507_33_powerpoint-slides_Ch17_DRCS.ppt

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Design of Reinforced Design of Reinforced Concrete StructuresConcrete Structures

N. Subramanian


Chapter 17Chapter 17

Design of StaircasesDesign of Staircases


Introduction

Reinforced concrete (RC) stairs are an important component of a building and often the only means of providing access between the various floors of a building.

The staircase essentially consists of landings and flights. Often, the flight is an inclined slab consisting of risers and treads (collectively called the going of staircase), whereas the landing is a horizontal slab (see Fig. 17.1).

From a structural point of view, a staircase consists of slab or beam elements.


Fig. 17.1 Components of a staircase (a) Plan of staircase (b) Terminology used (c) Part section


Definition of Terms

Tread or going of step: Tread is the horizontal upper portion of a step where the foot rests. Going of step is the horizontal distance of the tread minus the nosing.

Nosing: Sometimes, the tread is projected outwards for aesthetics or to provide more space; this projection is called the nosing. Many times, the nosing is provided by the finishing over the concrete tread (see Fig. 17.1c).

Riser and rise: Rise is the vertical distance between two consecutive treads and riser is the vertical portion of the step.


Flight or going of stair: Flight is a series of steps provided between two landings. Going of stair is the horizontal projection of the flight.

Landing: Landing is the horizontal slab provided between two flights. It is provided every 10–14 steps for comfort in climbing. Landing is also provided when there is a change in the direction of the stairs.

Overlap: The amount by which the nosing of a tread (or landing) over sails the next lower tread (or landing) is called the overlap.

Waist: It is the least thickness of a stair slab.

Definition of Terms


Winder: The radiating or angular tapering step is called winder.

Soffit: It is the bottom surface of a stair slab.

Headroom: The vertical distance of a line connecting the nosings of all treads and the soffit is referred to as the headroom.

Steps may be of three types as follows (see Fig. 17.2): (a) Brick or concrete steps on inclined slab(b) Tread-riser steps(c) Isolated steps

Definition of Terms


Fig. 17.2 Type of steps (a) Steps on waist slab (b) Slabless tread-riser (c) Isolated steps


Types of StaircasesSome of the most common geometrical configurations are shown in Fig. 17.3, which include the following:

1. Straight flight stairs with or without intermediate landing (Figs 17.3a and b)

2. Quarter-turn stairs (Fig. 17.3c)3. Half-turn stairs, also referred to as dog-legged or scissor-type

stairs (Fig. 17.3d)4. Branching stairs (Fig. 17.3e)5. Open-well stairs (half-turn) (Fig. 17.3f) and quarter-turn landing

(Fig. 17.3g)6. Spiral stairs (Figs 17.3h and i)7. Helicoidal stairs (Fig. 17.3j)


Types of Staircases

Fig. 17.3 Plan views of various types of stairs (a) and (b) Straight flight stairs (c) Quarter-turn stairs (d) Half-turn stairs (e) Branching stairs (f) Open-well (half-turn) stairs (g) Open-well stairs with quarter-turn landing (h) Part-circular stairs (i) Spiral stairs (j) Helicoidal stairs


Types of Staircases

Spiral, helical, circular, and elliptical stairs are also referred to as geometrical stairs.

The type of stair and its location are selected based on architectural considerations, such as accessibility, function, comfort, lighting, ventilation, and aesthetics, as well as structural and economic considerations.

Free-standing stairs, which are similar to dog-legged stairs in plan, but with their landing unsupported, provide an elegant appearance. They are three-dimensional structures and have to be fixed at both the top and bottom ends for stability, as shown in Fig. 17.4.


Free-standing Stair

Fig. 17.4 Typical free-standing stair (a) Plan (b) Section (c) Isometric view


Free-standing Stair at BWI Airport, USA


Structural Classifications

For design purposes, stairs are classified into the following two types, depending on the predominant direction in which the slab of the stair deflects in flexure:

1. Transversely supported (transverse to the direction of movement in the stair)

2. Longitudinally supported (in the direction of movement)


Transversely Supported Stairs

Transversely supported stairs include the following types:

1. Simply supported steps supported by two walls or beams or a combination of both (see Fig. 17.5a)

2. Stairs cantilevering from a central spine beam (see Fig. 17.5b)

3. Steps cantilevering from a wall or a beam (see Fig. 17.5c). The detailing of stair slab when concrete or brick step is adopted is also shown in Fig. 17.5(d). It has to be noted that the tread-riser type of arrangement is also employed as cantilevers.


Fig. 17.5 Transversely supported stairs (a) Supported between two stringer beams or walls (b) Doubly cantilevered from a central spine beam (c) Cantilevered from spandrel beam or wall (d) Detailing of cantilever stair for concrete and brick steps



When the slab is supported at the two sides by stringer beams or walls as shown in Fig. 17.5(a), it should be designed as simply supported. The stringer beam in Fig. 17.5(a) may also be provided as an upstand stringer.

The spandrel beam (see Fig. 17.5c) is subjected to equilibrium torsion in addition to bending moment and shear. Although the slab may be spanning transversely, the spandrel and spine beams of Figs 17.5(a) and (c), respectively, span longitudinally between the supporting columns.



When the slab is doubly cantilevered from the central spine beam as in Fig. 17.5(b), it is better to check for the case of loading on one side of the stair slab, which may induce torsion in the spine beam. This condition may also dislodge the slab from the beam if proper detailing is not provided.

The detail as shown in Fig. 17.5(b) may prevent such a separation, as the stirrups of the beam will anchor the slab into the beam, provided the stirrups are designed to take into account torsion as well.


Longitudinally Supported Stairs

These stairs span between the supports at the top and bottom of a flight and are unsupported at the sides.

Longitudinally supported stairs may be supported in any of the following ways:

1. Internal beams at the ends of the flight in addition to beams or walls at the outside edges of the landings (see Fig. 17.6a)

2. Beams or walls at the outside edges of the landings (see Fig. 17.6b)


Fig. 17.6 Types of stairs spanning longitudinally (a) Support at top and bottom risers (b) Landing slab spanning in the same direction as stairs (c) Supported on the edge of landing slab


Longitudinally Supported Stairs3. Landings that are supported by beams or walls running in the

longitudinal direction (see Fig. 17.6c)

4. A combination of these three methods

5. Stairs with quarter landings associated with the open-well stairs

In all these cases, we may adopt either the waist slab (Fig. 17.2a) or the tread-riser type (Fig. 17.2b).

The slab thickness depends on the effective span, which should be calculated according to the boundary condition.


Longitudinally Supported Stairs

All these staircases require four columns in plan. Of these, two columns should be used to support the landing beam.

Stringers, treads, or complete flights and landings can be precast, depending upon the nature of the project, site, and size of the crane, as shown in Fig. 17.7.

Care should be exercised when detailing the junctions of in situ concrete with precast units to avoid unsightly finishes.


Fig. 17.7 Precast concrete flight, landing, or stringers (a) Precast flight (b) Precast concrete flight with landing (c) Precast stringers


Effective Span

Following are the rules for calculating the effective spans, depending on the way the stair slab is supported:

1. When the stairs span longitudinally and are supported at the top and bottom by beams, as shown in Fig. 17.6(a), the effective span is the distance between the respective centres of beams.

2. When the stairs span longitudinally with the landing slab also spanning in the same direction as the stairs, as shown in Fig. 17.6(b), the effective span is the centre-to-centre distance (c/c) between the supporting beams or walls.


Effective Span

3. When the stairs span longitudinally and are supported by landings on top and bottom, which span in the transverse direction (perpendicular to the stairs), as shown in Fig. 17.6(c), the effective span is to be taken as the total going of the stair plus half the width of the landing on each end or one metre, whichever is smaller.

4. In the case of stairs spanning transversely (horizontally in the transverse direction), as shown in Fig. 17.5, the effective width of the stair is taken as the effective span.


Stair with Landing Supported on Three Sides

The landing slab running at right angles to the direction of the flight and supported by walls or beams on three sides, as shown in Fig. 17.8, are common in residential buildings. The Indian code does not have provisions for this case.

There are two critical locations for the flexural design of such stairs: (a) The mid-span location for positive moment(b) The kink location, where the landing slab meets the inclined waist slab, for negative moment.

The effective length may be taken as the going of this type of stair, as IS 456 provisions are very conservative[Ahmed, et al.(1995,1996)]


Effective Span

Fig. 17.8 Stair with landing slab supported on three sides


Loads on Stair Slabs

The dead load to be considered on the stairs includes the following:1. Self-weight of stair slab (waist slab, tread-riser slab, or individual

steps)2. Self-weight of step 3. Self-weight of finish

The imposed loads are assumed to act as uniformly distributed loads on the horizontal projection of the flight, that is, on the going of staircase, as well as on the landing.


Loads on Stair Slabs

Type of Staircase Imposed Load (kN/m2)

Service stairs for maintenance in watertanks, catwalks, etc.

1.5

Staircase in residential buildings 3.0

Staircase in offices and public buildings 5.0

Staircase with isolated steps 1.3 kN/step*

* This concentrated load should be applied at the free end of each cantilever step.

Table 17.2 Imposed load on staircases as per IS 875(Part 2):1987


Distribution of Loads on StairsAccording to Clause 33.2 of IS 456, the following distribution of loads may be taken:

1. In the case of stairs with open wells, when a staircase takes a right-angled turn, the load on areas common to any such span (usually in landings) may be taken as 50 per cent in each direction, as shown in Fig. 17.9(a).

2. When a longitudinally spanning flight or landing is embedded by at least 110 mm into walls, the loading may be assumed to act on a reduced width of flight, due to partial two-way action. The code permits this reduction in width as 150 mm, as shown in Fig. 17.9(b). It also suggests increasing the effective breadth of the section by 75 mm.


Fig. 17.9 Distribution of loading on stairs (a) Open well stairs (b) Stairs built into the walls


Design of Stair Slabs Spanning TransverselyIsolated Tread Slabs

These slabs are designed as cantilever slabs.

It is important to anchor the top bars into the support.

In earthquake zones, equal amount of bottom bars with adequate anchoring has to be provided to resist stress reversals.

It is necessary to provide proper chairs for the main bars so that they remain at the top face during concreting operations.


Cantilevered Stairs


Design of Stair Slabs Spanning Transversely

Slabless StairsIn these stairs, each tread-riser unit, consisting of the riser slab and one half of tread slab on either side, is assumed to act independently as a beam having a Z-section, as shown in Fig. 17.10 (Next Slide).

The main bars are placed at the top or bottom of the riser portion, depending upon whether the system is cantilevered or simply supported.

Nominal distributors in the form of stirrups are provided.


Transversely Spanning Tread-riser Stair

Fig. 17.10 Transversely spanning tread-riser stair (a) Typical tread-riser arrangement (b) Tread-riser unit taken for design as Z-section (c) Detailing of tread-riser stair


Design of Stair Slabs Spanning TransverselyStairs with Waist Slab

In this type of stair, the longitudinal axis of the flight is inclined to the horizontal, and the steps form a series of triangles on top of the waist slab.

If the steps are also made of concrete, nominal reinforcement, in the form of stirrups, are provided in the steps to prevent the cracking of nosing.

The main bars are provided transversely, at the top or bottom, depending upon whether the slab is cantilevered or simply supported.


Design Of Stair Slabs Spanning Longitudinally

Slabless stairsThe aesthetic appeal of tread-riser stair is lost if the slab thickness exceeds the riser, R. Hence, the effective span for these stairs is usually kept below 3.5 m.

The bending moments are considered to occur in the longitudinal direction in the riser as well as treads. Each tread slab is subjected to a bending moment combined with shear force, whereas the riser slab is subjected to a constant bending moment and an axial force (see Fig. 17.11(a).

The reinforcement detailing is shown in Fig. 17.11(b) can resist the negative bending moment near the supports, arising out of any partial fixity ..

`


Longitudinally Supported Tread-riser Stairs

Fig. 17.11 Longitudinally supported tread-riser stairs (a) Bending moment and shear force diagram (b) Detailing of reinforcement


Design Of Stair Slabs Spanning LongitudinallyStairs with Waist Slab

The slab is designed as a simply supported slab.

The reinforcements are placed longitudinally as shown in Figs 17.12 and 17.13 (Figure 17.12 shows the detailing for stairs supported at the ends of landing and Fig. 17.13 shows the detailing for stairs supported at the ends of flights).

Detailing of bars should be properly done at the junction of the flight and landing slab.

The distributor bars are provided in the transverse direction, along the width of the waist slab.


Fig. 17.12 Detailing of dog-legged stair supported at the ends of landing


Fig. 17.13 Detailing of dog-legged stair supported at the ends of flights


Design Of Stair Slabs Spanning Longitudinally

Free-standing StairsThe lower flight is subjected to axial compression, bending, and torsion, whereas the upper flight has to resist axial tension, bending, and torsion.

The landing slab has to be stiff in its own plane in order to connect the two out-of-plane flights effectively.

The flight reinforcement should be well anchored into the supporting beams at the top and bottom floor levels, and these beams should be designed and detailed carefully to resist the forces and moments introduced by the flights.


Free-Standing Stair at Marlins BallPark, Miami, Florida


Helicoidal Staircases

A Helicoidal stair is a stair describing a helix around a central void and the shape is generated by moving a straight line touching a helix such that the moving line is always perpendicular to the axis of the helix (see Fig. 17.14).

The critical stress resultants acting on the girder are the bending moment about the two principal planes and torsional moment, transverse shear, and axial thrust.

The connecting slabs or beams at the floor levels must be designed to provide the required fixity at the ends of the Helicoidal girder.


Helicoidal Staircases

Fig. 17.14 Helicoidal staircase (a) Elevation (b) Plan (c) In a hospital-cum- residence in Panrutti, Tamil Nadu


Fig. 17.15 Helicoidal stair notations

Where ϕ is the angle measured from the middle point of the curve of the slab


Earthquake Considerations

In general, RC staircases are built integrally with the structural system of the building, even though they are analysed as isolated systems.

The elements of these staircases, such as the flight slabs and landing slab, act as diagonal braces and attract large lateral forces during an earthquake, thereby incurring damage (Fig. 17.16a).

The provision of a sliding support will prevent the stair slab from acting as diagonal bracing (Fig. 17.16b and c).

Stairs with landings are not normally reinforced to act as compression braces and can be expected to fail in a very brittle manner.


Failure of Staircases during Earthquakes

Gujarat earthquake, 26th Jan. 2001

Christchurch Earthquake, 22nd Feb. 2010



Fig. 17.16 Earthquake effects (a) Damage locations due to diagonal bracing effect (b) Location for sliding support (c) Detail at the sliding support



The beams supporting the landing slab of dog-legged stairs will also cause the secondary effect of short columns, in addition to causing the twist of the building due to stiffness irregularity in plan, if they are not located centrally.

Short-column effect results in enhanced shear demand with additional stiffness introduced at intermediate levels.

Axial load also increases in these columns due to increased rigidity of the particular bay. This increase in both axial and shear forces may result in brittle failure of these short columns. Hence, it is important to include the stairs in the modelling of the structure.


Separated Staircases

The staircases are completely separated and built on a separate RC structure by providing adequate gap between the staircase tower and the building to ensure that they do not pound each other during shaking caused by a strong earthquake (see Fig. 17.17).

The opening at the vertical joints between the floor and the staircase may be either covered with a tread plate attached to one side of the joint and sliding on the other side or covered with some appropriate material that could crumble or fracture during an earthquake without causing structural damage.


Separated Staircases

Fig. 17.17 Separated staircases (a) Plan (b) Section at X–X (c) Detail at A


Built-in Staircase

This is done by providing rigid walls at the stair opening, as shown in Fig. 17.18. Under such circumstances, the joints as provided in separated staircases will not be necessary.

The two walls enclosing the staircase should extend through the entire height of the stairs and to the building foundations.


Built-in Staircase

Fig. 17.18 Rigidly built-in staircase (a) Plan (b) Section Y–Y (c) Section at X–X


Staircases with Sliding Joints

This strategy is used where it is not possible to provide rigid walls around stair openings; to adopt the separated staircase, sliding joints should be provided as shown in Fig. 17.16(b) so that they will not act as diagonal bracing.

As the stairs provide vital link of communication and services, they should be designed for higher safety factor when com pared with the other structural elements.

An important factor of 1.5 must be applied to the staircases located in earthquake zones.


Fire Protection

The fire protection rating for the staircase should be at least 30 minutes more than that assigned to the building.

It is better to provide a cover not less than 25 mm. The minimum thickness of slabs in the staircase should be 110 mm.

More importantly, the fixtures and railings must be fireproof.

Fire-resistant fibreglass covers should be used for the railings and steps instead of plastic covers.


Poor Quality of Concrete in Stairs


Case Study


Case Study (continued)

Source: http://www.stuff.co.nz/national/christchurch-earthquake/5733033/Stair-work-not-done-when-quake-hit


Thank You!

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