Combined Footing: Types I Shape I When to Provide I Design Steps as per IS 456-2000

Introduction

A thick reinforced concrete slab that is provided under the two columns is called combined footing. It is provided when the distance between the two columns is small and soil has low bearing capacity.

Shape

  • Rectangular and Trapezoidal footing.

When to provide combined footing?

  • IF the position of the column is nearer with each other, then there is a chance of overlap of footing.
  • IF the anyone of the columns 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 the nearest footing.
  • When 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.
  • IF the Dimension of one side of the footing is restricted and to accommodate a permissible bearing capacity of the soil, the other side of footing is required to increase, so in that case, is to be combined.

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Types of Combined footing

 Rectangular combined footing

A reinforced concrete slab that is provided beneath the two columns is called rectangular combined footing.

Combined Footing

 When to provide a 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.

Trapezoidal Footing

 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.

Strap Footing

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.

Strap Footing

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.

Design Steps of Combined Footing

Following are the steps of combined Footing as per IS 456-2000.

  1. Calculate the Area of Footing.
  2. Calculation of net bearing soil pressure.
  3. Evaluate the Net factored soil pressure.
  4. Find out the effective depth of footing.
  5. Find out the critical section for bending moment and shear forces.
  6. Check for punching shear stress (two-way shear).
  7. Calculation of bending moment and shear forces.
  8. Calculate the Area of steel, Number of Bar, Diameter, and Spacing.
  9. Draw the S.F.D. and B.M.D.
  10. Calculate the bearing strength of column and footing concrete.
  11. Check the anchorage of the reinforcement.

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