The column lapping zone in RCC is one of the most important reinforcement detailing aspects in reinforced concrete structures. Correct column lapping ensures safe transfer of axial load, bending forces, and seismic forces from one storey to another. Improper lapping location or insufficient lap length can drastically reduce column strength and may lead to serious structural failures.
This comprehensive guide explains column lapping zone in RCC as per IS code, including correct location, lap length, seismic provisions, best construction practices, and common mistakes.
What Is Column Lapping in RCC
Column lapping is the process of overlapping two vertical reinforcement bars in a column to maintain continuity when a single bar is not long enough to cover the full height. The overlapping length allows load to be transferred safely through bond between steel and concrete.
Column lapping is also called lap splicing and is preferred over welding in most RCC buildings due to economy and ease of construction.
Why Column Lapping Zone Is Critical
Columns carry vertical loads of the entire structure. Any weakness in column reinforcement can lead to:
- Loss of load-carrying capacity
- Cracking and buckling
- Sudden collapse in extreme cases
Correct column lapping zone ensures:
- Structural continuity
- Safe stress transfer
- Better durability
- Seismic safety
Correct Location of Column Lapping Zone in RCC
Ideal Column Lapping Zone
As per good RCC practice and IS code principles:
👉 Column lapping should be provided in the middle zone of the column height.
This zone is under comparatively lower bending stress and mainly axial compression, making it the safest region for lapping.
Zones Where Column Lapping Should Be Avoided
Column lapping should not be done:
- At beam-column junctions
- Near floor/slab levels
- At column base
- At column top where moment is high
These areas are highly stressed and sensitive to failure.
Column Lapping Zone in Multi-Storey Buildings
In multi-storey RCC buildings:
- Lapping is usually provided just above slab level, extending upward into the next storey
- But the actual lap zone is kept away from the beam-column joint region
This helps maintain joint integrity and avoids congestion of reinforcement.
Column Lapping Zone as per IS Code
IS 456:2000 Clause 26.2 provides guidance for lap splices.
Key points:
- Lap splices shall not be provided at sections of maximum stress
- Laps shall be staggered
- Bars larger than 36 mm shall not be lapped (they must be welded or mechanically coupled)
- Lapping must satisfy development length
IS 13920:2016 gives additional rules for seismic zones.
Lap Length for Column Lapping in RCC
Lap length depends on bar diameter, grade of steel, and stress condition.
Lap Length in Compression (Most Columns)
As per IS code:
Lap length in compression ≥ Development length or 24d, whichever is greater
Common site practice:
- Fe415 → 24d to 30d
- Fe500 → 30d or more
Lap Length in Tension (Rare in Columns)
Where columns experience tension (seismic zones or uplift):
Lap length ≥ Development length or 40d to 50d
Staggered Lapping in RCC Columns
All bars should not be lapped at the same level.
IS code recommends:
- Maximum 50% bars lapped at one section
- Remaining bars should continue without splice
This prevents formation of a weak plane in the column.
Column Lapping Zone in Seismic Zones
As per IS 13920:2016:
- Lapping is not allowed in plastic hinge zones
- Plastic hinge zones occur:
- Near beam-column joints
- Near column ends
- Lapping must be shifted to the central region of column height
- Special confining reinforcement (closer ties) must be provided in lap zone
These measures ensure ductility during earthquakes.
Construction Best Practices for Column Lapping
To ensure safe column lapping:
- Follow structural drawings strictly
- Provide full lap length
- Tie bars properly in lap zone
- Maintain required clear spacing
- Ensure adequate concrete cover
- Provide closer ties in lap zone
- Compact concrete thoroughly
Common Mistakes in Column Lapping Zone
Avoid these serious site errors:
- Lapping at beam-column joint
- Insufficient lap length
- All bars lapped together
- Poor bar alignment
- Inadequate ties in lap zone
- Welding without structural design
These mistakes can severely compromise column safety.
Column Lapping vs Beam Lapping
| Column Lapping | Beam Lapping |
|---|---|
| Mainly axial load | Mainly bending load |
| Done at mid-height | Done in low moment zones |
| Less sensitive to location | Highly sensitive to location |
| Shorter lap length | Longer lap length |
Understanding this difference is critical in RCC detailing.
Frequently Asked Questions on Column Lapping Zone in RCC
Where is the safest column lapping zone
At the middle height of the column.
Is lapping allowed at slab level
Directly at slab-beam junction is avoided; lap zone must be above or below it.
Can column bars be welded instead of lapping
Only when specified by design or when bar diameter exceeds 36 mm.
Is column lapping mandatory
Yes, wherever full bar length is not available, lapping or coupling is mandatory.
Is column lapping same for all buildings
No, seismic and non-seismic zones have different requirements.
Conclusion
The column lapping zone in RCC plays a vital role in ensuring the strength, stability, and safety of reinforced concrete structures. Correct placement of lap zones, adequate lap length, staggered splicing, and adherence to IS code provisions are essential for durable and earthquake-resistant buildings.
