35
CHAPTER
Stairs
CHAPTER OUTLINE 35.1 STAIR FUNDAMENTALS
35.4 CONCRETE STAIRS
35.2 WOOD STAIRS
35.5 FREESTANDING CANTILEVERED STAIRS
35.3 STEEL STAIRS A stair, defined as a series of ascending (or descending) steps, is an important element that allows occupants to move vertically in a building. Architectural historians claim that the stair remained a purely functional element (without artistic overtones) until the end of the fifteenth century. The beginning of the sixteenth century, inspired by Leonardo da Vinci’s sketches, however, “signaled a new era of expression for the staircase” [35.1]. From then on, the staircase played an increasingly important visual role, often becoming a sculptural feature in a space, an imperial entrance to a public building or a significant facade element. The birth of the elevator—and, subsequently, the escalator—reduced the importance of the stair. More recently, the requirement to make buildings accessible to persons with disabilities further eroded its significance. Because a stair cannot be used by people in a wheelchair, it is no longer a mandatory feature of an entrance lobby. (Increasingly, entrance lobbies in contemporary public buildings are designed without a stair.) Consequently, stairs are reverting to their purely functional role—fulfilling the requirement as exit stair or standby vertical circulation in the event of electrical outage or mechanical interruption. However, despite the stair’s decreasing significance, in many contemporary buildings the rhythm and repetitive features of a stair have been transformed into an important aesthetic component of the interior space, as shown by the images in Figure 35.1. 35.1 . This chapter begins with a general introduction to stairs, followed by the details of construction of simple wood, steel, and concrete stairs.
861
Part 2 Materials and Systems of Construction
(a)
(b)
FIGURE 35.1 Images showing the aesthetic potential of stairs; see also Figure 35.20. (a) An entrance lobby stair with structural steel beams (stringers) and concrete-filled sheetsteel tread pans (see Figure 35.13). (b) A highly transparent glass wall showcasing the stair in the Madison Museum of Contemporary Art, Madison, Wisconsin. (c) Stair in the entrance lobby of the Madison Museum of Contemporary Art, Madison, Wisconsin with structural steel beams (stringers) and glass treads. (c) Architect: Cesar Pelli and Associates. 862
.
Tread width
Tread width
Tread width
Tread Nosing projection not to exceed 1-1/4 in.
Riser Riser height
(a)
Inclined projection not to exced 1-1/4 in.
(c)
(b) FIGURE 35.2 Tread, riser, and nosing configurations in a stair.
35.1 STAIR FUNDAMENTALS
NOTE
Because a stair provides vertical transportation, it is part of the means-of-egress (exit) system of a building. It is also a relatively hazardous element because injuries due to falls from stairs are not uncommon. For this reason, stair design is stringently controlled by building codes.
T READ , R ISER ,
AN D N OSING
There are two main components of a stair: treads and risers. A tread is the horizontal surface on which one walks. The riser is the vertical component that separates one tread from another. Generally, a stair has several treads and risers. For the sake of safety, the dimensions of treads and risers must be uniform in a stair. Building codes allow a small dimensional variation because perfect uniformity is unachievable. In walking on a horizontal or an inclined surface, an average person can comfortably traverse a distance of 24 to 25 in. in one step. Therefore, a rule of thumb generally used in proportioning the treads and risers of a stair is 2(riser height)
+
tread width
=
Treads and Risers of a Stair The treads and risers of a stair must meet the following dimensional requirements: Minimum tread width Riser height
Thus, if the risers in a stair are each 5 in. high, the tread width should lie between 14 and 15 in. The most commonly used dimensions for an interior public stair are 12- to 13-in. treads and 6-in. risers. Outdoor stairs generally have a smaller riser and hence a w ider tread. Building codes generally require a riser height between 4 in. and 7 in. and a minimum tread width of 11 in. In most stairs, the tread is a simple flat surface, and the riser is a solid vertical surface, Figure 35.2(a). Where space is limited, the effective tread width can be increased somewhat by inclining the risers, Figure 35.2(b), or by projecting the front edge of the tread beyond the riser, Figure 35.2(c). The front edge of a tread is referred to as the nosing. When an inclined riser or a projected nosing is used, the code-required minimum width of a tread does not change. In other words, the width of a t read is considered the horizontal distance between the vertical planes of the foremost projections of adjacent tre ads, as shown in Figure 35.2(b) and (c). The nosing of a tread is subjected to the maximum abrasion. In public stairs with heavy traffic, the treads should consist of a strong, dense material such as granite, highstrength concrete, or steel. Alternatively, a separate nosing (approximately 2 12 in. wide) consisting of an abrasion-resistant and skid-resistant material is epoxied or embedded into the tread. Stairs can also be constructed without risers, referred to as open-riser stairs, Figure 35.3. Because of safety concerns, open-riser stairs are subject to more stringent code restrictions than stairs with solid risers. For example, open-riser stairs are generally not allowed as exit stairs. Additionally, the clear vertical distance between the treads of open-riser stairs cannot exceed 4 in.
11 in.
4 to 7 in.
Residential Stair Building codes are less restrictive for a stair within a dwelling unit: Minimum tread width Riser height
24 to 25 in.
=
=
=
4 to 734
=
10 in.
in.
A 10-in. tread is allowed in a dwelling unit stair, provided that a nosing projection of 34 to 114 in. is used. If a nosing pro jection is not used, an 11-in. minimum tread width is required. Some local building codes may allow a riser height of greater than 7 34 in. for a residential basement stair.
S TAIR S HAPES The most commonly used stair shape is a U-shaped stair (in plan). It consists of two flights of stairs between floors with a midfloor landing (or simply a midlanding or
FIGURE 35.3 A stair with open risers. 863
landing), Figure 35.4. In addition to the U-shaped stair, some of the other commonly used stair shapes are
Floor level
• Straight-run stair with one or two flights, Figure 35.5: A straight-run stair with more than two flights can be used, but this is uncommon. • L-shaped stair, Figure 35.6(a): Where the space is limited, the landing of an L-shaped stair can be used for steps, yielding trapezoidal (pie-shaped) treads, referred to as winders, Figure 35.6(b). Stairs with winders are not as safe as those with rectangular treads, and their use in an exit stair is strictly controlled by building codes. • Circular stair: A circular stair may consist of all winders and can take many shapes. A spiral stair is a special type of circular stair in which the treads twist around a central column and are cantilevered from it, Figure 35.7. It is generally an open-riser stair. Again, building codes have several restrictions on the use of a spiral stair. A helical stair is a circular stair without a central supporting column (see Section 35.5).
Landing
Structural support generally required under landing.
Floor level
Thus, each flight is supported at floor level and at landing level; see also the freestanding stair in Figure 35.20. FIGURE 35.4 A U-shaped stair.
O PE N
AN D C LOSED S TAIRS
Stairs are also described as either open or closed. An open stair is exposed to the area below on one or more sides, whereas a closed stair is fully enclosed with a stair enclosure (stair shaft) and is usually accessed through a doorway.
W IDTH
OF S TAIR
The minimum width of a stair is determined by its purpose. When it is used as an exit stair, its width depends on the number of occupants it serves (occupant load) but is not less than 44 in. clear (between handrail and handrail) for an open exit stair or 48 in. for an enclosed exit stair. An exit stair for an occupant load of less than 50, or a stair within a dwelling unit, has a minimum width of 36 in.
f o t e h s i g i R l f
t h g i l f f o e s i R
f o t e h s i g i R l f
(a) A single-flight, straight-run stair
The rise of one flight of stair is generally limited by codes to a maximum of 12 ft.
(b) A two-flight, straight-run stair
FIGURE 35.5 Straight-run stairs; see Fig. 35.1(a).
f o t e h s i g i R l f
f o t e h s i g i R l f
(a)
Pie-shaped treades called winders
(b)
FIGURE 35.6 L-shaped stairs (a) without winders and (b) with winders. 864
t h g i l f f o e s i R
Chapter 35 Stairs
A spiral stair with a handrail on the left allows a right-hand grip on the handrail when walking down (preferred by some designers).
Floor
Floor Headroom 80 in. minimum
Ceiling
A spiral stair with a handrail on the right allows a right-hand grip on the handrail when walking up. FIGURE 35.7 A spiral stair.
FIGURE 35.8 Headroom in a stair.
H EADROOM The headroom in a stair is the minimum clearance between a tread and a projection above, Figure 35.8. Building codes generally require the headroom to be a minimum of 80 in. at any point on the stair.
G UARD U NI T , H ANDRAIL, B ALUSTERS ,
AN D N EWEL P OS T
The edge of a stair exposed to a change in height (i.e., not protected by the wall of the enclosure) must have a guard unit to protect against falling. The minimum height of a guard unit is 42 in., Figure 35.9(a). The clear distance of openings in a guard unit must not exceed 4 in. 865
A guard unit can be solid or open. A guard unit with an opening may consist of horizontal or vertical members or both. The clear dimension of an opening in a guard unit must be less than 4 in. Minimum guard unit height = 42 in.
Vertical members of a guard unit, called balusters Guardrail Handrail t h g . i n e i h l 8 i 3 a r o t d n 4 a 3 H
t h g i e . h n l i i m a r . d n r i - a 2 u 4 g
Newel post Baluster
(a) Guard unit and handrail in a stair
(b) Balusters and newel post in a wood stair
FIGURE 35.9 Guard unit, handrail, balusters and newel post.
The height of a handrail in a stair is generally required to lie between 34 in. and 38 in. The cross-sectional profile of a handrail is controlled by building codes to give it the required graspability. In some wood stairs, the first and/or the last vertical member of the guard unit (referred to as a baluster) is highlighted by using a more ornate design. Such a baluster is referred to as a newel post, Figure 35.9(b).
S TAIR L AYOUT
AN D S TAIR P LA N
In preparing a stair layout, we first determine the floor-to-floor height and then calculate the number of risers and treads. Assume that the floor-to-floor height in a building is 10 ft 8 in., that is, 128 in. Assume further that we would like the riser height to be appro ximately 6 in. Dividing 128 by 6 gives us the number of risers: Number of risers
21.3 6 Because the number of risers must be a whole number, assume 21 risers. Dividing 128 in. by 21 gives the exact riser height, 6.1 in. From the tread-riser relationship given earlier, the tread width is 24 (or 25)
-
=
2(6.1)
=
=
11.8 to 12.8 in.
We will use a tread width of 12.0 in. Assume further that a U-shaped stair is desired and the width of the stair is 4 ft. By code, the minimum width of the landing must be the same as the
EXPAND
YOUR KNOWLEDGE
Summary of Stair-Design Criteria 1. Tread depth and riser height must be dimensionally uniform throughout. 2. Minimum tread width = 11 in. Riser height = 4 to 7 in. (See Section 35.1 for exceptions for residential stairs.) 3. Nosing projection … 114 in. 4. Stair width = function of the occupant load, but not less than 48 in. for an enclosed exit stair, 44 in. for an open exit stair, or 36 in. for a stair serving an occupant load of less than 50 or a residential stair.
866
5. 6. 7. 8. 9.
Width of landing Ú stair width. Rise of one flight … 12 ft. Headroom Ú 80 in. Use of winders is restricted. Height of guardrail = 42 in. minimum. Height of handrail = 34 to 38 in. 10. Handgrip portion of the handrail must have a circular cross section between 114 in. and 258 in. Noncircular profiles must provide equivalent graspability.
Chapter 35 Stairs
LANDING
12
11 10
13
9
14
8
15
7
16
6
17 18
5 4
19
3
20
2
21
1 UP
Width of landing > Width of stair
Stair enclosure wall
It is a good drafting practice to number the risers in the plan of a stair.
FIGURE 35.10 Plan of a U-shaped stair (at the second floor level) that extends only from the first floor to the second floor.
Guard unit
width of the stair, that is, 4 ft. With these data, a plan of the stair can be drawn, as shown in Figure 35.10.
S TAIR -D RAWING C ONVENTIONS There are standard conventions for how stairs are shown in building plans. Figure 35.10 is the plan at the second-floor level of a U-shaped stair that extends from the first to the second floor only. The plan of the same stair at the first-floor level is shown in Figure 35.11(a). If the same stair were to extend over several floors, then the plan of the stair at a typical floor would generally be drawn as shown in Figure 35.11 (b).
Omitting a riser at midlanding and at a floor allows the handrail to turn without a pronounced vertical step. DN UP
(a) Plan of the stair in Figure 35.10 at the first-floor level
UP
(b) Plan of a multifloor stair at a typical floor
FIGURE 35.11 Stair plans at different levels. 867
F IRE -R ATED S TAIR ( S HAFT ) E NCLOSURE
Part 2 Materials and Systems of Construction
PRACTICE
Because most stairs in a building are used as exit stairs, they need to be enclosed by vertical enclosures (also referred to as shafts or shaft enclosures ). Generally, a shaft enclosure is required to be 1-h rated for a building up to four stories tall and 2-h rated for a building with five stories or more. Shafts are not required for individual single-family dwellings (up to four stories tall). Building codes also contain several other exceptions to the requirement of shaft enclosures. For example, shafts are not required if the stair connects only two floors and is not used as an exit stair.
QUIZ
Each question has only one correct answer. Select the choice that best answers the question. 1. An approximate formula generally used in determining the tread dimension (T) and riser dimension (R) in a stair is a. 2T + R = 24 to 25 in. b. 2T + 2R = 24 to 25 in. c. 2R + T = 24 to 25 in. d. R + T = 24 to 25 in. e. none of the above. 2. The minimum tread width required by building codes for a nonresidential stair is a. 11.0 in. b. 11.5 in. c. 12.0 in. d. 12.5 in. e. 13.0 in. 3. The minimum riser height required by building codes for a nonresidential stair is a. 6.0 in. b. 5.5 in. c. 5.0 in. d. 4.5 in. e. 4.0 in. 4. The maximum nosing projection allowed for a stair is a. 3.0 in. b. 212 in. c. 2 in. d. 112 in. 1 e. 14 in.
7. The rise of one flight of stair is generally limited by building codes to a. 7 ft. b. 8 ft. c. 10 ft. d. 12 ft. 8. Given a multistory building with a floor-to-floor height of 10 ft and an optimal riser height of 7 in., how many treads would you use for a U-shaped stair with a midlanding between floors? (The landing is not counted as a tread.) a. 15 b. 16 c. 17 d. 18 e. None of the above 9. A handrail and a guardrail in a stair are synonymous. a. True b. False 10. The minimum height of a guardrail in a stair is a. 34 in. b. 36 in. c. 38 in. d. 40 in. e. none of the above. 11. A stair constructed without risers is generally called a a. no-riser stair. b. closed-riser stair. c. open-riser stair. d. hollow stair.
5. A riser must be vertical. It cannot be inclined. a. True b. False
12. A stair with treads cantilevered from a central column is a a. circular stair. b. U-shaped stair. c. L-shaped stair. d. spiral stair. e. helical stair.
6. A U-shaped stair has been provided between the first floor and the second floor of a building with a midlanding. This stair has a. one flight. b. two flights. c. three flights. d. four flights.
13. The minimum width of a stair in a dwelling unit is a. 2 ft 6 in. b. 3 ft. c. 3 ft 6 in. d. 4 ft. e. none of the above.
35.2 WOOD STAIRS The most important parts of a wood stair are the carriages (also called rough stringers ). Carriages are the structural elements of a stair (inclined beams) and are specially cut to support the treads. Figure 35.12 shows a commonly used method of framing a wood stair.
P REFABRICATED W OO D S TAIRS There are several manufacturers who supply prefabricated wood stairs per the architect’s design. Prefabricated wood stairs are usually transported in a knocked-down (KD) version, where each part is uniquely numbered for assembly on site. They are commonly used for more ornate stairs requiring detailed millwork and craft, which are not usually possible at the site.
868
Floor frame
Floor frame
Stringer (or finish stringer); see detail sketch below. Gypsum wallboard
Wood ledger support for carriages. Alternatively, use joist hangers.
Wall frame Riser Tread Carriage (rough stringer), generally of 2-by lumber (or equivalent LVL member). The number of carriages required depends on the width of the stair and the spanning capability of the material used for the treads. For most residential stairs, three carriages are common. Landing frame supported on stud walls. Wood ledger support for carriages. Alternatively, use joist hangers. Carriage
Stud wall
Thrust blcok
(a) Framing of a typical wood stair
Gypsum board Stringer (or finish stringer), generally of 1-by finish lumber, nailed to wall frame over gypsum drywall Space between finish stringer and rough stringer is covered over by treads and risers. 2-by nailer block nailed to wall frame along the slope of the carriage
Carriage (rough stringer) nailed to nailer block
(b) Detail of rough stringer and finish stringer FIGURE 35.12 A commonly used framing system for a wood stair.
869
A 15 14 UP 13 12 11 10 9 8 7 6 5
Stud wall
Finished wood tread Finished wood riser Guard unit
Rough riser Rough tread
Omitting a riser at the landing (as shown here) allows the guardrail to turn without a pronounced vertical step
4 3 2 1
A
Rough stringer Finished wood flooring applied over rough treads and risers
Handrail
Finished wood tread Finished wood riser
(c) Plan of stair
Finished stringer height to match wall base Stud wall
Finished wood treads and risers applied directly over rough stringers
Handrail Nailer block between studs to support handrail
P Chamfer front edges of rough treads to allow carpet to wrap over neatly
Carpet applied over rough treads and risers
(e) Three alternative details at P
(d) Section A-A FIGURE 35.12 (continued ) A commonly used framing system for a wood stair.
35.3 STEEL STAIRS Stairs in public buildings are generally constructed of steel or concrete. Because steel stairs can be shop fabricated and brought to the site ready for installation, they are far mo re commonly used than concrete stairs. Another reason for the lack of use of concrete stairs is that their formwork is complicated and expensive. Prefabricated steel stairs are used in all types of public buildings, that is, steel- and concrete-frame buildings and load-bearing masonry buildings. They are particularly popular for exit stairs.
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Chapter 35 Stairs
One-piece, sheet steel bent to form a tread-riser unit Tread pan (in a thread-riser unit) to be site-filled with concrete Stringer. In this case, a structural steel channel section has been used, but the use of a steel plate is also common FIGURE 35.13 A typical prefabricated steel stair consists of two stringer beams (stringers) to which tread-riser units made from sheet steel are welded; s ee Figure 35.14 for details. (In this stair, the guard unit and handrail have not yet been installed.)
Site-filled concrete in tread pan; 1-1/2-in.-thick concrete fill is typical Sheet steel bent to form tread-riser unit. Sheet thickness is a function of stair width
A
Weld
Site-filled concrete in tread pan
Section A-A
A
Stringer (generally a steel channel; a steel plate may also be used); depth of stringer is a function of stringer span
FIGURE 35.14 Typical detail of tread-riser units welded to stringers. In this detail, tread pans are site-filled with concrete; see Figure 35.15 for alternatives.
A typical prefabricated flight of a steel stair consists of two stringer beams (stringers) to which tread-riser units made of sheet steel are welded, Figure 35.13. The tread pan is generally site filled with concrete, Figure 35.14 . For good wear resistance, a concrete strength of 5,000 psi is generally specified. Other tread finishes include a precast-concrete drop-in tread with a slip-resistant broom finish, Figure 35.15(a), and sheet steel with a raised, diamond-shaped checkered pattern, Figure 35.15(b). Factory-installed epoxy-aggregate fill or wear- and slip-resistant coatings can also be used.
S TRINGERS Stringers in a steel stair function as inclined beams, spanning from the floor to the landing and from the landing to the next floor. They generally consist of a structural-steel channel 871
Precast concrete tread (5,000psi concrete with broom finish for slip resistance, wire mesh reinforcing) adhered to steel tread with epoxy cement
Sheet steel with checkered or raised diamond pattern for slip resistance, bent to form treadriser combination
Stringer
(a)
(b)
FIGURE 35.15 Two alternative details of tread-riser units in a steel stair (precast concrete treads and checkered steel treads); another detail is shown in Figure 35.14.
or steel plate (16 in. or 14 in. thick is typical). The depth of stringers is a function of the stringer span and the structural loads required by codes. The t read-riser units span between the stringers. Figures 35.16 and 35.17 show typical det ails of support connections between the stringers and the floor of the building.
L ANDING F RAME The landing of a steel stair is generally framed with structural steel members as a unit, called a landing frame . Typical details of connections between stringers and landings are shown in Figure 35.18. The finish on the landing is generally the same as that on the treads. Thus, where site-cast concrete is used on treads, the landing is also topped with concrete. The landing frame may be supported on a beam (specially introduced for the purpose) between the upper and lower floors of the building, on (masonry or concrete) stair-enclosure walls, or on columns independent of the structural frame of the building. In most buildings, however, the landing frame for a prefabricated steel stair is supported by suspending it from the upper-level floor beams with steel hanger bars, Figure 35.19. A major advantage of a suspended landing is that it allows adjustment of the height of the landing with a few turns of the nuts. Additionally, the entire stair can be erected before constructing the walls of the stair enclosure.
35.4 CONCRETE STAIRS Although concrete stairs can be precast and prefabricated, their use is limited because they are heavy, which increases the cost of transportation and installation. Most concrete stairs are site cast. As previously stated, the formwork for concrete stairs is intricate, which increases the cost and causes construction delays. Their use is, therefore, infrequent, even in buildings with a reinforced-concrete structural frame.
872
Block-out in floor beam to be filled with concrete after installing stair Steel angle end support welded to stringers
Steel angle embedded in reinforcedconcrete floor beam
Stringer Stringer
(a) Connection between stair stringer and upped reinforcedconcrete floor beam
Block-out in floor beam to be filled with concrete after installing stair
Steel angle end support welded to stringers
(b) Connection betwen stair stringer and lower reinforcedconcrete floor beam
Steel angle embedded in reinforced-concrete floor beam
FIGURE 35.16 Typical details of the connection between stringers and a reinforced-concrete floor.
35.5 FREESTANDING CANTILEVERED STAIRS In the various stair types discussed so far, each flight is supported at the floor and landing levels. The use of steel and reinforced concrete, however, allows the stairs to be constructed without any supports at the landings (designed as cantilevers and supported only at the floors). Cantilevered stairs, also referred to as self-supporting stairs, can either be U-shaped or circular in plan. A cantilevered, self-supporting, U-shaped reinforced stair is shown in Figure 35.20(a). Figure 35.20(b) shows a cantilevered steel stair. In this stair, the stringers (of structural-steel channels) function as continuous spatially bent beams that are rigidly connected to the floor beams at both floors. Tread-riser units that span between stringer beams are made of structural-steel plate. Self-supporting circular steel or concrete stairs can be constructed with or without landings. Called helical (or helicoidal ) stairs, they are fairly common in steel, concrete, and wood. A helical stair is similar to a spiral stair but has no central column support.
873
Steel angle end support welded to stringers
Stringer Floor beam
(a) Connection between stair stringer and upper structural steel floor beam Stringer Steel angle end support welded to stringers Floor beam
(b) Connection betwen stair stringer and lower structural steel floor beam
FIGURE 35.17 Typical details of the connection between stringers and a steel-framed floor.
Metal deck supported by landing frame and topped with concrete (2-1/2-in. concrete fill typical)
Structural steel channel as front header of landing frame
Landing frame headed by structural steel channel
Stinger
Stinger
Structural steel channel as front header of landing frame Landing frame headed by structural steel channels
FIGURE 35.18 Typical details of the connection between stringers and the landing frame. 874
Metal deck supported by landing frame and topped with concrete (2-1/2-in. concrete fill typical)
Hanger bar for landing support Stringer
Landing frame
FIGURE 35.19 A typical steel stair with a suspended landing —a commonly used landing support system for exit stairs in concrete and steelframe buildings. (Guard units and handrail have not yet been installed.)
(a)
(b)
FIGURE 35.20 Two examples of cantilevered freestanding stairs, which are supported on the upper and lower floor structures, with no supports provided at midlanding levels. 875
PRACTICE
QUIZ
Each question has only one correct answer. Select the choice that best answers the question. 14. In a typical wood stair, inclined beams that are cut to allow for the support of treads are called a. rough stringers. b. finish stringers. c. balusters. d. all of the above. e. none of the above. 15. In a typical wood stair, the number of stringers required is determined by the a. width of the stair. b. spanning capacity of the carriage material. c. floor-to-floor height. d. thickness of the treads. e. all of the above.
c. floor-to-floor height. d. spanning capacity of the tread-riser units. e. all of the above. 18. In a typical prefabricated steel stair, the stringers are cut to accommodate treads and risers. a. True b. False 19. The stringers in a typical prefabricated steel stair are generally made of a. wide-flange sections. b. channel sections. c. plates. d. (a) and (b). e. (b) and (c). 20. The landing frame in a typical prefabricated steel stair is generally hung from the building’s structural frame. a. True b. False
16. In a prefabricated steel stair, treads and risers are generally two separate components. a. True b. False
21. A self-supporting cantilevered stair is supported on intermediate landings only. a. True b. False
17. In a typical prefabricated steel stair, the number of stringers required is determined by the a. width of the stair. b. spanning capacity of the stringers.
22. A self-supporting cantilevered stair can be made only of reinforced concrete. a. True b. False
REVIEW QUESTIONS 1. Provide the approximate expression used in proportioning the dimensions of the treads and risers of a stair. What is the basis for this expression? List building codes’ restrictions on the dimensions of treads and risers. 2. With the help of at least two sketches, explain what a flight of stairs implies. What is the code-mandated maximum height of a flight of stairs? 3. Using a sketch, explain the difference between a handrail and guardrail of a stair, and give their code-mandated heights. 4. List the factors that determine the width of a stair. 5. Using sketches, explain how a steel stair’s landing frame can be supported. Which one of these support methods is most commonly used? 6. Explain why a prefabricated steel stair is most commonly used even in buildings that are built with a reinforcedconcrete frame structure. 7. Using a sketch, describe a cantilevered freestanding stair.
876
