In reinforced concrete construction, lap length or overlapping length refers to the length over which two reinforcement bars in a column are spliced or overlapped to maintain structural continuity. This connection technique, known as lap splicing, is essential to securely transfer loads from one bar to another, ensuring the strength and stability of the column.
Since the maximum commercially available length of reinforcement bars is generally 12 meters, it becomes necessary to join two or more bars to achieve longer reinforcement lengths required in large structures. This connection is typically performed by lapping or splicing bars together. Lap splicing can be achieved through methods such as staggered laps, welding, or couplers, each suited for different structural requirements and conditions.
Lapping Zones in Columns According to IS 13920:1983
The Indian standard code IS 13920:1983 classifies columns into three zones based on the moment distribution under lateral loading conditions. Understanding these zones helps in determining the appropriate location for lap splicing.
- Zone-A: Located at the top and bottom of the column, where the bending moment is maximum.
- Zone-B: The middle portion of the column where the moment gradually reduces to zero.
Because Zone-A is subjected to higher stresses, lap splicing is avoided here. Instead, Zone-B is identified as the ideal region for lap splicing because it experiences relatively lower moments, reducing the risk of failure in the splice region.
The zones are dimensioned relative to the clear height of the column (denoted as hc). Zone-A occupies one-fourth of the height at both ends, while Zone-B spans the middle half (hc/2). This zoning system also aids in the application of special confining reinforcement for ductile detailing, which is crucial for earthquake-resistant structures.
Lap Splices for Columns (According to IS 456-2000 and IS 13920-1993)
When detailing lap splices in columns, several important rules must be followed to ensure safety and performance:
- Bar size limitation: Lap splicing is not recommended for bars larger than 36 mm in diameter. For such bars, welding or mechanical couplers are preferred for joining.
- Lap splicing zone: Laps should not be provided in the tension zones (Zone-A) due to high stresses.
- Lap length: The lap length should be the greater of the development length (Ld) or 30 times the bar diameter (30d). If the structural designer does not specify a lap length, this general rule is applied.
- Lateral reinforcement: Close spacing of lateral ties or links is mandatory in the lap zone to provide confinement and enhance ductility. According to IS 13920-1993, special confined reinforcement should be placed within a distance lo, which is the lesser of hc/6 or 450 mm from the top and bottom of the column.
- Different bar diameters: When lapping bars of different diameters, the lap length corresponding to the smaller diameter bar is used.
- Avoid lapping at column-beam junction: Because this region experiences maximum stress, lap splicing is prohibited there.
- Staggering laps: Lap splices of adjacent bars should be staggered to avoid weakening any cross-section.
- Maintaining cover: Proper care should be taken so that the lap splicing does not compromise the clear concrete cover of the column.
- Minimum lap length: The lap length should never be less than 75 mm to ensure adequate load transfer.
Splicing with Offset Cranked Bar (Joggle) in Columns
Sometimes, bars need to be spliced with an offset or crank to maintain the clear cover of the column without affecting its structural integrity. This bending technique is called a joggle and is detailed in SP 34:1987, Handbook on Concrete Reinforcement and Detailing.
The crank length refers to the length of the bend provided to the reinforcement bar to maintain the clear cover.
The maximum slope of the crank is usually kept at 1:10, with a minimum crank length of 300 mm.
The crank length can be calculated using the formula:
Crank length=(d1​+d2​+5)×10mm
where d1​ is the diameter of the smaller bar and d2 is the diameter of the larger bar being lapped.
This bending technique helps in maintaining uniform concrete cover and ensures the reinforcement is properly embedded in the column.
Frequently Asked Questions (FAQs)
Q1: What is the lap length of column reinforcement?
Lap length is the overlapping length where two reinforcement bars are spliced to maintain continuity along the length of a column.
Q2: What is the purpose of lap length in columns?
Since bars are available only up to 12 meters in length, lap splicing allows connecting multiple bars to cover longer lengths, enabling secure load transfer.
Q3: What is crank length for reinforcement?
Crank length is the length of the bend in a reinforcement bar that helps maintain clear concrete cover. It is calculated by the formula:
Crank length=(d1​+d2​+5)×10mm
Q4: How to lap two bars of different diameters in columns?
The lap length is calculated based on the smaller diameter bar and the bars are lapped accordingly.
Conclusion
Proper lap splicing of reinforcement in columns is critical to structural integrity, especially for tall or heavily loaded columns. By adhering to the IS codes and best practices regarding lap length, zones, lateral reinforcement, and splicing methods, engineers can ensure a safe and durable structure that effectively transfers loads without failure.