In RCC structures, the placement of a single reinforcement bar may not meet the desired length of the concrete section. Hence, it becomes necessary to overlap two bars, so that the desired length is met. This length overlapped by two bars in concrete sections is called lap length or overlapping length, which are calculated separately for beams, columns and slab sections.
Calculation of Lap Length or Overlapping Length
The lap length is calculated for those concrete sections whose length is greater than 12m. Mostly, 12m is the stock length which is fixed to facilitate transportation to the site and for the ease of handling.
1. Lap lengths for Different Splices
The lap splices can be mainly of two types:
- Contact Splices
- Non-Contact Splices
Contact Splices
In this type, the reinforcement bars are kept in contact and wired together. This can be of three types:
1. Trench Mesh Laps
The figure-2 below shows the trench mesh lap provided in concrete reinforcement. The minimum length to be overlapped is 500mm. These overlap formed in T or L intersections as shown in the figure must overlap at a width of the trench mesh.
2. Square Mesh Laps
Figure below shows the square mesh lap which is carried out by an overlapping length not less than 225mm.
3. Reinforcement Lap
Reinforcement lap must be carried out by a lapping length not less than 500mm as shown in figure-4 below.
Also Read: Methods of Splicing Reinforcement Bars
2. Lap length in Tension
The lap splice length in tension reinforcement is calculated as per ACI 12.15. In a tension lap, the force is transferred from the reinforcement bars to the concrete by the bond which in turn force back to the reinforcement. This hence results in a continuous line of reinforcement.
Hence, the lap splice, in this case, shows varied interaction depending on the concrete strength, grade of steel reinforcement, location, bar size ,and spacing. As per ACI 318, the tension lap splices are classified into two classes Class A and Class B ( Table R12.5.2, ACI 318) for which the lap length are as follows:
Class A Splice: Lap Length ls = 1.0 ld
Class B Splice: Lap Length ls = 1.3ld
Where, ld= development length, which is calculated as per ACI 12.2 ( Without taking into consideration the modification factor).The minimum lap length is 12 inches
3. Lap Length for Compression Bars
The lap length for compression lap splice is calculated as per ACI 12.16.1 as;
ls = 0.0005fy (d) ; [ For fy less than or equal to 60,000psi]
ls = (0.0009fy - 24)d; [ For fy > 60,000 psi]
The minimum lap length for compression bars must be 12inches. When reinforcement bars of different sizes are bundled as compression bars, the lap length chosen must be larger of the following cases:
- Compression splice length of the least diameter bar or
- Compression development length of the largest bar