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Vibration of WF Beam Girders Supporting Steel Joists for Banquet Hall

mfstructural

mfstructural

Structural
Joined
Feb 1, 2009
Messages
232
Location
US
All,
I'm working on a project where the client purchased a building without consulting a structural engineer regarding the floor system. The building is two stories, no basement. There are multiple column lines spaced 29' in one direction, with HSS columns spaced at 25' and 40' in the other direction. the 40' span is in the middle of the building. The steel joists are 18k4 @ 2' o.c. spanning in the 29' directions supported by W24x62 girders. The second story of the building was originally designed for a LL of 75 psf per the designed drawings from 1999. The owner wants to convert the building from store/office use to a banquet hall, which is 100 psf LL. The joists showed that the top chord, bottom chord, and 3 web members at the ends required reinforcing for strength, which is pretty straightforward. The issue then becomes when the floor is evaluated for vibration from dancing. I reviewed AISC Design Guide 11 and used the floorvibe software to evaluate the floor. The software output states that the existing system doesn't work which makes sense based on LL also increasing.

1752075923749.png

Assuming the WF girders and joists are reinforced approximating the section properties below, the system still does not work for vibration.

1752076066145.png

I wanted to see if anyone else has come across something like this. I understand the criterion, but from a judgement perspective I am trying to understand if performing all the reinforcing will work and not yield excessive vibration. The acceleration is worse with less LL, presumably because there is less load "weighing" down the joists and girders.

Will a W24x62 really vibrate to that extent under load?

If I reduce the girder span to 13' (place a column underneath) it works but that is not an option per the client.

Thanks,
 
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Maybe the owner would be willing to let it go until they actually get a complaint.
 
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I thought about that but what if they get complaints after finishing the entire banquet hall spending a couple hundred grand and then have to go back and reinforce. I am trying to avoid that situation. From a strength perspective we can reinforce, but from a vibration it's hard to quantify how the vibration will feel. According to the AISC Design Guide 11 criteria it fails but what does that actually translate to in terms of real world perceptible vibration?
 
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Natural frequency is overwhelmingly dominant for rhythmic groups. 5.19 Hz isn't very high. The second harmonic of the force is up to 5.4 Hz according to Design Guide 11 Table 5-2. If you can get the natural frequency a bit above 5.4 Hz, you'll have a decent shot.

You could try to make the girders fully composite as shown in Design Guide 11 Figure 8-1(d).

And, by all means, don't hope for the best and wait to see if there's a complaint. If vibe is an issue, it could be a nightmare to fix in service.
 
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mfstructural said:
I thought about that but what if they get complaints after finishing the entire banquet hall spending a couple hundred grand and then have to go back and reinforce. I am trying to avoid that situation. From a strength perspective we can reinforce, but from a vibration it's hard to quantify how the vibration will feel. According to the AISC Design Guide 11 criteria it fails but what does that actually translate to in terms of real world perceptible vibration?
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I don't think there's any way to answer your last sentence, other than to guess. You might get lucky and then again you might not. I'd make sure my floor is satisfies the check. The Chapter 5 rhythmic groups method has been around a long time. The industry has about 30 years of experience with it.
 
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We're facing a similar situation. When the client saw the cost and complexity of reinforcing and damping the floor to mitigate potential vibration or acoustic issues, they chose to take the risk and adopt a wait and see approach. The floor hasn't been built yet, so the outcome remains to be seen, I’m keen to see how it performs.

Make sure they fully understand that the floor may not be suitable for dancing.
 
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Tomfh said:
the floor may not be suitable for dancing
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What does that mean though? It turns into the Tacoma Narrows bridge or a couple of Brads or Karens get a bit freaked out about the movement?
I imagine this is only an issue for very specific dance situations (maybe Raves)?
 
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I will divide the question inte two parts:

First you have the load, you have a rhythmic activity (dancing) on a floor not designed for that. In my experience, that is a nasty load, so excessive vibrations are to be expected. But still, the acceleration in the first calculation is huge (120 % of gravity). Does the floor meet criteria for walking, since that is the intended use, correct?

And then you have the criteria, 2 % of g. When I look in my copy of DG 11, I see that number for "Dining and Dancing". But I would say that both "Dining" and "Dancing" can be very different things. The people dancing will probably not disturb either themselves nor others dancing at that vibration level. But people dining close the dance floor may feel the vibrations. Based on my experience I would say that 2% of g, or 0.2 m/s^2 is on the high side.

But the problem is that you don't have 2% of g, the second calculation results in ~12% of g, and that is more than twice the recommended value for an outdoor footbridge. So you currently don't meet any criteria. Does the client understand that there may be a problem? The building may not be suitable for the intended use.

I am not comfortable with USC Units so I skip any comments on the calculations. But I would start with checking the floor for walking, does it work for the intended use? And if it does, why do you get such high vibration levels for dancing? And if it doesn't, what does that mean?

If you can't find anything strange in the calculations I would do some tests on the floor. Test the vibration levels for known excitations.
 
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XR250 said:
What does that mean though? It turns into the Tacoma Narrows bridge or a couple of Brads or Karens get a bit freaked out about the movement?
I imagine this is only an issue for very specific dance situations (maybe Raves)?
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Went out to a club at the University of Virginia during an ASCE conference/competition while I was in school. Wasn't exactly a rave, but it was a second floor room with wood joists from the mid 1800s spanning a good 20 feet. That floor was moving up and down at least 6 inches.
 
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I have worked on two collapsed wood framed floors - both at Frat Houses.
 
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ThomasH said:
I will divide the question inte two parts:

First you have the load, you have a rhythmic activity (dancing) on a floor not designed for that. In my experience, that is a nasty load, so excessive vibrations are to be expected. But still, the acceleration in the first calculation is huge (120 % of gravity). Does the floor meet criteria for walking, since that is the intended use, correct?

And then you have the criteria, 2 % of g. When I look in my copy of DG 11, I see that number for "Dining and Dancing". But I would say that both "Dining" and "Dancing" can be very different things. The people dancing will probably not disturb either themselves nor others dancing at that vibration level. But people dining close the dance floor may feel the vibrations. Based on my experience I would say that 2% of g, or 0.2 m/s^2 is on the high side.

But the problem is that you don't have 2% of g, the second calculation results in ~12% of g, and that is more than twice the recommended value for an outdoor footbridge. So you currently don't meet any criteria. Does the client understand that there may be a problem? The building may not be suitable for the intended use.

I am not comfortable with USC Units so I skip any comments on the calculations. But I would start with checking the floor for walking, does it work for the intended use? And if it does, why do you get such high vibration levels for dancing? And if it doesn't, what does that mean?

If you can't find anything strange in the calculations I would do some tests on the floor. Test the vibration levels for known excitations.
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The recommended limit if the floor only supports dancers would be 4-7%g. They will tolerate pretty high accelerations.

Seated people tolerate much less. Thus, if diners are a possibility, the limit is 1.5-2.5%g.

If Design Guide 11 is saying 118%g, then the floor would probably have severe problems. I worked on a project years ago that wasn't nearly that bad, but they still almost closed the facility after a Friday night event that almost caused a panic. The movements were so extreme that air pressure changes were opening and closing double doors and making noises as the ceiling tiles were being pushed up and down. It was a nightmare for the EOR.

At 12.7%g, the chances of a problem are pretty high, but one might get lucky. LOL
 
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phamENG said:
Went out to a club at the University of Virginia during an ASCE conference/competition while I was in school. Wasn't exactly a rave, but it was a second floor room with wood joists from the mid 1800s spanning a good 20 feet. That floor was moving up and down at least 6 inches.
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Surely you're using hyperbole!

50%g at 5 Hz equates to the following displacement amplitude: (0.5 * 386) / (2*pi*5)^2 = 0.2 in.
 
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271828 said:
Surely you're using hyperbole!
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No. Not in the least. I was pretty terrified. Everyone else was having a great time (and also probably drunk). Everyone was dancing in unison and I could barely walk across the floor. I got to side of the room where the joists were bearing and walked along it to the door and got out of there.
 
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Yeah, I get that the motion was extreme, but 6 in.?

50%g at 5 Hz would be 0.2 in.
 
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More likely 0.5 to 1 Hz, which is closer to the 6" Pham said. Dancing at 5 Hz would be pretty exhausting
 
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The floor will vibrate mostly at its natural frequency. 5 Hz is a fairly low value for that.

The lowest (not quite) believable natural frequency is 2 Hz. The displacement amplitude corresponding to 50%g would be (0.5*386 in./s^2) / (2 * pi * 2 Hz)^2 = 1.2 in.

Let's come at this from the opposite direction. If the floor had a displacement amplitude of 6 in., then its acceleration would be the following for a few different frequencies:

(6 in.)*(2 * pi * 2 Hz)^2 * (1g / 386 in./s^2) = 2.5g
5 Hz: 15g
8 Hz: 39g

I'm almost suspecting you guys are messing with me. LOL
 
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