In construction, lap length is the overlap of reinforcement(steel) bars, when the length of the bar is shorter than the requirement, used to transfer the load between bars. In column and beam lapping length is essential to enhance load transfer and prevent structural failure.
What is lapping?
It is the process of overlapping the reinforcement bars side by side to obtain the desired design length.
What is lapping length?
Lap length is the length of the overlap of bars required to safely transfer stress from one bar to another. The lap length of steel is taken as 50d.
What is the lap length formula is 456?
Lap length formula= Development Length Coefficient × Diameter of Reinforcing Bar
Lap length= (45 to 50) x d
Why is lapping length provided?
- Lapping is required to take the load from the upper level to the lower story.
- Lapping is provided where minimum bending stress occurs.
- Maximum production length of steel bar=12m. For example, if a building is constructed where the length of the bar is required to be more than 12 m. However, the bars are not available larger than 12 m. So, we need to lap the bars.
- Multi-story building /High rise building
- For Example:
- Increase the strength of the building.
What is the lap length in the column and beam?
In column Lap length As per IS code 456-2000:
- Overlapping length should not be less than 75mm.
- In the case of columns generally, we take 24d-40d where d is the diameter of the bar.
In beam Lap length As per IS code 456-2000:
- Flexural tension: Minimum lapping length is Ld or 30d (whichever is greater)
- For different diameter bars: Minimum lapping length is 2Ld or 30d (whichever is greater)
- Minimum straight length of lapping: 15d or 20mm
The lapping length should not be less than 24d for the beam. As we calculated the compression of the lapping length which is must equal to the development length.
For Different Diameter Bars:
When the bars of different diameters are to be spliced, the lapping length is calculated considering the smaller diameter bar.
How to identify the lapping Zone?
Now, we discuss the lapping zone in the column and beam for the stability of the structure.
Lapping Zone in the column:
In the case of the column make the lapping process to understand easy for you. Let’s consider a slab column junction.
- Column Zones: First, divide the column into three zones that is zone A, zone C zone C.
- Critical zones: Zones A and C are considered the most critical zones in compression due to the lateral loads.
- Intermediate Zone: Between the critical zone is intermediate zone which is zone B.
The reason is that due to later loads that are seismic or wind loads these end zone is subjected to maximum bending moment in compression to the intermediate zone. That is zone B for illustration.
Lapping Zone in beam:
Now, we discuss the what should be the position of the lap in the case of beams. Before deciding the lapping zone of a beam we need to identify the points where the member is subjected to maximum bending moment. So, simply we identify the tension zone of the beam.
Let’s consider a continuous beam that is subjected to uniformly distributed load due to this loading the beam will tend to bend in a way that the maximum bending moment will be generated at the center of the span.
Tension zone identification:
There will be positive moments that appear in the center at negative moments at each support. Due to the maximum bending of the top fiver at the beam of the end and bottom fiber in the mid-span will experience maximum expansion. Hence they called the tension zone of the beam and we can’t overlap the bars in the tension zones. So, the bars come out from the concrete due to the maximum stress.
Divide the span into equal lengths:
- Lapping top bars:
So, the top bars can be overlapped in the mid-span of the beam due to minimum tension in the top fiber of the mid-span
- Lapping Bottom Bars:
The bottom bars should overlap at the column junctions with a distance of L/4 from each end.
Note: For the bottom bars, we can’t overlap the bears at the mid-span of the beam due to the maximum tension in the bottom fiber in the beam.
Why lapping provided in the intermediate zone in a column?
Let’s consider a column that is subjected to having the 3 zones.
- Zone A: When the load is applied in zone A it will tend to bend the column in such a way that the maximum moment is generated at zone c.
- Zone B: If the load is applied in zone B both the end zones will experience a maximum bending moment in the compression to the intermediate zone C.
- Zone C Similarly, if the load is applied in zone C the maximum moment will be generated at zone A.
Hence, in all cases the end zone will experience maximum bending moment which means maximum tension therefore it would not be safe to overlap the bars in critical zones. these are tension zones rather they should be provided in the intermediate zone.
What are the general Rules for Different lapping zones?
- Remember that not more than 50% of the bar of the bottom bars in a beam overlap in the same zone, column, or any clear span.
- Maintain a clear cover of 40 mm for columns to the main bar.
- Ensure an equal distance between the clear covers.
- Calculate the lapping length for the column center.
- Lapping should be done in a staggered manner; these laps should not be given at the same level to avoid buckling as per 13920.
- The stirrups shall be closely spaced in the lapping portion because when we provide lapping in a concrete member the strength of the member slightly reduces.
What lapping/ Welding is required?
Generally, there are two ways of lapping.
- Overlapping: For steel bars of dia.<36mm
- Welding: For steel bars of dia. ≥36mm
Note: Bars ≥36mm: Weld if possible; if welding is not possible then permit lapping but with additional spirals around the lap pass.
Lap Length for 1:2:4 Nominal Mix Concrete:
- Tension lapping length for the mild steel (MS) bar is 58d.
- Eliminating the anchorage value (where 9d is hook allowance up to 25mm and k=2) then
Lap length=58- 2*9d
Lap Length For M20 concrete:
- Columns: 45d.
- Beams: 60d
- slabs: 60d.
Lap length for neck column to footing
Geberally, the lap length for neck column to footing is 16d.
Cutting length of cracked bar:
Lpa length for cracked bars=L+0.42D+0.42D
Example for 20mm diameter column bars:
- Lap length=45 x 20
Frequently Asked Questions:
What are the different diameters of steel bar used in house construction?
|8mm, 10mm, 12mm
What happens if we do not provide the lap length?
- Load transfer mechanism will fail
- Reinforcement bar split and crack
Adequate practice for lap length is essential for building structures. If you ignore lap length requirements and rules, you compromise the strength of the structure. However, attention must be paid to understanding the lap length rules for structural strength and stability.