stair

SARRERA DESBERDINA:

Stairs

Stairs are a structure designed to bridge a large vertical distance between lower and higher levels by dividing it into smaller vertical distances. This is achieved as a diagonal series of horizontal platforms called steps which enable passage to the other level by stepping from one to another step in turn. Steps are very typically rectangular. Stairs may be straight, round, or may consist of two or more straight pieces connected at angles.
Types of stairs include staircases (also called stairways) and escalators. Some alternatives to stairs are elevators (also called lifts), stairlifts, inclined moving walkways, ladders, 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]
This is an excerpt from Staircase.
The concept of stairs is believed to be 8000 years old, and are one of the oldest structures in architectural history.^[2] The oldest example of spiral stairs dates back to the 400s BC.^[3] Medieval architecture saw experimentation with many different shapes, and the Renaissance even more so with varied designs.^[4]
A stair, or a stairstep, is one step in a flight of stairs. 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.^[5]
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 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.^[5]
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.^[5]
Ascending a set of stairs to a higher floor is often referred to as going "upstairs",^[6] the opposite being "downstairs".^[7] The same words can also be used to mean the upper or lower floors of a building, respectively.^[6][7]
Each step is composed of a tread and a riser. Some treads may include a nosing.
The balustrade is the system of railings and balusters that prevents people from falling over the edge.
Handrails may be continuous (sometimes called over-the-post) or post-to-post (or more accurately newel-to-newel). For continuous handrails on long balconies, there may be multiple newels and tandem caps to cover the newels. At corners, there are quarter-turn caps. For post-to-post systems, the newels project above the handrails.
Another, more classical, form of handrailing that is still in use is the tangent method. A variant of the cylindric method of layout, it allows for continuous climbing and twisting rails and easings. It was defined from principles set down by architect Peter Nicholson in the 18th century.
The dimensions of a stair, in particular the rise height and going of the steps, should remain the same along the stairs.^[15]
The following stair dimensions are important:
Stairs can take a large number of forms, combining straight runs, winders, and landings.
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 a 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.^[16]
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. The use of landings and a possible change of direction have the following effects:
Other forms include stairs with winders that curve or bend at an acute angle, three flights of stairs that join at a landing to form a T-shape, and stairs with balconies and complex designs.
A "mono string" staircase is a term used for a staircase with treads arranged along a single steel beam. A "double string" staircase has two steel beams, one on either side, and treads spanning between.

The term "helical stair" has many synonyms:^[17]: 53
The helical stair is also called the spiral stair, winding stair, circular stair, elliptical stair, oval stair, geometric stair, vis, vice, vis de Saint Gilles, St. Gilles screw, belfry stair, turret stair, caracole, turnpike, cochlea, cockle, corkscrew, and ascensorium. Helical is possibly the least poetic but also the most accurate generic title.
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, with a continuously changing radius. The mathematical term for the 3-dimensional curve traced where the locus progresses at a fixed radius 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.^[18][19] True spiral staircases would be nonfunctional flat structures, although functional hybrid helical spiral staircases can be constructed. This article attempts 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).
A fundamental advantage of helical stairs is that they can be very compact, fitting into very narrow spaces and occupying a small footprint.^[17]: 54 For this reason, they can often be found in ships and submarines, industrial installations, small loft apartments, and other locations where floorspace is scarce.^[17]: 81 However, this compactness can come at the expense of requiring great craftsmanship and care to produce a safe and effective structure.^[17]: 54 By contrast, grand helical stairs occupying wide sweeps of space can also be built, to showcase luxurious funding and elegant taste.^[17]: 56 Architects have used the twisting curvilinear shape as an embellishment, either within or outside of their buildings.^{[17]: 58–59}
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:
"Open well" helical or circular stairs designed by architects often do not have a central pole, but there usually is a handrail at both sides of the treads. These designs have the advantage of a more uniform tread depth when compared to a narrow helical staircase. Such stairs may also be built around an elliptical or oval footprint,^{[17]: 68–72} or even a triangular or pentagonal core. Lacking a central pole, an open well staircase is supported at its outer periphery, or in some cases may be a completely self-supporting and free-standing structure.
An example of perimeter support is the Vatican stairwell or the Gothic stairwell. The 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).^[20]
Double helix staircases are possible, with two independent helical stairs in the same vertical space, allowing one person to ascend and another to descend without ever meeting, if they choose different helices.^[17]: 74 For examples, the Pozzo di San Patrizio allows one-way traffic so that laden and unladen mules can ascend and descend without obstruction, while Château de Chambord, Château de Blois, and the Crédit Lyonnais headquarters ensure separation for social purposes.
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.
The earliest known helical staircases appear in Temple A in the Greek colony Selinunte, Sicily, to both sides of the cella. The temple was constructed around 480–470 BCE.^[21]
When used in Roman architecture, helical stairs were generally restricted to elite luxury structures. They were then adopted into Christian ecclesiastic architecture.^[22] During the Renaissance and Baroque periods, increasingly spectacular helical stairways were devised, first deleting walled enclosures, and then deleting a central post to leave an open well.^{[17]: 60–74} Modern designs have trended towards minimalism, culminating in helical stairs made largely of transparent glass, or consisting only of stair treads with minimal visible support.^[17]: 67
There is a common misconception that helical staircases in castles rose in a clockwise direction, to hinder right-handed attackers.^[23][24] 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.^[25] 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"^[25] 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".^[23]
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.
Where there is insufficient space for the full run length of normal stairs, "alternating tread stairs" may be used (other names are "paddle stairs", "zig-zag stairs", or "double-riser stairs").^[17]: 19 Alternating tread stairs can be designed to allow for a safe forward-facing descent of very steep stairs (however, designs with recessed treads or footholds do not have this feature). The treads are designed such that they alternate between treads for each foot: one step is wide on the left side; the next step is wide on the right side. There is insufficient space on the narrow portion of the step for the other foot to stand, hence the person must always use the correct foot on the correct step. The slope of alternating tread stairs can be as high as 65° as opposed to standard stairs, which are almost always less than 45°.
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.^[26] 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.^[27]
Alternating tread stairs have been in use since at least 1888.^[28] Today, the design is used in some loft apartments to access bedrooms or storage spaces.^[26]
Local building codes often dictate the number of emergency exits required for a building of a given size, including specifying a minimum number of stairwells. For any building bigger than a private house, modern codes invariably specify at least two sets of stairs, completely isolated from each other so that if one becomes impassable due to smoke or flames, the other remains usable.
The traditional way to satisfy this requirement was to construct two separate stairwell stacks, each occupying its own footprint within each floorplate. Each stairwell is internally configured into an arrangement often called a "U-return" or "return" design.^[29] The two stairwells may be constructed next to each other, separated by a fireproof partition, or optionally the two stairwells may be located at some distance from each other within the floorplan. These traditional arrangements have the advantage of being easily understood by building occupants and occasional visitors.^[29]
Some architects save floor footprint space while still meeting the exit requirement, by housing two stairwells in a "double helix" or "scissors stairs" configuration whereby two stairwells occupy the same floor footprint, but are intertwined while being separated by fireproof partitions along their entire run.^[29] However, this design deposits anybody descending the stack into alternating locations on each successive floor, and this can be very disorienting.^[29] Some building codes recommend using a color-coded stripe and signage to distinguish otherwise identical-looking stairwells from each other, and to make following a quick exit path easier.
Ergonomically and for safety reasons, stairs must have certain dimensions so that people can comfortably use them. Building codes typically specify certain clearances so that the stairs are not too steep or narrow.
Nicolas-François Blondel in the last volume of his Cours d'architecture^[30] (1675–1683) was the first known person to establish the ergonomic relationship of tread and riser dimensions.^[31] He specified that 2 × riser + tread = step length.^[32]
It is estimated that a noticeable misstep occurs once in 7,398 uses and a minor accident on a flight of stairs occurs once in 63,000 uses.^[33] Stairs can be a hazardous obstacle for some, so some people choose to live in residences without stairs so that they are protected from injury.^[34]
Stairs are not suitable for wheelchairs and other vehicles. A stairlift is a mechanical device for lifting wheelchairs up and down stairs. For sufficiently wide stairs, a rail is mounted to the treads of the stairs, or attached to the wall. A chair is attached to the rail and the person on the chair is lifted as the chair moves along the rail.
(overview of Approved document K – Stairs, Ladders and Ramps)^[35]
The 2013 edition "approved document K" categorises stairs as private, utility and general access
When considering stairs for private dwellings, all the specified measurements are in millimetres.
Building regulations are required for stairs used where the difference of level is greater than 600
Steepness of stairs – rise and going
Any rise between 150 and 220 used with any going between 220 and 300
Maximum rise 220 and minimum going 220 remembering that the maximum pitch of private stairs is 42°. The normal relationship between dimensions of the rise and going is that twice the rise plus the going (2R + G) should be between 550 and 700
Construction of steps
Steps should have level treads, they may have open risers but if so treads should overlap at least 16mm. Domestic private stairs are likely to be used by children under 5 years old so the handrail ballister spacing should be constructed so that a 100mm diameter sphere cannot pass through the opening in the risers in order to prevent children from sticking their heads through them and potentially getting stuck.
Headroom
A headroom of 2000mm is adequate. Special considerations can be made for loft conversions.
Width of flights
No recommendations are given for stair widths.
Length of flights
The approved document refers to 16 risers (steps) for utility stairs and 12 for general access. There is no requirement for private stairs. In practice there will be fewer than 16 steps as 16 x 220 gives over 3500 total rise (storey height) which is way above that in a domestic situation.
Landings
Level, unobstructed landings should be provided at the top and bottom of every flight. The width and length being at least that of the width of the stairs and can include part of the floor. A door may swing across the landing at the bottom of the flight but must leave a clear space of at least 400 across the whole landing
Tapered steps
There are special rules for stairs with tapered steps as shown in the image Example of Winder Stairs above
Alternate tread stairs can be provide in space saving situations
Guarding
Flights and landings must be guarded at the sides where the drop is more than 600mm. As domestic private stairs are likely to be used by children under 5 the guarding must be constructed so that a 100mm diameter sphere cannot pass through any opening or constructed so that children will not be able to climb the guarding. The height for internal private stairs should be at least 900 mm (35.4 in) and be able to withstand a horizontal force of 0.36|kN/m|.
American building codes, while varying from State to State and County to County, generally specify the following parameters:^[36]
Religious shrines and memorial structures are often approached via stairs, sometimes numbering hundreds or thousands of steps. Many Neoclassical buildings feature prominent wide stairs ascending to an elevated platform or plinth where the main entrance is located. In recent years, increasing concerns about accessibility have encouraged architects to retrofit discreet lower-level public entrances or elevators to ease wheelchair access.
Large open-well helical stairs have been used as a central feature in public buildings from the Renaissance until modern times.^[17]: 81 A 21st-century version is the helical glass-enclosed, glass-treaded stairways at the center of several flagship Apple Stores.^[42][43]
Vessel is an artistic structure and visitor attraction which was built as part of the Hudson Yards Redevelopment Project in Manhattan, New York City. Built to plans by the British designer Thomas Heatherwick, the elaborate honeycomb-like structure rises 16 stories and consists of 154 flights of stairs, 2,500 steps, and 80 landings for visitors to explore.
Stairs may also be a fanciful physical construct such as the "stairs that go nowhere" located at the Winchester Mystery House. The Penrose stairs, devised by Lionel and Roger Penrose, are a famous impossible object. The image distorts perspective in such a manner that the stairs appear to be never-ending, a physical impossibility. The image was adopted by M. C. Escher in his iconic lithograph Ascending and Descending.

^ Ryder, Charles (2011). The spiral stair or vice: its origins, role and meaning in medieval stone castles (PhD). University of Liverpool. p. 120.

^ ^{a b} Guy, Neil (2011–2012), "The Rise of the Anticlockwise Newel Stair" (PDF), The Castle Studies Group Journal, 25: 114, 163

^ ^{a b} Ryder, Charles (2011). The spiral stair or vice: its origins, role and meaning in medieval stone castles (PhD). University of Liverpool. pp. 293–294.