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Amechanical floor,mechanical penthouse,mechanical layer ormechanical level is astory of ahigh-rise building that is dedicated tomechanical andelectronics equipment. "Mechanical" is the most commonly used term, but words such asutility,technical,service, andplant are also used. They are present in all tall buildings, including theworld's tallestskyscrapers, with significantstructural,mechanical andaesthetics concerns.

While most buildings havemechanical rooms, typically in thebasement, 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 sufficientredundancy in the services they provide. As arule of thumb, skyscrapers require a mechanical floor for every 10 tenant floors (10%), although this percentage can vary widely (seeexamples 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'sfloor numbering (this is required by somebuilding 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 inNew York City.^[1] Sometimes buildings are designed with a mechanical floor located on thethirteenth floor, to avoid problems in renting the space due tosuperstitions about the number.

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 usingoutriggertrusses. The triangular shape of thestruts precludes the laying of tenant floors, so these sections house mechanical floors instead, typically in groups of two. Additional stabilizer elements such astuned mass dampers also require mechanical floors to contain or service them.

This layout is usually reflected in the internalelevator 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 orskylobby 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 mechanicalpenthouse, which can also contain communications gear and window-washing equipment. On most building designs, this is a simple "box" on theroof, while on others it is concealed inside a decorativespire. 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.

Besides structural support and elevator management, the primary purpose of mechanical floors is services to househeating, ventilation, and air conditioning (HVAC) equipment.^[2] They may containair handling units,cooling towers (in mechanical penthouses),electrical generators,chiller plants, andwater 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, equipmentcooling, and basicfirefighting throughsprinklers (especially important since ground-based firefighting equipment usually cannot reach higher than a dozen floors or so). It is inefficient, and seldom feasible, forwater 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, sinceoil is used as either afuel orlubricant in those elements.

Mechanical floors also contain communication and control systems that service the building and sometimes outbound communications, such as through a large rooftopantenna (which is also physically held in place inside the top-floor mechanical levels).

Modern computerizedHVAC control systems minimize the problem of equipment distribution among floors by enabling central remote control.

Most mechanical floors require external vents orlouvers forventilation and heat rejection along most or all of their perimeter, precluding the use of glasswindows. The resulting visible "dark bands" can disrupt the overallfacade design, especially if it is fully glass-clad. Differentarchitectural styles approach this challenge in different ways.

In theModern andInternational styles of the 1960s and 1970s whereform follows function, the vents' presence is not seen as undesirable. Rather it emphasizes the functional layout of the building by dividing it neatly into equal blocks, mirroring the layout of the elevators and offices inside. This could be clearly seen on the Twin Towers of theWorld Trade Center and can be seen on theWillis Tower. In theIDS Tower inMinneapolis, the lowest mechanical floor serves as a visual separation from the street- andskyway-level Crystal Court shopping center and the office tower above; the upper mechanical floor (above the 50th and 51st floors, the uppermost occupied floors) serves as a "crown" to the building.

Conversely, designers of the recentpostmodern-style skyscrapers strive to mask the vents and other mechanical elements in various ways. This is accomplished through such means as complex wall angles (Petronas Towers), intricatelatticeworkcladding (Jin Mao Building), or non-glassed sections that appear to beornamental (Taipei 101, roof of Jin Mao Building).

Some low-rise, residential (usually apartment buildings or dormitories), or non-residential buildings, especially built in an architecture style that promotes the use of elements such as sloped roofs and/or bell towers, may have mechanical floors disguised as attics or towers. In this case, the ventilation systems of the mechanical floor are seen as gable vents, dormers, orabat-sons (louvers in a bell tower). Examples include some buildings inUCLA, like Dodd Hall, which has a mechanical floor disguised as an attic and a bell tower.

These are examples of above-ground mechanical floor layouts for some of the world's tallest buildings. In each case, mechanical penthouses and spires are counted as floors, leading to higher total floor counts than usual.

• Taipei 101: Floors 7–8, 17–18, 25–26, 34, 42, 50, 58, 66, 74, 82, 87, 90, 92 to 100 in the penthouse – total 17/102, 17%. The official count of 11^[3] corresponds to the number of groups in the office section. Floors 92–100 contain communications equipment^[4] and so are not typically counted as mechanical since they do not service the building itself.
• One World Trade Center: Floors 2–19, 92–99, and 103–104 – total 28/104, 26%.
• Previous WTC (Twin Towers): Floors 7–8, 41–42, 75–76, and 108–109 – total 8/110, 7%. The 110th Floor of 1 WTC (North Tower) housed television and radio transmission equipment. Some sources erroneously mention 12 floors, in groups of three, due to the height of the vents (actually the ceilings there were higher) and because levels 44 and 78 wereskylobbies which in many buildings sit directly on top of the mechanical floors. However, the twin towers had oneoccupied office floor under each skylobby, accessible throughescalators.
• Willis Tower (FormerlySears Tower): Levels 29–32, 64–65, 88–89, 104–108, 109 (penthouse), and 110 (penthouse roof) – total 15/110, 13%.
• Petronas Towers: Floors 6–7, 38–40, 43, 84, 87–88 – total 9/88, 10%^[5]
• Jin Mao Building: Floors 51–52, and 89–93 in the penthouse – total 7/93, 7.5%^[6]
• Burj Khalifa: Floors 17–18, 40–42, 73–75, 109–111, 136–138, 155, and 160–168, in the penthouse – total 25/168, 15%
• John Hancock Center: Floors 16–17, 42–43, 93, 99–100 (penthouse) – total 7/100, 7%
• Empire State Building: Floors 87–101 – total 15/102, 14%
• Shanghai Tower: Floors 6–7, 20–21, 35–36, 50–51, 66–67, 82–83, 99–100, 116–117, 122–124, and 128 – total 20/128, 16%
• Shanghai World Financial Center: Floors 6, 18, 30, 42, 54, 66, 78, 89, and 90 – total 9/101, 9%
• Lotte World Tower: Floors 3–4, 13, 21–23, 39–41, 59, 60, 72–75, 83, 84, 102–106, 115, and 116 – total 24/123, 20%

• Case study for Hong Kong'sCentral Plaza by the Department of Architecture ofHong Kong University, Energy Features section

• Skyscrapers
• Heating, ventilation, and air conditioning
• Rooms
• Floors

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