Types of Shallow Foundation I When to Use I Where to Use?

Introduction

Foundation is the part of the structure which transfer the load of the superstructure to the ground. Foundation is the interlink between structure and ground. Shallow Foundation is constructed below the ground level in order to increase the lateral stability of the structure.

All the weight of structure rest on the soil and its properties. One thing is important in foundations i.e. SBC. Soil Bearing Capacity (SBC) is defined as the amount of weight per unit the soil can bear. Soil Bearing Capacity depends upon the depth of soil. Higher the depth, greater will be the SBC.

Why we need Foundation?

As the train needs rail, vehicles need a road, sheep needs water to walk similarly building must need a foundation.

Depending upon the superstructures, different foundations are used. Foundation ensures that the load of the is distributed over the ground under the building.

  • It provides stability to the superstructure.
  • Prevents the structure from the settlement.
  • It gives comfort to the people.
  • It ensures the load distribution of the superstructure to the ground.

Main Function of Shallow foundation

  • Distribution of load to the foundation.
  • Increment of lateral stability.
  • Retain the earth material.
  • Stability of structure.
  • Reduces the differential settlement.
  • Prevent sliding and overturning.

Shallow Foundations

A shallow foundation is defined as the depth of the footing is less than the width of the footing is called Shallow foundations. The type of foundation is decided to be use is dependent upon the soil bearing capacity.

Types of Shallow Foundation

  • Isolated footing
  • Stepped footing
  • Sloped  footing
  • Eccentric or Shoe footing
  • Combined footing
  • Raft or Mat foundation
  • Strap footing
  • Strip foundation

How to select which foundation is suitable?

There are three parameters that decide which foundation is suitable for.

  • Load coming from the superstructure.
  • Safe Bearing capacity of the soil (SBC).
  • Location/Property line/Neighboring structure.

Design criteria of shallow foundation

We design shallow foundation for two criteria;

  • Shear failure criteria: Foundation must be safe against shear failure.
  • Settlement criteria: The settlement of the foundation must be within the permissible limit.

When we provide Shallow foundation?

If the load coming from superstructure is low or high but if the soil has sufficient bearing capacity at low depth then the shallow foundation is preferable.

 Now I am going to explain different types of shallow foundations and also I will be discussing their advantage, disadvantage, when to use, and where to use.

Isolated footing

A footing that is provided under an individual column is known as isolated footing.

Shape

  • Rectangular, square.

When to provide and where to provide Isolated?

  • It is used when a safe bearing capacity is very high.
  • Loading coming from superstructure is low.
  • It is located generally within the property line.

Advantage of Isolated footing

  • Most economical because of the placement of reinforcement.
  • Easy to construct.
  • Not highly skilled labor is required.

Stepped footing

A footing where footing is stacked upon each other in the forms of steps. This type of footing is not constructed nowadays.

Shape

  • Rectangular, square.

When to provide step footing?

  • This type of footing is used when safe bearing capacity is high but load coming from a column in footing is more. So there is a chance of punching failure so to prevent the isolated footing from punching failure we provide steps.

Slope footing

Slope Footing is one of the types of shallow foundation which is the combination of rectangular and trapezoidal shape. The slope of trapezoidal is 45 degrees from all sides.  Due to a 45 degree slope reduces the amount of concrete and reinforcement.

Shape

  • Trapezoidal.

When to use Slope footing?

  • If the safe bearing capacity of the soil is more but load coming is more. If the load is more then there is a chance of happening punching failure, so in that situation, slope footing plays an important role.
  • This footing shape is more efficient because the column load also deviates at an angle 45 degrees when it reaches to the footing, so there is proper load transfer.

Eccentric or shoe footing

A footing that is standing in only one side of the centerline of the column is called Eccentric footing. It constructed at the property line and half cut out of the original footing.

Shape

  • It is half cut out Rectangle, Trapezoidal, and stepped footing.

When to provide Eccentric footing?

  • If the position of the column is located on the property line and there is no provision of extra area for foundation, so in that situation, we provide the footing whose projection is within the existing property line.

Combined Footing

A footing which is provided under the two columns is called combined footing.

Shape

  • Rectangular and Trapezoidal footing.

When to provide combined footing?

  • IF the position of the column is near each other, then there is a chance of footing overlap.
  • IF the anyone of the column is located in the property line then footing in the direction cannot be extended so to make the footing stable against overturning, we combine the footing with near footing.
  • The Safe bearing capacity of the soil is very less then the area required for individual footing is more, so in that situation, there is a chance of footing overlap.

Types of Combined footing

 Rectangular combined footing

 When to provide rectangular combined footing?

  • A rectangular combined footing is provided if the width of one footing is restricted and safe bearing capacity less then it required expanding the area of one side of the footing.

Trapezoidal Combined footing

The trapezoidal footing is provided when the load of one column is higher than other column loads. IF the projection of both footings beyond the face of the column is restricted.

Raft or mat foundation

A footing that is provided under more than one column is called a raft foundation. It is a thick concrete slab that is resting on the soil. It supports the columns, shear walls, and transfer a load of superstructure to the soil. In mat footing, the individual’s footings cannot overlap with each other because the footing area is more. When more than one column is provided in a row as, like combined footing, such types of footing is called raft footing.

Shape

  • Rectangular, square, irregular shape.

 When to provide Raft or mat foundation?

  • If the Safe bearing capacity of the soil is less than 100 KN/m2 (Assumption only).
  • The load coming from the superstructure is very high.
  • IF the area of isolated footing is 60 percent more than the area covered by the building then we go for a raft foundation.
  • If we provide a shear wall either for a lift or for water, a raft foundation must be required.

Strap Footing

If the two footings are combined with the Reinforced concrete beam is known as strap footing and the beam is called a strap beam. Strap beam connects both footings in such a way that it acts as a single unit.

Shape

  • Rectangular, Trapezoidal.

Function of Strap

  • The function of the strap is to transmit the moment caused by an eccentricity to the interior column footing that helps to resist the footing from overturning.

When to provide strap footing?

  • If the position of the column is located on the property line, then column footing is projected on only one side of the centerline of the column. As a result, load coming from superstructure is eccentrically loaded with the axis that causes the overturning to the eccentric footing, so to restrict the overturning; we connect the beam with another footing.
  • We can use Strap footing instead of the combined footing if the distance between the columns is large and not required more area of the footing.

Strip Foundation (or wall footing)

A footing that supports the Load-bearing wall or shear wall is known as strip footing. Strip footing is also known as wall footing. The length of the strip footing is greater than the width of the footing.

Width of footing is at least two times the width of the load-bearing wall. The width of strip footing depends upon the bearing capacity of the soil.

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