What Is Chair Bar? | Cutting Length
Calculation Of Chair Bars | Uses
What Is Chair Bar?
Chair reinforcement or
simply chair rods are small structural elements that are used to place the
reinforcement bars in the correct position and maintains the correct space
between the top and bottom reinforcements.
Chair bars are used
primarily on the slab and flooring. The diameter of the chair bar should not be
less than 12 mm.
Application Of Chair Bars
Why Chair Bars Are Used In
Reinforcement?
1. Maintain the required spacing between the top and
bottom reinforcement cage in footings, slabs, raft foundations, etc.
2. Maintaining a clear cover for reinforcement during
concrete vibration.
3. To protect the upper and lower cages from
displacement and sagging during concreting due to movement of trolleys, walking
of workers, dead load of new concrete etc.
4. Provide additional support for reinforcement bars.
5. To increase the tension property of concrete. This
helps to prevent structural collapse due to failure in the tension zone.
Given Data:
Height of footing = 550
mm
Top and bottom main bars Diameter = 16 mm
Top and bottom distribution bars Diameter = 12 mm
Clear cover (top & bottom) = 25 mm
Area of Footing = 2sqm
Before calculating the cut length of chair bar, we need
to calculate the length of each section of the chair bar.
Sections
Of Chair Bar:
Head of chair: The top horizontal part of the chair bar where the top
cage reinforcement is located is called the head of the chair.
Chair height: The vertical distance of the chair bar is called the
height of the chair.
Leg of chair: The bottom
vertical part of chair bar is known as leg of chair.
Height of chair:
Height
of chair = Height of footing – ( 2 x Clear cover) – (Dia of
bottom main bar) – (Dia of top main bar + Dia of top distribution Bar)
= 550 – ( 2 x 25) – 12 – (16 + 12)
= 460 mm
Head Of Chair:
Length
of Head = (2× Spacing of distribution bar) + (2×25)
= (2 x 150) + ( 2 x 25)
= 350 mm
Leg of chair:
Leg
of chair = (2 x Spacing of bottom main bar) + 50
= ( 2 x 150) + 50
= 350 mm
Cutting
length of chair bar:
Cutting length of chair bar = chair Head+ (2 x chair
height) + (2 x chair leg) – 4 bends of 90°
= Head of chair + (2 x height of chair) + (2 x leg of
chair) – 4 x 2d
= chair Head+ (2 x chair height) + (2 x chair leg) – (4 x
2d)
= 350 + (2 x 460) + (2 x 350) – 4 x 2 x 12
=
1874 mm = 1.9 m
Number
of chair bars required in 2sqm
NOTE: Consider giving 1nos. Chair bar per
square meter.
The no. of chair bars = Area of structural reinforcement
in sqm / chair bar per sqm. = 2sqm/1sqm = 2nos.
Total
Cutting Length Of Rod = 2nos x 1.9 m = 3.6m
FAQ
What
should be the diameter of chair bars used in construction?
Typically, chair bars are made of reinforcement with a
diameter of 12 mm and above. The most common dia bars are 12mm, 16mm, and in
some cases 20mm.
In rare cases, it is
advisable to use 10 mm bars on the slabs when there is no additional external
load other than the reinforcement cage load.
The reason for not using
dia Bars lesser then 12 mm, which bend or hook to each other for loads, due to
which the main purpose of providing the chair bar is useless.
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