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Question on Vibrations
3

Question on Vibrations

Question on Vibrations

(OP)
I am going through Design Guide #11 and I have a few questions.  Does everyone perform a vibrations check for every building they do by hand or do you let something like RAM do it?  In reading through the manual, it seems like so many things are left to the discretion of the designer that by the time you compare your criteria to the suggested limits in the Design Guide, it doesn't seem like it would be as reliable as, say the buckling strength of a beam.  Does anyone else feel (or did you when you first started out) a little overwhelmed by this subject?
Also, where do you find vibrations info for concrete, I know it has a lot of mass and as a result, the accelerations are low and likely not a problem, but I am sure it must still be checked.  
I have info for wood, although it has more to do with deflection limitations to conrol vibrations.

RE: Question on Vibrations

Structural EIT,

I can remember at a conference they said that in vibration sensitive buildings you would design for vibrations first, then check for strength after, usually it was the opposite.

Normally I would only check the most flexible members in a frame.

In my experience, vibration for concrete is only checked for vibration sensitive areas such as hospitals, precision industries e.t.c. This is very complicated subject that is probably best left to specialists IMO.

I have seen a few good references on timber floor vibration, I recommend you look up the following website:

http://www.awc.org/HelpOutreach/faq/FAQfiles/Vibration.html?areaID=1

csd

RE: Question on Vibrations

3
For steel buildings, I recommend checking every representative "typical" bay using FloorVibe.  It only takes a couple of minutes per bay.

You are kinda right about the lack of reliability, for a few reasons.

It is almost impossible to estimate damping with any reasonable precision.  That's the one that folks already know.

Natural frequency prediction is the easy one.  However, they can only be reliably estimated within about 20% using the best finite element modeling techniques (design model, not a reseach model that has been "adjusted" or "updated" to match experimental results) that we have now, much less the simplified methods in DG11 or the UK SCI DG.  Even with the best FE models, prediction is not good.  I've personally done full modal tests of two real buildings and I've read papers from others who have done the same.  Real buildings have multiple vibration modes, sometimes closely spaced.  Finite element models get in the ballpark of the frequencies, but the modes will be shaped a little different and will be in totally the wrong order!!  The FE model will also have the modes much tighter spaced than the measurements indicate.  Sometimes, there will be 10 modes between 6.0 Hz and 6.5 Hz, for example.  Any claim of nailing the first several modes using a FE model with any reliability casts serious doubt on the engineer's credibilty, IMO.  Now go back to the simplified methods and consider the luck one will have in nailing it.

Here's the big one: Human walking forces are almost totally unpredictable.  I've measured many of these myself and have studied the literature.  These are always broken down into four Fourier amplitudes.  For example, the first will be about 0.5*bodyweight, the second 0.2, third 0.1, and fourth 0.05--for example.  The problem is that a plot of these amplitudes looks about like a shotgun blast!!  There is literally NO pattern in the 3rd and 4th harmonics of the walking force and these are the ones usually used in design.  For example, you might *think* the 3rd harmonic is 0.1*bodyweight, but the data shows A LOT of points anywhere from 0.01 to 0.16.  It literally looks like a shotgun blast when plotted versus step frequency.  Until we can get people to walk "better" there's no hope of really nailing an acceleration prediction.

All that being typed, DG11 Chapter 4 was developed by a couple of guys with 30-40 years each of experience, so has been tweaked and fudged so that it works reasonably well.  You can use it with a fair amount of confidence.  It won't nail the prediction, but your floor will almost certainly be ok if you correctly apply chapter 4 and it predicts the floor to be ok.

For the long haul, the most sophisticated techniques will be probabilistic, but these are several years away.  For example, future criteria will allow you to tell an owner that he'll have a 90% chance of no complaints in a given area.

For concrete, you can adapt DG11 or use the new UK SCI DG General Assessment chapter which is directly applicable.  It's fairly difficult and requires FE modeling and a solid grasp of vibration theory to be able to use it at all, though.  Vibe is almost never an issue for concrete unless you have sensitive equipment.

RE: Question on Vibrations

StructuralEIT:

Concur with 271828. We also use FLOORVIBE for steel framed buildings. Very quick and easy to check out several possible layouts and combinations of floor thickness, beam spacing, beam size, etc.

We check floor vibration for every building. A few minutes a bay to check vibrations is time well spent.

Normally, I design 'typical' floor bay(s) for strength and deflection criteria, and then check vibration on FLOORVIBE before going too much further, to see if I need to tweak assumptions. Best to do this early on. Its a bummer to get a month into a project and realize that your assumed floor thickness isn't going to work.   

RE: Question on Vibrations

If a structural floor is built of steel, with composite or non-composite deck and concrete topping, I check vibration.  I use Design Guide # 11.

Otherwise I don't check vibration.  I think, in general, experience indicates that the following floor systems will not have problems with vibration:
* PCC plank on steel beams
* reinforced concrete (flat slabs, one way joists, waffle slabs)
* wood

That is my opinion--I look forward to hearing others' comments!

DaveAtkins

RE: Question on Vibrations

With most offices using the "open concept" layouts (half height partitions, etc.) today, I always do a vibration analysis for every office building we design. As stated previously, I have found some inconsistancy between the calcs and reality, but I try to muddle my way thru it. With large open floors and long spans, I often add an inch of extra concrete to help dampen the floor from transient vibration.

Hospitals with micro surgery and special production facilities require more of an expert.

RE: Question on Vibrations

DaveAtkins:

I concur. Precast planks, hollow cores, Double Tees and CIP concrete floor systems generally are not succeptible to vibration problems.

Generally, I see most lab buildings, reasearch centers, etc with sensative equipment in them designed with CIP concrete for the exact reason that these floors have good vibration characteristics.

My statement above should have read ' We check floor vibration for every STEEL FRAMED building...'

RE: Question on Vibrations

My response was relative to steel framed buildings. I agree with Dave Atkins with one exception. I know of a building (not one of mine fortunately) that had a vibration problem with hollowcore plank but they were using it as a dance floor. I assume it was not ballroom dancing!

RE: Question on Vibrations

Jike,

Good point, Dance floors, stadiums and gymnasiums need special consideration above and beyond normal vibration checks.

csd

RE: Question on Vibrations

I use FloorVibe for simple floors framed with steel.  I use STAAD-pro for more complex configurations.

RE: Question on Vibrations

archeng59, how do you use STAAD-pro for this?  Just to get frequencies?

RE: Question on Vibrations

yes, the natural frequency of the system.  

RE: Question on Vibrations

(OP)
Thank you all for the responses.  271828, I know that you have extensive knowledge in this area.  Is it a correct statement, generally speaking, to say that for a given span, the more a beam deflects, the smaller its frequency will be?
Is it correct to state that, generally speaking, in order to stiffen the floor system it is best to stiffen (and raise the frequency(ies) of the least stiff members (and hence member(s) with the lowest frequency(ies))?

RE: Question on Vibrations

Q1: Yes, in fact if you apply the tributary mass to the beam as a weight per length and calc the deflection, you can get the frequency from that.  In US Customary Units, if you calc this deflection in in., the natural frequency is fn=0.18*sqrt(g/Delta) where g=386 in/sec.^2  Fundamentally, the natural frequency is proportional to sqrt(stiffness/mass) or more brief: fn~sqrt(k/m).

Q2: Yes.  Using DG11 methods, you'll calc the beam fn and the girder fn.  If the beam fn is lower, increase the beam until the beam and girder fn are similar.  If you need the frequency to be even higher, then increase them together after that.  "Least stiff" is not as precise as "lower frequency" because a beam can be very stiff, but still have a high mass--remember fn~sqrt(k/m).

RE: Question on Vibrations

(OP)
Excellent!! thanks.
I have a situation in which there are W40x264 spanning 75 feet with W14 spanning 22' framing into them.  The vibrations are really posing a problem and I can't increase the frequency of the girders enough to get it under control. Any suggestions you might have from previous experience would be greatly appreciated.

RE: Question on Vibrations

Several ideas, probably none of them desirable:

Make the girders even bigger, add columns, reframe to get the girders closer together, place a "column" from the center of the girder up to the next level to make them work together.  Try castra.. (oops) castellated beams to get more depth and still have room to install ductwork and pipes.

Put the girders the other way, so that you have 75 ft beams that you can make closer together.  That's probably my best idea.  For example, you might end up with W30x90 girders (mostly for depth--connections) and W36x150 or W40 beams at 4-5' apart.

RE: Question on Vibrations

Try adding mass (extra concrete).

RE: Question on Vibrations

jike's idea might help.  Adding mass is helpful because a=F/m, so will decrease acceleration at for force input at a given frequency.  However, it will be counterproductive if you're having trouble with your natural frequency being too low (likely in this case) because remember fn~sqrt(k/m).  It's worth a try to both increae and decrease mass using thicker or thinner slab, lightweight or normal weight concrete, etc.

RE: Question on Vibrations

Creating continuity and fixity at the ends can help, but it is much harder to analyse.

csd

RE: Question on Vibrations

(OP)
Alright, I definitely can't start adding concrete because the W40 girder is working VERY hard.  Unfortunately, I can't add columns either, because this is an area that MUST be column free below for a performance area.  
In reading the design guide. it says to only account for continuity if the girder is physically continiuous over the columns, not for moment connections.  Does this apply for calculating the girder panel weight only or also for the deflection calcs?  We are providing full moment connections, but Floorvibe takes the deflection as a simple span deflection regardless of what end conditions you specify in RAM.  If I can count on the moment connections for deflection, this well definitely help to raise the frequency by a pretty healthy margin since the fixed end deflection is 1/5 that of a simple span deflection.

RE: Question on Vibrations

Be careful with the continuity stuff.  I assume that you don't have a fixed-fixed girder, but have the potential of making the girder continuous with other girders.  Believe it or not (nobody does at first) a two-span (or 3,4,...) equal-span, equal-stiffness, equal-load continuous beam has the same frequency as the simply supported one.  Yeah, I know--nobody believes it, but it's true, LOL.

With your last paragraph you're getting into the area of FE modeling of your floor instead of using FloorVibe.  

We're typing about typical walking loads and typical occupancies, right?

How 'bout the idea of having a column to the next level above?  Seems like that idea has some potential to me.

RE: Question on Vibrations

Make it into a frame so that the columns take out some of the deflection?

RE: Question on Vibrations

Is there some reason you are running your girder for the long spandirection, and the beams in the short span direction?

For 'normal' situations, it is usually better from an economic and vibrational point of view to run the beams in the long span direction and the girders in the short span direction.

Possiblly there may be some reason that precludes this that is not apearent to those of us who cannot see the exact condition.

Regards.

RE: Question on Vibrations

I agree that the beams should run in the long direction.  I was able to make a floor work for vibration once with long joists, short girders, and a lot of concrete over the deck.

DaveAtkins

RE: Question on Vibrations

(OP)
This isn't a simple, rectangular building.  There are very large cantilevers everywhere.  This is the framing scheme we needed to get it to work.  We did try many other schemes.

RE: Question on Vibrations

You might consider fitting tuned vibration absorbers, if you have really run out of other options. However it will almost certainly be more satisfactory, and probably cheaper to improve the structure.

Cheers

Greg Locock

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

RE: Question on Vibrations

Is there a reason you are restricting the depth of girders to 40"?  Sounds like you need more depth, possibly use trusses or 27128's castellated beams.

RE: Question on Vibrations

Greg, we try those from time to time with some hit-and-miss success.  Do you have a specific type in mind?

The downside is that the mass of the structure changes over time (partitions move, more/less furniture, more/less storage, etc.), so they get out of tune.  The tuning process ends up needing repeated down the road.

RE: Question on Vibrations

We use them all over cars. I agree they are a bit hit and miss. Usual mistakes are - too /much/ damping, not enough mass, not mounted in the right place.

Cheers

Greg Locock

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

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