Floor Vibration - Design Guide 11

[ivorm]

I'm wondering if anyone could provide some help on a question I have regarding floor vibration and Design Guide 11. I have a steel framed floor, all beams and girders, the beams are all 33 feet long but I have different length girders depending on where I am in the building. Some girders are 33 feet long, while others are 16.5 feet long. When I design the floor using the long span girder to target V1/3 of say 2000mips I get a beam size of W24x68 and a heavy girder at W33x201. If I then try to use that same beam (W24x68) on the bay with the shorter girder (say a W24x55 girder for geometry reasons), I end up with a a higher vibration because of the effective panel weight of the girder is lower and leads to a lower combined effective panel weight. There is a little bit of info about unequal beams and joists at the top of page 26 but I think that's aimed more at girders within the same bay where the joists they support have different spans.

I don't see a way around this problem other than upsizing the beams in the short girder bay to increase the fundamental frequency of the beams and lower the vibration or increasing the short girder size significantly. These solutions will obviously look pretty strange on the plans since the beams are all the same span and have the same tributary width, someone is also going to question a short heavy girder.

Am I missing something here regarding this calculation?

The other confusing thing is the note on page 26 regarding "floor width and floor length". It says "Floor width is the distance perpendicular to the span of the joists over which the structural framing (beam or joist and girder size, spacing, length, etc.) is identical or nearly identical within adjacent bays". How are we supposed to deal with situation like this one where we have very different girder spans?

 

[JoshPlumSE]

I think you're interpretation is basically correct. It can be a little unintuitive. Sometimes larger beams or shorter spans can produce worse vibrations.

One alternative proposition is that you could use the FEM solution method if you feel your situation is a little different from what the conventional DG-11 calculations were calibrated to work for. The FEM solution is only in the 2nd edition of DG-11, of course. I don't even remember the chapter. But, they've got a good example in there to help figure out how to do it.

Not saying I think this is a good idea. It is actually a lot of work. However, it does work pretty well when you have something outside the normal situations where DG-11 applies.

Caveat: I'm mentioning the FEM method partly because the company I work for (Computers and Structures Inc, which produces the SAP2000, ETABS and SAFE software) added a way to automate a good portion of this FEM procedure and I spent a good amount of time testing and documenting the feature.

 

[ivorm]

I think you're interpretation is basically correct. It can be a little unintuitive. Sometimes larger beams or shorter spans can produce worse vibrations.

One alternative proposition is that you could use the FEM solution method if you feel your situation is a little different from what the conventional DG-11 calculations were calibrated to work for. The FEM solution is only in the 2nd edition of DG-11, of course. I don't even remember the chapter. But, they've got a good example in there to help figure out how to do it.

Not saying I think this is a good idea. It is actually a lot of work. However, it does work pretty well when you have something outside the normal situations where DG-11 applies.

Caveat: I'm mentioning the FEM method partly because the company I work for (Computers and Structures Inc, which produces the SAP2000, ETABS and SAFE software) added a way to automate a good portion of this FEM procedure and I spent a good amount of time testing and documenting the feature.

Thanks for the reply Josh. I was looking at chapter 7 of DG11 previously to see how much work it would be to model the floor. We have a few ETABS licenses, I was excited when I saw they added the new DG11 analysis, but unfortunately I think it uses steady state cases which requires an ETABS Ultimate license. I haven't looked into if it's possible to analyze the floor using ETABS Plus, maybe it's just less automated and more time consuming?

 

[RWW0002]

Just FYI - Some tools for vibration analysis:

Floorvibe has been a very affordable program to look at vibrations per Design Guide 11

There are also some tools through AISC and SJI that can be used for vibration checks.

Floor/Diaphragm Systems

www.steeltools.org

Floor Vibration Analysis - Steel Joist Institute

The Steel Joist Institute has partnered with Dr. Thomas H. Murray, Professor Emeritus at Virginia Polytechnic Institute and State University to provide a vibration analysis program that combines years of experience in vibration testing with the latest research in the field. The latest vibration...

steeljoist.org

 

[JoshPlumSE]

Yes, it uses steady state analysis to do the DG-11 calcs. I am not 100% sure (I don't do any sales), but I think the Plus version includes this feature. I can check with someone if you want more clarity.

FWIW, it takes a while to really understand how to model your floor to get good results. I had to go over Brad Davis' example multiple times and multiple ways before I was confident in how I was modeling.

One thing that's interesting is I then tried to reproduce the results of one of the other DG-11 examples. One which was done with the traditional calculation methods. And, it came out pretty close. It don't remember it being exactly the same. But, I remember it was pretty close. Anyway, after that I felt really good about the FEM method and my ability to model and get accurate results.