What is Development Length for Steel in Reinforced Concrete?

Development length can be defined as a length ofreinforcing bars into embedment of concreterequired to develop full tensile strength of them. 

This length depends on eithersplitting or pull-out failure. According to figure 01 the moment is obviously maximum at point a (not considered weight of beam); maximum moment means maximum stress on steel. As the beam is simply supported, end restraint is zero which means zero moment at support. Let’s try to analyze this problem. 

If the stress in bar at a is f_s and area of bar is A_b , the tension force in bars is A_bf_s; this means, this force has to be transferred to concrete from bar. To transfer this force by bond force, l length of bar is required.

The strength of bar is A_bf_y; to develop this strength, the length of transferring force, (here l) must be equal or more than development length. If development length is l_d, when l ≥ l_d , bond failure occur after mobilizing full strength of bar (A_bf_y) i.e. premature failure is avoided.

Thus, the beam may fail due to other type of failure like shear or bending, but bond failure is restricted.The beam is safe even when local slip is occurred near cracks across small regions on the beams.

Notice that, the main safety requirement for failure due to bond is only length of reinforcing bar. This length from a point of given stress in steel (f_s or anything ≤ f_y) to nearest free end should be equal or more than development length of it.

 Bond force per unit bar length, 

Where jd=Lever arm between resultants of compression and tension force.

v= Shear force

If proper development length is provided, magnitude of nominal flexure bond force, as in equation (1), is not very important as integrity of beam is ensured; this is valid even under local minor bond failures.

When actual length provided is not adequate i.e. full development length, however, cannot be provided,special anchorage like by hooks, should be provided.