Understanding Concrete Foundation Design: Bearing

Designing a foundation is difficult.

But once you understand the logic behind how loads travel and how the soil reacts, the entire process becomes intuitive.

What Does a Foundation Actually Do?

Any foundation acts as a medium between your superstructure (columns, walls, beams) and the soil below it.

1. The structure applies vertical loads, horizontal forces, and moments.
2. The soil resists these loads by providing equal and opposite reactions.
3. These soil reactions govern stability, bearing, sliding, overturning, and structural design of the foundation.

How Loads Affect a Footing?

When loads act on a footing, it naturally wants to:

1. move downwards (due to gravity loads),
2. slide (due to lateral loads), or
3. overturn (due to moments).

But soil has resistance.

This resistance creates reaction pressures under the footing.

These pressures are what we check during design.

In simple words:

The structure wants to move. The soil stops it. The result is reactions.

Simple Isolated Footing 

Bearing Capacity

Let us consider only vertical force is acting on the footing. 

Under this load, the footing tries to crush the soil below it. 

But the soil has some strength of its own, known as "Bearing strength of soil" or simply "Bearing capacity of soil."

If the applied load exceeds the soil’s strength, the soil loses its stability and soil failure occurs.

Common types of soil failures:

1. General Shear Failure

2. Local Shear Failure

3. Punching Shear Failure

 (We will discuss these in another blog.)

If the soil has enough capacity, it will resist the applied load, and an equal and opposite reaction will act on the base of the footing.

This reaction is obtained by distributing the structural loads onto the bottom surface of the foundation.

Why Pressure Becomes Unequal?

The lateral forces and moments try to lift one side of the foundation and increase pressure on the other. 

This causes the reaction distribution to become non-uniform. This unequal distribution of pressure on the footing results in each corner having different bearing pressures. 

If we assume a linear distribution of the pressure, we can calculate bearing pressures for each corner of the footing considering the applied maximum moments and then use simple linear relation between each corner to get a bearing pressure plane for the footing.

If the maximum bearing pressure on the footing is more than the bearing capacity of the soil, we say the foundation is safe in bearing.

Vertical + lateral loads generally produce a trapezoidal pressure distribution

Calculation for Bearing Check:

1. Soil Bearing Capacity

Before designing any footing, we must know how much pressure the soil can safely take.

The bearing capacity of the soil is calculated by geotechnical analysis usually involving various tests on the soil like SPT test (Standard Penetration Test), Plate Load Test, ect.

SBC = Soil Bearing Capacity and should be > Bearing Pressure on the footing

Once we have the bearing capacity, we calculate the actual bearing load on the footing and compare the two.

2. Bearing Capacity under Vertical Loads

For vertical loads, the bearing pressure can be calculated using simple stress = Force/Area formula.

Bearing Pressure = F/A

This assumes uniform pressure under the entire footing.

3. Bearing Capacity with Moments

If any moment or shear force is coming than we can use the bending moment formula to calculate bending stresses

Bending Pressure = M/I

If the footing experiences bending moments, pressure becomes non-uniform.

where M = moment on the section, y is distance of the furthest fiber under bending stress, and I is moment of inertia for that section

Pressure Distribution on the footing in a general loading case

A General formula for combined axial load in Z Direction, and biaxial bending in both X and Y directions can be considered as well,

Bending Pressure = P/A ± Mx*yx/Ix ± My*yy/Iy

This gives you the pressure at each corner of the footing.

If the moment is large enough, one edge of the footing may experience zero pressure.

This is where Teng’s graphical method (from Foundation Design) is used to modify the pressure distribution and calculate Bending pressure.

Final Thoughts:

Foundation design is not just formulas—it’s understanding how forces want to move the structure and how soil resists that movement.

This is the first blog in the series.

Next, we will discuss:

1. Loss of Contact in Concrete Foundations

2. Teng's Method for Modified Bearing Pressure

Stay Tuned.