Question on Vibrations 3

[Lion06]

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.

 

[271828]

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.

 

[csd72]

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

 

[lkjh345]

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.

 

[DaveAtkins]

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

 

[Lion06]

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.

 

[GregLocock]

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 for tips on how to make the best use of Eng-Tips.

 

[hokie66]

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.

 

[271828]

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.

 

[GregLocock]

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 for tips on how to make the best use of Eng-Tips.