What is overlap?
What is overlap?
During placing the steel in RC structure if the required length of a bar is not sufficiently available to make a design length then lapping is done. Lapping means overlapping of two bars side by side to achieve required design length.
Suppose, we need to build a 100 feet tall column. But practically 100 ft long bar is not available and it is also not possible to cage. Therefore we need to cut the bars. Now, we need to transfer the tension forces from one bar to the other at the location of discontinuity of bar.
So we have to provide the second bar closed to the first bar that is discontinued and overlapping is to be done. The amount of overlapping between two bars is known as lap length. In case of RCC structure, if the length of reinforcement bars need to be extended, splicing is used to join two reinforcement bars for transferring the forces to the joined bar.
Calculation of lap length for various splices
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. The lap splices can be mainly of two types:
1. Contact Splices
2. Non-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-1 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.
Fig.1. Trench Mesh Laps
2. Square Mesh Laps
Figure 2 shows the square mesh lap which is carried out by an overlapping length not less than 225mm.
Fig.2. Square Mesh Lap
3. Reinforcement Lap
Reinforcement lap must be carried out by a lapping length not less than 500mm as shown in figure-3.
Fig.3. Reinforcement Bar Lap
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.The minimum lap length is 12 inches. On the other hand, ld is given by the following equation:
where fc is concrete’s compression strength, fy is steel bar’s yield stress and 𝛼, 𝛽 and 𝛾 are constants based on the standard and each type of bars. Moreover, for bars with diameters less than 20, 𝛾 is equal to 0.8 and for other types is equal to 1. (c+ktr/db) ratio is also determined by the spacing of the bars and their laps.
Lap Length for Compression Bars
The lap length for compression stress 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 than the following cases:
Compression splice length of the least diameter bar or
Compression development length of the largest bar