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Natural Science Forum / Physics / Acoustics / September 2005Building design against Vibrations
In recent years, the installation of vibration sensitive equipment in multi-story buildings has become commonplace. We all know that placing such equipment on the ground floor, on terra firma, is the best choice, But that is not the way that humans act and think. As a result many vibration sensitive devices (e.g. MRI's) and instruments (e.g. confocal microscopes) are routinely installed and operated on the upper floors of buildings. The trend is anything is increasing; more are wanted and built despite our warnings that it is not the best technical practice. The issue is that there is no such thing as a rigid building. All beams, panels and floor sections have bending modes and resonances. Typical house wood floor joists have their first resonant mode at around 6 Hz. A reinforced concrete floor slab may have a similar resonance anywhere from 4 Hz ("long-span" between columns) to 20 Hz (old poured concrete and brick and tile structures circa 1900). As exciters, during working hours people walk around (2 Hz footstep rate) and slam doors (wide band delta function). So what do we advise our clients (architects, structural engineers, owners) to do to "get their satisfaction" after building occupation? Discipline in door slams and elevator behavior can be controlled, but the last item, footsteps seems nigh impossible to eliminate. Coupled with the fact that practical vibration isolation tables, for microscopes, etc., work at high frequencies and down to about 5 Hz, loose their effectiveness and can actually amplify 2 Hz vibrations. Few of the users of such equipment will allow to be placed on the ground floor. There is only a very limited space there in most buildings. To build a first, second and third floor deck of stiffness that will not allow measurable floor deflections below about 20 Hz is a structural challenge to say the least. I estimated in one case that an appropriate floor stiffness would be that it shall not have more than 1/64th of an inch deflection at its center, under design load. All this to provide a first panel resonance between columns at or above 20 Hz. I believe that architects and structural engineers (and owners that must pay the bill) will shudder at the thought of designing, building and paying for such a building. I wonder what the experience of this astute audience has been in this arena, and I welcome any constructive comments as to what to advise. Angelo Campanella > In recent years, the installation of vibration sensitive equipment in >multi-story buildings has become commonplace. We all know that placing [quoted text clipped - 39 lines] > > Angelo Campanella I work in buildings for medical research. I carry a device for measuring electrostatic or magnetic interference. I always wanted to add a microphone to it to listen to building vibration, as well as a photodiode to detect light noise. I would also like a sound display showing the modes of vibration or how they look exactly in 3D. The buildings vibrate at 60 Hz, and other freqs. I'm listening to the ground floor of a 12 story old sturdy building. There isn't much LF but still has upper frequiencies present. Newer buildings have less weight and things are really bad. They are also more electrically noisey. At least we influenced having our new buildings using aluminized drywall to help reduce RF noise. Most Lab tables probably have good damping at a couple Hz, but I might look into that. Some people put slabs on inner tubes, not too great, allthough with the incorporation of added air lines, damps out the shakes. Its an enormous job to keep things in order. Another thing, these places are not stable, everybody and labs get moved about every so often. Pushing for ground floor space is not much of an option in these types of institutions. greg >I work in buildings for medical research. I carry a device for measuring >electrostatic or magnetic interference. I always wanted to add a microphone >to it to listen to building vibration, as well as a photodiode to detect light I guess I might expand my all purpose detector, as having electostatic, magnetic, light, vibration, and low frequency audio, or ventillation pressure changes. I'll leave radiation out, but would be nice! greg >>I work in buildings for medical research. I carry a device for measuring >>electrostatic or magnetic interference. I always wanted to add a microphone >>to it to listen to building vibration, as well as a photodiode to detect light > I guess I might expand my all purpose detector, as having electostatic, magnetic, > light, vibration, and low frequency audio, or ventillation pressure changes. I'll > leave radiation out, but would be nice! You response is illuminating. I find a similar posture in talking to the building owner. Having E and H detectors is nice for evaluating spaces where open electrodes are used to ply living matter. The effects of sound is a more perplexing, as it is often confused with "vibration". The normal concern is about floor vibration since the effects the apparatus supported by said floor. There is indeed an interaction with airborne low frequency sound, since flat surfaces of experimental apparatus can be pushed about by such sound waves. I often must distinguish the two in contemplating structural solutions to these problems. The inner tubes are an ersatz version of "air supports", which incorporate heavy rubber boots fitted with pressure regulators to stabilize and maintain a level apparatus posture. The latest twist is active vibration isolators which promise to overcome the nominal 2Hz (or wherever it is placed) resonance amplification, and could extend to much lower frequencies. I suspect that these capabilities are in the R&D phase right now. Angelo Campanella >The buildings vibrate at 60 Hz, and other freqs. I'm listening to the ground floor >of a 12 story old sturdy building. When you look (listen) closer you will notice that buildings have vibration peaks at about 58 Hz and the half frequency 39 Hz in USA, 48 and 24 Hz in Europe, respectively. That is caused by large asynchronous machines, mostly by air conditioning. They run slighty slower than line frequency would drive synchronous motors. Need good vibration measuring equipment to resolve those effects. w. In article <aiBXe.276415$5N3.134691@bgtnsc05-news.ops.worldnet.att.net>, > In recent years, the installation of vibration sensitive equipment in > multi-story buildings has become commonplace. The biggest problem I've had with vibration-sensitive equipment in buildings is that no one (manufacturers, sales-reps or customers) could tell me what the "acceptable" vibration limits were for a specific piece of equipment. I usually made due with VC curves or criteria based on similar equipment, but then I've had to over-specify the mitigation to make sure I didn't miss anything. There have been a few cases where I've been supplied with vibration criteria from the manufacturer-supplied data (usually MRI and MEMS-manufacturing machines). Of course, I've also found that in existing installations of these same equipment, ambient vibration levels usually exceed the manufacturer's spec with no adverse effects(!). <Barbie voice> "Vibration is hard!" </Barbie voice> Herb Of course, I've also found that in >existing installations of these same equipment, ambient vibration levels >usually exceed the manufacturer's spec with no adverse effects(!). I had the same experience more than once. The manufacturers put enough of reserve into their specs, they know their customers -) w. |
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