Splicing of bars and conditions

Splicing of bars is the process of joining two bars end to end so that stress is transferred from one to the other. The usual methods are (i) the lap splice, where the bars overlap and the force passes through bond with the surrounding concrete, (ii) the welded splice, and (iii) the mechanical splice with couplers.

Primary conditions for splicing (IS 456 cl. 26.2.5)

1. Location: splices should be kept away from sections of maximum tensile stress. They are best placed where the bending moment is small, e.g. column bars just above floor level, or beam bottom bars near the supports.
2. Staggering: splices should be staggered, and not more than half of the bars should be spliced at one section. Bars in a lap are placed in contact, or with a clear gap of not more than $4\phi$, so that stress is transferred properly.
3. Lap length (including the anchorage value of hooks):
   • flexural tension: not less than $L_d$ or $30\phi$, whichever is greater;
   • direct tension: not less than $2L_d$ or $30\phi$, whichever is greater;
   • compression: not less than $L_d$ (in compression) or $24\phi$, whichever is greater.
4. Bar size: lap splices are not used for bars larger than 36 mm. Such bars are welded or joined with mechanical couplers, and the strength of the joint must be at least that of the bar.
5. Transverse steel: closely spaced stirrups or ties are provided around the lap zone to confine the concrete and prevent splitting.
6. In members subjected to repeated or reversed loading, mechanical splices are preferred to laps. In seismic zones (IS 13920), lap splices in columns are placed only in the central half of the column height and are enclosed by closely spaced hoops.