Spread footing is types of footing that are constructed at a shallower depth. There are many types of shallow foundation constructed rest the different types of structures.
The nature of the structure and the applied loads and loading arrangement consider in determining the type of foundation. The main type of spread footings are as follows.
- Isolated pad footings
- Combined footings
- Strip footings
- Strap footings
- Raft foundations
Isolated Pad Footings
The most commonly constructed spread footing type is the isolated pad footings. Pad footing are easy construct and simple when compared to the other types of footings.
Most of the low-rise buildings are constructed on the shallow foundation. Our to those pad foundation are more frequently constructed.
Design and construction of the pad footings are simple. When the allowable bearing capacity from the soil investigation report is known and after the column axial loads is calculated footing area can be calculate from the following equation.
Area of footing = Column Axial Load / Allowable bearing capacity
With this we can determine the dimension of the footing. So that required footing thickness and area of reinforcement can be calculated.
Combined Footings [Spread Footing]
When the columns are located very closely or when the area of pad footing is not adequate to carry the column loads and when there is column at the edge and need to combine with internal column foundation, we construct the combined footings.
Design of the combined footing is also same at the pad footing. However, it is little complicated when compared with isolated pad footing.
Strip Footings
Strip footings are more common in building construction and when there is structure applied line load on the ground.
Having low bearing capacity increase the footing area. However, there are limitations in increasing the footing area as the overlapping the pressure bulbs of each footing.
Having strip footing combining set of columns increase the area of footing and suitable for low bearing soils.
Strap Footings
We cannot always place the column at the center of the footing. Especially when there are blind walls, column will be at the edge of the boundary. Footing cannot be shifted to the other boundary. In those situations, the spread footing type strap footings are constructed.
Design of the strap footings is bit complicated and more knowledge in footing design is required as it involves certain calutrons to do.
Stra footings is connected to the internal column by the strap beam which required to be design together with the footings. Combination of the strap footing and beam acts together to transfer the loads.
Raft Foundations
Raft foundations are also known as the mat foundation constructed as one of the main spread footing types when we are unable use the other types of shallow foundations.
When compared to the pad footings, combined footings or strip footings, raft foundation can bear more loads.
Construction of the raft foundations are challenging as there are lots of reinforcement and complicated detailing at column connection, lift connections, etc. Further, design of the mat foundation is also required much attention when compared to the pad footings.
Design for serviceability requirements such as cracking of concrete, etc. also need to be considered during the design in addition to checking the structure for the bending, punching shear, vertical line shear, etc.
If the structure cannot be constructed on the raft foundation, the structure will be constructed on the pile foundation.
In addition to the above discussed spread footing types, there are different forms of footings that derived from the above types such as irregular types of footings.
