Structural Design for Pad Footing (Eurocode 2)

A pad footing can be categorized as either an axially loaded footing or eccentrically loaded footing. For an axially loaded footing, the distribution of soil bearing pressure is uniform. This is not the case for an eccentrically loaded footing.

Based on the soil pressure under serviceability limit state, we can determine the dimension of footing, to ensure the induced pressure does not exceed the permissible bearing pressure of soil.

To proceed with structural design of footing, we need to know the soil pressure under ultimate limit state. This pressure is then used to calculate the design bending moment and shear force in critical sections. The reinforcement area required can be determined afterwards. Other than bending moment and shear force, we also need to check the section resistance against punching shear. For this process, we need to focus on both column face and basic control perimeter.

In this worked example, we need to conduct structural design for a pad footing subjected to permanent and variable actions of 800kN and 400kN respectively.

First, by knowing the soil bearing capacity, we can determine the pad area required under SLS. Based on the result, we can set the value for footing width and length.

Then, we apply factor of safety on the action and calculate the design force under ULS. This force along with the footing area calculated from footing dimension we set earlier on, can give us the soil pressure developed under ULS.

With the ULS soil pressure, we can determine the bending moment developed in footing. The part of footing protrudes from column face is treated as a cantilever, and bending moment shall be determined accordingly. After knowing the design value, we can proceed with longitudinal reinforcement design. This step should be conducted for bending about both x-x and y-y axes. In this example, H16-200 reinforcement is used in both directions.

For section shear check, we first identify the location of critical section, which is 1.0d away from column face. Again, by using the soil pressure and protruding part beyond the critical section, we may determine the design shear force. For comparison, we treat the concrete section as unreinforced for shear force, and calculate its resistance. Should the resistance is greater than the design shear, then we do not need to provide shear reinforcement in the footing.

Punching shear check for the footing is conducted at the column face and basic control perimeter (located 2.0d from column face). The design punching shear force is compared with plain concrete shear resistance. Similarly, reinforcement should be provided when the unreinforced concrete is not enough to resist the force.