In reinforced concrete construction, lapping zone in column plays a crucial role in ensuring structural integrity and stability. Lapping refers to the process of overlapping two reinforcing bars to safely transfer stress from one bar to another. When a single steel bar length is insufficient to cover the required span of a column, lapping becomes necessary. Understanding the correct lapping zone in column is vital for achieving strength, durability, and compliance with design standards.
What Is Lapping in Column?
Lapping in column is the method of connecting two steel bars by overlapping them for a specific length to ensure proper load transfer. It’s essential because steel bars are manufactured in standard lengths, usually up to 12 meters. However, in tall structures or long columns, longer bars are needed. Instead of using mechanical couplers or welding, lapping provides a cost-effective and simple method for continuity of reinforcement. The lap length depends on factors such as grade of concrete, diameter of bar, type of stress (tension or compression), and concrete cover and bonding quality. In general, the lap length in compression is smaller than in tension due to better bond characteristics under compression.
Why Is Lapping Zone Important in Columns?
The lapping zone in column determines where the overlaps should be placed to minimize structural weakness. Incorrect lapping can cause cracks, poor load transfer, and even failure at critical points. Proper lapping ensures uniform stress transfer between bars, structural safety and serviceability, compliance with IS 456:2000 and other building codes, and economic reinforcement detailing. A column carries high axial loads; hence, the position of lapping directly affects its performance and stability.
Location of Lapping Zone in Column
1. Lapping Zone in Short Columns
For short columns (less than 3 meters), the lapping should be located at the center of the column height because the middle portion experiences lesser bending moments, reducing the risk of cracks or stress concentration.
2. Lapping Zone in Long Columns
For long columns, which may experience both axial load and bending moments, the lapping should be done away from regions of maximum stress, typically placed near the mid-height or 1/3rd of the column height from the top or bottom.
3. Avoiding Lapping at Column-Beam Junction
Never lap the bars at the beam-column junction. This zone experiences the highest bending and shear stresses. Overlapping here weakens the joint and can cause failure. Always provide a clear length of full bar (development length) above and below the beam-column connection before starting the lap.
Lap Length in Column as per IS Code
According to IS 456:2000 (Clause 26.2.5.1), the lap length in compression should not be less than 24 times the diameter of the bar (24d) or 300 mm, whichever is greater. For example, for a 16 mm diameter bar, lap length = 24 × 16 = 384 mm. Since 384 mm > 300 mm, the lap length will be 384 mm. For tension members (if column reinforcement is subjected to tension due to lateral loads or moments), the lap length is taken as L = 47 × bar diameter (47d).
Rules and Guidelines for Lapping in Columns
Only 50% of bars should be lapped at a particular section. Lapped bars should be staggered by a minimum of 1.3 times the lap length to avoid stress concentration. Ensure proper concrete cover (usually 40 mm for columns) to protect reinforcement and ensure proper bond strength. Before lapping, the bars must be clean and free from rust, oil, or dust. The lapped portion should be tied tightly with binding wire to prevent slippage during concreting. Lapping in regions with high bending moments (like near supports) should be avoided; mechanical couplers are preferred in such cases.
Types of Lapping in Column
1. Straight Lapping
The most common method where two bars overlap in a straight line for a required lap length. Suitable for compression members.
2. Cranked Lapping
Bars are bent or cranked before overlapping to increase the bond and anchorage. This is mainly used when the space is limited or in sloped members.
3. Welded Lapping
Bars are joined by welding instead of overlapping. It provides a strong connection but is costlier and requires skilled labor.
4. Coupler Connection
Modern high-rise structures often use mechanical couplers instead of lapping. They provide 100% strength transfer and avoid congestion of steel in heavily reinforced columns.
Common Mistakes in Lapping Columns
Common mistakes include lapping all bars at the same height, providing insufficient lap length, lapping near the beam-column junction, using corroded or unclean steel bars, improper binding, or ignoring staggered laps. Each of these errors can significantly reduce the column’s compressive and load-bearing capacity.
Practical Example of Lapping Zone in Column
Suppose you have a 3000 mm high column with 16 mm diameter main bars. According to IS 456:2000, lap length = 24 × 16 = 384 mm. To ensure structural safety, provide lap at the middle 1000 mm zone (between 1000 mm and 2000 mm from the base). Stagger the laps between adjacent bars by at least 500 mm. Avoid any lap within 450 mm of the beam-column junction.
Advantages of Proper Lapping Zone
Proper lapping enhances column stability and strength, ensures better load transfer between reinforcement bars, reduces stress concentration, improves durability, and minimizes crack formation. It also maintains continuity of reinforcement without using costly alternatives. Correct lapping detailing simplifies site work and reduces steel wastage.
When to Use Couplers Instead of Lapping
Couplers are ideal when the bar diameter exceeds 36 mm, when the structure is subjected to high axial or dynamic loads, or when there is limited spacing for reinforcement. They eliminate overlapping, provide full strength transfer, and comply with modern construction standards for seismic resistance.
FAQs About Lapping Zone in Column
What is the minimum lap length for columns?
As per IS 456:2000, the minimum lap length for compression bars is 24d or 300 mm, whichever is greater.
Where should lapping not be done in a column?
Avoid lapping near the beam-column junction or regions with high bending stress. It should ideally be placed at mid-height of the column.
Can all bars in a column be lapped at the same level?
No, only 50% of the bars should be lapped at a single section to maintain column strength.
What happens if lapping is not properly done?
Improper lapping can lead to cracks, bond failure, and structural collapse under high loads.
Can we weld bars instead of lapping?
Yes, welding or mechanical couplers can be used as alternatives to lapping, especially for larger diameter bars or high-stress zones.
Conclusion
The lapping zone in column is a critical aspect of reinforced concrete construction. A properly designed and executed lap ensures safe stress transfer, durability, and compliance with structural codes. Always follow IS 456:2000 guidelines for lap length and placement, avoid congested or highly stressed areas, and ensure that laps are staggered. For heavy or high-rise structures, mechanical couplers can replace lapping for improved performance and reliability.
