Mullion Column in Blockwork - Construction Guide
Mullion Column in Blockwork: Construction Guide
Published by ConstructionGo
Author: Rehan A. Qamar
In modern construction practices, especially with infill block walls or partition systems, **mullion columns** have become critical structural and architectural components. A mullion column in blockwork serves as a **vertical stiffener**—providing lateral stability, supporting openings like windows and doors, and enhancing seismic performance. Though seemingly simple, their design, placement, and detailing require deep engineering understanding to avoid future cracks, load transfer issues, or architectural mismatch.
1. What is a Mullion Column?
A mullion column, also called a vertical stiffener or masonry pier, is a **reinforced vertical structural element** placed between or adjacent to non-load bearing blockwork. These columns are typically made of **RCC (Reinforced Cement Concrete)** and integrated within block masonry walls.
They serve multiple purposes:
- Provide **lateral stiffness** to tall or wide walls
- Support **window and door frames**
- Act as **seismic energy dissipators** in infill walls
- Prevent large-scale **cracking** or wall displacement
2. Typical Dimensions and Spacing
The dimensions of a mullion column depend on the wall height, thickness, building zone, and loading condition. Typical dimensions are:
- Width: 100 mm to 150 mm (matching wall thickness)
- Depth: 150 mm to 300 mm
- Spacing: Usually 2.5 m to 3.5 m c/c (center-to-center)
Note: For high seismic zones, mullions are provided at **shorter intervals** with additional tie beams or bands.
3. Structural Purpose of Mullion Columns
Though non-load-bearing walls don't typically support vertical loads, they do need lateral restraint and boundary support. Mullion columns:
- **Restrict horizontal displacement** due to wind or earthquake
- **Reduce slenderness** of masonry walls
- **Transfer concentrated loads** from small lintels or chajjas
- **Enhance wall panel integrity**
4. Materials Used
Mullion columns are generally made with:
- Concrete grade: M20 or M25 (depending on load)
- Steel reinforcement: Fe500 TMT bars
- Blockwork interface: Keying or mechanical anchoring
- Cover: 20 mm nominal cover
5. Detailing and Reinforcement
Reinforcement detailing is essential to ensure proper anchorage and bonding. Common details include:
- 4 nos. 12 mm dia bars vertically (minimum)
- 8 mm dia stirrups at 150 mm c/c (spacing reduced near joints)
- Overlap of bars at lintel or floor levels if continuation needed
- L-shaped dowels for connection with horizontal tie beams
Important: Mullions should be **anchored to floor slab or beam** above and below to complete the load path.
6. Placement of Mullions in Blockwork Walls
Mullion columns are ideally placed at:
- Ends of long partition or infill walls
- At the edge of window and door openings
- Junctions between perpendicular walls
- Every 3 meters in long walls without openings
The placement pattern should form **vertical zones** within which the blockwork is confined.
7. Mullion Columns in Seismic Zones
In seismic-prone areas (Zone III–V), mullion columns are critical components of non-structural seismic resistance. They:
- Absorb and distribute seismic energy across walls
- Prevent brittle collapse of unconfined walls
- Allow safe cracks at joints rather than structural failure
**IS 4326** and **IS 13920** recommend the use of vertical and horizontal reinforcement bands in masonry walls, often integrated using mullion columns and lintel bands.
8. Blockwork Around Mullion Columns
Special care is required when constructing block masonry near mullions:
- Ensure proper bonding using toothing or keying
- Stagger vertical joints near mullion face
- Apply wire mesh across junction to prevent shrinkage cracks
- Use polymer-modified mortar in seismic zones
Adhesive-based AAC block systems also require proprietary joint mesh and sealant along mullion edges.
9. Mullion vs Pier vs Stiffener
The terminology varies across regions:
- Mullion: Vertical support between or at ends of openings (windows/doors)
- Pier: Wide masonry column supporting load or acting as a buttress
- Stiffener Column: Vertical RCC embedded in non-structural wall
In most site references, these terms are **used interchangeably**, but understanding their subtle differences helps in correct BOQ, drawing, and execution.
10. Case Study – RCC Mullion Columns in Hospital Partition Walls
In a G+7 hospital project in Maharashtra, 100 mm thick AAC partition walls were supported using 230 mm × 150 mm RCC mullions at every 3-meter spacing. Key outcomes:
- Improved crack resistance after air conditioning duct installation
- Window edges held without deformation
- No vertical shrinkage cracking observed even after 1 year
(...continued in next part)
11. Construction Sequence of Mullion Columns
The step-by-step execution of mullion columns should be integrated with blockwork planning. The standard sequence is:
- Mark the layout and mullion centerlines on slab/floor
- Fix vertical bar reinforcement using approved bar bending schedule (BBS)
- Install shuttering box (wood or metal) aligned with block wall
- Pour concrete of desired grade and vibrate well
- Allow curing for at least 3 days or until gain of sufficient strength
- Begin blockwork using proper joint mesh and bonding at column edges
12. Mullion Columns in AAC and Hollow Block Construction
In AAC (Autoclaved Aerated Concrete) or hollow block walls, mullion columns become even more essential:
- Blocks are lightweight and fragile under horizontal loads
- Seismic resistance is achieved only with confining members
- Window or door openings need side mullions and lintels
Note: Most AAC block manufacturers recommend columns at ≤3 m spacing with pre-defined RCC detailing templates.
13. BOQ and Rate Analysis of Mullion Columns
For estimation, mullion columns are measured in cubic meters (m³). A typical BOQ item would be:
“Providing and casting RCC mullion columns of size 230mm × 150mm with M20 grade concrete, including shuttering, steel reinforcement (Fe500), curing, finishing, and alignment.”
Rate analysis includes:
- Concrete (material + mixing)
- Steel (weight as per BBS)
- Shuttering (fixed cost per column)
- Labour, curing, and overheads
Typical Cost Range: ₹7500 to ₹9500 per m³ depending on quantity and site conditions.
14. Mistakes to Avoid During Execution
Site teams must be cautious to avoid the following:
- Incorrect spacing – leading to wall cracking
- No mechanical anchoring at slab – column becomes ineffective
- Poor alignment – causes aesthetic and junction issues
- No wire mesh between block and column – leads to shrinkage cracks
- Premature load on block wall before mullion curing – may result in hairline cracks
15. Difference Between Load-Bearing and Mullion Columns
| Aspect |
Mullion Column |
Load-Bearing Column |
| Purpose |
Stiffening & crack prevention |
Transfer vertical loads to footing |
| Position |
Within non-load bearing blockwork |
As part of structural frame |
| Size |
150×230 mm typical |
230×450 mm and above |
| Steel Requirement |
Minimal (12 mm bars) |
High (16–25 mm bars) |
| Design Drawing |
Often on architectural or blockwork drawings |
Structural drawing required |
16. Design Guidelines from Codes
While specific IS codes do not define “mullion” columns distinctly, the following documents guide wall design and integration:
- IS 1905: Code of Practice for Structural Use of Unreinforced Masonry
- IS 4326: Earthquake Resistant Design of Masonry Buildings
- IS 13920: Ductile Detailing of RCC Structures Subjected to Seismic Forces
- NBC Part 6: Structural Design Section on Masonry
17. Architectural Integration of Mullions
Though mullion columns are structural, they impact finishes and alignments:
- Can be **hidden within plaster or POP finish**
- Need to align with **door/window jambs**
- Should be **symmetrical in elevation** if exposed
- Consider paint or cladding differences due to material variation
Interior designers may conceal these with gypsum board or cabinets in drywalls.
18. Mullion Columns in Curtain Walls and Glazing
In façade engineering, the term "mullion" also refers to vertical framing elements of glass walls. While different from RCC mullions, they share similar principles:
- Act as **vertical load bearers for panels**
- Transfer wind loads to horizontal transoms
- May be made of **aluminum, steel, or RCC** in heavy-duty frames
Conclusion: Whether in blockwork or glass façades, mullions are about vertical support and load management.
19. Summary of Mullion Column Characteristics
| Parameter |
Typical Value |
| Material |
M20 or M25 RCC with Fe500 Steel |
| Typical Size |
150 mm × 230 mm |
| Spacing |
2.5–3.5 meters center to center |
| Minimum Reinforcement |
4 nos 12 mm bars + 8 mm stirrups @150 mm |
| Connection |
Anchored to slab or beam at top and bottom |
(...continued in final part)
20. Mullion Columns in High-Rise Buildings
In high-rise residential and commercial buildings, non-load bearing walls must endure higher wind pressures and building movements. Mullion columns in such contexts:
- Act as vertical restraint against out-of-plane buckling
- Distribute stresses uniformly across multiple floors
- Minimize damage to finishes like paint, POP, or tiles
Engineering Practices in High-Rises:
- Use higher grade concrete (M25 or M30)
- Design RCC mullions as part of ductile detailing (IS 13920)
- Provide expansion joints and control joints aligned with mullions
- Ensure accurate BBS, anchorages, and construction sequence
21. Mullions in Precast and Modular Construction
In modular or precast systems, mullions are either:
- Precast as separate columns and joined onsite
- Cast monolithically with precast wall panels
Precast mullions require careful treatment at joints with grouting compounds and post-installed rebar connections to maintain seismic integrity.
Advantages in modular systems:
- Speed of construction
- Dimensional accuracy and clean finish
- Reduced onsite congestion
22. Maintenance and Inspection of Mullion Columns
Once installed, mullions require minimal maintenance, but regular inspection helps prevent future issues. Key points to check:
- Cracks at block-mullion junction
- Rust stains indicating rebar exposure
- Plaster debonding at vertical joints
- Movement due to shrinkage or thermal expansion
In case of damage, injection grouting and fiber mesh finishing are common repair methods.
23. Frequently Asked Questions (FAQs)
Q1: Are mullion columns structural?
A: They are semi-structural. Though not designed for vertical load-bearing, they provide lateral restraint and seismic resistance.
Q2: Is reinforcement necessary in every mullion column?
A: Yes. Even minimal vertical reinforcement (4 bars of 12 mm) and ties are required for strength and ductility.
Q3: Can I omit mullions in AAC block walls if the wall height is under 2.4 m?
A: Possibly, depending on seismic zone and wall thickness, but it is safer to include one at wall ends.
Q4: Should mullion columns be placed before or after blockwork?
A: Preferably before. However, they can be cast along with wall masonry using shuttering gaps if needed.
Q5: Do mullion columns affect building elevation design?
A: Yes. If exposed or misaligned, they can break elevation symmetry. Coordination with the architect is essential.
24. Checklist for Site Execution
- ✅ Mullion layout marked as per architectural block plan
- ✅ Bar bending schedule approved and steel placed with cover blocks
- ✅ Vertical continuity ensured with dowels
- ✅ Concrete poured without honeycombing
- ✅ Blockwork bonded and keyed properly
- ✅ Wire mesh or chicken mesh applied at column-joint interface
- ✅ Plastering aligned flush with column face
- ✅ All windows and door openings anchored to adjacent mullions
25. Future Trends in Mullion Column Systems
With innovation in construction and prefabrication, mullion technology is evolving:
- FRP-Reinforced Mullions: Lightweight and corrosion-resistant for coastal zones
- Hybrid Mullions: Combining steel frame with AAC block infill
- Smart Mullions: Sensors embedded to detect wall movement or structural stress
- BIM-Based Layout: Digital planning and clash detection before slab casting
26. Conclusion
Mullion columns in blockwork may seem minor in size, but their impact on the durability, strength, and seismic performance of walls is profound. Whether you’re constructing a ground floor partition or a 40-storey tower, neglecting these vertical stiffeners can result in:
- Cracking of finishes
- Failure of infill panels during earthquakes
- Unsafe edge conditions for windows and doors
By integrating well-detailed and properly constructed RCC mullions, engineers ensure not only code compliance but also long-term serviceability and aesthetics.
Let mullion columns be treated not as "just a column"—but as **invisible protectors of your wall systems**.
27. Credits & References
- IS 1905 – Code of Practice for Structural Use of Unreinforced Masonry
- IS 4326 – Earthquake Resistant Design of Buildings
- IS 13920 – Ductile Detailing of Reinforced Concrete Structures
- National Building Code of India (Part 6)
- Engineering field notes and case studies from ConstructionGo projects
Author: Rehan A. Qamar, B.E. Civil
ConstructionGo.in – Learn Site, Lead Site.
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