Lap length refers to the length of overlap provided in reinforcing bars to ensure proper transfer of load from one bar to another. The calculation of lap length depends on factors like the type of structure, concrete strength, and the diameter of the reinforcing bars.
In this article today we discuss very important topic lap length used in civil construction line. People may get confused with lap length with the development length and Anchorage length. All three are quite difference. When we place a steel in RCC structure member like column, beam and slab we need to join one rebar to another rebar bar or overlapping of two rebar due to to shorten size of one rebar. Amount of length of overlapping of two rebar is known as lap length. It is provided for safely transfer of stress load from one rebar to another rebar.
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Development length and Anchorage length is provided in column, beam and RCC slab at end point of steel as L shaped or as hook to safely transfer the load to the other concrete structure from one structure member to another structure member.
What is Lap length? How to calculate it for column, beam and slab?
What is lap length? This is amount of length overlapping between two rebar is known as lap length, it is also called as Lapping lenth, Lapping, overlap length and lap splices. Lapping is usually done where minimum Shear forces is encountered or minimum bending stress occur. In general lamp length is 50D which means 50 times the diameter, if both bar of same diameter then a smaller diameter should be selected for calculating lap length.
Lapping meaning:- lapping meaning is the length provided to overlapping between two rebars in order to safely transfer load from one bar to another bar and alternative to this is to provide mechanical couplers. It is also known as lap splices.
Why lapping is provided in RCC structure? Suppose we have to construct 30 metre height column, there is no 30m size of single reinforcement available in market, the reinforcement made by different company of Steel manufacturer available is 12metre or 40 feet long. So you need to join 3 rebar to get specific Desire height, so overlapping should be done between two rebar in order to safely transfer stress load from one rebar to next rebar.
Why rebar available in size only 12 metre? Various Steel manufacturing company design rebar bar of size 12 metre or 40 feet long for removing any technical difficulty and easily transportation,storage and avoiding manufacturing difficulty.
What will happen if we don’t provide lap length? As we know that lapping should be provided between two rebar in order to safely transfer shear forces from one bar to next bar, if we don’t provide lap length between two rebar the load is not safely transferred to one bar to next bar, causing structure will be failed, if not failed it cause cracking, and permanent collapsing of a structure. For preventing from any cracking, structure failure and permanent collapsing, lapping should be provided in steel when we placed Steel in RCC structure.
Lap length formula
The lap length formula for a beam and a slab is: lap length = 60D and for a column: lap length = 45D, where D is the diameter of the steel bars. The lap length formula is used to calculate the lapping length for column, beam and slab.
The lap length formula for a beam is: lap length = 60D, where D is the diameter of the steel bars. If the diameter of the steel bar used in the beam is 12 mm, its overlap or lapping length is 12 × 60 = 720 mm (2.36 ft).
The lap length formula for a slab is: lap length = 60D, where D is the diameter of the steel bars. If the diameter of the steel bar used in the slab is 8 mm, its overlap or lapping length is 8 × 60 = 480 mm (1.57 ft).
The lap length formula for a column is: lap length = 45D, where D is the diameter of the steel bars. If the diameter of the steel bar used in the column is 16 mm, its overlap or lapping length is 16 × 45 = 720 mm (2.36 ft).
How to calculate lap length?
As we know different RCC structure like column beam and slab experiences compression and tension. When load is applied on beam and slab it will experiences compression and tension both that’s why it is known as flexural member of concrete structure. And when loading applied on column it with experiences compression only, so column is compressive member of concrete RCC structure.
When load is getting on beam, it will experiences negative and positive bending moment. Top concrete fibre of beam and slab will experiences compressive stress, in which bending is maximum at the both support end and minimum at the middle of a span in compression zone. Bottom concrete fibre of beam and slab will experiences tension stress in which bending stress is maximum at middle of span and minimum at both support end in tension zone. So more quantity of steel is provided near two support end in of top concrete fibre in compression zone and middle span of bottom concrete fibre in tension zone.
Lap length as per IS code 456:- RCC structure like column beam and slab will experience compression and tension, according to IS code 456 tension have two cases:- for one flexural tension lap length should be equal to development length of rcc structure or 30d is provided and whatever greater should be taken. In general lap length should be taken as 41d, where d is diameter of rebar. For direct tension lap length should be equal to twice of development length 2Ld or 30d. For compression, lap length should be taken as equal to development length and it should not be taken less than 24d.
How to calculate lap length for column? In general, column is provided with lap length 45d, where d is diameter of rebar, assuming column size 9″×9″ using 4 number of 12mm of Fe500 bar with m20 grade of concrete,their lap length = 45d, 45×12 = 540mm (1.77feet).
How to calculate lap length for beam?
In general, beam is provided with lap length 60d, where d is diameter of rebar, assuming beam size 9″×9″ using 4 number of 12mm of Fe500 bar with m20 grade of concrete,their lap length = 60d, 60×12 = 720mm (2.36feet).
How to calculate lap length for slab?
In general, slab is provided with lap length 60d, where d is diameter of rebar, assuming beam size 9″×9″ using 10mm of Fe500 bar with m20 grade of concrete,their lap length = 60d, 60×10 = 600mm (1.97feet).
Lapping zone for beam and column
Lapping zone for column:– when load is applied on column, it will experiences compressive stress from top and bottom or from both end. Maximum compressive stress in column present at both end top and bottom of L/4 region and minimum compressive stress at mid span of column. So lapping zone for column is mid-span. Lapping should be provided in middle span of column due to minimum compressive stress and it will not provided in both support end, where compressive stress is high.
Lapping zone for beam:- when load is applied on beam, top concrete fibre will experiences compression and bottom concrete fibre will experiences tension. Compressive stress is maximum at both end support L/4 region of top concrete fibre and minimum at middle span, so lapping zone for top concrete fibre of beam is middle of span of compressive member. Maximum tension at middle span of bottom concrete fibre and minimum near end support L/4 region, so lapping should be provided near two support end in bottom concrete fibre. Lapping should be not provided at column beam joint. Lapping zone for beam is middle span of top concrete fibre and both end support L/4 region in bottom concrete fibre.
Minimum lap length:- minimum lap length for direct tension is equal to development length,it should be not less than 15d or 20cm. And for compression, minimum lab length should be taken as not less than 24d. Where d is diameter of rebar.
Calculation of lap length depends on which factors
Calculation of lap length depends on which factors:– value of lap length used for beam slab and column depends on diameter of rebar which are going to used, amount of stress forces and different types of concrete grade used for RCC construction. Lap length should be not provided where high shear forces occurs, it will be provided where there is minimum shear forces.
Lap length for different grade of concrete and steel
Lap length for different grade of concrete:- lap length or overlapping length depends on Grade of concrete and grade of Steel which are used in RCC construction.
1) RCC structure having m15 grade of concrete provided with Fe250 grade of Steel, minimum lap length 55d provided in tension zone and 45d in Compression zone.
2) RCC structure having m15 grade of concrete provided with Fe415 grade of Steel, minimum lap length 57d provided in tension zone and 47d in Compression zone.
3) RCC structure having m15 grade of concrete provided with Fe500 grade of Steel, minimum lap length 68d provided in tension zone and 57d in Compression zone.
4) RCC structure having m20 grade of concrete provided with Fe250 grade of Steel, minimum lap length 46d provided in tension zone and 37d in Compression zone.
5) RCC structure having m20 grade of concrete provided with Fe415 grade of Steel, minimum lap length 47d provided in tension zone and 38d in Compression zone.
6) RCC structure having m20 grade of concrete provided with Fe500 grade of Steel, minimum lap length 57d provided in tension zone and 46d in Compression zone.
7) RCC structure having m25 grade of concrete provided with Fe250 grade of Steel, minimum lap length 39d provided in tension zone and 32d in Compression zone.
8) RCC structure having m25 grade of concrete provided with Fe415 grade of Steel, minimum lap length 41d provided in tension zone and 33d in Compression zone.
9) RCC structure having m25 grade of concrete provided with Fe500 grade of Steel, minimum lap length 49d provided in tension zone and 39d in Compression zone.
Lap length for m15 concrete:– minimum lap length is 68d in tension zone and 57d in compression zone is provided for m15 grade of concrete if Fe500 grade of steel are used, if Fe415 grade of steel used then minimum lap length is 57d in tension zone and 47d in compression zone.
Lap length for Fe500 grade steel:- minimum lap length is 68d in tension zone and 57d in compression zone is provided for Fe500 grade of steel if m15 grade of concrete are used, if m20 grade of concrete are used then minimum lap length is 57d in tension zone and 46d in compression zone.
Lap length for Fe415 grade steel:– minimum lap length is 57d in tension zone and 47d in compression zone is provided for Fe415 grade of steel if m15 grade of concrete are used, if m20 grade of concrete are used then minimum lap length is 47d in tension zone and 38d in compression zone.
Lap length for Fe250 grade steel:- minimum lap length is 55d in tension zone and 45d in compression zone is provided for Fe250 grade of steel if m15 grade of concrete are used, if m20 grade of concrete are used then minimum lap length is 56d in tension zone and 37d in compression zone.
Lap length for m20 concrete:- minimum lap length is 57d in tension zone and 46d in compression zone is provided for m20 grade of concrete if Fe500 grade of steel are used, if Fe415 grade of steel used then minimum lap length is 47d in tension zone and 38d in compression zone.
Lap length for m25 concrete:- minimum lap length is 49d in tension zone and 39d in compression zone is provided for m25 grade of concrete if Fe500 grade of steel are used, if Fe415 grade of steel used then minimum lap length is 41d in tension zone and 33d in compression zone.
Lap length for m30 concrete:- minimum lap length is 45d in tension zone and 36d in compression zone is provided for m30 grade of concrete if Fe500 grade of steel are used,where d is diameter of rebar which are going to use in construction.
Lap length for m35 concrete:- minimum lap length is 40d in tension zone and 32d in compression zone is provided for m35 grade of concrete if Fe500 grade of steel are used,where d is diameter of rebar which are going to use in construction.
Lap length for m40 concrete:- minimum lap length is 36d in tension zone and 36d in compression zone is provided for m40 grade of concrete if Fe500 grade of steel are used, where d is diameter of rebar which are going to use in construction.
How much lap length is provided in column, beam and slab?
Steel lapping length formula:– steel lap length formula as per IS code 456 for RCC structure like column beam and slab for one flexural tension, lap length should be equal to development length of rcc structure or 30d. For direct tension lap length should be equal to twice of development length 2Ld or 30d. For compression, lap length should be taken as equal to development length and it should not be taken less than 24d.
Lap length formula for slab:- lap length formula for slab for one flexural tension, lap length should be equal to development length of rcc structure or 30d and for compression, lap length should be taken as equal to development length and it should not be taken less than 24d. In general it should be taken as 60D for slab, where D is diameter of rebar which are going to use.
Lap length in tension:- when load is applied RCC structure like column, beam and slab will experience tension. For flexural tension, lap length should be equal to development length of rcc structure or 30d and for direct tension lap length should be equal to twice of development length 2Ld or 30d, where d is diameter of rebar which are going to used.
Lap length in compression:- when load is applied RCC structure like column, beam and slab will experience compression. For compression, lap length should be taken as equal to development length and it should not be taken less than 24d, where d is diameter of rebar which are going to used.
How much lap length provided in column, beam and slab? As per IS code 456, 30d lap length is provided in tension and minimum 24 d in compresson in column, beam and slab. But in general, 45d lap length are used for column and 60d are used for beam and RCC slab.
Lap length for column:- when load is applied on RCC column, it will experience compression, as per IS code 456 for RCC column for one flexural tension, lap length should be equal to development length of rcc structure or 30d and for compression, lap length should be taken as equal to development length and it should not be taken less than 24d. In general lap length 45d is used for column, where d is diameter of rebar which are going to used.
Column reinforcement lapping details:- there is two types of column, rectangular column is provided with minimum 4 number of 12mm bar and circular column is provided with minimum 6 number of bar. Details of column reinforcement lapping is taken as 45d, where d is diameter of rebar which are going to used.
Lap length for beam:- As per IS code 456, 30d lap length is provided in tension and minimum 24d in compresson for beam. But in general, 60d lap length are used for rcc beam,where d is diameter of rebar which are going to used.
Lap length for slab:- As per IS code 456, 30d lap length is provided in tension and minimum 24d in compresson for slab. But in general, 60d lap length are used for rcc roof slab,where d is diameter of rebar which are going to used.
Lap length for footing:- If it’s a dowel reinforcement then lap length for footing shall be 12D and If reinforcement is subjected to tension (within the tension chord) then lapping length shall be 50D. Actual lap length depends on diameter, coating, steel grade, cover, concrete grade and actual steel stress that needs to be transferred.
Lap length for different grade and size of Steel bar
Lap length for 8mm bar:- assuming 8mm bar of Fe500 provided in RCC roof slab, their lap length should be 60d, where d is diameter of rebar which are going to be used, calculation of lap length for 8mm bar = 60×8= 480mm (1.58feet), so lap length for 8mm bar is 480mm (1.58feet).
Lap length for 10mm bar:- assuming 10mm bar of Fe500 provided in RCC roof slab, their lap length should be 60d, where d is diameter of rebar which are going to be used, calculation of lap length for 10mm bar = 60×10= 600mm (0.60m) or 1.97feet, so lap length for 10mm bar is 600mm (0.60m) or 1.97feet.
Lap length for 12mm bar used in beam:- assuming 12mm bar of Fe500 provided in RCC roof slab, their lap length should be 60d, where d is diameter of rebar which are going to be used, calculation of lap length for 12mm bar = 60×12= 720mm (0.72m) or 2.36feet, so lap length for 12mm bar used in beam is 720mm (0.72m) or 2.36feet.
Lap length for 12mm bar used in column:- assuming 12mm bar of Fe500 provided in RCC column, their lap length should be 45d, where d is diameter of rebar which are going to be used, calculation of lap length for 12mm bar = 45×12= 540mm (0.54m) or 1.77feet, so lap length for 12mm bar used in column is 540mm (0.54m) or 1.77feet.
Lap length for 16mm bar used in column:- assuming 16mm bar of Fe500 provided in RCC column, their lap length should be 45d, where d is diameter of rebar which are going to be used, calculation of lap length for 16mm bar = 45×16= 720mm (0.72m) or 2.36feet, so lap length for 16mm bar used in column is 720mm (0.72m) or 2.36feet.
Lap length for 16mm bar used in beam:- assuming 16mm bar of Fe500 provided in RCC beam, their lap length should be 60d, where d is diameter of rebar which are going to be used, calculation of lap length for 16mm bar = 60×16=960mm (0.96m) or 3.15feet, so lap length for 16mm bar used in beam is 960mm (0.96m) or 3.15feet.
Lap length for 20mm bar:- assuming 20mm bar of Fe500 provided in RCC column, their lap length should be 45d, where d is diameter of rebar which are going to be used, calculation of lap length for 20mm bar = 45×20= 900mm (0.9m) or 2.95feet, so lap length for 20mm bar used in column is 900mm (0.9m) or 2.95feet.
Lap length for 25mm bar:- assuming 25mm bar of Fe500 provided in RCC column, their lap length should be 45d, where d is diameter of rebar which are going to be used, calculation of lap length for 25mm bar = 45×25= 1125mm (1.125m) or 3.69feet, so lap length for 25mm bar used in column is 1125mm (1.125m) or 3.69feet.
Lap length for 32mm bar:– assuming 32mm bar of Fe500 provided in RCC column, their lap length should be 45d, where d is diameter of rebar which are going to be used, calculation of lap length for 32mm bar = 45×32= 1440mm (1.44m) or 4.73feet, so lap length for 32mm bar used in column is 1440mm (1.44m) or 4.73feet.
General rule and regulation to provide lap length in column beam and slab
1) When the bars of different diameters are to be spliced the lap length is calculated considering the smaller diameter bars.
Suppose you are constructing a column, from bottom 20 mm diameter bar is coming and from here 16 mm diameter bar has to be spliced then for calculating lap length 16 mm diameter should be considered and not 20 mm.
2) lapping should be not done in Steel bar size which are greater than 36 mm, instead of lapping, welding should be provided on that rebar for continuity and increasing in length in order to safely transfer load from one rebar to next rebar.
3) lapping should not be provided where maximum shear force exist in concrete fibre, it should be provided where shear forces is minimum.
4) actual design of lapping length calculated on the basis of design structure of column, beam and slab according to grade of Steel, grade of concrete and amount of stress forces exist.
How to calculate lap length for column?
In general, column is provided with lap length 45d, where d is diameter of rebar, assuming column size 9″×9″ using 4 number of 12mm of Fe500 bar with m20 grade of concrete,their lap length = 45d, 45×12 = 540mm (1.77feet).
How to calculate lap length for beam?
In general, beam is provided with lap length 60d, where d is diameter of rebar, assuming beam size 9″×9″ using 4 number of 12mm of Fe500 bar with m20 grade of concrete,their lap length = 60d, 60×12 = 720mm (2.36feet).
How to calculate lap length for slab?
In general, slab is provided with lap length 60d, where d is diameter of rebar, assuming beam size 9″×9″ using 10mm of Fe500 bar with m20 grade of concrete,their lap length = 60d, 60×10 = 600mm (1.97feet).
Lap length for m30 concrete:- minimum lap length is 45d in tension zone and 36d in compression zone is provided for m30 grade of concrete if Fe500 grade of steel are used,where d is diameter of rebar which are going to use in construction
Lap length for m35 concrete:- minimum lap length is 40d in tension zone and 32d in compression zone is provided for m35 grade of concrete if Fe500 grade of steel are used,where d is diameter of rebar which are going to use in construction.