Free Pad Footing Foundation Calculator

All structures require foundations to stand and these foundations resist the dead load (self-weight) of the structure and any applied live loadings. Footing are typically designed for either low rise buildings where the generally weaker layers of ground found at the surface level are sufficient to resist the imposed forces on the foundation. In some cases of hard ground or rock a shallow footing is used as digging or piling to deeper layers is not necessary to achieve the necessary bearing capacity. Many temporary structures also use shallow footings as they are generally easier to install.

Bearing capacity is the maximum pressure a soil can support before failure. Geotechnical Engineers use their understanding of bearing capacity to design foundations to safely transfer loads (such as self-weight of the structure) from building foundations into the underlying soils.

Pad foundations and shallow footings require several different checks including both geotechnical and structural assessments for a complete design. There are several main considerations when designing pad footing foundations:

The bearing capacity of shallow pad foundations can be assessed using the Terzaghi bearing capacity equations, using closed form calculations and correlations between the geotechnical design parameters, foundation geometry (shape factors) and applied loading.

Horizontal sliding of a building foundation can be assessed by considering the total vertical forces applied to the foundation, including the self-weight, dead load and imposed service loads and multiplying these by a friction coefficient based upon the friction angle of the soil beneath the foundation. The overall stability of the foundation can then be assesed by comparing the applied horizontal force to the horizontal resistance. It is also possible to consider the strength of the vertical wall of the foundation against the soil if friction alone is not sufficient.

Overturning of a pad foundation is often one of the governing critial cases for assessment. This calculation compares the applied destabilising design bending moments applied on the foundation to the stabilising moments. Stabilising moments are calculated from the self-weight of the foundation multiplied by it's lever arm from the point of rotation, axial compressive force applied to the foundation multiplied by it's lever arm from the point of rotation and the soil overburden if available and it's centre of gravity over the foundation from the point of rotation.

Several structural checks for building foundations are required including checks on the transverse shear force applied on the joint between the pad foundation base and the column itself. Another structural check required is of the punching shear effect of the column on the foundation base. This requires analysis at the column face and at a distance 1d and 2d from the column face.