Stairs

Stairway of the Opéra Garnier (Paris, France)
Stairways in Persepolis, Iran, built ca. 550 B.C.
A straight stairway with tiled treads, a double railing and two landings in the "Porta Garibaldi" station, Milan
The simplest form of staircase, down to Man o'War Cove, Dorset, England
Stairs-street in Cefalù, Italy

A stairway, staircase, stairwell, flight of stairs, or simply stairs is a construction designed to bridge a large vertical distance by dividing it into smaller vertical distances, called steps. Stairs may be straight, round, or may consist of two or more straight pieces connected at angles.

Special types of stairs include escalators and ladders. Some alternatives to stairs are elevators (lifts in British English), stairlifts and inclined moving walkways as well as stationary inclined sidewalks (pavements in British English).

Components and terms

A stair, or a stairstep is one step in a flight of stairs.[1] 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.[1]

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 degree angle landing. Stairs may also return onto themselves with 180 degree 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.[1]

Stairs may be a required component of egress from structures and buildings. Stairs are also provided for convenience to access floors, roofs, levels and walking surfaces not accessible by other means. Stairs may also be a fanciful physical construct such as the stairs that go nowhere located at the Winchester Mystery House. Stairs are also a subject used in art to represent real or imaginary places built around impossible objects using geometric distortion, as in the work of artist M. C. Escher.

"Stairway" is also a common metaphor for achievement or loss of a position in the society; or as a metaphor of hierarchy (e.g. Jacob's Ladder, The Battleship Potemkin).

Step

Each step is composed of tread and riser.

Tread
The part of the stairway that is stepped on. It is constructed to the same specifications (thickness) as any other flooring. The tread "depth" is measured from the outer edge of the step to the vertical "riser" between steps. The "width" is measured from one side to the other.
Riser
The vertical portion between each tread on the stair. This may be missing for an "open" stair effect.
Nosing
An edge part of the tread that protrudes over the riser beneath. If it is present, this means that, measured horizontally, the total "run" length of the stairs is not simply the sum of the tread lengths, as the treads overlap each other.
Starting step or Bullnose
Where stairs are open on one or both sides, the first step above the lower floor or landing may be wider than the other steps and rounded. The balusters typically form a semicircle around the circumference of the rounded portion and the handrail has a horizontal spiral called a "volute" that supports the top of the balusters. Besides the cosmetic appeal, starting steps allow the balusters to form a wider, more stable base for the end of the handrail. Handrails that simply end at a post at the foot of the stairs can be less sturdy, even with a thick post. A double bullnose can be used when both sides of the stairs are open.
Stringer, Stringer board or sometimes just String
The structural member that supports the treads and risers in standard staircases. There are typically three stringers, one on either side and one in the center, with more added as necessary for wider spans. Side stringers are sometimes dadoed to receive risers and treads for increased support. Stringers on open-sided stairs are called "cut stringers".
Winders
Winders are steps that are narrower on one side than the other. They are used to change the direction of the stairs without landings. A series of winders form a circular or spiral stairway. When three steps are used to turn a 90° corner, the middle step is called a kite winder as a kite-shaped quadrilateral.
Trim
Various moldings are used to decorate and in some instances support stairway elements. Scotia or quarter-round are typically placed beneath the nosing to support its overhang.

The railing system

A multi-flight stairway with handrails
Stone stairs and wooden balustrade
Two flights of stairs joined by a landing
Example of winder stairs with a simple handrail supported by three newel posts

The balustrade is the system of railings and balusters that prevents people from falling over the edge.

Banister, Railing or Handrail 
The angled member for handholding, as distinguished from the vertical balusters which hold it up for stairs that are open on one side; there is often a railing on both sides, sometimes only on one side or not at all, on wide staircases there is sometimes also one in the middle, or even more. The term "banister" is sometimes used to mean just the handrail, or sometimes the handrail and the balusters or sometimes just the balusters.[2]
Volute 
A handrail end element for the bullnose step that curves inward like a spiral. A volute is said to be right or left-handed depending on which side of the stairs the handrail is as one faces up the stairs.
Turnout 
Instead of a complete spiral volute, a turnout is a quarter-turn rounded end to the handrail.
Gooseneck 
The vertical handrail that joins a sloped handrail to a higher handrail on the balcony or landing is a gooseneck.
Rosette 
Where the handrail ends in the wall and a half-newel is not used, it may be trimmed by a rosette.
Easings 
Wall handrails are mounted directly onto the wall with wall brackets. At the bottom of the stairs such railings flare to a horizontal railing and this horizontal portion is called a "starting easing". At the top of the stairs, the horizontal portion of the railing is called a "over easing".
Core rail 
Wood handrails often have a metal core to provide extra strength and stiffness, especially when the rail has to curve against the grain of the wood. The archaic term for the metal core is "core rail".
Baluster 
A term for the vertical posts that hold up the handrail. Sometimes simply called guards or spindles. Treads often require two balusters. The second baluster is closer to the riser and is taller than the first. The extra height in the second baluster is typically in the middle between decorative elements on the baluster. That way the bottom decorative elements are aligned with the tread and the top elements are aligned with the railing angle.
Newel 
A large baluster or post used to anchor the handrail. Since it is a structural element, it extends below the floor and subfloor to the bottom of the floor joists and is bolted right to the floor joist. A half-newel may be used where a railing ends in the wall. Visually, it looks like half the newel is embedded in the wall. For open landings, a newel may extend below the landing for a decorative newel drop.
Finial 
A decorative cap to the top of a newel post, particularly at the end of the balustrade.
Baserail or Shoerail 
For systems where the baluster does not start at the treads, they go to a baserail. This allows for identical balusters, avoiding the second baluster problem.
Fillet 
A decorative filler piece on the floor between balusters on a balcony railing.

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 which 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 earliest spiral staircases appear in Temple A in the Greek colony Selinunte, Sicily, to both sides of the cella. The temple was constructed around 480–470 BC.[3]

Other terms

Floating staircase in Minneapolis
Staircase in Ford plant in Los Angeles with double bullnose and two volutes. An intermediate landing is part of this U-shaped stair.
Apron 
This is a wooden fascia board used to cover up trimmers and joists exposed by stairwell openings. The apron may be moulded or plain, and is intended to give the staircase a cleaner look by cloaking the side view.[4]
Balcony 
For stairs with an open concept upper floor or landing, the upper floor is functionally a balcony. For a straight flight of stairs, the balcony may be long enough to require multiple newels to support the length of railing. In modern homes, it is common to have hardwood floors on the first floor and carpet on the second. The homeowner should consider using hardwood nosing in place of carpet. Should the carpet be subsequently replaced with hardwood, the balcony balustrade may have to be removed to add the nosing.
Flight 
A flight is an uninterrupted series of steps.
Floating stairs 
A flight of stairs is said to be "floating" if there is nothing underneath. The risers are typically missing as well to emphasize the open effect. There may be only one stringer or the stringers otherwise minimized. Where building codes allow, there may not even be handrails.
Landing or Platform 
A landing is the area of a floor near the top or bottom step of a stair. An intermediate landing is a small platform that is built as part of the stair between main floor levels and is typically used to allow stairs to change directions, or to allow the user a rest. A half landing is where a 180° change in direction is made, and a quarter landing is where a 90° change in direction is made (on an intermediate landing).[5] As intermediate landings consume floor space they can be expensive to build. However, changing the direction of the stairs allows stairs to fit where they would not otherwise, or provides privacy to the upper level as visitors downstairs cannot simply look up the stairs to the upper level due to the change in direction. The word 'landing' is also commonly used for a general corridor in any of the floors above the ground floor of a building, even if that corridor is located well away from a staircase.
Mobile Safety Steps
Can be used as temporary, safe replacements for many types of stairs
Runner 
Carpeting that runs down the middle of the stairs. Runners may be directly stapled or nailed to the stairs, or may be secured by a specialized bar that holds the carpet in place where the tread meets the riser, known as a stair rod.
Spandrel 
If there is not another flight of stairs immediately underneath, the triangular space underneath the stairs is called a "spandrel". It is frequently used as a closet.
Staircase 
This term is often reserved for the stairs themselves: the steps, railings and landings; though often it is used interchangeably with "stairs" and "stairway". In the UK, however, the term "staircase" denotes what in the U.S. is called "stairway", but usually includes the casing – the walls, bannisters and underside of the stairs or roof above.
Stairway 
This primarily American term is often reserved for the entire stairwell and staircase in combination; though often it is used interchangeably with "stairs" and "staircase".
Stairwell 
The spatial opening, usually a vertical shaft, containing an indoor stairway; by extension it is often used as including the stairs it contains.
Staircase tower 
A tower attached to, or incorporated into, a building that contains stairs linking the various floors.

Measurements

The measurements of a stair, in particular the rise height and going of the steps, should remain the same along the stairs.[6]

The following stair measurements are important:

A quite unusual "variable rise" stairway, which also distorts visual perspective (at The Duomo in Urbino, Italy)

The easiest way to calculate the rise and run is to use a stair stringer calculator,[7] the amount of spindles you need is calculated based on the width of the spindles and the fact that the distance between each spindle should be 99mm or less.

Forms

Stairs with heavy pedestrian traffic

Stairs can take a large number of forms, combining winders and landings.

The simplest form is the straight flight of stairs, with neither winders nor landings. It is not often used in modern homes because:

Another form of straight staircase is the space saver staircase, also known as paddle stairs or alternating tread staircases, that can be used for a steeper rise, but these 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. Although the rhythm of stepping is not interrupted in a straight run, which may offset the increased fall risk by helping to prevent a misstep in the first place, many stairs will require landings or winders to comply with safety standards in the Building Regulations.[8]

Straight stairs can have a mid-landing incorporated, but it's probably more common to see stairs that use a landing or winder to produce a bend in the stairs as a straight flight with a mid-landing will require a lot of linear space and is more commonly found in commercial buildings. "L" shaped stairways have one landing and usually change in direction by 90 degrees. "U" shaped stairs may employ a single wider landing for a change in direction of 180 degrees, or 2 landings for two changes in direction of 90 degrees each. A Z-shaped staircase incorporates two parallel 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:

Other forms include stairs with winders that curve or bend at an acute angle, three flight stairs that join at a landing to form a T-shape, and stairs with balconies and complex designs can be produced to suit individual properties.

A mono string staircase is a term used for a steel spine staircase with treads.

A Double string staircase has two steel beams on either side and treads in the center.

Spiral and helical stairs

Spiral stairs in Art Deco style. Nebotičnik skyscraper, Ljubljana.
The external spiral staircase of Palazzo Contarini del Bovolo, Italy
Gothic spiral staircase in St. Elisabeth Cathedral in Košice (Slovakia)

Spiral stairs wind around a newel (also the central pole). They typically have a handrail on the outer side only, and on the inner side just the central pole. A squared spiral stair assumes a square stairwell and expands the steps and railing to a square, resulting in unequal steps (larger where they extend into a corner of the square). A pure spiral assumes a circular stairwell and the steps and handrail are equal and positioned screw-symmetrically. A tight spiral stair with a central pole is very space efficient in the use of floor area. Spiral stairs have the disadvantage of being very steep - only if they are tight or are otherwise not supported by a center column. This is because of two reasons. The wider the spiral, the more steps can be accommodated per spiral. Therefore, if the spiral is large in diameter, via having a central support column that is strong (invariably large in diameter) and a special handrail that helps distributes the load, each step may be longer and therefore the rise between each step may be smaller, (equal to that of regular steps.) Otherwise, the circumference of the circle at the walk line will be so small that it will be impossible to maintain a normal tread depth and a normal rise height without compromising headroom before reaching the upper floor. To maintain headroom some spiral stairs have very high rises to support a very short diameter. These are typically cases where the stairwell must be a small diameter by design or must not have any center support by design or may not have any perimeter support. An example of perimeter support is the Vatican stairwell shown in the next section or the gothic stairwell shown to the left. That stairwell is only tight because of its design in which the diameter must be small. Many spirals, however, have sufficient width for normal size treads (8 inches) 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. The photo on the right are self-supporting stairs. This means the spiral needs to be necessarily steep to allow the weight to distribute safely down the spiral in the most vertical manner possible. Spiral steps with center columns or perimeter support do not have this limitation. Building codes may limit the use of spiral stairs to small areas or secondary usage if their treads are not sufficiently wide or have risers above 8 or 9 inches.

The term "spiral" has a more narrow definition in a mathematical context, as a mathematical spiral lies in a single plane and moves towards or away from a central point. The mathematical term for motion where the locus remains at a fixed distance from a fixed line whilst moving in a circular motion about it is "helical". The presence or otherwise of a central pole does not affect the terminology applied to the design of the structure.

Spiral stairs in medieval times were generally made of stone and typically wound in a clockwise direction (from the ascender's point of view),[9] to place attacking swordsmen (who were most often right-handed) at a disadvantage. This asymmetry forces the right-handed swordsman to engage the central pike and degrade his mobility compared with the defender who is facing down the stairs. Extant 14th to 17th-century examples of these stairways can be seen at Muchalls Castle, Crathes Castle and Myres Castle in Scotland. Exceptions to the rule exist, however, as may be seen in the accompanying image of the Scala of the Palazzo Contarini del Bovolo, which winds up counter-clockwise.

Developments in manufacturing and design have led to the introduction of kit form spiral stairs. 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.

Helical or circular stairs do not have a central pole and there is a handrail on both sides. These have the advantage of a more uniform tread width when compared to the spiral staircase. Such stairs may also be built around an elliptical or oval planform.

The 'Bramante' staircase, Vatican museums, showing the two access points at the bottom of the stairs

Both double spiral and 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. For examples, the Pozzo di S. 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. Fire escapes, though built with landings and straight runs of stairs, are often functionally double helices, with two separate stairs intertwined and occupying the same floor space. This is often in support of legal requirements to have two separate fire escapes.

Both spiral and helical stairs can be characterized by the number of turns that are made. A "quarter-turn" stair deposits the person facing 90 degrees from the starting orientation. Likewise, there are half-turn, three-quarters-turn and full-turn stairs. A continuous spiral may make many turns depending on the height. Very tall multi-turn spiral 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.

Alternating tread stairs

An alternating tread stair (center) between a half-width stair (left) and full-width stair (right), built with Duplo blocks.
An alternating tread stair climbing the steep slope of a pinnacle in Pinnacles National Park, California, United States.

Where there is insufficient space for the full run length of normal stairs, alternating tread stairs may be used. Alternating tread stairs allow for a safe forward-facing descent of very steep stairs. 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 degrees as opposed to standard stairs which are almost always less than 45 degrees. The advantage of alternating tread stairs is that people can descend face forward. The only other alternative in such short spaces would be a ladder which requires 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 not frequently accessed.

The image on the right illustrates the space efficiency gained by an alternating tread stair. The alternating tread stair appears in the image's center, with green-colored treads. The alternating stair requires one unit of space per step: the same as the half-width step on its left, and half as much as the full-width stair on its right. 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" steps.

There is often (here also) glide plane symmetry: the mirror image with respect to the vertical center plane corresponds to a shift by one step.

Alternating tread stairs have been in use since at least 1888.[10]

Ergonomics and building code requirements

On a rarely used staircase from before 1754, the Crown Princess of Sweden ascends from the Stockholm Palace garden with her husband and baby in 2013.
Staircase

Ergonomically and for safety reasons, stairs must have certain measurements so that people can comfortably use them. Building codes typically specify certain measurements so that the stairs are not too steep or narrow.

Nicolas-François Blondel in the last volume of his Cours d'architecture[11] (1675–1683) was the first known person to establish the ergonomic relationship of tread and riser dimensions.[12] He specified that 2 x riser + tread = step length.[13]

It is estimated that a noticeable mis-step occurs once in 7,398 uses and a minor accident on a flight of stairs occurs once in 63,000 uses.[14] Some people choose to live in residences without stairs so that they are protected from injury.

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.

UK requirements

Staircase between levels at Cabot Circus shopping centre, Bristol, England, United Kingdom. A sitting area is provided on the right of the staircase proper.

(overview of Approved document K – Stairs Ramps and Guards)[15]

Approved document K categorises stairs as ‘Private’, ‘Institutional or assembly’ and ‘other’

When considering stairs for private dwellings

Building regulations are required for stairs used where the difference of level is greater than 600 mm (23.6 in)

Steepness of stairs – Rise and Going – Any rise between 155 and 220 mm (6.1 and 8.7 in) used with any going (tread) between 245 and 260 mm (9.6 and 10.2 in) or any rise between 165 and 200 mm (6.5 and 7.9 in) used with any going between 223 and 300 mm (8.8 and 11.8 in)

Maximum Rise 220 mm (8.7 in) and Minimum Going 220 mm (8.7 in) 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 mm (21.7 and 27.6 in)

Construction of steps – Steps should have level treads, they may have open risers but if so treads should overlap at least 16 mm (0.6 in). 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 100 mm (3.9 in) diameter sphere cannot pass though the opening in the risers in order to prevent children from sticking their heads through them and potentially getting stuck.

Headroom – A headroom of 2 m (78.7 in) 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 stairs in shops or assembly building. There is no requirement for private stairs. In practice there will be fewer than 16 steps as 16 x 220 mm (8.7 in) gives over 3.5 m (137.8 in) total rise 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 mm (15.7 in) 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 600 mm (23.6 in). As domestic private stairs are likely to be used by children under 5 the guarding must be constructed so that a 100 mm (3.9 in) 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 (2.1 lbf/in).

Industry guidance on designing a staircase to UK Regulation and Standards.

US requirements

American building codes, while varying from State to State and County to County, generally specify the following parameters:[16][17][18]

Stairs and art

As much as stairs are very functional, stairs are can be very decorative and an impressive part of a building. Especially at the entrance of a large building stairs play an important role in the first impression of a building. In large buildings such as banks this is very popular. Modern companies and construction utilize the opportunities of functional stairs to actually upgrade buildings. Large utilities such as banks as well as residential buildings such as penthouses (e.g. in St George Wharf Tower) have modern and luxurious installations.

Notable sets of stairs

The world's longest stairway at the Niesenbahn funicular in Switzerland has 11,674 steps
A wide shot of the massacre on the "Odessa Steps" from The Battleship Potemkin (1925).

See also

An early kind of burglar alarm - trip stairs. The idea was that when a robber broke in, he would try to sneak quietly up the stairs in the dark, not knowing that on the way up there would be steps close together (like the second and third from the top), causing him to hopefully trip and wake someone up.

References

  1. 1 2 3 R.E. Putnam and G.E. Carlson, Architectural and Building Trades Dictionary, Third Edition, American Technical Publishers, Inc., 1974, ISBN 0-8269-0402-5
  2. ↑ "Banister". Merriam Webster dictionary. Retrieved 2 November 2010.
  3. ↑ Ruggeri, Stefania (2006) Selinunt, Affinità Elettive, Messina, ISBN 88-8405-079-0, p. 77
  4. ↑ "Staircase glossary". www.wonkeedonkeerichardburbidge.co.uk.
  5. ↑ Terminology. Stairplace.co.uk. Retrieved on 2013-11-11.
  6. 1 2 3 4 Neufert, Ernst; Neufert, Peter (2000). Architects' Data (3rd ed.). Blackwell Science. p. 191. ISBN 0-632-05771-8.
  7. ↑ Stair stringer calculator. Homeconstructionimprovement.com (2009-03-29). Retrieved on 2013-11-11.
  8. ↑ http://www.legislation.gov.uk/uksi/2010/2214/pdfs/uksi_20102214_en.pdf
  9. ↑ "Stairs". BBC.co.uk. 23 January 2001. Retrieved 2 November 2010.
  10. ↑ James H. Monckton (1891). Moncktons One Plane Method Of Hand Railing and Stair Building, Copyright 1888. John Wiley & Sons. p. Plate 2, Figure 4.
  11. ↑ "Vol. 4&5 (see pages 692&693)". Retrieved 14 August 2012.
  12. ↑ "Staircase". 1911encyclopedia.org. Archived from the original on 2004-12-16.
  13. ↑ "Generativeart.com Koutamanis, GA".
  14. ↑ "Stair Safety: A Review of the Literature and Data Concerning Stair Geometry and Other Characteristics". Retrieved 2 November 2010.
  15. ↑ The Building Regulations 2000: Protection from falling, collision and impact. Office of the Deputy Prime Minister. inbalance-energy.co.uk
  16. ↑ "A-Mezz Engineered IBC Stairs". Retrieved 2 November 2010.
  17. ↑ Codes. Amezz.com. Retrieved on 2013-11-11.
  18. ↑ Stairway Manufacturers' Association. Stairways.org. Retrieved on 2013-11-11.
  19. ↑ Section 3231(c) of the State of California Building Code specifies a rise between 4 and 7.5 inches (100 and 190 mm) and a minimum run of 10 inches (250 mm).
  20. ↑ "Popular Mechanics homepage". Retrieved 2 November 2010.
  21. ↑ NFPA 101 Life Safety Code Handbook Tenth Edition 2006, Coté and Harrington, ISBN 0-87765-697-5, pg.167
  22. ↑ "ADA 2010". ADA Compliance.
  23. ↑ "Guinness World Records".
  24. ↑ "Welcome to Lysefjord". Retrieved 2 November 2010.
  25. ↑ National Commission on Terrorist Attacks (July 22, 2004). The 9/11 Commission Report (PDF) (first ed.). W. W. Norton & Company. p. 294. ISBN 0-393-32671-3.
  26. ↑ "Monument". www.cityoflondon.gov.uk. City of London. 28 April 2009. Retrieved 28 April 2009.
  27. ↑ The Cascade Complex. Cmf.am. Retrieved on 2013-11-11.
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