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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Mullion Column in Blockwork