Foundations are one of the most essential substructures on which a building stands. It is beneath the ground soil and the ground on which it rests is called foundation bed. They transfer the load from the structure constructed on it to the soil. Foundations can be broadly classified as shallow or deep. The choice of the type of foundation to use for a particular building should only be made after consideration of some elements, such as the study of adjacent buildings as well as their type of foundation and state of the same, the characteristics and the mechanical properties of the soil on-site, the groundwater level, the values of the loads to be transmitted to the foundation, type of material that makes up the superstructure, the cost, some technical aspects for carrying out the work, the types of foundation on the market and others factors. Direct foundation is the first type of foundation to be searched and just presenting advantage when the area occupied by the Foundation cover between 50% to 70% of the available area.
Isolated footings are a type of shallow foundation. They are commonly used for shallow foundations in order to carry and spread concentrated loads, caused for example by columns or pillars. Isolated footings can consist either of reinforced or non-reinforced material. For the non-reinforced footing however, the height of the footing has to be bigger in order to provide the necessary spreading of load.
The isolated footing is used to support individual columns. They can be either of steeped type or have projections in the concrete-base. In the case of heavily loaded columns, steel reinforcement is provided in both the directions in a concrete bed.
Generally, a 15 cm offset is provided on all sided of the concrete bed. In the case of brick masonry columns, an offset of 5 cm is provided also on all the four sides in regular layers. When the footing of concrete columns maybe a slab, steeped or slope type.
Advantages of Isolated Footing
- Isolated footing is economical.
- It is used when the soil bearing capacity is high.
- It is suitable when the loads on footing are less.
- It is also very suitable if the columns of a building are not close space.
Disadvantage of Isolated Footing
- The soil is needed to be stable all around the base of the structure.
- To cope with the high load, it can be very large in size
- Separate foundations make this design weak against the differential settlement that may affect the building.
Categories of Isolated Footing
- Flat or Pad or Plain Footing
- Sloped Footing
- Stepped Footing
This kind of footings is generally square or rectangular or circular in shape and is provided under each column independently. Flat or Pad Footing is one of the Shallow Foundations. It is circular, square or rectangular slab of uniform thickness.
Sloped isolated footing requires less concrete and reinforcements bars than pad footing. It is constructed cautiously in order to maintain a 45-degree inclination from all sides. The concrete mix used must be stiff in order to avoid sustaining viscous deformation.
This type of footing was constructed in olden days. From the name, footings are stacked upon one another as steps. Three concrete cross sections are stacked upon each other and forms as a steps. This type of footing is also called as a Step foundation. Stepped footing is used generally in residential buildings.