Welcome  to  the  HOT  TOPIC in  Earthquake  Engineering # 9 !
By Valentin Shustov
     Seismic isolators, if properly used, can partially cut off the earthquake energy transmission to a structure (Topic 4). Further, after the earthquake waves which penetrated through a shield of base isolators enter the structure, there are three ways to soothe their damaging effect, namely:
    a) to dissipate the seismic waves energy with the properly engineered damping devices; 
      b)  to disperse the seismic waves energy between a much wider range of natural frequencies by adequately configuring the building elevation thus preventing the resonant amplifications (Topic 5); 
      c)  to absorb the resonant portions of the whole seismic waves band with the help of  mass dampers. 
   Devices of the last kind have been installed in high-rise buildings for a quarter of a century. They are, usually, located at one of the upper stories or distributed along the building height. Those devices have the following major deficiencies
    1. They are targeting only one, fundamental frequency and cannot control the higher modes of vibration. 
    2. They require a special space inside the building for their accommodation. 
    3. They are often very intricate and need a permanent maintenance that makes them a rather expensive. 
     To increase the shielded range of forcing frequencies while keeping the construction costs as low as possible, the concept of  Multi-Frequency Quieting Building System (MFQBS) was developed. 
      Each MFQBS includes horizontal rigid floor or roof diaphragms enclosed by an assembly of extended cantilevers of varying natural periods, connected in parallel and running perpendicular to the diaphragm boundaries along its perimeter. The cantilevers connected to each side of the diaphragm are pliant to vibrations in the directions of those sides (see Figure above). 
   MFQBS is a kind of a distributed system.  However, in contrast with the system described, for instance, in Kitada and Park, 1994, it represents a more sophisticated structural control technique in which each individual mass damper is substituted with a mass damping floor diaphragm substructure consisting of a multitude of differently tuned mass dampers. 
      Being tuned, with the help of their geometry and material properties, to the main natural frequencies of the building, the MFQBS cantilevers act as partial tuned mass dampers that increase the earthquake/wind mitigation of the system and permit an architectural design both attractive and functional. 
      The MFQBS cantilevers should not be necessarily some sorts of solid beams. They can be constructed as 3D frames like, e.g. the roof diaphragm cantilevers in Figure to the right.
    Normally, the cantilever beams are staying still in a waiting mode. When swaying of a building begins, one or several beams gets involved in a vigorous resonant vibration which absorbs the propagating energy of the major resonant waves in the building structure thus preventing lateral displacement amplifications in the building structure during an earthquake or wind.
      Kitada, Y. and Park, K.C., 1994, "A Comparative Study of Distributed vs.Concentrated Hybrid Mass Damper Systems in High-Rise Buildings",  Proc.1st World Conf. on Structural Conrtrol,  FA2-23, Vol.3, Pasadena, CA.
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  This page was last updated on 14 April 2000.