Development length and need of splices

Development length

Development length $L_d$ is the length of embedded bar needed to develop its design stress $0.87f_y$ by bond with the concrete:

$$L_d=\frac{0.87f_y\,\phi}{4\,\tau_{bd}}$$

For example, for Fe415 deformed bars in M20 concrete, $\tau_{bd}=1.2\times1.6=1.92$ N/mm$^2$ and $L_d=47\phi$.

Why splices are required

Reinforcing bars are available only in standard lengths (usually 12 m, and often shorter in practice because of transport). Splices (joints) are therefore needed for the following reasons:

1. Limited length of bars: members longer than the available bar length, such as long beams, tall columns and walls, need bars to be joined.
2. Transport and handling: long bars are difficult to carry, store and fix on site.
3. Construction stages: column bars are left projecting above each floor and lapped with the bars of the next lift (at construction joints).
4. Continuity of load path: splices transfer the full force from one bar to the next, so the reinforcement acts as a continuous bar.
5. Economy: short offcuts of bars can be used instead of being wasted.

Types: lap splices (most common), welded joints and mechanical couplers. Laps should be staggered, be at least $L_d$ or $30\phi$ (flexural tension), and should not be placed at sections of maximum stress.