What is Development Length | Calculation as per IS:456

To give you an idea of the significance of development length we will do a simple demonstration. Ask your friend to hold a pen or pencil in closed fist and you try to pull that out.You will feel the more length of the pen that is inside in his/her fist, the more you need to apply effort to get that out. This is what the concept of development length.

Thus, the development length may be defined as the length of embedment necessary to develop the full tensile strength of the reinforcement, controlled by either pull-out or splitting.In other words, a certain minimum length of the bar, called the development length, has to be provided on either side of a point of maximum steel stress to prevent the bar from pulling out under tension.

Now, let’s have an idea about the anchorage. In the case of plain bars, the bond is due to the chemical adhesion and mechanical friction between steel and concrete, but it is easily broken when a large load is applied. End anchorage is provided to prevent this. Such a beam will not collapse even if the bond is destroyed over the entire length, provided the anchorage is sufficient.

Why to provide development length ?

  • The stress at the end of a bar is zero; it builds up gradually along its length through bond with the surrounding concrete. To ensure this development length is required.
  • The extra reinforcement length will help transfer the stresses developed in the junction, for example, in the beam-column joints.

Importance of Ld

  • No premature bond failure will occur if the actual length L is equal to or greater than Ld.
  • Then the beam is supposed to fail in bending or shear failure rather than bond failure.
  • In the case of the bond, we are considering the overall mechanism of failure rather than the limiting stresses to govern the design.

Calculation if development length as per IS :456

According to Clause 26.2.1 of the Indian code, the calculated tension or compression in any bar at any section shall be developed at each side of the section by an appropriate development length, given by :

$$
L_{\mathrm{d}}=\frac{d_b f_{\mathrm{s}}}{4 \tau_{\mathrm{bd}}}
$$

where,

  • db is the nominal diameter of the bar,
  • fs is the stress in the bar at the section considered at design load (for fully stressed bar, fs = 0.87 fy)
  • τbd is the design bond stress as per Table below.
Grade of concreteM20M25M30M35M40 and above
Design bond , MPa1.21.41.51.61.9
For fully deformed bars1.181.371.541.711.87
  • For deformed bar in tension, τbd values can be increased by 60 %
  • For bars in compression, the value of bond stress in tension can be increased by 25 %
  • In case nominal reinforcement is provided, τbd is taken as 1.0 MPa.

$$
L_{\mathrm{d}}=\frac{0.136 d_b f_{\mathrm{y}}}{\tau_{\mathrm{bd}}}
$$

Reduction in development length due to excess reinforcement

The Indian and US codes follow for the reduction in development length by the ratio (Ast required )/ (Ast provided) when excess reinforcement is provided to resist the factored applied moment in a flexural member (SP: 24: 1983).

$$
{L’_{\mathrm{d}}}=L_{\mathrm{d}} \frac{A_{st-required}}{A_{st-provided}}
$$

where L’d is the actual development length and Ld is the development length of the fully stressed bars.

Note : Such a reduction us not allowed for tensile lap splices, development of positive moment reinforcement at supports and development of shrinkage and temperature reinforcement. This reduction is also not permissible for reinforcement in structures located in regions with high seismic risk.

Development length of bars in compression

The real performance of bond in compression is something which requires research. The development length required is shorter for bars in compression than in tension because of the absence of tension cracking in concrete and the beneficial effect of end bearing of the bars in compression. Increase the value of bond stress in tension by 25 % – as per IS code.

Development Length of Bundled bars

For situations requiring heavy concentration of reinforcement, bundles of bars can save space and reduce congestionfor placement and consolidation of concrete. Bundling of bars in column will result in better locating and orienting of the reinforcement for increased column capacity, also fewer ties are required if column bars are bundled. Equivalent diameters of bundled bars are given in table below :

Bar diameterEquivalent diameter, mm
$$d_{eq} = d_{b} \sqrt{n_b}$$
Two bar bundleThree bar bundleFour bar bundle
1216.9720.7824
1622.6327.7132
2028.2834.6440
2231.1138.1144
2535.3643.3050
2839.6048.5056
3245.2555.4364

Increased development length for individual bars within a bundle, whether in tension or compression, is required when bars are bundled together. The additional length is required because there is no core of concrete between the bars to provide resistance to slipping.

Factors affecting Ld

  • Compressive strength of concrete
  • The density of concrete
  • Rebar clear cover
  • Rebar centre to centre spacing
  • Coating of rebar
  • Rebar diameter
  • Type of bundled reinforcement
  • Type of reinforcement

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