Summary
- An eccentric footing supports a column placed away from the footing centre.
- Property boundaries and limited space are among the most common causes of the use of eccentric footings.
- The offset column leads to unbalanced soil pressure and other structural considerations.
- Design involves checks for soil bearing pressure, flexural, shear, stability, and reinforcing calculations.
- Various types of eccentric footings such as isolated, combined, and strap footings may be suitable in various circumstances.
- Selection of the footing depends on the load from columns and other factors.
An eccentric footing is designed where the column is positioned off-centre from the footing. The eccentric footing is quite common when used along boundary walls. They are also useful when there is not much room available in the structure for providing foundations.
The eccentric footing design takes into consideration the soil pressure and loads imposed by structures. Bending, shear, settlement, and reinforcement requirements should also be evaluated. This guide explains how to design eccentric footings for the Indian scenario.
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What Is an Eccentric Footing in Civil Engineering?
The eccentric footing assists in carrying the load of a column, which has been placed off the centre of the footing. The distance between the point of action of the load and the centre of the footing is termed eccentricity.
The impact of eccentricity resulting in the formation of the bending moment is determined through the following formula: M = P × e, where P is the column load, whereas e is the eccentricity.
In addition to this, the eccentricity is responsible for the formation of an unequal distribution of soil pressure, wherein there would be high pressure at one end and low pressure at the other end.
Why and When Is Eccentric Footing Needed?
Eccentric footing is used where it is impossible to locate the column in the centre of the footing. Some of the main factors that make the use of eccentric footing unavoidable are as follows:
- Space constraint: If a column is situated near the property line, then a centred footing would face problems.
- Architectural requirement: Sometimes, it might become a necessity to place the column at a particular point due to architectural considerations, even if it is not the centre of the footing.
- Constraints of other buildings: Other buildings or footings may make it difficult to place the footing at the centre.
- Irregular building site: At times, an irregular building site might require an eccentric footing.
Types of Eccentric Footing
The various types of eccentric footings include the isolated eccentric footing, combined footing, and the strap footing. These types will be considered appropriate depending on the location of the columns, space available, loads of the columns, and the soil conditions.
| Type | What it is | Where it is commonly used |
|---|---|---|
| Isolated eccentric footing | A single footing supports one column placed away from its centre. | In case there is a column that is located near the edge, but the eccentricity can still be controlled within the area provided for the footing. |
| Combined Footing | A footing used to support two or more columns. | When the columns are closely spaced, or their footings will interfere with each other. |
| Strap Footing | Two footings joined by a reinforced concrete strap. | When there is a need to provide support for a boundary column but cannot extend beyond the property line. |
Concentric vs Eccentric Footing: Key Differences
A concentric footing has the column load acting through the centre of the footing, while an eccentric footing has the load positioned away from the centre. This changes how pressure is distributed through the soil and affects the footing's structural design.
| Feature | Concentric Footing | Eccentric Footing |
|---|---|---|
| Column placement | The column sits near the middle of the footing. | The column is shifted toward one side of the footing. |
| Load transfer | The column load reaches the soil through the central area of the footing. | Because the column is off-centre, the load reaches the soil unevenly. |
| Soil pressure | The pressure below the footing is relatively uniform when the load is centred. | One side of the footing carries more pressure than the other. |
| Bending moment | No additional moment develops from an offset column load. | The offset load produces a bending moment that must be considered in the design. |
| Typical situation | Generally used when the column can be kept away from property boundaries and other restrictions. | Particularly useful when a column is close to a property boundary, and the footing cannot extend equally on both sides. |
A footing may also be eccentric in two directions because of the placement of the load on the column in two directions. Here, the pressure distribution must be considered in both directions. A design of the eccentric footing must take into account the actual positioning of the column, load, soil, and space available.
Design Considerations for Eccentric Footing
In the design of an eccentric footing, there is a need to evaluate the effect of the eccentric loading of the column on the ground and the footing structure. The following are the factors that should be considered:
- Soil pressure: Check the pressure on the ground below the footing and make sure it does not exceed the soil bearing capacity.
- Load and eccentricity: Evaluate the different loads on the footing and the moment of the eccentricity of the column.
- Size of the footing: Calculate the appropriate size of the footing.
- Reinforcement: Reinforcement for bending must be designed.
- Shear: Check the footing for one-way and punching shear.
- Indian Standards: Follow the relevant requirements of IS 456 and other applicable Indian Standards.
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Step-by-Step Eccentric Footing Design Process
Step-by-Step Eccentric Footing Design Process - illustrative photo for an Indian home
The eccentric footing design process starts with site and structural data and ends with detailed structural drawings. Here are the steps to follow:
1. Collect Site and Structural Data
The first step to be taken in this regard is the collection of all the necessary information. This includes data on the column load, dimensions of the column, location of the column, moment applied, soil bearing capacity, site boundaries, grade of concrete, and reinforcement grade. The location of the column is especially important.
2. Calculate the Required Footing Area
Estimate the required footing area using the column load and allowable soil bearing pressure. The preliminary relationship is:
Footing area ≈ Load / Allowable soil pressure
The final area should also account for the applicable load basis, footing self-weight, soil conditions, and settlement requirements.
3. Decide on the Footing Type
Select the most suitable arrangement based on the site constraints. Options may include an eccentric isolated footing, combined footing, or strap footing. A boundary column may require a strap or combined arrangement when an isolated footing cannot provide a suitable pressure distribution.
4. Calculate Eccentricity
Determine the distance between the column load line and the footing centre. For a simple loading case:
e = M / P
Where e is eccentricity, M is the applied moment, and P is the vertical load. For a corner column, eccentricity may need to be considered in two directions.
5. Check Soil Pressure
Calculate the pressure below the footing based on the applied load, footing area, and eccentricity. The pressure will not be uniform when the load is eccentric. Check the maximum and minimum pressure against the applicable soil bearing requirements.
6. Check Shear and Bending
The footing must be checked for bending, one-way shear, and punching shear. These checks help determine whether the selected footing dimensions and thickness can safely resist the applied forces.
7. Design the Reinforcement
Provide reinforcement based on the calculated bending requirements. The design should also consider bar spacing, development length, anchorage, and concrete cover. The reinforcement layout should reflect the actual eccentric footing geometry.
8. Prepare Structural Drawings
Complete the eccentric footing detail after the design checks are finished. The drawings should show the footing dimensions, column position, eccentricity, thickness, reinforcement, spacing, cover, and any strap beam or connection details.
What Are the Common Mistakes in Eccentric Footing Design?
Problems in eccentric footing design can have consequences in terms of the soil pressures, strength of the structure, and footing stability. These problems include:
- Inaccurate calculation of loads: Inaccuracies in the calculation of column loads or overlooking load combinations can produce either an inappropriate footing dimension or incorrect eccentricities.
- Not considering soil conditions: Failure to consider accurate soil bearing capacity data when designing can produce inaccurate footing dimensions and increased soil pressures.
- Improper reinforcement placement: The reinforcement must comply with the structural design. Improperly spaced or placed bars can decrease the strength of the footing.
- Neglecting column position: Column positioning is critical in calculating the eccentricity. If the column position changes, then the footing design must be changed according to the new position.
- Using a generic design file: A generic design file for eccentric footing in Excel, PDF, or online format cannot be used as the final design. This should be customised for the specific design situation.
- Moving footing on site: Any changes in the footing or column position during construction without structural approval will change the loads.
Eccentric Footing vs Other Foundation Types
An eccentric footing is used when a column cannot be positioned at the centre of its footing. Combined and strap footings can also address restricted foundation layouts, but their arrangements are different.
The suitable option depends on the column layout, available space, loads, and soil conditions.
| Foundation type | Arrangement | Suitable when |
|---|---|---|
| Eccentric isolated footing | One column is supported by a footing with the column positioned away from its centre. | A single column is offset because of a boundary or space constraint. |
| Combined footing | Two columns are supported by one common footing. | Columns are close enough that separate footings may overlap or a shared footing is more practical. |
| Strap footing | Two separate footings are connected with a strap beam. | A boundary column needs to be connected to an interior footing while keeping the exterior footing within the available space. |
Next Steps in Eccentric Footing Design and Implementation
Once the eccentric footing design is prepared, the next step is to verify the design and ensure it is suitable for the actual site conditions.
Use this checklist before construction:
- Consult a structural engineer: The proposal for the footing size, loads, reinforcement, and detailing should be confirmed with an experienced structural engineer.
- Site condition verification: Verify whether the site data, including soil data, column location, boundary limits, and foundation area, are similar to the design considerations.
- Indian Standards: The design should be confirmed with respect to IS 456 and other Indian Standards before construction.
- Verify the details: The design of the footing using RCC, eccentric footing, and beam detailing should be verified with the structural drawings.
- Construction confirmation: Verify whether the footing and reinforcement are placed based on the approved drawings. It should not be altered without proper approval.
Build Your Footing With the Right Construction Materials
Once the footing design is finalised, the next step is sourcing the materials needed for construction. Cement, TMT steel, and waterproofing materials all play an important role in building a strong and durable foundation. HomeRun helps you source these construction essentials from trusted brands and get them delivered to your site.
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Frequently Asked Questions
What is an eccentric footing?
An eccentric footing supports a column placed away from the centre of the footing. This offset creates eccentricity and changes the soil pressure beneath the footing.
Why is an eccentric footing used?
An eccentric footing is commonly used when a column is close to a property boundary or when limited space prevents a centrally placed footing.
What is the difference between eccentric and isolated footing?
An isolated footing supports a single column. Eccentric describes the position of the column load relative to the footing centre. Therefore, an isolated footing can also be eccentric.
How is eccentricity calculated in footing design?
For a simple loading case, eccentricity can be calculated using e = M/P, where M is the applied moment, and P is the vertical load. The complete footing design requires additional soil and structural checks.
Is an eccentric footing safe?
An eccentric footing can be safe when it is designed for the actual column loads, soil conditions, eccentricity, and structural requirements. The final design should be reviewed by a qualified structural engineer.
What is an eccentric footing with a beam?
An eccentric footing with a beam typically refers to a footing connected to another footing through a strap beam. This arrangement is commonly considered for boundary columns where the footing cannot extend outside the property.
What is a biaxial eccentric footing?
A biaxial eccentric footing has eccentricity in two directions. This can occur with a column near a plot corner, requiring the footing to be checked along both axes.
What factors affect eccentric footing design?
The main factors include column load, eccentricity, soil bearing capacity, footing dimensions, bending, shear, reinforcement, and settlement. The applicable Indian Standards should also be followed during design.