INTRODUCTION :
Staircases provide means of movement from one floor to another in a structure. Staircases
consist of a number of steps with landings at suitable intervals to provide comfort and safety for
the users.
PARTS OF STAIRCASE :
- Tread : horizontal upper
portion of a step. - Riser : vertical portion of a step.
- Rise : vertical distance between two
consecutive treads. - Flight : a series of steps provided
between two landings. - Landing : a horizontal slab provided
between two flights. - Waist : the least thickness of a stair slab.
- Winder : radiating or angular tapering steps.
- Soffit : the bottom surface of a stair slab.
- Headroom : the vertical distance from a line
connecting the nosings of all treads and the
soffit above. - Nosing : The edge of the tread projecting
beyond the face of the riser and the face of a cut
string.
R.C.C. STAIRS :
The various advantages of reinforced concrete stairs are given below :
• They have requisite fire resisting qualities to a great extent
• They are durable,strong, pleasing in appearance and can be
easily rendered non-slippery.
• They can be designed for greater widths, longer spans and any height.
• They can be moulded in any desired form to suit the requirements of the architect.
• They can be easily cleaned.
• The cost of maintenance is almost nil.
• They can be pre-cast or cast-in-situ.
DIFFERENT STAIR ARRANGEMENT IN CASE OF R.C.C. :
• Single straight flight stairs
• Inclined slab stairs with half space landings
• String beam stairs
• Cantilever stairs
• Spiral stairs
1) SINGLE STRAIGHT FLIGHT STAIRS:
Although simple in design and construction, is not popular because of the plan space it
occupies The flight behaves as simply supported slab, spanning from landing to landing.
The effective span/total horizontal going is usually taken as landing edge to edge by providing a
down stand edge beam to each landing. If these edge beams are not provided, the effective span would betaken as overall of the
landings, resulting in a considerably increased bending moment and hence more reinforcement.
INCLINED SLAB STAIRS WITH HALF SPACE
LANDINGS :
This type of stairs gives more compact plan layout
and better circulation than the single straight flight
stairs. The half space or 180o turn landing is introduced at
the midpoint of the total rise, giving equal flight
spans, thus reducing the effective span and hence
reducing bending moment considerably. In most designs, the landings span crosswise on to a
load bearing wall or beam and the flights span from
landing to landing.
STRING BEAM STAIRS :
In this category, the slab is supported on one side
by side wall side by a stringer beam I.e. a string or edge beam is used to span from
landing to landing to resist bending moment with
the steps spanning horizontally. In this case, the waist slab is thinner and an
overall saving in the concrete volume required
can be achieved, but this saving in material is
usually offset by the extra formwork cost
required for string beam. The string beam can be either up stand or down
stand in format and can be on both sides if stairs
are free standing. Each step is designed as spanning horizontally with the
bending moment equal to wll/8, where w is the uniformly
distributed load per unit area on the step, inclusive of the self
weight. Sometimes, for wider steps, a central string beam spanning
between the end walls or column is provided on which the
stairs slab is supported. The waist slab is designed as slab cantilevering from both the
sides of the string beam.
CANTILEVER STAIRS:
They are also called
spine wall stairs. They consist of a
central vertical wall
from which the Space
the flights and half
space landing are
cantilevered. The wall provides a degree of fire resistance between the flights and is therefore used mainly for the
escape stairs. Since both flights and landings are cantilevered the reinforcement is placed in the top of the flight slab
and in the upper surface of the landing to counteract the induced negative moment. The plan arrangement can be a single straight flight or two flights with half space landings.
R.C.C.SPIRAL STAIRS :
Mainly used as accommodation stairs in the
foyers of prestige buildings such as theatres,
banks, commercial complexes etc. Can be expensive to construct- normally at
least seven times the cost of conventional
stairs. The plan shape is generally based on a circle;
it is also possible to design an open spiral
stair with an elliptical core, which is known
as helical stairs. The spiral an be designed around a central
large diameter circular column, where the steps are cantilevered from
that, or in case of helical stairs, can be
designed as open circular well. A large amount of steel reinforcement is used to
resist the bending moment, shear force and
torsional moment. The continuous slab varies in thickness from top
to bottom- less at top and increasing at the
bottom. There are two or three sets of reinforcement with
top and bottom layer in each: Continuous bars running the length of
the spiral Cross or radial bars Diagonal bars laid tangential in two
directions to the inner curve.
PRE-CAST STAIRS :
Most of the concrete stair arrangements are possible to produce as precast concrete components
which can be:
• Individual steps units
• Complete flight with number of steps required
Common types of precast steps units :
• Rectangular cantilever steps
• Spandril cantilever steps
• Sector shaped cantilever units.
TOP:
In situ concrete staircase, the stairs are
supported on soft pads at discrete
points, a joint separate wall and stair.
MIDDLE:
Cast-in place concrete stair shaft with
prefabricated stair flights, the bottom
and top support for the flight include
sound insulation, the landings are
laterally supported on soft pads,
flights separated from wall by a joint.
BOTTOM:
Staircase made completely of
prefabricated elements, the flights are
extended to include the landings,
support points with sound insulation
layer, stair is separated from wall
elements.
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