How do you calculate foundation load and stress?

1 Types of foundations

Foundations are classified into two main categories: shallow and deep. Shallow foundations are those that transfer the load to the soil near the surface, such as strip, pad, raft, or mat foundations. Deep foundations are those that transfer the load to a deeper layer of soil or rock, such as piles, piers, or caissons. The choice of foundation type depends on the soil conditions, the load magnitude and distribution, and the design requirements.

2 Soil properties

Soil properties are essential for determining the bearing capacity, settlement, and shear strength of the foundation. Some of the soil properties that affect foundation load and stress are density, moisture content, porosity, permeability, cohesion, friction angle, and modulus of elasticity. These properties can be obtained from field tests, such as standard penetration test (SPT), cone penetration test (CPT), or pressuremeter test (PMT), or from laboratory tests, such as triaxial test, direct shear test, or consolidation test.

3 Applied loads

Applied loads are the forces and moments that act on the foundation from the structure and the environment. They include dead load, live load, wind load, seismic load, thermal load, and hydrostatic load. These loads can be calculated using the structural analysis and design codes, such as ASCE 7, ACI 318, or AISC 360. The applied loads can be classified into vertical, horizontal, and moment loads, depending on their direction and point of action.

4 Foundation load

Foundation load is the total load that the foundation has to support from the applied loads and its own weight. It can be calculated by summing up the vertical, horizontal, and moment loads at the base of the foundation. For example, if a strip foundation supports a wall with a dead load of 100 kN/m, a live load of 50 kN/m, and a wind load of 20 kN/m, the foundation load per unit length is 170 kN/m.

5 Foundation stress

Foundation stress is the intensity of force per unit area that the foundation exerts on the soil. It can be calculated by dividing the foundation load by the contact area between the foundation and the soil. For example, if a strip foundation has a width of 1 m and a length of 10 m, the contact area is 10 m2. If the foundation load per unit length is 170 kN/m, the foundation stress is 17 kPa.

6 Methods of calculation

Various methods of calculation for foundation load and stress exist, depending on the type of foundation, soil conditions, and complexity of the problem. Elastic methods, such as Boussinesq's theory, Westergaard's theory, and Mindlin's theory, assume that the soil behaves as a linear elastic material and use the theory of elasticity to derive equations for foundation stress and settlement. Empirical methods, like Meyerhof's method, Terzaghi's method, and Peck's method, use empirical formulas or charts based on experimental data or field observations to estimate the foundation stress and settlement. Numerical methods, such as finite element method (FEM), finite difference method (FDM), or boundary element method (BEM), use numerical techniques to model the soil-foundation interaction and solve equations for foundation stress and settlement; examples include PLAXIS, ABAQUS, and ANSYS.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?