Lapping is very important for reinforcement usually done to increase the length of bar. In this article, I will discuss lap length with example.
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 of two bars side by side to achieve required design length. (Refer to below image)
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
1. For flexural tension – Ld or 30d whichever is greater.
2. 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 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.