Let us go through some of the FAQs to gain basic knowledge of the lap length or lap splice of the reinforcement bars.
1. What is lap length?
The minimum length that should be provided when we overlap two reinforcement bars ( rebars ) so that the tension force & designed loads are transferred safely at the joints from one bar to another.
Lap length is also known as lap splice.
2. Why we provide lap length for rebars?
The maximum length of the reinforcement bars is limited to 12.2m ( or 40 ft.). This limited standard length is to facilitate the smooth manufacturing process and to avoid rebar transportation problems.
When we construct the multistorey buildings, the length of the single bar is insufficient to reach the top building height from the footing level. So, providing a lap length becomes necessary for the safe transfer of the load.
3. What happens if we don't provide sufficient lap length?
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| Cracks in RCC structure. |
The provision of specified lap length is a must for the reinforcement bars to keep the structure intact in all types of load conditions. The RCC structure may develop cracks due to the splitting of rebars at the insufficient lap length zones. This ultimately leads to the failure of the structural member as the load transfer mechanism fails when the rebars of insufficient lap length cannot bear the superimposed load over the structure.
4. What are the general rules while providing lap length?
1. Lapping should be provided alternately in a staggered pattern so that the lapping zone will not fall at the same level. This rule helps to avoid buckling of the structure.
2. The bars should be staggered with not more than 50% of bars in any section of the lapping zone. The minimum clear distance between the lapped rebars shall be 0.25 times the lap length.
3. In any case, the staggering distance between the lapping of bars should not be less than 75mm.in any type of bars and structures.
4. When the bars of different diameters are lapped, the lap length is calculated for the smaller diameter bar.
5.For the bars whose diameter is greater or equal to 36mm., lapping should not be done. Instead of lapping, these bars should be welded. If welding is not possible, then you can lap these bars with an additional spiral of 6mm bar at the splice length.
6. The stirrups (lateral ties ) should be closely placed at the lapping portion of the rebar.
7. For flexural tension, Ld or 30d whichever is greater is considered.
8. For direct tension, 2Ld or 30d whichever is greater is considered.
9. The straight length of the lapping bars in the tension zone should not be less than 15d or 20cm.
10. In the case of compression, the lap length should be equal to the Ld calculated in compression, but in any case not less than 24d.
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5. What is the main function of lapping in the RCC structures?
1. Lapping helps to provide continuity to the reinforcement bars in all types of RCC structures.
2. Lapping is needed to safely transfer the loads (or stress) from one reinforcement bar to another.
3. Lapping helps to design the RCC structures with the required strength.
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