Structural Foundation Types

Posted on April 5th, 2021
Posted in Newsletter Volume 1    Tags:

When determining the proper foundation for your structure, multiple site-specific factors must be evaluated in the planning stage. First and foremost, when determining which foundation type to be used, it is essential to understand the soil of the ground below the proposed design. This is accomplished by a Geotechnical Engineer that will provide recommendations based on the soil investigation as well as applicable seismic parameters for the area. Once the ground is evaluated, foundation types can be divided into two groups: shallow and deep. Understanding the type of structure and the loads to be applied to the foundation are equally imperative when designing the foundation.

Shallow Foundations are typically lightly loaded and for relatively low structures. They are just as they sound in that they are usually several feet deep – at most – and designed to frost depth to prevent frost heave, where applicable.

  • Pad/Spread: Relatively small foundation which evenly distributes individual point loads across the foundation. The size is dependent upon the soil bearing capacity for the required loading.
  • Strip: Excellent choice for continuous loads and closely spaced columns, usually found in building walls.
  • Strap: Strap footings are when two or more column footings are connected by a strap that ties them together. This distributes individual column loads to other footings, as well as increases overall stability. Commonly used in areas with soils of low bearing capacity and assists with preventing differential settlement.
  • Mat: Commonly used with greater structural loads requiring increased weight distribution in low allowable bearing locations. The numerous point loads are distributed throughout the entire concrete mat instead of using deep foundations.

Deep Foundations are usually used with more heavily loaded and taller structures. These structures are more affected by both wind and seismic forces than lower lying and lightly loaded structures. This requires the applied loads to be transferred directly to the soils beneath the foundation. Deep foundations are also used where shallow layers of soil are weak and need to be penetrated for the applied loading.

  • Pile: Usually drilled or driven steel, concrete or wood columns placed deep into the soil strata to support heavy loads until sufficient bearing capacities are achieved. The bearing capacities are achieved through both friction and end bearing.
  • Pier: Drilled shafts and/or piers are drilled and placed using concrete and reinforcing steel of larger diameter than piles. This type of foundation’s bearing capacity is dependent on it’s end bearing and sized accordingly. It is common to install numerous piers to support heavier loads.

Each type of foundation has its advantages and disadvantages that need to be analyzed when choosing a final design. These range from the required loading from the structure, soil conditions and the loading capability of the soil, as well as the cost to construct and maintain. A Professional Engineer should always be consulted in the foundation design and modification of all structure types to keep the structure sound and safe to occupy.