Are you planning to build a new home on a corner plot? Or does your plot not have a setback area? Then you must consider an eccentric footing design for your house. This type of footing helps safely transfer structural loads to the soil without affecting adjacent properties. Through the detailed structural plan of experts, this footing can balance all uneven loads, giving proper stability and safety to the structure. It is applicable in both urban residential and commercial projects, with limited land availability.
Read on to learn more about eccentric footings, their types, design process, advantages, and disadvantages.
What is an Eccentric Footing?
The footing design where the centre of gravity of two building elements, like a column and a footing, are not aligned is called eccentric footing or shoe footing. This footing design is useful when the land area is not sufficient for placing the column at the centre of the footing, and there is a lack of a setback area. It is commonly constructed at the corner of a plot, where the exterior column is very close to the property boundary. This footing is suitable when the soil bearing capacity is 24 kN/m².
Types of Eccentric Footings
There are two major types of eccentric footings: uniaxial and biaxial.
Uniaxial Eccentric Footings
The footing where the axial load will be acting on one of the footing’s centre lines (i.e x-axis or y-axis) is called a uniaxial eccentric footing.
Biaxial Eccentric Footings
On the contrary, the footing where the axial load will act on both the centre lines, i.e., the x-axis and y-axis, is called a bi-axial footing.
When Is Eccentric Footing Required?
- Property boundary constraints
- Adjacent buildings and shared walls
- Column near plot edge
- Load transfer issues in urban construction
Eccentric Footing Design Method
Step 1: Determine Column Load and Moments
Gather axial load (P) and moment (M) by conducting structural analysis.
qmax = Q/BL + 6M/B²L
qmin = Q/BL – 6M/B²L
Where,
Q – Vertical load
M – Moment on the foundation
B – Width of the foundation
L – Length of the foundation
Step 2: Calculate Eccentricity
The offset distance between the centre of the applied load (from a column) and the geometric centre (centroid) of the footing is called eccentricity. It can be calculated using the formula
e = M/P
Where,
M – Momentum
P – axial Column load (P)
Step 3: Check Soil Bearing Capacity
Check the safe bearing capacity (SBC) of the soil using the formula
Maximum soil pressure: qmax = (P/A)* 1.5
The maximum pressure on the soil does not exceed the safe bearing capacity. Adjust the footing size if necessary to keep the eccentricity within acceptable limits.
Step 4: Calculate the Maximum Bending Moment
You can calculate the maximum bending moment at different critical sections using the formula according to the plot country code.
M = (WL)² / 2
Step 5: Calculate the Depth of Footing
The depth of footing can be calculated by using the maximum moment calculated.
d = sqrt(Mulim/(0.138*fck*b))
Where,
- 𝑑: Effective depth of the section (distance from extreme compression fiber to centroid of tensile reinforcement)
- Mulim: Limiting ultimate moment of resistance (maximum factored bending moment the concrete section can withstand without compression failure).
- 𝑓𝑐𝑘: The characteristic compressive strength of the concrete, typically measured in N/mm2
- 𝑏: Width of the rectangular section.
The depth of footing should be increased 1.5 to 2 times for shear consideration.
Step 6: Find the Required Footing Area.
You need to find the required area of the footing using the formula
Area = Total Load on soil ÷ Safe bearing capacity of Soil (SBC)
Step 7: Reinforcement Design
The percentage of steel required will be calculated, and after that, reinforcement details should be prepared according to the moment calculated and the formula provided by the country code.
Mu/(b*d^2) and Pt will be calculated.
Area of steel required, Ast = (Pt/100)*b*d
This value must be greater than the minimum area of steel i.e., 0.12% of the footing area.
The number of Bars can be calculated as = (Ast required)/ (Ast of 1 bar)
Step 8: Stability and Safety Checks
Finally, the designed footing should be checked for one-way shear (Vu= Load * Shear), two-way shear, and development length. If all the checks are passed, then a summary of footing materials with reinforcement should be prepared with a suitable diagram.
Eccentric Footing vs Centric Footing
| Parameters | Eccentric Footing | Centric Footing |
| Load Application | The load will be applied away from the centre of the footing. | The load will be applied exactly at the centre of the footing. |
| Consideration of eccentricity | Considered for footing design (e = M/P) | Here, e=0 |
| Soil Pressure Distribution | Non-uniform; maximum at one edge, minimum at the other | Uniform across the footing area |
| Moment Generation | Produces a moment due to eccentricity | No moment generated due to concentric load |
| Stability | Comparatively less stable; requires special design for stability | More stable; no risk of overturning |
| Design Complexity | Higher checks for uplift, shear, and bending are critical | Simpler; mainly checks for bearing and shear |
| Reinforcement Requirement | Higher due to bending and shear | Lower, mainly for bearing and minor shear |
Advantages and Limitations of Eccentric Footing
What are the Benefits of Eccentric Footings
Suitable for Minimal Land Area: This footing design is useful when the land area is insufficient for placing the column at the centre of the footing, and there is a lack of a setback area.
Design Flexibility: The shoe footing design allows for extensions adjacent to existing buildings or placement near existing infrastructure like pipes, making it suitable for sites with limited area.
Load Management: These footings are important to maintain the stability of the structure, especially under the application of uneven or eccentric loading conditions.
What are the Limitations of Eccentric Footings?
Complex Design: The design calculations involved are comparatively trickier than other types of footing design.
Higher Reinforcement Requirement: The steel quantity required for reinforcement is comparatively more than that of regular footing designs.
Common Design Mistakes to Avoid
- Ignoring soil pressure variation
- Incorrect eccentricity calculation
- Inadequate reinforcement anchorage
- Poor strap beam integration
Eccentric footing is a practical and efficient solution for constructing buildings on corner plots or sites with limited land and no setback area. By carefully balancing uneven loads, it ensures structural stability and safety while protecting adjacent properties. Though its design is more complex and requires additional reinforcement compared to centric footings, the benefits, such as flexibility in placement, effective load management, and suitability for urban residential and commercial projects, make it a reliable choice. For expert construction services and precise footing designs in Bengaluru, trust Brick & Bolt to bring your dream home to life with quality and safety.
Frequently Asked Questions (FAQs)
What is eccentricity in footing design?
Eccentricity in footing design is the offset distance between the center of the applied column load and the centroid of the footing. It causes uneven soil pressure and induces bending in the footing.
What are the benefits of eccentric footings?
Eccentric footings allow for efficient use of space near property boundaries and can accommodate loads near existing infrastructure. They provide flexibility in constrained sites without requiring larger combined footings.
What is the eccentricity limit for footings?
The eccentricity limit is typically L/6, where L is the footing dimension in the direction of eccentricity. Exceeding this limit can cause uplift and non-uniform pressure, risking instability.
How to calculate eccentricity of foundation?
Eccentricity is calculated as e = M/P, where M is the moment from the column load and P is the axial load. This value helps determine the distribution of soil pressure under the footing.
Can eccentric footing be used for residential buildings?
Yes, eccentric footings can be used for residential buildings, especially when columns are near property lines or adjacent to existing structures. They are suitable if properly designed for stability and soil pressure.