Types of foundation failure depend on the load it is subjected to. A foundation can fail in three different ways under loads. They are as follows:
- Punching shear failure of the foundation
- One-way shear failure of the foundation
- Flexure failure of the foundation.
The above three modes of failure should be checked during the design stage of a concrete foundation for the given load. Guidelines provided by standard codes of practice should be followed so that the foundation does not fail in any of the failure types as mentioned under any possible load combinations when the structure is occupied.
Types of foundation failure are discussed in detail below:
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Punching shear failure of foundation:
Punching shear failure is also known as diagonal tension failure of the foundation. In this mode of failure, the foundation fails due to the formation of inclined cracks around the perimeter of the column.
The critical section for punching shear failure is taken at d/2 from the face of the column, where d is the effective depth of footing.
To avoid punching shear failure, the ultimate upward shear force at this section in the foundation should be less than the shear resistance of concrete for the given percentage of concrete. Additional reinforcement should be provided to resist punching shear in case of shear resistance of concrete with reinforcement provided is not sufficient. The failure of foundation in this mode appears as truncated cone or pyramid around the column, stanchion or pier as shown in the figure below:
One Way shear failure of foundation:
Foundations in one-way shear failure fail in inclined cracks across a full width of the footing that intercept the bottom of the footing slab at a distance d from the face of the column (called critical section), where d is the effective depth of footing slab.
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In the case where steel base plate is used under column directly on the footing slab, the distance d is measured from a line halfway between the face of the column and the edge of the base plate.
To avoid one-way shear failure of foundations, the shear stress at the critical section of footing should be less than the shear strength of concrete with a given percentage of reinforcement used.
One way shear failure of the footing is shown in the figure below:
Flexure failure of foundations:
During the design of footing, Mu/bd2 is calculated to get the percentage of reinforcement for the moment the foundation is exposed to. Mu is the ultimate or factored moment; b is the width of footing. The critical section for flexure is considered at distance d from the face of footing. The standard codes take care of flexure failure during design by providing the percentage of reinforcement required to resist the bending moment. But in case, when the bending moment increases in footings, then footing fails as shown in the figure below:
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