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Stairs =LINK=

Types of stairs include staircases (also called stairways), ladders, and escalators. Some alternatives to stairs are elevators (also called lifts), stairlifts, inclined moving walkways, and ramps. A stairwell is a vertical shaft or opening that contains a staircase. A flight (of stairs) is an inclined part of a staircase consisting of steps (and their lateral supports if supports are separate from steps).[1]



A stair, or a stairstep, is one step in a flight of stairs.[2] In buildings, stairs is a term applied to a complete flight of steps between two floors. A stair flight is a run of stairs or steps between landings. A staircase or stairway is one or more flights of stairs leading from one floor to another, and includes landings, newel posts, handrails, balustrades and additional parts. A stairwell is a compartment extending vertically through a building in which stairs are placed. A stair hall is the stairs, landings, hallways, or other portions of the public hall through which it is necessary to pass when going from the entrance floor to the other floors of a building. Box stairs are stairs built between walls, usually with no support except the wall strings.[2]

Stairs may be in a "straight run", leading from one floor to another without a turn or change in direction. Stairs may change direction, commonly by two straight flights connected at a 90 angle landing. Stairs may also return onto themselves with 180 angle landings at each end of straight flights forming a vertical stairway commonly used in multistory and highrise buildings. Many variations of geometrical stairs may be formed of circular, elliptical and irregular constructions.[2]

The simplest form is the straight flight of stairs, with neither winders nor landings. These types of stairs were commonly used in traditional homes, as they are relatively easy to build and only need to be connected at the top and bottom. However, many modern architects may not choose straight flights of stairs because:

Another form of straight staircase is the "space saver staircase", also known as "paddle stairs" or "alternating tread staircases". These designs can be used for a steeper rise, but they can only be used in certain circumstances, and must comply with regulations.

However, a basic straight flight of stairs is easier to design and construct than one with landings or winders. The rhythm of stepping is not interrupted in a straight run, which may offset an increased fall risk by helping to prevent a misstep in the first place. However, many long straight runs of stairs will require landings or winders to comply with safety standards in building regulations.[11]

Straight stairs can have a mid-landing incorporated, but it is probably more common to see stairs that use a landing or winder to produce a bend in the stairs. A straight flight with a mid-landing will require a lot of straight length, and may be more commonly found in large commercial buildings. "L" shaped stairways have one landing and usually a change in direction by 90. "U" shaped stairs may employ a single wider landing for a change in direction of 180, or two landings for two changes in direction of 90 each. A "Z" shaped staircase incorporates two opposite 90 turns, creating a shape similar to that of the letter "Z" if seen from above. Use of landings and a possible change of direction have the following effects:

The term "spiral" has a more narrow definition in a mathematical context, as a curve which lies in a single plane and moves towards or away from a central point. The mathematical term for the 3-dimensional curve traced where the locus progresses at a fixed distance from a fixed line while moving in a circular motion around it is a "helix". Since the very purpose of a stairway is to change elevation, it is inherently a 3-dimensional path. Loose everyday usage conflates the terms helical and spiral, but the vast majority of circular stairs are actually helical.[13] True spiral staircases would be nonfunctional flat structures, although functional hybrid helical spiral staircases can be constructed. This article will attempt to preferentially use the terms "helix" and "helical" to describe circular stairways more clearly and precisely, while reserving the term "spiral" for a curve restricted to a flat plane.

Helical stairs, sometimes referred to in architectural descriptions as vice, wind around a newel (also called the "central pole"). The presence or absence of a central pole or newel does not affect the overall terminology applied to the design of the structure. In Scottish architecture, helical stairs are commonly known as a turnpike stair.

Helical stairs typically have a handrail along the outer periphery only, and on the inner side may have just a central pole. A "squared helical" stair fills a square stairwell and expands the steps and railing to a square, resulting in unequal steps (wider and longer where they extend into a corner of the square). A "pure helix" fills a circular stairwell, and has multiple steps and handrail elements which are identical and positioned screw-symmetrically.

Helical stairs have a handedness or chirality, analogous to the handedness of screw threads, either right-handed or left-handed helical shapes. Ascending a right-handed helix rises counter-clockwise, while ascending a left-handed helix rises clockwise (both as viewed from above).

Helical stairs have the disadvantage of being very steep if they are tight (small radius) or are otherwise not supported by a center column. The cylindrical spaces they occupy can have a narrow or wide diameter:

An example of perimeter support is the Vatican stairwell or the Gothic stairwell. This latter stairwell is tight because of its location where the diameter must be small. Many helices, however, have sufficient width for normal size treads (8 inches (200 mm)) by being supported by any combination of a center pole, perimeter supports attaching to or beneath the treads, and a helical handrail. In this manner, the treads may be wide enough to accommodate low rises. In self-supporting stairs the helix needs to be steep to allow the weight to distribute safely down the structure in the most vertical manner possible. Helical steps with center columns or perimeter support do not have this limitation. Building codes may limit the use of helical stairs to small areas or secondary usage, if their treads are not sufficiently wide or have risers taller than 9.5 inches (240 mm).[14]

Emergency exit stairways, though built with landings and straight runs of stairs, are often functionally double helices, with two separate stairs intertwined and occupying the same floor footprint. This is often in compliance with legal safety requirements to have two independent fire escape paths.

Helical stairs can be characterized by the number of turns that are made. A "quarter-turn" stair deposits the person facing 90 from the starting orientation. Likewise, there are half-turn, three-quarters-turn and full-turn stairs. A continuous helix may make many turns depending on the height. Very tall multi-turn helical staircases are usually found in old stone towers within fortifications, churches, and in lighthouses. Winders may be used in combination with straight stairs to turn the direction of the stairs. This allows for a large number of permutations in designs.

There is a common misconception that helical staircases in castles rose in a clockwise direction, to hinder right-handed attackers.[17][18] While clockwise helical staircases are more common in castles than anti-clockwise, they were even more common in medieval structures without a military role, such as religious buildings.[19] Studies of helical stairs in castles have concluded that "the role and position of spirals in castles ... had a much stronger domestic and status role than a military function"[19] and that "there are sufficient examples of anticlockwise stairs in Britain and France in [the 11th and 12th centuries] to indicate that the choice must have depended both on physical convenience and architectural practicalities and there was no military ideology that demanded clockwise staircases in the cause of fighting efficiency or advantage".[17]

Developments in manufacturing and design have also led to the introduction of kit form helical stairs. Modular, standardized steps and handrails can be bolted together to form a complete unit. These stairs can be made out of steel, timber, concrete, or a combination of materials.

An advantage of alternating tread stairs is that people can descend while facing forward, in the direction of travel. The only other alternative in such short spaces would be a ladder, which requires a backward-facing descent. Alternating tread stairs may not be safe for small children, the elderly, or the physically challenged. Building codes typically classify them as ladders, and will only allow them where ladders are allowed, usually basement or attic utility or storage areas infrequently accessed.

The block model in the image illustrates the space efficiency gained by an alternating tread stair. The alternating stairs (3) requires one unit of space per step: the same as the half-width stairs (2), and half as much as the full-width stairs (1). Thus, the horizontal distance between steps is in this case reduced by a factor of two, reducing the size of each step. The horizontal distance between steps is reduced by a factor less than two if for construction reasons there are narrow "unused" step extensions.

These stairs often (including this example) illustrate the mathematical principle of glide plane symmetry: the mirror image with respect to the vertical center plane corresponds to a shift by one step.

Alternating tread stairs are sometimes referred to as "witches stairs", in the supposed belief that they were created during an earlier era as an attempt to repel witches who were thought to be unable to climb such stairs.[20] Such a fanciful origin of the term has since been disproved, with experts finding no mention in any historical literature of stairs that were believed to prevent access by witches.[21] 041b061a72


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