When mechanical components are installed in buildings located in seismic active areas, the Uniform Building Code "Earthquake Regulations, section 2312 states: "Every building or structure and every portion thereof shall be designed and constructed to resist stresses produced by lateral forces [of earthquakes] .....". Thus, the attachment and securing of most building components such as fans, pipes, valves and suspended ceilings, must be especially robust. The seismic risk is assigned as a "Seismic Zone" value on a scale from 1 to 10 (I to X), with I to V being slight to none and X being intense as near Irkutsk, Siberia. The western US is VI and above, IX in Southern California. South Central US and New England is VI to VIII. Florida, Texas and Western Great Lakes areas are V and below. We have provided seismic analysis reports for large safety-related centrifugal fans now installed in nuclear power plants around the world. The chief concern here is that these fans survive and be able to operate in a timely manner to assure public safety after an earthquake. The safety item of concern is that a negative HVAC static pressure must be continuously maintained within the building and around reactor components to prevent the inadvertent release of radioactive gases to the surrounding communities (e.g. the "Three-Mile Island" incident). Building lateral accelerations of concern range up to 1g or more. The design of attachment bolts and sway braces become the primary focus. For larger solid objects, whose dimensions are sufficient to present internal mechanical resonance modes that may be excited by seismic energy, two vibration frequency ranges are of interest. The first is the "rigid frequency" - typically 20HZ or 30Hz, above which the acceleration spectrum is constant ("plateau"), and where it is considered that very little earthquake vibration energy exists. The second and more important range includes the "response spectrum" for a given floor location. This spectrum level is a calculated summary of the vibration responses that might occur for a one degree-of-freedom system, resonant at each successive analysis frequency, attached to that floor. One normally designs to (derives maximum stress at) the frequency where maximum vibration amplitude occurs. Unfortunately, this frequency will decrease, and the peak acceleration at that frequency can increase (insufficient damping), from ground level to the top floor of a multistory building. This is because the building structure acts as a band-pass filter, mode converter and resonator. Thus, a ground level response spectrum peaking at 0.2g around 12 Hz can incite damaging top floor vibrations approaching 1g at around 3 or 4 Hz. A multistory building will require a floor-by-floor mechanical seismic analysis. Vibration isolation measures that are intended to normally soften mechanical vibration to building are of serious concern. On the one hand, one wants to isolate vibrating items such as fans. On the other hand, such an isolated system is very limp and can easily be driven into its stops or crash into adjacent equipment during an earthquake. Such thrashing can systematically destroy any number of elements in a few seconds during an earthquake. Thus, "snubbers" must be designed and installed which provide soft landings for such spring-isolated systems. Every car has snubbers, where the soft "feather" motion of the sprung car body ultimately encounters a more firm rubber biscuit somewhere on the frame. Expensive cars have a wider scope and do a nicer job of snubbing - as contrasted with trucks and inexpensive cars which often exhibit "harshness" negotiating bumpy roads. Generally, car and truck body motions are more severe than any earthquake motion of building components. But since little or no protection is provided in buildings (no seat belt or roll cage), damage and injury in and around these "permanent" man-made structures is severe during earthquakes. Economics (construction costs and isolation/snubber installation labor and maintenance) usually drive the ultimate selection. Usually, the best location for sensitive equipment is at ground (grade) level. If "thrashing" is problematic, it is best to hard mount them (bolt firmly in place).
If you have acoustical or noise control design problems you would like to discuss, feel free to call or FAX us any time.
Campanella Associates
For more information on... Angelo J. Campanella, P.E., Ph.D. (Principal)
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