Steel Floor Vibrations - General Discussion

[DEL2000]

I have the latest RamSteel with the floor vibration software, the AISC Design guide 11, plus several articles written by Thomas Murray (co-author of Design Guide 11).

With all of this information at my hands, I still question whether I am using the design recommendations correctly, at least for the types of buildings that I am currently working on. Everything that I am confused about, luckily, can be discussed without having to get too involved in the equations/calculations.

First and foremost, he almost always recommends a Beta modal damping factor of 2 to 2.5% for schools, 2% for paperless offices, 3% for modern offices, and 5% for offices of 30 years ago. What I have found in looking at floors, simply changing my "assumed" Beta by a very small amount has a huge effect on the calculated result.

Confusion # 1 - What confuses me the most is that the buildings that I typically do do not exactly fall into the categories that are described in the design guide. This vagueness coupled with the very large effect of changing B by a very small amount just doesn't give me very much confidence in using the design guide. The examples given in the design guide don't help much, because they are too idealized in comparison to the buildings that I do (see confusion #2 below).

Example 1 - Office building with three main girder lines (basically only 2 bays deep), 2 of these girder lines have 10' tall brick veneer cladding, while the center girder line has joists on each side. Simply from the presence of the cladding on the exterior girders enough to eliminate vibration on this building from ever being a concern (ie, B= 5%)? Or can the interior girder with the joists still control.

Example 2 - The office building of example 1, but say directly above the center girder we have a full height, fire rated corridor/area separation wall. Is this one wall enough to jump us to a B=5%, vibration not an issue for the Center girder?

Example 3 - School, with all of the fire-rated, full height area separation walls (corridor walls, walls between classrooms), basically having at least one wall per bay. Is this enough to jump up the modal damping number to 5%. How many full height partition walls does it take to go from 2-2.5% modal damping (which Murray specifically recommends for schools), to 5% modal damping. It seems like most schools would have all sorts of full height partitions (like all around every classroom).

Confusion number 2 - the examples in the design guide . . . as I said before, I thought they were too idealized to be of much use, except to show the simple mechanics of the calculations. They give you a bay in the middle of some building which might be 10 bays in each direction of uniform framing. And then they say that the entire floor has no partitions, and is fully open (B=2%). If you envision this examples . . . this paperless office would be a football field size area of partial height partitions and computers (and I am sure that the design guide works splendidly in this condition). The buildings I typically am doing are much smaller work areas of 30'x50' areas between walls.


So what do I want to know? I just need to know that I am applying the provisions in design guide 11 correctly, and if there are any shortcuts (or layout tricks) which would allow you to rule out (or eliminate) vibration alltogether. My "confusion #1" is the only thing that I am seeking input on. "Confusion #2" is basically just added for basic commentary.

 

[WillisV]

I will try to address some of these:

Confusion No. 1 - Example No. 1:

First of all vibration is not simple a "girder" or a "beam" problem it is a "full bay" problem. It is the combination of the vibrational characteristics of all these elements that can cause a floor to be too lively. So the question of if the exterior girder being dampened by the facade can eleminate concern versus the interior girder controlling is really not valid - in your case you simply have 2 bays each consisting of the one interior girder, one exterior girder, and joist running between.

Directly addressing the exterior girder concern, in general the exterior girder only supports 1/2 of the mass of an interior girder and is therefore more succeptible to vibrations. See the discussion on page 20 of the design guide "interior floor edges" - which states that for interior edges special considerations should be given to the edge beam, but for exterior girders, the lack of mass is typically canceled out by the addition of facade and therefore they are treated like any other beam.

So in this case you would run an analysis of one of the bays which an edge beam an interior beam and the joists. Ram's Floor Vibe software (originally written by Tom Murray) handles theis condition quite efficiently.

Confusion No. 1 - Example No. 2

If you have full height partitions running along the girders it would certainly help the girders, but offer no additional help at all to the joists. I would use a 3% damping maximum for this condition. The only cases in which I would consider using more than 3% are:

1) Full height partitions running parallel to the length of the beams every 20-30' - 5% damping.

2) At least one full height partition running perpindicular to the beam spans near the center of the beam span or two at approximately third points of the span - Vibration will not be a concern.

See the April 2004 issue of Modern Steel Construction for teh article titled "Office Fit-Out and Floor Vibrations" by Murray for additional information on damping ratio assumptions.

Confusion No 1. Example 3.

Use same recommendations as above.