A mechanical floor, mechanical penthouse, mechanical layer or mechanical level is a story of a high-rise building that is dedicated to mechanical and electronics equipment. "Mechanical" is the most commonly used term, but words such as utility, technical, service, and plant are also used. They are present in all tall buildings, including the world's tallest skyscrapers, with significant structural, mechanical and aesthetics concerns. While most buildings have mechanical rooms, typically in the basement, tall buildings require dedicated floors throughout the structure for this purpose, for a variety of reasons discussed below. Because they use up valuable floor area (just like elevator shafts), engineers try to minimize the number of mechanical floors while allowing for sufficient redundancy in the services they provide. As a rule of thumb, skyscrapers require a mechanical floor for every 10 tenant floors (10%), although this percentage can vary widely (see examples below). In some buildings, they are clustered in groups that divide the building into blocks, while in others they are spread evenly through the structure, and in still others, they are mostly concentrated at the top. Mechanical floors are generally counted in the building's floor numbering (this is required by some building codes) but are accessed only by service elevators. Some zoning regulations exclude mechanical floors from a building's maximum area calculation, permitting a significant increase in building sizes; this is the case in New York City. Sometimes buildings are designed with a mechanical floor located on the thirteenth floor, to avoid problems in renting the space due to superstitions about the number.
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, while 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.
Mechanical concerns
Besides structural support and elevator management, the primary purpose of mechanical floors is to house heating, ventilation, and air conditioning (HVAC) equipment. They may contain air handling units, cooling towers (in mechanical penthouses), electrical generators, chiller plants, and water pumps. In particular, the problem of bringing and keeping water on the upper floors is an important constraint in the design of skyscrapers. Water is necessary for tenant use, air conditioning, equipment cooling, and basic firefighting through sprinklers (especially important since ground-based firefighting equipment usually cannot reach higher than a dozen floors or so). It is inefficient, and seldom feasible, for water pumps to send water directly to a height of several hundred meters, so intermediate pumps and water tanks are used. The pumps on each group of mechanical floors act as a relay to the next one up, while the tanks hold water in reserve for normal and emergency use. Usually the pumps have enough power to bypass a level if the pumps there have failed, and send water two levels up. Special care is taken towards fire safety on mechanical floors that contain generators, compressors, and elevator machine rooms, since oil is used as either a fuel or lubricant in those elements. Mechanical floors also contain communication and control systems that service the building and sometimes outbound communications, such as through a large rooftop antenna (which is also physically held in place inside the top-floor mechanical levels). Modern computerized HVAC control systems minimize the problem of equipment distribution among floors by enabling central remote control.
Aesthetics concerns
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