Footing design guideline is the same for all type of footings. However, the design calculation is not the same. Those will be varying based on the relevant code of practices.
The following shall be done during the design of the footings.
- Firstly, we have to do a soil investigation to obtain the soil’s allowable bearing capacity. This shall be considered important. In small buildings like 2-3 stores, sometimes the structural engineer could be able to judge the probable bearing capacity of the soil based on his experience.
This could only be done when the condition of the ground is good. If there are weak soils, it is highly recommended to carry out a soil investigation.
- You have to estimate the loads on the foundation. When the building is smaller, there won’t be any later loads due to the wind. However, there could be lateral loads due to soil pressure, etc. Based on those loads, we have to estimate the loads on the foundation at serviceability limit state (SLS) and ultimate limit state (ULS)
- If you know the SLS load on the foundation and allowable bearing capacity of the soil, area of the footing can be calculated. In this calculation, approximate thickness for footing could be considered.
A = σ / F
Where A – area of the footing
σ – allowable bearing capacity of soil where the foundation is place
F – column SLS load
- If you know the area, dimensions of the footing can be calculated. If it is square footing,
Length of footing = √A
- Based on the applied load, we can predict a thickness for the footing, or you may use the punching shear criteria to obtained approximate thickness for footing.
- If we know the thickness of the footing, we can calculate the ultimate bearing pressure under the foundation.
- Now we know the pressure under the footing and so we can calculate the bending and shear forces wherever we need.
- Bending moment shall be calculated at the face of the column. Once you calculate the bending moment, reinforcement area can be calculated as per the relevant standards.
- Punching shear and vertical line shear need to check according to the relevant standards. The punching shear perimeter is varying from standards to standards.
- After that you have to look into the reinforcement detailing requirements.