What Is Chair Bar?
Chair reinforcement or simply chair bars are small structural elements used to properly place reinforcement bars in position and maintain the proper space between the top and bottom reinforcements.
Chair bars are mainly used in slab and footing. It is one of the most important parts of the raft foundations. The diameter of chair bar should not be less than 12 mm.
Why Chair Bars Are Used In Reinforcement?
The importance of chair bars is considerable. They are used for the following purposes.
1. To hold the reinforcement properly in position.
2. To maintain the distance between top reinforcement and bottom reinforcement.
3. To maintain the cover of top and bottom bar.
4. To provide vertical support to the upper cage reinforcement and lower cage reinforcement in footing and slab.
5. To provide strong supports to the bars.
6. To increase tension property of concrete. Thus it helps to prevent the structure from collapsing due to the failure in tension zone.
7. To hold the reinforcement properly so that the concrete may surround the reinforcement from all directions (from the bottom also).
8. To minimize the tidiness of work and to avoid messing up the reinforcement.
9. To hold the upper cage bar from collapsing during vibrating.
10. To withstand labour’s weight and fresh concrete weight.
11. Finally, when concrete hardens, chairs remain trapped in the concrete, making no problem at all for later.
Parts Of Chair Bar:
Head Of Chair: The top horizontal part of chair bar on which the upper cage reinforcement rests is called head of chair.
Height Of Chair: The vertical distance of chair bar is called height of chair.
Leg Of Chair: The bottom vertical part of chair bar is known as leg of chair.
How To Calculate Cutting Length Of Chair Bar:
Height of footing = 600 mm
Diameter of top and bottom main bars = 20 mm
Diameter of top and bottom distribution bars = 16 mm
Clear cover (top & bottom) = 50 mm
Before calculating the cutting length, we need to calculate each part length of chair bar.
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)
= 600 – ( 2 x 50) – 20 – (20 + 16)
= 444 mm
Head Of Chair:
Length of Head = (2× Spacing of distribution bar) + (2×50)
= ( 2 x 150) + ( 2 x 50)
= 400 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 = Head of chair + (2 x height of chair) + (2 x leg of chair) – 4 bends of 90°
= Head of chair + (2 x height of chair) + (2 x leg of chair) – 4 x 2d
444 + (2 x 400) + (2 x 350) – 4 x 2 x 12
= 1848 mm = 1.8 m
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