Define development length (short)

Development length is the length of a reinforcing bar, embedded in concrete, that is required to transfer the design stress in the bar to the surrounding concrete by bond, so that the bar will not pull out. It is given by $L_d=\dfrac{0.87f_y\,\phi}{4\,\tau_{bd}}$ (IS 456 cl. 26.2.1), where $\phi$ is the bar diameter and $\tau_{bd}$ the design bond stress.