Design of Combined Footing Excel Sheet
A reinforced concrete (RC) eccentric rectangular combined footing supports multiple columns, especially when loads are unequal or columns are near property lines.
The term "eccentric" indicates that the resultant load doesn't align with the footing's center, causing uneven pressure distribution.
Combined footings are provided to support two or more column loads. These may be continuous with rectangular or trapezoidal.A Combined footing is used to support the load of two or more adjacent columns.To have a uniform bearing pressure and to minimize differential settlement between columns.
Why Use Eccentric Rectangular Combined Footings?
- When columns are close to property boundaries, individual footings would extend beyond the site limit.
- To distribute unequal column loads effectively.
- This is a cost-effective solution compared to separate footings for closely spaced columns.
- To maintain structural stability and prevent differential settlement.
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| Download Design of Combined Footing Excel Sheet |
Design Principles
1. Load Analysis: Calculate all loads acting on each column, including dead, live, wind, and seismic loads.
2. Eccentricity Calculation: Determine the distance between the resultant load's line of action and the footing's centroid.
3. Footing Dimensions: Design the footing's length and width to ensure uniform pressure distribution within soil bearing capacity limits.
4. Pressure Distribution: Assess soil pressure to confirm it remains within safe limits, accounting for eccentric loading.
5. Bending and Shear Analysis: Compute bending moments and shear forces due to eccentricity and design reinforcement accordingly.
6. Reinforcement Details:
- Longitudinal bars along the footing's length.
- Transverse bars across the width.
- Top reinforcement near columns to handle negative moments.
7. Stability Checks: Verify against overturning, sliding, and bearing failures per relevant codes (e.g., ACI, IS, Eurocode).
Construction Method
1. Site Preparation: Excavate to the required depth, ensuring a level base and compacted soil.
2. Formwork Installation: Set up formwork accurately to maintain design dimensions and eccentricity.
3. Reinforcement Placement: Install reinforcement as per design, ensuring proper anchorage and spacing.
4. Concrete Pouring: Place concrete uniformly, using vibrators to eliminate air pockets.
5. Curing: Maintain moisture for at least 7 days to achieve desired strength.
6. Formwork Removal and Backfilling: Remove formwork after concrete gains sufficient strength and backfill with suitable material, compacted in layers.
Advantages
- Optimizes space, especially near property lines.
- Cost-effective for supporting multiple columns.
- Distributes unequal loads efficiently.
- Reduces differential settlement risks.
Disadvantages
- Design and analysis are more complex due to eccentricity.
- Requires more reinforcement to handle additional stresses.
- Construction demands precise execution to maintain design integrity.
- Higher costs may be incurred for high-load scenarios.
Common Mistakes to Avoid
- Neglecting the effects of eccentricity in design.
- Underestimating reinforcement requirements.
- Inaccurate formwork leading to misalignment.
- Using substandard concrete, compromising durability.
For practical application, a spreadsheet tool is available to assist in the design of RC eccentric rectangular combined footings
Download Design of Combined Footing Excel Sheet
