What is lap length?
Lap length is defined as the length provided to allow the overlap of the two reinforcement bars, thereby ensuring that the load is transferred safely and efficiently from one bar to another.
When the reinforcement bars are placed; the length of a single reinforcing bar may not be sufficient. In this case, the required design length is obtained by overlapping two bars. The overlap length is thus provided to allow such lapping of the side-by-side reinforcing bars. In other words, the length of the overlap between the two reinforcing bars is the length of the lap.
The length of the lap can also be provided when the diameter of the reinforcement bar needs to be changed along the length, especially when reinforcing columns. This process of overlapping the side of the reinforcing bar by side in order to obtain the desired design length is known as lapping.
The primary goal of providing this is to ensure an efficient transfer of axial force from the termination bar to the new connected bar along the same line of action in the joint.
NOTE:
It is general practice to produce 12 m size steel reinforcement bars to ensure ease of transport and handling. However, during the construction of reinforced concrete structures; Larger beams, columns and slabs may need more then that length. In this case, the reinforcing bars must be overlapped to achieve the desired length. Usually, such overlapping of bars is performed where the magnitude of the bending stress is less.
When two reinforcement bars are of Equal Diameter; Lap length can be calculated using the following formula:
Lap Length = 50d
Where,
d = diameter of the reinforcement bars
If you extend the top bar of a column from a lower (bottom) bar of Unequal Diameter, the lap length is calculated for the smaller diameter bar.
Example Calculation:
If the Topbar of 12mm dia. Is lapped with a lower (bottom) bar of 16mm dia and if the lap length is taken as 50d, then
Lap length 50d = 50 × 12mm = 600mm. ← Correct
Lap length 50d = 50 × 16mm = 800mm ← Incorrect
Lap Length in Tension
The following formula can be used to calculate the length of the lap, including the anchorage value of the hooks, for the lap length in tension,
1. Lap length For flexural tension = L x d or 30 x d (The greater value among the two values calculated is taken.)
2. Lap length For direct tension = 2 x L x d or 30 x d (The greater value among the two values calculated is taken.)
Where,
L = development length
In such a case, the straight lapping length of the reinforcement bars should not exceed 200 mm or 15 x d.
Lap Length in Compression
For the length of the lap in compression, the value of the lap length can be taken to be the same as the value of the growth length for development. However, in either case, the lap length cannot be less than 24 x d.
Lap Length Calculation For Different
Diameter Bars:
The lap length is calculated considering the
smaller diameter bar. When the bars of different diameters are to be spliced
Must read: General Rules For Lapping In Column Reinforcement
Importance Of Providing Correct Lap Length
Lap length is essential for reinforced concrete structures to allow tensile and compression loads to be transferred from one reinforcement bar to another by cutting or skin friction. Lack of lap length can disrupt the load transfer mechanism and lead to failure of the entire structure. Also, when the length of the provided lap is not sufficient, the reinforcement bars will split and thereby lead to cracks in the concrete. Thus, lap length is essential for reinforced concrete structures.
Lap Length of Columns, Slabs & Beams
1. Lap Length of Columns:
Procedures for calculating the lap length of columns in a reinforced concrete structure is provided by Clause 26.5.3 of IS 456: 200.
According to this code, the diameter of the bars should not be less than 12 mm. The number of longitudinal bars to be provided in a rectangular column should be four or more and more than or greater than Six in a circular column. The spacing of such longitudinal bars should be less than 300 mm when measured around the perimeter of the column. The lap length of the columns can be calculated using the following formula,
Lap Length of Column = 45d
Where,
d = diameter of the bar
2. Lap Length of Slabs:
Procedures for calculating the lap length of the slabs in a reinforced concrete structure is provided by Clause 26.5.1 of IS 456:200.
According to this code, the diameter of the reinforcement bars should be less than one-eighth of the total slab thickness.
The lap length of the slabs can be calculated using the following formula,
Lap Length of Slab = 60 x d
Where,
d = diameter of the bar
3. Lap Length of Beams:
Procedures for calculating the lap length of the beams in a reinforced concrete structure is given by Clause 26.5.2 of IS 456: 200.
According to this code, side reinforcement bars should be provided if the depth of the beam web is more than 75 cm. In such a case, the area of the reinforcement bars to be used should not exceed 0.1 per cent of the total area of the web. The reinforcement wires should be evenly distributed on both faces of the beam, i.e. the gap should not exceed 300 mm or the thickness of the mesh, whichever is less.
In beams, cross-reinforcement should be provided to withstand external tension and compression. In T-beams and I-beams, such reinforcement will run around longitudinal bars located near the outer face of the flange.
The lap length of the beams can be calculated using the following formula,
Lap length of beams = 60 x d
Where,
d = diameter of the bar
Also Read: What is Lapping Zones for Column and Beam | lapping Zones | Lceted
FAQ
What will cause improper lap length?
When the length of the provided lap is not sufficient, the reinforcement bars will split and thereby lead to cracks in the concrete. Thus, lap length is essential for reinforced concrete structures.
Why lap length is provided?
It is general practice to produce 12 m size steel reinforcement bars to ensure ease of transport and handling. However, during the construction of reinforced concrete structures; Larger beams, columns and slabs may need more then that length. In this case, the reinforcing bars must be overlapped to achieve the desired length.
What is the Minimum lap length?
What is lap length?
Lap length is defined as the length provided to allow the overlap of the two reinforcement bars, thereby ensuring that the load is transferred safely and efficiently from one bar to another.
What are the types of lapping?
Must
read: BBS
Basics: Lapping, bend, development and hook length calculation
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