Eccentric footing design is required when a column load does not act through the centre of the footing. This usually happens near property boundaries, plot edges, existing structures, or columns placed close to a wall. Because the load is off-centre, soil pressure below the footing becomes uneven, which can create bending, tilting, and settlement risks if not designed correctly. The design must consider column load, safe bearing capacity, eccentricity, soil pressure distribution, footing size, bending moment, shear, reinforcement, and site restrictions. This guide explains eccentric footing types, calculation logic, design steps, construction checks, and common mistakes.
Quick Summary
Eccentric footing design is used when a column is not placed at the centre of the footing, often due to a property line or space restriction. The off-centre load creates uneven soil pressure, so the footing must be checked carefully. Common solutions include isolated eccentric footing, strap footing, combined footing, trapezoidal combined footing, and raft footing, depending on load, soil condition, and site limitations.
What Is Eccentric Footing Design?
Eccentric footing design refers to the structural design of a footing where the column or wall load does not pass through the geometric centre of the footing. In a normal concentric footing, the load is central and soil pressure below the footing is usually more uniform. In an eccentric footing, the load is shifted from the centre, increasing soil pressure on one side and reducing it on the other.
This condition is common in urban plots where boundary columns are close to the property line. Since the footing cannot extend beyond the site boundary, the column may be placed near one edge of the footing. This creates eccentricity and requires proper design to prevent excessive soil pressure, rotation, settlement, or cracking.
Footing design should always follow approved structural drawings, soil investigation, relevant standards, and qualified engineer guidance.
Why Is Eccentric Footing Needed?
Eccentric footing is used when a footing cannot be placed symmetrically around a column. The most common reason is limited space near a boundary.
It may be needed in situations such as:
- Column located near a plot boundary
- Building wall close to neighbouring property
- Existing structure restricting footing extension
- Basement wall or retaining wall support
- Unequal column loading
- Corner columns with limited footing area
- Urban construction with tight setback conditions
- Irregular column layout
Instead of extending the footing outside the property line, the designer modifies the foundation system to keep it within the site while maintaining stability.
Concentric Footing vs Eccentric Footing
| Feature | Concentric Footing | Eccentric Footing |
| Load position | Column load passes through centre | Column load is away from centre |
| Soil pressure | More uniform | Uneven or trapezoidal |
| Common use | Interior columns | Boundary or edge columns |
| Design complexity | Lower | Higher |
| Main risk | General settlement or shear | Uneven pressure, rotation, settlement |
| Typical solution | Isolated footing | Eccentric, strap, combined, or raft footing |
A concentric footing is preferred where space allows. An eccentric footing is used when site conditions restrict equal projection around the column.
How Eccentricity Affects Footing Behaviour
Eccentricity creates a moment at the base of the column. This moment changes the soil pressure distribution below the footing.
In a concentric footing, soil pressure is more evenly distributed. In an eccentric footing, pressure becomes higher on the side closer to the load and lower on the far side. If eccentricity becomes too large, part of the footing may theoretically lose contact with the soil. This is unsafe because soil cannot resist tension.
For rectangular footing under one-way eccentric loading, the common soil pressure relation is:
qmax = P/A × (1 + 6e/L)
qmin = P/A × (1 – 6e/L)
Where:
- P = vertical service load
- A = footing area
- e = eccentricity
- L = footing length in the direction of eccentricity
- qmax = maximum soil pressure
- qmin = minimum soil pressure
For full soil contact, eccentricity is usually kept within the middle third of the footing, which means e should not exceed L/6 in that direction. If e > L/6, the footing usually needs redesign or a different foundation solution.
Service Load vs Factored Load in Footing Design
Soil pressure and safe bearing capacity checks are generally performed using service loads. These checks help verify whether the pressure transferred to soil remains within allowable limits and whether settlement risk is acceptable.
RCC design checks such as bending moment, one-way shear, punching shear, and reinforcement design may require factored loads as per applicable design standards. This distinction is important because soil bearing checks and reinforced concrete strength checks are not always performed using the same load level.
A structural engineer must decide which load combinations and factors apply based on the design code, soil report, and project requirements.
One-Way and Two-Way Eccentricity
In one-way eccentric footing, the load is eccentric in one direction only. The pressure distribution can be checked using the common qmax and qmin formula for that direction.
In two-way eccentric footing, the load is eccentric in both directions. This is more complex because soil pressure varies across both length and width. Do not apply the one-way formula blindly in both directions without proper design verification. Two-way eccentricity should be checked by a structural engineer using appropriate structural and geotechnical analysis.
Types of Eccentric Footing
Isolated Eccentric Footing
An isolated eccentric footing supports a single column whose load is not at the centre of the footing. It may be used when eccentricity is small and the soil pressure remains within safe limits.
Best Used For
- Lightly loaded boundary columns
- Small residential structures
- Good soil bearing capacity
- Limited eccentricity
Limitation
If eccentricity is high, an isolated footing may create excessive soil pressure or uplift tendency on one side. In such cases, strap or combined footing may be more suitable.
Strap Footing
A strap footing connects an eccentric exterior footing to an interior footing through a rigid strap beam. The strap beam helps balance the overturning effect caused by eccentric loading.
Best Used For
- Boundary columns near property lines
- Exterior column connected to an inner column
- Sites where footing cannot extend beyond boundary
- Medium-load structures
Limitation
The strap beam must be properly designed. It usually transfers moment between footings and should not be assumed to rest on soil unless specifically designed that way.
Combined Footing
A combined footing supports two or more columns on one common base. It is often used when columns are close together or when a boundary column footing becomes eccentric due to space restrictions.
Best Used For
- Two nearby columns
- One boundary column and one interior column
- Unequal column loads
- Limited footing space
Limitation
The footing shape and size must be designed so the resultant load acts properly within the footing area.
Trapezoidal Combined Footing
A trapezoidal combined footing is used when two column loads are unequal. The footing shape is adjusted to align the resultant load closer to the footing centroid.
Best Used For
- Unequal column loads
- Heavy boundary columns
- Space-restricted plots
- Efficient area distribution
Limitation
It requires careful design and accurate setting out on site.
Raft or Mat Foundation
A raft foundation may be used when several columns are close, soil bearing capacity is low, or isolated footings overlap. It distributes loads over a larger area.
Best Used For
- Low bearing capacity soil
- Closely spaced columns
- Basement structures
- Multi-storey buildings
- High settlement-risk areas
Limitation
Raft foundations are more complex and should be designed only by qualified structural engineers.
Eccentric Footing Design Considerations
Eccentric footing should not be designed using guesswork or thumb rules. The following factors must be checked:
- Column load and moment
- Service load and factored load combinations
- Safe bearing capacity of soil
- Settlement limits
- Footing size and shape
- Eccentricity in one or both directions
- Maximum and minimum soil pressure
- One-way shear
- Two-way or punching shear
- Bending moment
- Footing depth
- Reinforcement requirement
- Rotation risk
- Boundary restrictions
- Adjacent foundations
- Groundwater and drainage
- Construction feasibility
If maximum soil pressure exceeds allowable bearing capacity, the footing size, shape, or type must be revised.
Step-by-Step Eccentric Footing Design Process
Step 1: Collect Site and Structural Data
Start with column load, column size, soil bearing capacity, plot boundary, groundwater condition, soil report, and architectural layout. Do not design the footing without correct load and soil data.
Step 2: Find Required Footing Area
The basic required area is estimated as:
Area = Service load / Safe bearing capacity
This gives an initial footing size. For eccentric footing, the area may need to be increased because soil pressure is not uniform.
Step 3: Decide Footing Type
Check whether an isolated eccentric footing is possible. If pressure distribution is unsafe or eccentricity is high, consider strap footing, combined footing, trapezoidal footing, raft foundation, pile foundation, or another engineered solution.
Step 4: Calculate Eccentricity
Eccentricity is the distance between the column load line and the centroid of the footing.
e = Distance between load centre and footing centre
For two-way eccentricity, eccentricity must be checked in both directions using proper analysis.
Step 5: Check Soil Pressure
For one-way eccentricity:
qmax = P/A × (1 + 6e/L)
qmin = P/A × (1 – 6e/L)
The design should ensure:
- qmax does not exceed safe bearing capacity
- qmin does not become negative
- Footing remains stable against overturning and settlement
Step 6: Check Shear and Bending
The footing must be checked for bending moment, one-way shear, and punching shear around the column. These checks influence footing depth and reinforcement.
Step 7: Design Reinforcement
Reinforcement is provided based on bending moments in both directions. Extra reinforcement may be required near the column face and high-pressure zone. Development length, cover, bar spacing, anchorage, and concrete cover must be checked.
Step 8: Prepare Structural Drawings
The final drawing should show footing size, depth, reinforcement details, column position, clear cover, concrete grade, steel grade, strap beam details if any, and construction notes.
Basic Eccentric Footing Calculation Example
Assume:
- Column load, P = 600 kN
- Safe bearing capacity, SBC = 150 kN/m²
- Adopted footing size = 2.5 m × 2.0 m
- Footing area, A = 5.0 m²
- Eccentricity along length, e = 0.20 m
- Length in eccentric direction, L = 2.5 m
Average soil pressure:
P/A = 600 / 5 = 120 kN/m²
Maximum soil pressure:
qmax = 120 × (1 + 6 × 0.20 / 2.5)
qmax = 120 × (1 + 0.48)
qmax = 177.6 kN/m²
Minimum soil pressure:
qmin = 120 × (1 – 0.48)
qmin = 62.4 kN/m²
Result:
- qmax = 177.6 kN/m², which exceeds the assumed SBC of 150 kN/m²
- qmin = 62.4 kN/m², so soil contact remains positive
- Since qmax is too high, the footing size or foundation type must be revised
This example explains the soil pressure check only. Final eccentric footing calculation must also include bending, one-way shear, punching shear, reinforcement, settlement, load combinations, and code-based checks.
Eccentric Footing Reinforcement Basics
Reinforcement in eccentric footing may not always be symmetrical because soil pressure is higher on one side. The design must reflect bending and shear requirements.
Key reinforcement checks include:
- Main steel direction
- Distribution steel direction
- Extra bars near high bending zones
- Steel near column face
- Development length
- Clear cover
- Bar spacing
- Punching shear requirement
- Strap beam reinforcement, if used
The reinforcement layout should be prepared by a structural engineer and followed exactly during construction.
Eccentric Footing vs Combined Footing vs Strap Footing
| Feature | Eccentric Footing | Combined Footing | Strap Footing |
| Supports | Usually one off-centre column | Two or more columns | Two footings connected by strap beam |
| Common use | Small eccentricity | Nearby columns or boundary columns | Boundary column with interior column |
| Load balancing | Through footing size and pressure check | Through common base | Through strap beam action |
| Soil pressure | Uneven | Designed to be more balanced | Balanced through strap connection |
| Complexity | Moderate | Higher | Higher |
| Best for | Limited eccentricity | Closely spaced or unequal columns | Property line columns |
The right foundation option depends on load, column spacing, soil capacity, site restrictions, and structural design.
Diagram Suggestions for Publishing
Because eccentric footing design is easier to understand visually, add diagrams in the final blog layout.
| Diagram | Suggested Alt Text |
| Concentric vs eccentric footing | “Eccentric footing design comparison with concentric footing” |
| Soil pressure distribution | “Eccentric footing design showing qmax and qmin soil pressure distribution” |
| Middle-third rule | “Middle third rule for eccentric footing calculation” |
| Strap footing | “Strap footing for boundary column eccentric foundation design” |
| Combined footing | “Combined footing for property line column load balancing” |
| Reinforcement layout | “Foundation reinforcement layout for eccentric footing design” |
These visuals can improve user understanding and reduce confusion around load eccentricity and soil pressure.
Construction Checks for Eccentric Footing
Before concreting, check:
- Footing size and layout
- Column position from boundary
- Excavation depth
- Soil level and bearing surface
- PCC bed thickness
- Reinforcement spacing and cover
- Column starter bars
- Strap beam reinforcement, if any
- Formwork alignment
- Concrete grade
- Water removal from excavation
- Anti-termite treatment, if specified
- Engineer approval before concrete pour
Even a well-designed footing can fail if it is constructed in the wrong position or with incorrect reinforcement.
Common Mistakes to Avoid
Avoid these mistakes in eccentric footing design and construction:
- Treating eccentric footing like normal isolated footing
- Ignoring property line restrictions
- Not checking maximum soil pressure
- Allowing qmin to become negative
- Using soil bearing capacity without soil testing
- Confusing service load and factored load checks
- Applying one-way eccentricity formulas blindly to two-way cases
- Reducing footing size to save cost
- Ignoring strap beam design
- Placing column incorrectly on site
- Using wrong reinforcement spacing
- Not checking punching shear
- Constructing without structural drawings
- Starting work without approval
Eccentric footing is sensitive to load position, so layout accuracy is critical.
When Is Eccentric Footing Not Suitable?
Eccentric footing may not be suitable when:
- Eccentricity is very high
- Soil bearing capacity is low
- qmax exceeds allowable pressure
- Footing rotation risk is high
- Adjacent foundations interfere
- Column load is heavy
- Settlement control is critical
- Multiple nearby columns are present
- Basement or retaining loads are involved
In these cases, a combined footing, strap footing, raft foundation, pile foundation, or another engineered system may be needed.
Conclusion
Eccentric footing design is required when a column load does not pass through the centre of the footing, commonly near property boundaries or space-restricted plots. Since eccentric loading creates uneven soil pressure, the footing must be checked for maximum pressure, minimum pressure, bending, shear, settlement, load combinations, and reinforcement. Small eccentricity may be handled with an isolated eccentric footing, while higher eccentricity may require strap, combined, raft, or pile foundation. Always follow structural drawings and engineer guidance before construction.
FAQs
- What is eccentric footing design?
Eccentric footing design is the process of designing a footing where the column load does not pass through the centre of the footing. It requires checks for soil pressure, eccentricity, bending, shear, settlement, load combinations, and reinforcement. - Where is eccentric footing used?
Eccentric footing is commonly used near property boundaries, corner columns, existing buildings, basement walls, and sites where the footing cannot extend equally around the column. It is common in urban plots with limited space. - What are the main types of eccentric footing?
The main types include isolated eccentric footing, strap footing, combined footing, trapezoidal combined footing, and raft foundation. The correct type depends on column load, soil bearing capacity, eccentricity, column spacing, and boundary restrictions. - How is soil pressure calculated in eccentric footing?
For one-way eccentricity, soil pressure is commonly estimated using qmax = P/A × (1 + 6e/L) and qmin = P/A × (1 – 6e/L). Maximum pressure should not exceed safe bearing capacity, and minimum pressure should not become negative. - What happens if eccentricity is more than L/6?
If eccentricity is more than L/6, the theoretical minimum soil pressure may become negative, meaning part of the footing may lose contact with soil. Since soil cannot take tension, the footing must be redesigned or replaced with a different foundation type. - What is the difference between strap footing and combined footing?
A strap footing connects two separate footings with a strap beam to balance a boundary column load. A combined footing uses one common footing slab to support two or more columns and distribute their loads together. - Is eccentric footing safe for multi-storey buildings?
Eccentric footing can be safe for multi-storey buildings only if designed by a qualified structural engineer using soil reports, accurate loads, settlement checks, and applicable standards. Heavy loads may require combined, raft, or pile foundations instead. - Who should design an eccentric footing?
A qualified structural engineer should design an eccentric footing. The design depends on column loads, soil report, safe bearing capacity, footing size, eccentricity, bending moment, shear, reinforcement, settlement, and local building requirements.
Sowjanya Siddanoor