What is Loss of Contact in Concrete Foundation Design

Now that we have discussed what bearing pressure in foundations is, we can move to another side of the same coin— Loss of Contact in Foundations.

When a building transfers load to the soil, we typically assume that the entire base of the foundation remains in full contact with the ground.

But in real-world conditions, especially under eccentric vertical loads, lateral forces/moments, tension, or seismic actions, this may not happen.

Loss of contact can cause incorrect bearing pressure calculations, unsafe designs, excessive settlements due to redistribution of pressure, etc.

 (Refer Bearing in Concrete Foundation Design)

What is Loss of Contact?

When a foundation is subjected to vertical loads plus lateral forces or overturning moments, a bending pressure develops at the soil–footing interface. 

One side of the footing experiences higher compression whereas the opposite side experiences reduced compression or even tension. 

A compressive and tensile effect is due to bending pressure caused by overturning moments whereas evenly distributed tensile/compressive pressure also acts on the footing due to applied vertical loads.

If the tensile component exceeds the compressive component, uplift occurs.

Because soil cannot take tension, the portion of the footing experiencing uplift detaches from the soil.

All this can cause the resultant of the load to shift away from the centroid of the footing.

In simple explanation, 

Part of the footing stops touching the ground due to uneven loading.

When moment acts, one edge compresses and the other lifts. If vertical compressive load is not sufficiently large to keep the lifted region in compression, Loss of Contact occurs.

Due to this, the remaining soil in contact now carries all the load, causing redistribution of bearing pressure on the foundation. This phenomenon is explored in detail in Teng’s “Foundation Design”.

Why does Loss of Contact Occurs?

1. Eccentric Loading

Sometimes the vertical load acts away from the centerline of the column and this causes a non-uniform soil pressure distribution (a moment is introduced due to eccentricity). 

This non-uniform distribution can cause Loss of Contact.

2. Overturning Moment

Sometimes lateral loads like wind, seismic, or machine loads can cause the foundation to overturn and push one side down while lifting the other resulting in Loss of Contact.

3. Lateral Loads on Block Foundations

These are part of the Dynamic Forces on Equipment Foundations. Basically, due to presence of Unbalanced Forces on rotating equipment (due to uneven balancing of mass) some uplift can occur leading to Loss of Contact.

4. Rigid Footing Assumption

With linear pressure distribution, any negative pressure value at the edge is interpreted as tension which results in Loss of Contact. (Refer Bearing in Concrete Foundation Design)

Simple Isolated Footing 

How Loss of Contact Affects Foundations?

Modification in Bearing Pressure

Due to loss of contact, the bearing pressure gets modified due to change in pressure distribution as well as reduction in available bearing area.

Nonlinear Soil Response

The Loss in Contact can also change the assumed Rigid Body Assumption to fail by creating a local zone of high bending/curving of the footing.

Reduced Sliding Resistance

Sliding is directly linked to the compressive normal force on the footing. If the compressive force reduces, sliding resistance also reduces. (More on this in the upcoming blog on Sliding Failure in Foundations).

More Settlement for Compressive Zone

Stress concentration on one side due to Loss of Contact may cause local settlement in that region, tilting the structure further.

How Engineers Check Loss of Contact in Practice?

Bearing Pressure Equations

The linear distribution of bearing pressure can be used to check Loss of Contact too. If any area has negative or zero bearing pressure, we can say that the area is under Loss of Contact. 

(Refer Bearing in Concrete Foundation Design)

Vertical + lateral loads producing a triangular pressure distribution with loss in contact

Middle Kern Rule

Any footing can be exposed to vertical loadings along with lateral loadings. The moments can be expressed in the form of eccentricity to the centerline of the footing (refer figure above), and this leads us to the middle kern rule. 

If the eccentricity caused by the moments is within the kern (middle third), no Loss of Contact occurs and if it is outside, Loss of Contact is present. 

If eccentricity exceeds half the footing dimension, the resultant lies outside the footing and foundation is theoretically unstable.

It is a graphical approach to the Bearing pressure equation method.

Middle Kern Shown on a Cross-section of Footing

Soil Spring Models

In FEM Analysis, each node can be modelled as a spring of stiffness similar to the stiffness of soil in that region. This spring will compress (use springs as compression-only) on application of load.

Springs with tensile reactions have uplift whereas springs with zero or negative reaction make the Loss of Contact Zone.

Is Loss of Contact Always Bad?

No.

Controlled uplift is acceptable if it is small and within specified percentages of the total area (usually less than 20% of area) 

Only if the uplift area is too large, that makes the foundation unstable or overstressed can cause trouble.

Top View of Isolated Footing showing Loss in Contact

How to Reduce or Avoid Loss of Contact?

Increase Footing Size

This reduces eccentricity of the Load and Distributes the pressure on the larger area resulting in lesser Loss of Contact

Increase footing dimensions

Add Tie Beams or Grade Beams

Beams help in distribution of loads between various foundations and thus reduces chances of isolated uplift.

Tie beams are added to result in load distribution

Mat or Deep Foundation instead of Isolated Footings

Mats and piles can handle moments and uplift better than isolated footings.

Increase Dead Load

Increases overall compression on the soil and reduces uplift

Calculation for Loss of Contact Check:

1. Kern Check

Bearing Pressure for the Combined axial load in Z Direction, and biaxial bending in both X and Y directions can be shown as,

Bearing Pressure = 

If we considered the same footing now with only the axial load acting at some eccentricity in X and Y directions, the Bending Pressure can be written as,

Bearing Pressure = 

Comparing both equations we get, 

Mx = 

My = 

Resultant eccentricity is shown due to acting moments

For no loss of contact, Bearing Pressure > 0.

on simplifying we get

This can be plotted on the cross section of the column to get the region where if resultant eccentricity of loads occurs, no loss of contact will occur.

Also, if the eccentricity more than the half of column edge sizes, the point of loading lies outside the column, and the foundation is considered to have theoretically flipped and unstable.

The resultant vertical load acts outside the footing

2. Bearing Pressure Equations

The General formula for combined axial load in Z Direction, and biaxial bending in both X and Y directions, given that footing is symmetrical, can be considered as well,

Bearing Pressure = 

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

(Refer Bearing in Concrete Foundation Design)

If the Bearing Pressure is Zero or Negative, the corner is said to be in Loss of Contact. Assume a linear Distribution of Pressure, we can find the exact point beyond which the footing is not in contact of soil. 

Joining these points of Zero Bearing Pressure, we can find the exact region and use plane geometry to calculate percent of Loss in Contact

Final Thoughts:

Loss of contact is a common but critical behavior in foundation engineering. Understanding it ensures accurate bearing pressure design, stability checks, and safe structural performance.

This is the second blog in the series.

Next, we will discuss:

1. Teng's Method for Modified Bearing Pressure

2. Sliding in Concrete Foundations.

Stay Tuned.