Engineer Zone by JSW Neosteel | Structural Design Support

Fundamentals of reinforcement design

Reinforcing steel of the same type and grade should be used as the main reinforcement in a structural member. However, using two different types or grades of steel simultaneously, one for main reinforcement and the other for secondary reinforcement is permissible.

Bars may be placed individually, in pairs, or bundled in groups of up to three or four. Bundled bars must be securely tied and enclosed within stirrups or ties. Bars exceeding 32 mm in diameter should generally not be bundled, except when detailed in columns.

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

Note

The development length includes the anchorage values of hooks in tension reinforcement.

For bars with non-circular sections, the development length must be adequate to develop the required stress in the bar through bond.

For deformed bars conforming to IS 1786, bond stress values must be increased by 60%.

For bars placed in compression, the standard bond stress values may be increased by 25%.

The development length for each bundled bar must be proportionally increased: 0% for two bars in contact, 20% for three bars in contact, and 33% for four bars in contact.

Anchoring reinforcement bars

Anchoring bars in tension

Deformed bars may be used without end anchorages, provided the required development length is satisfied.
Hooks should normally be provided for plain bars in tension.
Bends and hooks shall conform to IS 2502.
Bends: The anchorage value of a bend shall be taken as 4 times the bar diameter for each 45° bend, subject to a maximum of 16 times the bar diameter.
Hooks: The anchorage value of a standard U-type hook shall be equal to 16 times the bar diameter.

Anchoring bars in compression

The anchorage length of a straight bar in compression shall be equal to the development length specified for bars in compression.
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.

Spacing of reinforcement

Fs	Percentage redistribution to or from section considered
Fs	-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 values provided in the table do not apply to members exposed to highly aggressive environments, unless the moment of resistance is calculated with fs limited to 300 N/mm² in limit state design and σs limited to 160 N/mm² in working stress design.

Nominal cover to reinforcement

A. Nominal cover for durability

Exposure Condition	Minimum Nominal Cover (mm)
Mild	20
Moderate	30
Severe	40
Very Severe	70
Extreme	70

Note

For main reinforcement up to 12 mm diameter bars under mild exposure, the nominal cover may be reduced by 5 mm.
Unless specified otherwise, the actual concrete cover should not deviate from the required nominal cover by more than +10 mm.
For severe and very severe exposure conditions, a reduction of 5 mm may be permitted where the concrete grade is M35 or above.

B. Nominal cover to meet specified period of fire resistance

Fire Resistance	Nominal Cover
Fire Resistance	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