Eccentric footing

Eccentric Footing

Eccentric footing is a type of foundation used when the load from a column does not align with the center of the footing.

Eccentric Load

Refers to footings that are not concentrically loaded, meaning the load is applied off-center.

Common Causes of Eccentricity

Factors that lead to the load being off-center, including property lines, lateral forces, applied moments, and combined footings.

Property Lines

The footing must extend into the property to properly support the column, causing the load to be off-center.

Lateral Forces

Wind, seismic, or earth pressure can push horizontally on a structure, shifting the resultant load away from the footing's center.

Applied Moments

Bending moments from structural elements cause rotational effects that shift the load away from the footing's center.

Combined Footings

Shared footings between multiple columns often require eccentric placement to balance unequal loads or spacing.

Non-Uniform Soil Pressure

The primary effect of eccentricity is a non-uniform (trapezoidal) distribution of pressure under the footing, unlike the uniform distribution in a concentric footing.

Unequal Settlement

This uneven pressure can lead to unequal settlement of the footing, where one side sinks more than the other.

Increased Design Complexity

Eccentric loads require a more complex design approach to ensure stability and prevent failure.

Eccentric Isolated Footing

A single footing supports one column, but the column is placed away from the geometric center of the footing.

Strap Footing

Consists of two isolated footings (usually one eccentric and one central) connected by a strap beam.

Combined Footing with Eccentricity

Supports two or more columns, but one of them may be located eccentrically.

Wall Footing with Eccentric Load

Occurs when a load-bearing wall is not centrally placed on its footing.

Site Preparation

The initial phase of construction that includes survey & layout, clearing, and excavation.

Survey & Layout

Mark the column locations, property lines, and eccentric footing boundaries precisely using total station or theodolite.

Excavation

Dig foundation pit to required depth considering soil strata and safe bearing capacity. Side slopes/shoring if soil is loose.

Leveling

Dress the bottom of excavation flat and level.

Compaction

Compact the soil at the base using rammers/plate compactors.

Blinding Layer (PCC)

Lay a 75-100 mm thick plain cement concrete (lean mix, e.g., M10 or 1:4:8) to provide a clean, firm base.

Formwork

Fix shuttering according to footing shape (eccentric, trapezoidal, or rectangular) and ensure stability.

Reinforcement Placement

Place bottom reinforcement mesh (main + distribution steel) as per design and maintain proper cover blocks (usually 50-75 mm for footing).

Curing & Stripping

Protect footing from direct sun and rapid drying during the initial setting.

Formwork removal

Usually after 24-48 hours (depending on mix and temperature).

Curing

Keep footing continuously wet (ponding or wet gunny bags) for at least 7 days (longer if hot/dry weather).

Backfilling

Refill excavated soil in layers (150-300 mm thick), compact each layer properly.

Inspection first

Engineer checks footing size, reinforcement, alignment, and concrete quality before covering.

Drainage provision

If near property line, ensure water is diverted away to prevent softening of soil around footing.

Column construction

After footing gains strength (usually 7-14 days), start column concreting.

Check verticality

Since footing is eccentric, column alignment must be exact.

Space Utilization

Useful when the column is located near a property line or boundary wall.

Flexibility in Layout

Allows architects/engineers to place columns at edges without affecting load transfer.

Structural Feasibility

Provides a practical solution for supporting columns that can't be positioned at the center of the footing area.

Economical

Compared to a combined footing, a strap or eccentric footing can sometimes be more cost-effective.

Load Distribution

Strap beam helps balance the overturning effect due to eccentricity, keeping soil pressure uniform.

Uneven Pressure Distribution

If not designed properly, it can lead to non-uniform soil pressure beneath the footing, causing differential settlement.

Overturning Moment

Eccentricity generates bending moments and shear, which increases the complexity of analysis and design.

Structural Complexity

Requires a strap beam or tie beam in many cases to prevent rotation, making it more complicated than isolated footings.

Material & Cost

In some situations, strap beams or reinforcement requirements may increase material cost.

Not Suitable for Weak Soils

Weak soils can lead to excessive settlement or tilting.

Buildings Near Property Lines

Example: In a small city lot, a house is built very close to the boundary line. Application: The footing is made larger on the inside part of the property while keeping the column near the edge.

Boundary Walls or Fences

Example: A subdivision boundary wall is built exactly on the lot line. Application: The footing is extended inward, and the wall sits on one edge.

Industrial Buildings or Warehouses

Example: In a warehouse, large openings for trucks are placed along one side of the building. Application: Eccentric or strap footings are used to support these edge columns.

Use of Strap Footings

Example: Two columns are close together, but one is right next to the property line. Application: Engineers connect the eccentric footing of the edge column to the inner column footing using a strap beam.