All the shallow foundation types cannot be used in the construction due to the fact that they are always not compatible with the any type of construction.
Depending on the nature of construction and after assessing the condition of the ground, the suitable type of shallow foundation shall be selected.
Shallow Foundation Types
- Isolated pad footings
- Combined Footings
- Strip Footings
- Inverted T type footings
- Raft Foundations
Above are the main type of shallow foundations we used in the construction selection of the foundation type is also made following this order. The following image indicate the selection criteria for foundation type of buildings.
The above-mentioned criteria could be followed when selecting the foundation type. Brief summary of each foundation type is as follows.
Isolated Pad Footings
The most widely constructed shallow foundation type is the isolated pad footings.
It is simple to design and construction.
Having a single reinforcement net due to its behavior, reduce the cost of construction considerably when compared to the other types of footings having two reinforcement nets.
We design pad footings for bending and shear. Punching shear at punching shear perimeter 1.5d from the face of column or as applicable according to your relevant standards and vertical line shear stress at the face of the column will be checked.
In the punching shear design, requirement of the providing shear links is assess and most of the time, shear links are not provided for isolated pad footing by adjusting it thickness as required.
Combined Footings
Combined footings are mostly constructed when the gap between the columns is very close or when there are columns at boundary.
Design of the combined footing can be done using the rigid method that is using the conversational design calculation method or a software like safe could be used to model the footing.
Strip Footings
Strip footings are constructed when the shallow foundation type cannot be isolated pad footings.
This happens when there is low bearing capacity of the soil. When we need to increase the area of the footing, we could look for combined footings or strip footings.
Strip footings also can be considered as kind of combined footings.
A strip type footing is constructed combining all the column in a particular line. It increases the area of the foundation, and we can have a footing that apply low pressure to the soil.
Inverted “T” Type Footing
When the column load is increase, the thickness of the strip footings needs to be increased.
It will not be economical to increase the thinness as the cost of construction increases considerably. Further, thinner slab does not have that much of bending and shear capacity.
When the number of floor increase and with considerable high axial loads, strip footings are not economical.
In these situations, we add a beam to the strip footings and called as inverted “T” type footing.
Since beam is there, it has higher bending and shear capacity.
Further, thickness of the strip footing can also be reduced considerably as its spans in transverse direction with the introduction of the beam.
Raft Foundations
Raft foundations are also called as mat foundations.
When the column axial load increases, we have to increase the area of the footings. We can keep increasing the size of the isolated pad footing up to certain level.
Further increase in the size of the footings leads to overlapping of the pressure bulb of each foundation and form weak zone. Therefore, when this condition is reach, we proceed with the mat foundations.
Further, mat foundation can be used to constructed buildings up to around 10 stores. However, this will depend on the column axial load and the allowable bearing capacity of the soil.
