Lifting a structure a maximum of 0.4”(10mm) in the pad location and perhaps some immediate adjacent areas, is doable if provision is made for jacks. At some time, well beyond the lifetime of the architect, the structural engineer or the manufacturer, some future generation should not be left with an insoluble pad replacement problem. While we have no history of pads having gone bad and requiring replacement, we would suggest that if they are not made removable by using the same methods we will suggest a little further on for spring mountings, that there is provision for hydraulic jack pockets as needed. In isolating structures, this is not the general rule and pads are generally put in position and the structure proceeds. Therefore, we have not worked with deflections of more than 2”(50mm) at design loadings. This variation in elevation could be both a mechanical nuisance and undesirable. If the mountings had a deflection of 3”(75mm) at dead load, it would increase to 3.9”(99mm) when fully occupied. Live loads may be an additional 30% of dead load. The consideration of specified deflection, and when this specified deflection occurs, is of major importance as it controls the isolation performance.Īnother concern is the ratio of the dead and live loads of the structure. However, it is generally not practical to go to these numbers as the horizontal stiffness of the spring drops in proportion to the vertical and the building becomes more responsive to wind loads. Practical designs have deflections of as much as 4”(100mm), which corresponds to a frequency of about 1.5 Hz. Spring mountings can be manufactured to any capacity. It is our suggestion that all spring isolators have 10 Hz base isolation pads 2”(50mm) thick, manufactured to the same standards as in the earlier part of this discussion. While steel spring isolators do an excellent job of stopping vibration, it is very important that high frequency noise transmission is considered too. The extreme areas were supported on 6.5 Hz rubber bearings. Further away spring deflections were smaller and the frequency 3.5 Hz. In another example, that portion of the building over the railroad tracks was supported on 2.5 Hz spring isolators. The critical area was supported on spring mountings with a response of 3.5 Hz and the plaza area on 7 Hz rubber bearings. In one application, the building contained office space, a cinema and a live theater. The ground vibration dissipated with distance from the source, so lesser treatment would be satisfactory further from the railroad. We have been involved in a number of jobs where the acoustician recognized that not only did the building not respond as one mass, but that the requirements for isolation varied in different parts of the structure. Moving on to lower speed machinery at 30 Hz, we have had to use isolators with a response as low as 2 Hz (a frequency ratio of 15 to 1) before we could solve the problem. In mechanical isolation work we have found that when isolating vibration at frequencies as high as 250 Hz, we have had to use air springs with a response of 3 Hz before the isolation was effective as the vibration was in the audible noise range. There was no air-borne sound transmission, but the structure-borne noise radiating from the ceiling made it seem like they were directly overhead. The construction workers were using small chipping hammers about 150 feet from my office. We recently extended our own office building and some demolition was required. Concrete structures will transmit structurally induced noise over very large distances from the source with almost no attenuation. The frequency of the steel columns resting on the isolators might be very much higher. A lightly loaded structural floor might have a frequency as high as 7 Hz and fully loaded, 3 Hz. Just as in the isolation of structural floors from machinery, the selection of a practical deflection is dependent on the sensitivity or rigidity of the structure and the various resonances of the structural elements. It is also assumed that this rigid mass loads each isolator in accordance with the weight distribution as a lump mass at each location. The efficiency equation is based on the assumption that a structure is a lumped mass or a block of concrete that has no flexibility and an infinite resonance frequency. This would mean that the incoming frequency should not be lower than 16.5 Hz just to maintain the traditional but often unsatisfactory 90% theoretical efficiency. On page 2 we have indicated that the cut off point for rubber bearings is about 5.5 Hz. Not all problems can be solved with rubber isolation pads.
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