Technical details relating to TMT bars
as per BIS specifications

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REQUIREMENTS GOVERNING REINFORCEMENT AND DETAILING AS PER IS 456: 2000

1.General

Reinforcing steel of same type and grade is to be used as main reinforcement in a structural member. However, simultaneous use of two different types or grades of steel for main and secondary reinforcement respectively is permissible.

Bars may be arranged singly. or in pairs in contact, or in groups of three or four bars bundled in contact. Bundled bars shall be enclosed within stirrups or ties. Bundled bars shall be tied together to ensure the bars remaining together. Bars larger than 32 mm diameter shall not be bundled. except in columns

2.Development of Stress in Reinforcement

The calculated tension or compression in any bar at any section shall be developed on each side of the section by an appropriate development length or end anchorage or by a combination thereof.

Development Length of Bars
The development length Ld is given by

Ld = φσs/4τbd

where

φ = nominal diameter of the bar,
σs = stress in bar at the section considered at design load,
τbd = design bond stress

NOTES:
  • The development length includes anchorage values of hooks in tension reinforcement.
  • For bars of sections other than circular. The development Length should be sufficient to develop the stress in the bar by bond

For deformed bars, conforming to IS 1786 these values shall be increased by 60 per cent.

For bars in compression, the values of bond stress for bars in tension shall be increased by 25 per cent.

Bars bundled in contact, The development length of each bar of bundled bars shall be that for the individual bar, increased by 0 percent for two bars in contact, 20 percent for three bars in contact and 33 percent for four bars in contact.

3.Anchoring Reinforcing Bars

a. Anchoring bars in tension
  • Deformed bars may be used without end anchorages provided development length requirement is satisfied. Hooks should normally he provided for plain bars in tension.
  • Bends and hooks: Bends and hooks shall conform to IS 2502
  • Bends: The anchorage value of bend shall be taken as 4 times the diameter of the bar for each 45t' bend subject to a maximum of 16 times the diameter of the bar.
  • Hooks: The anchorage value of a standard U-type hook shall be equal to J6 times the diamet.er of the bar.
b. Anchoring bars in compression

The anchorage length of straight bar in compression shall be equal to the development length of bars in compression as specified in. The projected length of hooks, bends and straight lengths beyond bends if provided for a bar in compression, shall only be considered for development length.

4.Spacing of Reinforcement

Fs Percentage redistribution to or from section considered
-30 -15 0 15 30
Clear Distance between bars
N/mm2 mm mm mm mm mm
250 215 250 300 300 300
415 125 155 180 210 235
500 105 130 150 175 195
NOTE:

The spacing given in the table are not applicable to members subjected to particularly aggressive environments unless in the calculation of 'the moment of resistance, fs, has been limited to300N/mm2 in limit state design and σs limited to 16'N/mm2 in working stress design

5.Nominal Cover to reinforcement

a. Nominal Cover to meet durability requirements
Exposure Nominal Cover in mm not less than
Mild 20
Moderate 30
Severe 40
Very Severe 70
Extreme 70
NOTE:
  • For main reinforcement up to 12 nn diameter bar for mild exposure the nominal cover may be reduced by 5 mm.
  • Unless specified otherwise. actual concrete cover should not deviate from the required nominal cover by + I0 mm
  • For exposure condition 'severe' and 'very severe'. reduction of 5 mm may be made where concrete grade is M35and above.
b. Nominal cover to meet Specified Period of Fire Resistance
Fire Resistance Nominal Cover
Beams Slabs Ribs Column
Simply Supported Continuous Simply Supported Continuous Simply Supported Continuous
h mm mm mm mm mm mm mm
0.5 20 20 20 20 20 20 40
1 20 20 20 20 20 20 40
1.5 20 20 25 20 35 20 40
2 40 30 35 25 45 35 40
3 60 40 45 35 55 45 40
4 70 50 55 45 65 55 40
Source: IS 1786 (2008), IS 456 (2000)