Stair is the non-structural member having series of steps suitably arranged for the purpose of connecting different floor levels in the building. It is also called as vertical connectors. It provides easy, comfortable and safe upward and downward movement between the two different floor levels. It may be constructed of timbers, stone, steel or reinforced cement concrete. The room or space of the building in which stair is located is known as staircase.
Function of stair:
Provides a suitable means for the movement from one floor to another.
Provides vertical circulation in a building
Provides escapes route in case of fire, earthquakes and other emergency
Provides a degree of insulation
Elements of Stair:
Flight: series of steps without any platform break or landing in a direction
Landing: flat platform or resting provided at the head of series of steps.
Steps: portion of stair which permits accent and decent (Riser + Tread)
Riser: vertical portion of a stair providing support to the tread
Tread: horizontal upper surface of steps on which foot is placed
Strings: The slopping member which support the steps in the stairs. It is usually provided one at the outside and other adjacent to the wall.
Slope or Pitch: The angle between the horizontal line and line of nosing.
Soffit: under-surface of a stair
Baluster: vertical member that support handrail
Handrail: the member of stair at the top of baluster and acts as the support of hand of passerby.
Head room: the clear distance between the tread of a step and soffit of the flight or ceiling of landing immediately above.
Rise: the vertical distance of a flight (sum of riser)
Going: the horizontal distance between two consecutive rise (sum of tread of a flight)
Point to be known before stair-case design:
It should be located so as to provide easy access to the occupant of the building. It should be well lighted and ventilated directly from the exterior.
Residential building – central portion of building
Public building – Near the entrance
Apartment building – At central portion and other by the side building for easy emergency exit.
2. Width of stair:
The stairs should be wide enough to carry the user without much crowd and inconvenience. The minimum width of stairs for different types building are;
Residential building – 0.9 m
Institution building – 2.0 m
Educational building – 1.5 m
And other Public building – 1.5 m
Note: For reduced use staircases such as accesses to lofts, attics and terraces or for secondary staircases, the minimum width shall be 0.6 m.
3. Pitch /slope of stair:
The slope of stair should be just enough to be comfortable to the users. Generally the pitch of stair is taken between 25 and 40 degree. The stairs should be made flatter by introducing landing.
4. Head Room: The clear space between the tread and soffit of the stairs above should be more than 2 m
5. The width of landing should be more than or equal to the width of flight.
6. The number of riser per flight should not more than 15 and less than 3.
7. Winders should be avoided as far as possible. Winder are tapering steps which are provided for changing the direction of stairs.
8. Material of construction:
The materials used for construction should be such as to provide sufficient strength and fire resistance.
9. Steps proportions
The riser and tread of each steps should be of uniform dimension throughout. The rise and going should be of such dimensions to provide comfort to the users.
T + 2R = 600 – 640 mm
T * R = 400 – 450 cm2
T+R = 400 – 450 mm
10. Standard values:
For residential building: Tread (T) = 220 – 270 mm and Riser (R) = 150 – 170 mm
For public building: Tread (T) = 250 – 300 mm and Riser (R) = 100 – 150 mm
Types of Staircase:
On the basis of methods of construction
- Straight stairs
- Quarter turn stairs
- Dog-legged Stairs / Half-turn Stairs
- Open Well / Open Newel Stairs
- Spiral Stairs
- Bifurcated Stairs
On the basis of materials used for construction:
- Timber Stairs
- Steel Stairs
- Concrete stair
- Cast-in-situ stairs
- Pre-cast stairs
- Stone stairs
- Brick Stairs
Design of Staircase:
It is the calculation of number of steps with pre-assumption of comfortable access to desired floor. The size of steps should be assumed in such a way that stair can be accommodated in the given space with highest level of comfort. Stair is the center of focus in building and attention should be given for comfort and aesthetic beauty.
- Given, the level difference between two floors (ceiling height plus thickness of floor slab)
- Assume the types of building and stairs
- Assume the width of the stairs
- Consider the size of landing L equal to the width of stair
- Find out the height of one flight H1F
- Assume the height of riser
- Find the number of risers in a flight NR = H1F / size of riser
- Find the number of treads in a flight NT = NR -1
- Width of tread T = 600 – 2R
- Find out the length of stair = NT * T + L
- Adjust the entry space.
After the design of space occupied by the stair (ie. staircase), the stair is designed for strength. The design of stair is same as one way slab design.
Effective span of stairs:
The effective span of stairs can be calculated from IS 456:2000 clause 33. The effective span of stairs without stringer beam shall be taken as the following horizontal distances:
- Where supported to top and bottom riser by beam spanning parallel with the riser, the distance center to center of beams;
- Where spanning on the edge of landing slab where span parallel with the riser, a distance equal to the going of the stairs plus at each end either half width of the landing or one meter, whichever is smaller; and
- Where the landing slab spans in the same direction as the stairs, they shall be considered as acting together to form a single slab and the span determined as the distance center-center of the supporting beam or walls, the going being measured horizontally.
Load distribution on stairs:
[As per IS 456:2000 clause 33.2]
In the case of stairs with open well, where spans partly crossing at the right angle occur, the load on areas common to any two such spans may be taken as one-half in each direction. Where flights or landing are embedded into walls for a length of not less than 110 mm and are designed to span in the direction of the flight, a 150 mm strip may be deducted from the loaded area and the effective breadth of the section increases by 75 mm for the purpose of design.
The depth of section shall be taken as the minimum thickness perpendicular to the soffit of the staircase.