School Floor Vibration Values
School Floor Vibration Values
(OP)
I wanted to do a survey of those of you that have designed a school composite steel framed floor for vibration and see what values you are using for the Design Guide 11 method.
Which Beta, Dead Load, Live Load, and Damping Ratio values are you using and why?
From my own estimate, I would use Beta= 0.50%g, Dead Load = 4psf, Live Load = 4 psf, Damping = 0.20 for churches, but this is killing my weights.
Thoughts?
Which Beta, Dead Load, Live Load, and Damping Ratio values are you using and why?
From my own estimate, I would use Beta= 0.50%g, Dead Load = 4psf, Live Load = 4 psf, Damping = 0.20 for churches, but this is killing my weights.
Thoughts?






RE: School Floor Vibration Values
I ran into a vibration problem too. I ended up using 3" deck with 4 1/2" normal weight concrete topping (7 1/2" total). The last 1" of concrete was becasue of the vibrations.
RE: School Floor Vibration Values
RE: School Floor Vibration Values
Now that I have that off my chest, LOL:
0.5%g is ok.
DL = 4 psf is probably OK. The bottom line is that you need your absolute best estimate of the mass.
LL = 4 psf seems low. DG11 recommends 8 psf for an electronic office which contains almost nothing. Might be OK--might need increased to 8 psf depending on what you know is there.
Damping = 0.020 (definitely not 0.2!) is probably OK assuming you don't have full-height partitions. Might be a little on the low side. In reality, nobody anywhere, period, can tell you that 0.02 is right and 0.03 is wrong. The data doesn't exist.
RE: School Floor Vibration Values
RE: School Floor Vibration Values
I have found sometimes that adding mass makes vibrations worse. Has anyone else seen this?
It is true that a=F/m, but the a/g equation given in DG#11 has the natural frequency in it. I have found that sometimes the natural frequency is a bigger player and can trump the mass, such that adding mass makes things worse.
I would be interested in hearing if anyone else has ever come across this.
RE: School Floor Vibration Values
Mike McCann
MMC Engineering
RE: School Floor Vibration Values
The natural frequency ends up in that equation by the following:
1. ap/g is a modified steady-state response to sinusoidal force. If you have Chopra, see the section for "Response to Harmonic and Periodic Excitations" with omega = omegan -- 3.2.2 in my 1995 model. If you take the steady-state maximum amplitude and multiply it by omega^2 to get to acceleration and then cancel out some things, you end up with a = po / (2*zeta*m) which looks suspiciously like DG11 Eq. 2.3. The po and 2 get absorbed along with reduction factors into the numerator of Eq. 2.3.
2. The sinusoidal force amplitude is the amplitude of whatever footstep force harmonic matches fn, often the 3rd or 4th harmonic. See Eq. 2.1.
3. The sinusoidal force is less for higher harmonics than for lower ones. This decrease in the sinusoidal force results in Fig. 2.2 and is where the e^(-0.35fn) comes from. Note that higher harmonics are required to match higher fn, so that's how fn gets in there--we're matching h*fstep with fn. h*fstep is what really needs to be in the e^(), but fn is there because we're matching them and it's more convenient (and more confusing).
Po*e^(-0.35fn) is the harmonic force combined with some fudge factors. See the paragraphs above Eq. 2.3. Therefore, if you have a higher fn, then the input force is lower, so it makes sense that the response is lower. Because fn is inversely proportional to the sqrt of mass, one might jump to the conclusion that lower mass is better, but this is not always the case. If the mass is low enough, then the impulsive response from individual footsteps causes high accelerations like everyone's felt on a cheap wood floor at some point.