What is Overlapping or Lap Length in RCC?
Due to the limited length of the bar, lapping of bars are required in a member of more than 12m in length or where the length of bar required is more than 12m.
Lap Length:- Overlapping Length or Lap Length is provided to maintaining the continuity of bars in order to safely transfer the load from one bar to another bar. It depends on the Grade of Concrete, the grade of steel and dia. of the bar.
How Much Lap Length is Provided in Column?
As per IS Code 456-2000 overlapping length should not be less than 75mm. Lapping should be avoided in the tensile zone of construction members. In case of column generally, we take 24d – 40d where ‘d’ is dia of the bar.
Important Point Should be Remember for Column Lap Length:
1- Lapping should be provided at the centre of the column because bending moment at the midpoint is zero so try to lap at the midpoint.
2- Lapping of bars should be provided alternately. Lap should not be given at same point because buckling may occur.
3- Lateral ties should be provided closely spaced in Lapping zone.
4- Due to maximum stress, we can’t lap at the column/beam/slab joint. Lapping should not be given at L/4 distance from top and bottom of the slab.
How Much Lap Length is Provided in Beam?
In case of the beam we generally use 24d for compression zone and 45d for tensile/tension zone.
Important Point Should be Remember for Beam Lap Length:
1- Lapping (24d) in top bars avoided L/3 distance from both ends. For top bar, lapping should be at mid-span.
2- Lapping (45d) in bottom bars lap should be provided at column junction or L/4 distance from column face but should not be in mid-span of the beam.
3- Stirrups should be closely spaced near the columns and lose/normal at mid-span.
4- Lapping of bars should be alternately provided.
How Much Lap Length is Provided in Slab?
In the case of the slab, we use lap length 50d where ‘d’ is dia of the bar.
Important Point Should be Remember for Slab Lap Length:
1- RCC slabs function is similar to beams if the slab is designed as one way. It is ideal to lap bars at point of the least bending moment or at points of contra flexure.
2- Practically laps are provided beyond L/5 to L/3 ( L being the effective span) from support for bars at bottom of the slab. The top bars are generally short and no laps are necessary. However, at no point, the lapped bars shall exceed one-third of the total bars.
3- Bars will be considered staggered if an end to end distance b/w laps is at least (lap length +75mm)
