Types of Footings based upon Structure

Types of Footings based upon Structure

The foundation of a building is the part of a structure that transmits the load to ground to support the superstructure and it is usually the last element of a building to pass the load into soil, rock or piles.The primary purpose of the footing is to spread the loads into supporting materials so the footing has to be designed not to be exceeded the load capacity of the soil or foundation bed. The footing compresses the soil and causes settlement.The amount of settlement depends on many factors.Excessive and differential settlement can damage structural and nonstructural elements.Therefore, it is important to avoid or reduce differential settlement.To reduce differential settlement, it is necessary to transmit load of the structure uniformly.Usually footings support vertical loads that should be applied concentrically for avoid unequal settlement.Also the depth of footings is an important factor to decide the capacity of footings. Footings must be deep enough to reach the required soil capacity.

Types of Footings

The most common types of footing are strip footings under walls and single footings under columns.

Common footings can be categorized as follow:

1. Individual column footing

This footing is also called isolated or single footing. It can be square, rectangular or circular of uniform thickness, stepped, or sloped top. This is one of the most economical types of footing. The most common type of individual column footing is square of rectangular with uniform thickness.

2. Wall footing

Wall footings support structural or nonstructural walls. This footing has limited width and a continuous length under the wall.

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3. Combined footing

They usually support two or three columns not in a row and may be either rectangular or trapezoidal in shape depending on column. If a strap joins two isolated footings, the footing is called a cantilever footing.

4. Mat foundation

Mats are large continuous footings, usually placed under the entire building area to support all columns and walls. Mats are used when the soil-bearing capacity is low, column loads are heavy, single footings cannot be used, piles are not used, or differential settlement must be reduced through the entire footing system.

5. Pile footing

Pile footings are thick pads used to tie a group of piles together and to support and transmit column loads to the piles.

Footing types

Design Procedure

Wall footing

1. Compute the factored loads.

2. Assume the total footing thickness.

3. Compute the footing self-weight, the weight of earth on top of the footing.

4. Compute the effective allowable soil pressure for superimposed service loads.

5. Determine the soil pressure for strength design.

6. Compute the required footing width.

7. Assume the effective depth for the footing and shear check.

8. Compute the maximum factored moment.

9. Compute the required area of tension steel.

10. Check the ACI Code minimum reinforcement requirement.

11. Check the development length.

Individual column footing

1. Compute the factored loads.

2. Assume the total footing thickness.

3. Compute the footing self-weight, the weight of earth on top of the footing.

4. Compute the effective allowable soil pressure for superimposed service loads.

5. Compute required footing area.

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6. Compute the factored soil pressure from superimposed loads.

7. Assume the effective depth for the footing.

8. Check the punching shear and beam shear.

9. Compute the design moment at the critical section.

10. Compute the required steel area.

11. Check the ACI Code minimum reinforcement requirement.

12. Check the development length.

13. Check the concrete bearing strength at the base of the column.