mrsutton
Structural
- Jun 24, 2015
- 19
Hi, I have an increasingly common issue I'm at an analytical loss on how to deal with. I'm trying to develop quick, ballpark vibration values for slabs on grade for equipment.
I work in Silicon Valley where there is a lot of extremely sensitive equipment being installed in labs. I'm increasingly asked if the existing slabs on grade will meet various vibration criteria (VC-D, VC-E, etc etc) which are typically given in values of peak velocity/per frequency (developed by NIST back in 90's). I've done some online snooping and found some papers and journal articles discussing the general problem but none that give me a relatively straightforward analytical procedure to determine if a given slab on grade should meet a given vibration criterion. I have been able to determine slab on grade stiffness using both RISA and with some theoretical formulae found online--so that's not an issue.
I also find an academic paper which gave the formulae for determining response to cyclical, footfall loading for slabs on grade but stops short in discussing the analytical leap from time domain response to frequency domain response curves. I suspect I need to run a Fast Fourier Transform--but I can't recall how to do this without attempting to learn/relearn structural dynamics. Also in that particular paper's formula I'm having a tough time determining if they are considering a damped or undamped system--the functions I've plotted don't appear to decay at all but its tough to determine with cyclical excitation. I'm not looking for definitive solutions--especially given the uncertainty around the inputs like subgrade modulus, footfall pulse load and frequency, true concrete elastic modulus--but something where I can say the slab should be fine...or might warrant a more specialized vibration firm going out and doing on-site testing.
I figure some other engineer must run into this issue. Any guidance or resources would be much appreciated.
I work in Silicon Valley where there is a lot of extremely sensitive equipment being installed in labs. I'm increasingly asked if the existing slabs on grade will meet various vibration criteria (VC-D, VC-E, etc etc) which are typically given in values of peak velocity/per frequency (developed by NIST back in 90's). I've done some online snooping and found some papers and journal articles discussing the general problem but none that give me a relatively straightforward analytical procedure to determine if a given slab on grade should meet a given vibration criterion. I have been able to determine slab on grade stiffness using both RISA and with some theoretical formulae found online--so that's not an issue.
I also find an academic paper which gave the formulae for determining response to cyclical, footfall loading for slabs on grade but stops short in discussing the analytical leap from time domain response to frequency domain response curves. I suspect I need to run a Fast Fourier Transform--but I can't recall how to do this without attempting to learn/relearn structural dynamics. Also in that particular paper's formula I'm having a tough time determining if they are considering a damped or undamped system--the functions I've plotted don't appear to decay at all but its tough to determine with cyclical excitation. I'm not looking for definitive solutions--especially given the uncertainty around the inputs like subgrade modulus, footfall pulse load and frequency, true concrete elastic modulus--but something where I can say the slab should be fine...or might warrant a more specialized vibration firm going out and doing on-site testing.
I figure some other engineer must run into this issue. Any guidance or resources would be much appreciated.
