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How to Find the Cutting Length of Chair Bar in Footing | Why Chair Bars are Used in Reinforcement

In this Article today we will talk about the Cutting Length of Chair Bar | Why Chair Bars are Used in Reinforcement | Parts of Chair Bar | Chair Bar Segments | How to Calculate Chairs in Slab | Chair Bar Calculation Formula | Chair Bar in Footing | What is Chair Bar

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 Bar Segments

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.

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How to Find the Cutting Length of Chair Bar in Footing

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. Chair Bar Segments
  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 andllower 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 fai ure 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. How to Calculate Chairs in Slab

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. How to Calculate Chairs in Slab

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.


Before Calculating the Cutting Length of Chair Bar you must read this article:

Bar Bending Schedule Basic Formulas | BBS Formula | What is BBS


How To Calculate Cutting Length Of Chair Bar:

The chair bars are placed in the footings to support the top and bottom reinforcement cage as shown below.

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You keep it in your mind that, always the top head or supporting part of the chair should be tied to the bottom layer bar ( top distribution bar ) of the upper cage & the bottom leg part of the chair should be anchored to the bottom layer bar (bottom main bar ) of the lower cage.

For your understanding, I have given a drawing of a chair bar showing three parts in it, i.e. head, height & leg.

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Chair Bar Calculation Formula:

From the above drawing, we can say that

The cutting length of the chair bar

= [{(2nos.× height ) + (1no. × head ) + ( 2nos. × leg)} – (4nos. × 90° bend )]

= [ 2H + E + 2L – 8d ].
Let us now redraw the above-given footing with dimensions & specifications for the calculation purpose.

Example No.1:

Given Data:

The diameter of the chair bar = d =12mm.
Top & bottom clear cover = 50mm.
Depth of footing = 750mm.
Top & bottom main bar dia. = 20mm, spacing @ 150c/c.
Top & bottom distribution bar dia. = 16mm, spacing @ 150 c/c.

Now, we will calculate the length of the three parts of the chair bar separately.

1. Head ( E ):

The length of the head

= [(2nos.× c/c distance between the top distribution bar) + (extra length on either side of the top distribution bar)]
= [( 2nos. × 150mm. ) + ( 2nos. × 50mm. )]
{ Here, 50mm. is taken as the extra length. }
= [ ( 300mm. + 100mm. ]

400mm.

2. Height (H ):

Height of the chair

= [(footing depth ) – {(2nos. × clear cover ) + ( dia. of top main bar + dia. of top distribution bar + dia. of bottom main bar)}]
= [(750mm ) – {( 2nos. × 50mm.) + ( 20mm + 16mm + 20mm )}]
= [750mm – { 100mm + 56mm }]
= [ 750mm – 156mm ]

= 594mm.

3. Leg ( L ):

The length of the leg

=  [(2nos.× c/c distance between the bottom main bar) + (extra length on either side of the bottom main bar)]
= [( 2nos. × 150mm. ) + ( 2nos. × 50mm. )]
{ Here, 50mm. is taken as the extra length. }
= [ ( 300mm + 100mm. ]

400mm.

The cutting length of the chair bar:

= [ 2H + E + 2L – 8d ]

= [{(2nos.× height ) + (1no. × head ) + ( 2nos. × leg)} – (4nos. × 90° bend )]
= [{(2nos. × 594mm. ) + ( 1no. × 400mm.) + ( 2nos. × 400mm. )} – ( 4nos. × 2d )]
{Here, 2d is taken for the 90° bend, where d is the dia. of chair bar.}
= [ {1188mm. + 400mm. + 800mm.} -( 4nos. × 2 × 12mm )]
= [ 2388mm – 96mm ]
2292mm.
= 2.292m

Example No.2:

Evaluation of Cutting Length of Chair Bar:

Given Data:

Height of footing- 600mm

Diameter of top and bottom main bars-20mm

Diameter of top and bottom distribution bars-16mm

Clear cover(top and bottom)- 50mm

Solution:

At first we need to know about to calculation of each segment of length of chair bars-

Height of Chair = Height of footing – (2 * clear cover) – ( dia of bottom main bar) – ( dia of top main bar + dia of top distribution bar)

=600 – (2 * 50) – 20 – (20 + 16)

=444mm

Head of Chair: Length of head = (2 * spacing of distribution bar) + (2 * 50)

= (2 * 150) + (2 * 50)

= 400mm

Leg of Chair= (2 * spacing of bottom main bar) + 50

= (2 * 150) + 50

= 350mm

Cutting Length of Chair Bar = Head of Chair + (2 *  Height of Chair) + (2 * Leg of Chair) – 4 bend of 90 degree

= Head of Chair + (2 *  Height of Chair) + (2 *  Leg of Chair) – 4 * 2d

= 444 + (2 *  400) + (2 *  350) – 4 * 2 * 12

=1848mm

=1.8m

Note: 

  1. At least the head and the leg part of the chair should cover 3 nos. of rebars ( as shown in the above drawings ) to have a firm grip and proper positioning of the upper & lower cage of the footing.
  2. For the general calculation purpose 50mm. is taken as the extra length.

OTHER POSTS:

Basics of Bar Bending Schedule Formulas | BBS Formulas with Example

Bar Bending Schedule for RCC Box Culvert in Excel | Download Sheet

Bar Bending Schedule for Tie Beams/Strap Beams | BBS of Tie Beam

Bar Bending Schedule for footings | Steel Estimation for footings

Bar Bending Schedule (BBS) | BBS Step by Step Preparation | Excel Sheet


Conclusion:

Full article on Cutting Length of Chair Bar | Why Chair Bars are Used in Reinforcement | Parts of Chair Bar | Chair Bar Segments | How to Calculate Chairs in Slab | Chair Bar Calculation Formula | Chair Bar in Footing | What is Chair Bar. Thank you for the full reading of this article in “The Civil Engineering” platform in English. If you find this post helpful, then help others by sharing it on social media. If any formula of BBS is missing from this article please tell me in comments.

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