Functional requirements of foundation structures

Even if a foundation structure meets all the strength requirements, it should also satisfy another condition which is called serviceability.

Serviceability refers to the conditions under which a building is still considered useful. If those limits are exceeded, a structure that may not suffer structural damage but it would be considered unfit as the building can be unusable. Serviceability limit state design of structures includes factors such as durability, overall stability, fire resistance, deflection, cracking and excessive vibration.

A foundation must meet 6 basic requirements in order to be functional. Failure to do so would result in diminishing the usefulness of the whole building. Those requirements are the following:

1. Settlement

Settlement is defined as the vertical movement of the ground which is caused by stress alterations. It is the most important of all requirements and it occurs when a vertical load is applied to the foundation. It is usual for constructions to experience some degree of settlement but it must be maintained within the acceptable limits which are determined by regulation standards (1,2 cm to 5cm for common buildings, 2,5cm to 7,5cm for heavy industrial buildings and 5cm for bridges).

The most significant factor that determines the damage in a building is whether the settlement is uniform or not. When a structure sinks evenly no cracks and minor damage occur. However, if one side of the building settles more (differential settlement) the construction will suffer crucial damage.

2. Vibration

If a machine or a group of machines are installed in the building, foundation will have to address intense vibration without suffering damage. A method to tackle the issue is to isolate the foundation from the vibration source.

3. Lateral displacement

If the foundation of the structure is beneath the soil, lateral earth pressure may cause shear and moments loads. The lateral displacement must be restricted. The maximum tolerance for bridges constructions is 2,5 cm.

4. Ground heave

Ground heave is the vertical upward displacement of a building's foundation that is caused by soil's expansion. It is associated with the expansion of clay soils which swell when wet. As the soil generally cannot expand downwards or sideways, the result is that the exposed upper surface of the soil rises up. This phenomenon needs to be considered in design and construction phases of a structure.

5. Tilt

Tilt of a structure is caused when heave or settlement do not occur evenly on the foundation. It can cause significant damage to the framework of a building and therefore it needs to be restricted. In high buildings, tilt may not exceed 0.12 degrees from the horizontal level.

6. Durability 

A foundation must be functional for the life span of a project. Being placed underground, it needs to address severe issues like chemical, physical, and biological processes. Some methods to make the foundation durable include utilizing chemical resistant concrete, implementing a thicker layer of concrete to protect steel reinforcement or using insulating materials that will decelerate the weathering processes.