In reinforced concrete construction, proper spacing of reinforcement bars is crucial for achieving strength and durability. The steel chair for slab is a small but essential component used to maintain the correct distance between top and bottom reinforcement layers. Without steel chairs, reinforcement may shift during concreting, leading to reduced cover, weak zones, and cracks in the slab. By using steel chairs correctly, site engineers and contractors ensure that slab reinforcement remains in its designed position throughout concreting.
What is a Steel Chair in Slab?
A steel chair is a reinforcement accessory made from mild steel or high-yield steel bars, bent into a shape that supports the upper reinforcement mesh at the required height above the bottom reinforcement. It prevents sagging of top bars and ensures the slab cover is maintained as per IS code requirements. Steel chairs are typically fabricated on-site from 8 mm or 10 mm diameter bars.
Importance of Steel Chair in Slab Construction
The role of a steel chair for slab cannot be ignored because:
- It maintains proper spacing between reinforcement layers.
- Provides structural stability during concreting.
- Ensures correct cover thickness as per IS 456 guidelines.
- Reduces the chances of bar displacement while pouring and vibrating concrete.
- Enhances the durability of the slab by preventing corrosion issues due to inadequate cover.
Types of Steel Chair for Slab
Different types of steel chairs are used depending on slab thickness, reinforcement arrangement, and site requirements.
Simple Steel Chair
- Made from a single bent bar.
- Used in normal residential slabs with two reinforcement layers.
- Easy to fabricate and economical.
U-Type Chair
- Shaped like the letter “U” with a flat base.
- Provides better stability for heavier reinforcement.
- Commonly used in thicker slabs or flat slabs.
L-Type Chair
- One side is bent at 90 degrees for extra anchorage.
- Used in slabs where reinforcement needs additional locking.
Double Leg Chair
- Made with two vertical supports and a connecting bar on top.
- Stronger and suitable for slabs with higher loads.
Circular Chair
- Fabricated in round form to minimize obstruction during concreting.
- Often used in machine foundations and special structures.
Standard Size of Steel Chair for Slab
The size of a steel chair depends on slab thickness and cover requirements. Typical dimensions include:
- Height: 40 mm to 300 mm depending on slab depth.
- Diameter: 8 mm or 10 mm bar is commonly used.
- Base length: 80 mm to 150 mm for stability.
Steel Chair Spacing in Slab
The number of chairs required depends on the slab area and spacing of reinforcement. General guidelines are:
- Steel chairs should be placed at 1 m to 1.5 m spacing in both directions.
- For heavy reinforcement slabs, spacing may be reduced to 0.8 m.
- The IS code does not specify exact spacing, but site practice ensures sufficient chairs for support.
How to Install Steel Chair in Slab
Correct installation of steel chairs ensures effective performance. The steps include:
- Place bottom reinforcement mesh as per design.
- Position steel chairs at regular intervals across the slab.
- Ensure chairs are placed on cover blocks to avoid direct contact with shuttering.
- Tie the chairs to bottom bars with binding wire to prevent movement.
- Place top reinforcement mesh over the chairs and tie it securely.
- Check spacing and cover before concreting begins.
Site Checklist for Steel Chair Installation
- Verify slab thickness and reinforcement layers before deciding chair height.
- Use appropriate diameter steel (8–10 mm) for chair fabrication.
- Maintain minimum 20 mm clear cover between chair and shuttering.
- Avoid excessive chairs, as they may create congestion in reinforcement.
- Ensure chairs are fixed firmly and do not move during concrete pouring.
- Inspect spacing and positioning before giving clearance for concreting.
Advantages of Using Steel Chair in Slab
- Guarantees correct positioning of reinforcement.
- Minimizes human error in maintaining bar levels.
- Improves slab strength and load-carrying capacity.
- Prevents long-term durability problems caused by reduced cover.
- Cost-effective compared to repairing slab cracks later.
Disadvantages of Poor Chair Placement
- If chairs are not used, top reinforcement may sag.
- Excess chairs may cause concrete voids and weak points.
- Improper height may lead to less cover, increasing corrosion risk.
- Incorrect placement can cause uneven slab thickness.
Cost of Steel Chair for Slab
The cost depends on the diameter of bar used and labor for fabrication. On average:
- One steel chair weighs around 0.15 to 0.25 kg.
- If steel rate is ₹65 per kg, one chair costs around ₹10–15.
- For a 1000 sq.ft slab, around 150–200 chairs are typically required.
- Total steel chair cost ≈ ₹1,500–3,000 depending on slab size.
IS Code Reference for Steel Chair in Slab
IS 456:2000 does not directly specify steel chair design but emphasizes maintaining proper reinforcement cover and spacing. Steel chairs are used as a practical method to fulfill these requirements.
FAQs on Steel Chair for Slab
What is the purpose of a steel chair in slab?
Steel chairs are used to maintain the required spacing between top and bottom reinforcement layers in a slab.
Which size of bar is used for steel chair fabrication?
Generally, 8 mm or 10 mm diameter bars are used to fabricate steel chairs.
How many chairs are required in a slab?
On average, one chair is required for every 1 m² of slab area, but actual quantity depends on slab thickness and reinforcement layout.
Can I use plastic chairs instead of steel chairs?
Yes, plastic bar chairs are available and are corrosion-free, but they are more expensive compared to steel chairs.
What happens if steel chairs are not used?
Without chairs, the top reinforcement may sag during concreting, leading to insufficient cover, cracks, and reduced durability.
Conclusion
The steel chair for slab is a small yet critical component in reinforced concrete construction. It ensures correct spacing of reinforcement, maintains cover, and prevents displacement during concreting. Choosing the right type of chair, placing them at proper intervals, and installing them correctly improves the strength and durability of the slab. While the cost of steel chairs is minimal, their role in ensuring structural safety is significant. Following site best practices and IS code guidelines ensures that your slab performs well for decades without issues.
