What is chair bar? | Cutting Length Calculation Of Chair Bars | Uses | LCETED

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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