How to calculate the cutting length of chair bar in footings?/ Calculating the cutting length of bar chair in civil construction.

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

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.

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.

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.

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. 

  For BBS & cutting length of all types of structural members, click here.  

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