes of the concrete floor and then from the frequencies of the modes back calculate the velocities to compare with the guidelines set for in Steel Design Guide 11.

 

[lsmfse]

Try this Link.

http://moment.mit.edu/documentLibrary/Paper016/text016.html

 

[DaveAtkins]

Someone out there can correct me if I am wrong, but my engineering JUDGMENT is that a reinforced concrete floor system is unlikely to have vibration problems. I only check vibration when the floor system is concrete topped steel deck on steel joists or steel beams.

DaveAtkins

 

[fleisher]

DaveAtkins is correct. When you have a grid system consisting of a composite deck, then you need to check for the floor vibrations. Pure concrete slab need not be checked. If you look at the formulation of DG 11, you will notice it asks for entitiies like girders and beams and the direction which they span.

 

[BigInch]

If you are still curious, you can find some equations for predicting the natural frequencies for plates. If your deflection is ok and your loading frequency is outside say 0.8 to 1.2 times the natural frequency, you're usually going to be OK. If inside that range, thicken up the floor or, if you can't do that, then model it in some computer program to be sure the max amplitudes at the nat freq are within tolerance limits for humans or machines or whatever will occupy the floorspace. It is far better to avoid slabs with the same frequency as any machine installed there.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[blihp]

i generally agree with the notion that RC slabs in most situations generally are not prone to vibration from 'normal human activity' due to their relatively heavy mass giving good inherent damping, however there are situations when this DOES need to be checked, such as when there is sensitive equipment involved such as labratories.

If there is a driving dynamic load, which can be either from mechanical plant (like pumps etc) or even human activity (people jumping at a concert) then this needs to be checked explicitly for that loading.

A key question is thus, what is the type of loading may cause excitation? (humans walking, humans dancing, plant equipment?)

In terms of how to analyse a UK Published guide by the Concrete Society on Post Tensionsed Slabs gives guidance on calculating acceptance criteria (accelerations) of slabs based on people walking briskly across a floor plate.

 

[dik]

The new User's Guide to the 2005 National Building Code of Canada has a fairly detailed description of designing for vibration.

Dik

 

[NateTBA]

I too agree with DaveAtkins that the floor will most likely not have vibration problems. However my judgement leads me to want to at least look it into it and be sure that there will not be any excessive vibrations. I have still not found an abundant source of information, so what I am going to do is, while keeping in mind the amount of dead load I am dealing with in a 9" slab, I will mimmick the two way slab with a similar one way slab system with joist and joist girders or beams and see what the velocities come out to be. I will then compare these to the guidelines set forth by Guide 11 as well as the manufacturer of the laser eye equipment. If it the numbers are close I will delph further, if not I will leave it be and conclude no problems.

 

[NateTBA]

Thanks so much for all the opinions and advice

 

[WillisV]

I agree that for normal human activity a poured-in-place concrete slab will not generally be a problem but as mentioned above with lab buildings containing sensitive equipment it does need to be checked and will often control the design.

 

[gpendyala]

Mode shapes and displacements can be quickly checked with SAP 2000 package. For reference on vibrations, my choice is Structural Dynamics, Theory and Applications by Joseph W. Tedesco,William G. McDougal,C. Allen Ross.

I agree with the posts above that end use of the building is the important criterion to be taken into consideration.

Thanks,
Pendyala

Thanks,

Lakshmi G Pendyala

 

[ThomasH]

Hi

I'll go for the key question, what natural frequency does the floor have?

This be analyzed (calculated) with handcalculation och computer and it's often very simple.

Regards

Thomas

 

[GregLocock]

Gosh. I'd say after 25 years of working in dynamics, that natural frequencies of distributed systems are rarely simple.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ThomasH]

Greg

If that post was directed to me, my idea was a simple square concrete slab and it's natural frequency. That would only require a "simple" formula. But I might have missunderstood the problem.

The reason for the question is that I couldn't find any real information in the tread reqarding the size of the slab and in my experience the natural frequency is a good starting point for any dynamic analysis.

Regards

Thomas