Lapping is very important for reinforcement usually done to increase the length of bars. In this article, I will discuss lap length of bars with examples.
What Is Lap Length?
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 two bars side by side to achieve the required design length. (Refer to the below image)
Read: What Is Development Length Of Bar?
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 in every second story.
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.
Lap Length Formula
Lap length in Tension
The lapping length, including anchorage value of hooks, shall be
- For flexural tension – Ld or 30d, whichever is greater.
- For direct tension – 2Ld or 30d whichever is greater.
The straight length of lapping shall not be less than 15d or 20 cm.
Lap length In Compression
The lapping length in compression shall be equivalent to the development length in compression computed but not less than 24d.
For Different Diameter Bars
In case of bars having different diameter are to be spliced, the lapping length is calculated on the basis of smaller diameter bar.
Lap Splices
Lap splices should not be used for the bars having larger dia than 36 mm. In that case, welding should be done. But if welding is not practicable, then lapping may be permitted for the bars larger than 36 mm dia. Additional spirals should be provided around the lapped bars.
Lapping length For Concrete Of 1:2:4 Nominal Mix:
Lap length in tension (for plain Grade-1 MS bar) including anchorage value is 58d. So eliminating the anchorage value the lap length = 58 – 2*9d = 40d
where 9d = hook allowance of bars up to 25 mm and k=2
Lap length for compression bar is equal to the value of development length calculated i.e 43.5d.
Lap length Of Column, Bem & Slab (M20 Concrete)
- Columns – 45d
- Beams – 60d
- Slabs – 60d
So if we use 20 mm dia column bars, we have to provide a minimum lap of 45 * 20 = 900 mm.
Read – How To Check Quality Of TMT Bars
I hope you now have enough knowledge about lap length of rebars. If you have any questions, let me know in the comments.
Also Read
Rebar Couplers Vs. Lapping Vs. Welding
Difference Between Development Length & Lap Length
Types Of Rebar Used In Construction
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