How to Size and Design a Pad Footing? Explained in 1-minute

Pad Footings are one form of shallow foundations (spread footing) intended to directly support columns and transfer the building forces into the ground below. These can be axial loads and overturning moments.

Initial Sizing

The initial sizing may be based on the Allowable Bearing Capacity of the soil as well as the working loads (1.0*dead + 1.0*live) applied to the footing.

N is the unfactored dead + live loads, and qallow is the soil Allowable Bearing Capacity.

Pad footings are generally square unless there are some specific spatial constaints. The depth (thickness) of the Pad Footing is typically taken as around B/4 (where B is the footing width).

※ How to Calculate the Allowable Bearing Capacity of Soils? Explained in 1-minute.

Example Calculation

Consider a Pad Footing resting on a Clay soil with an undrained cohesive strength Cu of 100 kN/m2. The applied working load is 1600 kN. Calculate the required size of the footing.

Rebar Design (Bending)

We have sized the pad footing above to distribute the loads back to the ground. The pad is applying pressure on the ground, but conversely the ground is also effectively applying the bearing pressure to our footing.

Outside of the column "stem", the pad footing acts like a cantilever beam in bending, with a UDL applied to the underside from the ground bearing pressure. So the bottom of the pad footing experiences tension, which we need to provide rebar for. See diagram below.

For this design, we will consider Eurocode and convert our bearing pressure to a ULS ultimate bearing pressure by multiplying by 1.4. The UDL, w, per meter length of the footing on the cantilever area is therefore: 

w = 1.4*q_allow x