Column Bases & Column Splices
Column bases are structural elements provided at the bottom of steel columns to safely transfer loads from steel columns to concrete foundations. Since the compressive stress in concrete is much smaller than that in steel, a base plate is provided to spread the column load over a larger area and prevent overstressing of the concrete footing.
Introduction to Column Bases
Steel columns transfer heavy compressive loads to foundations. Direct transfer of load from a steel column to concrete footing may cause excessive bearing pressure. Therefore, steel base plates are used to increase the bearing area.
Types of Column Bases
- Slab Base
- Gusseted Base
1. Slab Base
A slab base consists of a thick steel plate attached to the base of the column. It is generally used for concentrically loaded columns carrying axial compression.
Features of Slab Base
| Feature | Description |
|---|---|
| Loading Type | Pure axial compression |
| Construction | Simple steel plate welded to column |
| Usage | Light and medium steel columns |
| Economy | Simple and economical |
2. Gusseted Base
Gusseted bases are used when the column is subjected to heavy loads or bending moments. Gusset plates and angles are provided to increase stiffness and improve force transfer.
Applications
- Heavy steel columns
- Columns carrying large moments
- Industrial structures
- Bridge supports
Design Procedure for Slab Base
Step 1: Bearing Strength of Concrete
Where:
- fck = Characteristic compressive strength of concrete
Step 2: Area of Slab Base
Where:
- A = Area of slab base
- P = Factored column load
Step 3: Size of Square Base Plate
Where:
- L = Length of base plate
- B = Width of base plate
Step 4: Bearing Pressure Under Base Plate
Where:
- w = Bearing pressure
- A0 = Area of slab base plate
Step 5: Thickness of Slab Base
Where:
- ts = Thickness of slab base
- w = Bearing pressure
- a = Larger projection beyond column
- b = Smaller projection beyond column
- fy = Yield stress of steel
Anchor Bolts in Column Bases
- Anchor bolts are used to fix the base plate to the concrete footing.
- Usually 2 or 4 holding-down bolts are provided.
- Anchor bolts resist uplift and lateral forces.
- For axial compression only, two bolts are generally sufficient.
Components of Column Base
| Component | Function |
|---|---|
| Base Plate | Distributes column load |
| Anchor Bolts | Fixes column to foundation |
| Concrete Footing | Transfers load to soil |
| Gusset Plates | Provides stiffness in heavy bases |
Advantages of Slab Base
- Simple fabrication and erection.
- Economical for light and medium loads.
- Easy load transfer mechanism.
- Suitable for concentrically loaded columns.
Advantages of Gusseted Base
- Suitable for heavy loads.
- Provides additional stiffness.
- Better moment resistance.
- Improved structural stability.
Applications of Column Bases
- Industrial steel structures
- Multi-storey buildings
- Transmission towers
- Bridge structures
- Steel sheds and warehouses
Conclusion
Column bases are essential structural elements used to safely transfer loads from steel columns to concrete foundations. Proper design of slab bases and gusseted bases ensures adequate load distribution, structural stability, and safety of steel structures. Understanding IS 800 provisions for column base design is important for every structural engineer.
References
- IS 800:2007 – General Construction in Steel
- Steel Structures by N. Subramanian
- Limit State Design of Steel Structures by S.K. Duggal
- Design of Steel Structures by Ramchandra
- SP:6 Structural Steel Handbook
Disclaimer
This article is intended for educational and informational purposes only. Engineers should refer to the latest IS codes and professional engineering standards before performing actual structural design calculations.
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