Steel Floor Vibration

[bookowski]

Here is the situation/structural system: This is a steel framed single family residential home. Beam spans are 35ft, slab is 1.5" composite deck w/2.5" NW topping (4" total assembly). Due to arch. demands the beams have been pushed down to heavy W14's that are cambered and are composite with the slab.

This layout does not work for recommended vibration criteria per AISC Design Guide. My thought however is that in a single family residence it would be very hard to excite that mass and have a vibration issue.... although I'm just saying that based on gut feel, the calcs seem to disagree. We are now being asked if this will in fact be a problem and my guess is that it would be ok but I don't have complete confidence in the answer. Any thoughts? It's just hard for me to imagine one guy walking through the house and causing that huge slab to move enough to impact another person in the home.

 

[spats]

You miss the point of floor vibration. The frequency and amplitude of the vibration dictates whether or not a vibration is barely perceptible or distinctly perceptible, but just someone walking across a floor can make others that are in the area feel uncomfortable. If you have never experienced it, you must live in a cave. Believe me, it's not hard to imagine!

 

[JoshPlumSE]

Just curious, is this a frequency issue or a amplitude issue?

I had a discussion with Dr. Murray a few years ago about Floor Vibration for sturctures with non-orthogonal framing. He told me that he doesn't see Floor Vibration problems for these types of floor layouts. Since so many of these custom homes tend to have odd / skewed beam layouts, this may apply in your situation. The idea is that if you're walking across 4 bays of framing where each bay has a similar frequency then that can lead to vibration problems. But, if you're walking across 3 or 4 bays where every bay has different framing angles and a different period for the panel, then vibrations won't normally be an issue.

Note: AISC DG-11 does have an example (see fig 7-2) which sort of contradicts what was said during that discussion. But, I think the key is that the issue with the skewed beams in Figure 7-2 was that it was an issue primarily with the beam's frequency.

 

[bookowski]

spats - thanks for your useless reply. Clearly I am posting because I acknowledge that I don't have the answer and am requesting input/help. Your valuable input that I don't understand and must live in a cave is extremely useful. I have some advice back to you in return, but I think my post would be removed if I state it here.

JoshP - I have an a/g of about 0.9% compared to the recommended 0.5%. The beams are clearly not the depth that we want them to be at W14 but this was pushed down our throat. Now that we are entering construction everyone is bringing back the issue of vibration and asking 'how perceptible/disruptive will it be'. The framing is somewhat discontinuous as you said, and this was another reason I thought a problem was unlikely. I'm just wondering in general if anyone has actually experienced a vibration problem with a similar mass/stiffness with such a low exciting population - i.e. one guy and his wife walking around. Whenever I've experienced vibration it is when standing/sitting still and there is a decent population providing the forcing function - like a big box store, airport etc.

 

[steellion]

I wouldn't worry too much about vibration of a concrete-on-steel deck for a single-family home. 1) The amount of time that one person would be walking around that may be disruptive to another person would be minimal. 2) Single family home is likely to have several heavy pieces of furniture to dampen the vibration as compared to an open-office building. 3) The inertia of this steel framed floor has to be much more than the wood joist and plywood framing typical of this construction.

 

[JoshPlumSE]

A single guy walking around is the basis of a lot of DG-11. Isn't it? Take a look at Table 4.1. The constant force Po of 65 lbs is, I believe, intended to represent the heel drop force of a single person walking.

A true expert may be able to take a look at your frequencies and your irregular framing layout and say confidently that you won't have a problem. But, I am not as confident. I know that irregular layouts tend to reduce any problem that would otherwise exist. But, I can't say confidently that it will eliminate them.

The presence of furniture, equipment, partitions, et cetera may reduce the response as well. You might take a look at the recommended damping levels. I haven't looked at this, but you MIGHT be able to assume greater levels of damping for residential construction.

 

[spats]

bookowski,

I said what I said because you were kissing the whole thing off because it was "hard for you to imagine". How else was I supposed to take it. Uncomfortable vibrations are a common occurance. I designed a 45' floor bay in a building, and I never will again... even though it was designed per the vibration criteria at the time. Sorry, I was being snide because I thought you were being lazy.

 

[bookowski]

JoshP - I agree, the intent is from a single person walking.

It seems like there is no straightforward answer, I was hoping that someone had a similar experience with residential single family. There are notes in the design guide about application, they use the example of a footbridge that is very lightly used and that the same criteria should not be applied. I guess that's kind of where my line of thinking was going. In a house you generally walk briefly or you are leaving the area. I had thought of the forcing function as something that takes several cycles to start mobilizing the mass and get it going.

Anyway, no need to beat this to death. Will post an update if it turns out it's a trampoline.

 

[strguy11]

What damping ratio are you using? Do the partition walls extend from floor to ceiling? If you have alot of them and no huge open spaces you might be able to assume a higher damping ratio.

 

[271828]

My thought however is that in a single family residence it would be very hard to excite that mass and have a vibration issue...

No, one person walking at resonance is plenty to get a structure that size moving. I've caused and felt vibrations due to one person on structures that dwarf this floor. It's all about resonance.

Don't discard vibrations because it's a house. If it's a steel framed house, then I'd be willing to bet the owners will be expecting imperceptible vibrations. "If I'd a wanted to feel vibrations when Janet walks across the floor, I would've paid for a wood floor!" LOL I'd stick with the 0.5 %g limit. If DG11 Ch. 4 applies, has been used correctly, and is predicting 0.9 %g, then there is a VERY high likelihood of complaints. I'd increase the framing size, or change some other parameter until Eq. 4.1 is coming up with 0.5 %g. No question about it.

had a discussion with Dr. Murray a few years ago about Floor Vibration for sturctures with non-orthogonal framing.

I don't want to put words in Dr. Murray's mouth, but I think he's stating observations only. Highly suggest that you don't discount vibration on this basis.

The amount of time that one person would be walking around that may be disruptive to another person would be minimal.

I have no doubt that this is false. If Joe is sitting in his chair and Janet walks across the floor 10x over the course of a day and shakes him a few of those... If anything, I think a residence is probably a more severe case than an office. At least in an office, you might get lucky and your desk in not near the middle of a bay, or people are actually working most of the time and not walking around LOL.

Single family home is likely to have several heavy pieces of furniture to dampen the vibration as compared to an open-office building.

Maybe, maybe not. Damping is mostly dependent on partitions. If this house has full height partitions jamming up against a floor above, then that's very, very helpful. If it has walls supporting only ceiling framing, then I doubt they'd do a lot. I've never quite understood why furniture would do anything to help damping. Damping is from things rubbing together, etc. A big chair or shelf just goes for the ride. A partition, ceiling, etc. has to slip and slide to let the floor move.

A single guy walking around is the basis of a lot of DG-11. Isn't it?

That's correct. One person walking at a frequency that will cause resonance.

Take a look at Table 4.1. The constant force Po of 65 lbs is, I believe, intended to represent the heel drop force of a single person walking.

Not true. See the 1993 AISC ENgineering Journal article by Allen and Murray, which is the basis of DG11 Ch. 4. Also, Murray derived Eq. 4.1 at this past year's NASCC. That presentation is in AISC's video proceedings. 65 lb is a "dummy" number that has three or four things wrapped into it and really means nothing on its own. Murray shows it in the presentation.

 

[rowingengineer]

What analysis method are u using? Sometimes the hand calcs can be conservative.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[PeterDebney]

The floor is big for a house but is reasonably small compared to an office, so I am not too surprised that there is the potential for a vibration problem. Domestic loads are also lighter than for offices and mass is the main resistance to human induced vibrations.

What is the floor’s fundamental frequency? If it gets much above 10 Hz then damping will not help you. Not that the old AISC guide considers transient response.

Is there any continuity that you can take into account? In my experience irregular and isolated floors have the worst vibration problems because they have the least modal mass.

I did a rough analysis model of your floor (with lots of guesses of course for beam centres, sizes, etc) and depending on the size of the W14 got accelerations ranging from 2 %g at the lightest end to 0.1 %g at the heaviest. Quite a range, but then not as wide as the stiffness range for W14s. This suggests that you should be able to get the floor to work with more detailed calculation.

PS I wrote a paper on the subject, which you can find here:

http://www.oasys-software.com/media/press/GSA/Sound&Vibration0911.pdf.

You might find it interesting.