Development Length (Ld)
Also known as: anchorage length, bond length, Ld
Development length is the minimum embedment a rebar needs in concrete to develop its full design stress through bond.
Development length (Ld) is the length of bar that must be embedded in concrete so the steel can reach its full design stress without slipping. IS 456 gives Ld = (φ × σs) / (4 × τbd), where φ is the bar diameter, σs is the steel stress, and τbd is the design bond stress.
For Fe415 deformed bars in M20 concrete, this works out to the well-known 47 times the bar diameter in tension. Higher concrete grades increase bond and shorten the required length; compression bars need less than tension bars.
Adequate development length at supports and member junctions is essential — under-anchored bars can fail in bond before the steel yields.