Pier construction for a new home foundation

Pier construction for a new home foundation

The primary objective of a structural engineer is to design a foundation that is properly anchored into the ground and stable once it is loaded with the structure that it has been designed for. Before designing a building foundation, the structural engineer needs to know the composition of the soil, the “load” that the foundation is being designed for, and the contour of the ground.

Soil composition challenges

In the San Francisco Bay Area and many other locations, one finds diverse soil properties. Most of the Bay Area is composed of clay and more dense bedrock, also referred to as Franciscan Melange, shale or serpentine. San Francisco is partially built on beach sand of various purities. In some areas of the city, the sand is mixed with “fill” of different compositions. East Bay soil is mostly expansive clay. In the presence of moisture, the clay will expand and when dry, contract.

Every soil composition presents different criteria for foundation design. For example, expansive soil presents a unique challenge, since clay has a tendency to want to uplift foundations and slabs. The foundation design has to accommodate these uplifts.

How ground contour is important in foundation design

The contour of the ground presents an additional important design factor. On flat land, a foundation design can be different than on steep grades. On hillsides, a foundation often needs to reside on a series of piers that have been drilled into the bedrock.

On stable, flat ground, a foundation can usually be L or T-shaped. The shape refers to the relationship of the footing to the stem of the foundation. The shape of a T footing is similar to the human foot, with a toe, heel and stem. The toe and the heel prevent the foundation from overturning. An L-shaped foundation does not have a heel. Typically, depending on soil conditions, the engineer will design a longer toe to compensate for the none-existent heel.

A rectangular-shaped foundation is usually called a Grade Beam. Often, a grade beam is positioned on a series of piers to create a stable grid anchored into bedrock.

Foundation design for seismic areas

In California, current applicable building codes require that all structures be designed to withstand a certain measure of seismic activity. Accordingly, all foundation design needs to take into consideration the possibility of seismic movement. The structural engineer has to consider not only the configuration of the foundation, but also the size and form of the reinforcing steel and strength of the concrete.

Concrete is basically composed of gravel, sand, Portland cement and other materials such as fly ash and water. The strength and performance of foundation concrete can be adjusted by controlling the amount of each component. Usually, high strength concrete has an increased amount of Portland cement.

If you would like to learn more about foundation design and what might be most appropriate for your next project, please contact SteelCore Builders.

About SteelCore Builders

As a division of Trush Construction Company, the SteelCore Builders crew and management have been doing structural concrete and strengthening since 1987. Due to our firm’s extensive engineering expertise and exceptional reputation, we are often retained to undertake very difficult and complex projects. Our continuous research and testing of new building materials and technologies enable us to address unique situations that other firms cannot.