Lap Length of Bars

Lap Length:

Development length and lap length are two important terms in reinforcement. But many of us get confused with the difference between development length and lap length. In our previous article we have already discussed what is development length of bars, today we will discuss what is lap length of bars. 

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 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 lap 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 20cm.

Lap Length In Compression:

The lap 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 lap 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.

Lap 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 For M20 Concrete:

Columns – 45d

Beams – 60d

Slabs -60d.

So if we need to lap 20mm dia column bars, we have to provide a minimum lap of 45 * 20 = 900mm.