How to Calculate Lap Length In RCC Structures
Hai engineers i am back with another important topic once again.In this article you will get to know concept regarding what is the definition of lap length and how to calculate lap length in RCC structures.
In this topic i would l would cover
1.What is Lap Length
2.Definition Of Lap Lenght In RCC Structures
3.How to Calculate Lap Length In RCC Structures ( Tension & Compression )
4.Formula to calculate lap length
5.Finally lap lenght for M20 concrete
What is Lap Length ( Definition)
Lapping can be defined as the overlapping of two bars side by side to upto the design length.Lap length is one of the important term in the reinforcement. This is usually confused with another important term called development length and anchorage length.
Why Lap Length Is Provided
During the placement of steel in Reinforced concrete structure, if the required length of single bar may fall short. To get the desired design length, lapping of two bars side by side is done. An alternative to this is to provide mechanical couplers.
Example Explanation :
For example, imagine there is a need to build a 100ft tall column. But it’s not practically available. Hence the bars are cut every second story.
steel bars overlapped
Then the tension forces are required to be transferred from one bar to the other bar at the location of discontinuity of the bar. So the second bar is kept closely to the first bar and overlapping is done. This amount of overlapping between two bars is called “lap length”. Lapping is usually done where minimum bending stress is encountered. In general, lap length is 50d which means 50 times the bar diameter, if both bars are of same diameter.
Lap length of bars with same diameter
Formula To Calculate Lap Length
Lap Length In Tension
The lap length including anchorage value of hooks shall be
- For flexural tension – Ld or 30d whichever is greater is considered.
- For direct tension – 2Ld or 30d whichever is greater is considered.
The straight length of lapping of bars shall not be less than 15d or 20cm.
Lap Length In Compression
The lap length is equal to the development length calculated in compression but not less than 24d.
Lap Length For 1:2:4 Nominal Mix
The Lapping length in tension (MS bar – mild steel bar) including anchorage value is 58d.
So eliminating the anchorage value, the lap length = 58d – 2*9d = 40d. (Where 9d = hook allowance up to 25 mm and k=2)
Lap Length For M20 Grade Concrete
- Columns – 45d
- Beams – 60d
- Slabs – 60d
This infers that if we need to lap 20 mm diameter column bars, a minimum lap of 45 * 20 = 900 mm is provided.
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