Structural Concerns
Some skyscrapers have narrow building cores that require stabilization to prevent collapse. Typically this is accomplished by joining the core to the external supercolumns at regular intervals using outrigger trusses. The triangular shape of the struts precludes the laying of tenant floors, so these sections house mechanical floors instead, typically in groups of two. Additional stabilizer elements such as tuned mass dampers also require mechanical floors to contain or service them.
This layout is usually reflected in the internal elevator zoning. Since nearly all elevators require machine rooms above the last floor they service, mechanical floors are often used to divide shafts that are stacked on top of each other to save space. A transfer level or skylobby is sometimes placed just below those floors.
Elevators that reach the top tenant floor also require overhead machine rooms; those are sometimes put into full-size mechanical floors but most often into a mechanical penthouse, which can also contain communications gear and window-washing equipment. On most building designs this is a simple "box" on the roof, on others it is concealed inside a decorative spire. A consequence of this is that if the topmost mechanical floors are counted in the total, there can be no such thing as a true "top-floor office" in a skyscraper with this design.
Read more about this topic: Mechanical Floor
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