By Gregory Havel
As I discussed in Part 1 of this series, different types and sizes of buildings use different types and sizes of foundations. The type of foundation and its strength are determined by the type and weight of the structure that it will support.
Other factors involved in the design of a foundation are the load-bearing capacity of the soil underneath, the slope on which the building will be constructed, and the building dimensions and the spacing of the load-bearing walls and columns.
A single structure may have a separate foundation for each column. In Photo 1, each of the four columns in this bridge support has its own foundation: a pad of reinforced concrete poured on top of pilings.
(1) Photos by author.
Photo 2 shows 16 steel pilings that were driven into soil near the river and filled with concrete to support one of the columns of a bridge support (like the one shown in photo 1). It is often more cost-effective—and as strong—to drive pilings to bedrock (or to a depth where friction with the soil prevents the piling from being driven down further) than it is to excavate to bedrock and pour the entire excavation full of concrete. This type of concrete foundation supported by pilings is used for buildings, bridges, and other structures.
Photo 3 shows the huge footing for a building on a gravel hillside. This footing is 18 inches (45.7 cm) thick with two mats of reinforcing steel (rebar) in it. This great strength and width are needed because of the load-bearing capacity of the gravel soils, the slope of the hill, the size of the gymnasium (60 × 100 feet or 18.29 × 30.48 meters) on this lower level and the height (18 feet or 5.49 m) of the reinforced concrete wall that it will support. This building is two levels and is built on a steep hillside. The gymnasium is on the lower level, with exits at grade on the downhill side. The upper level has offices, classrooms, and assembly rooms; exits at grade on the uphill side; and appears to be the second floor from the downhill side.
Sometimes, a building’s foundation will require more support than a traditional concrete footing and foundation poured onto undisturbed or compacted soil, but less support than that of steel pilings filled with concrete. A “rammed aggregate pier system” of compacted gravel or crushed stone can be constructed below the footings. (GeoPier, as shown on the machines, is one patented system.)
Photo 4 shows the machines that are needed to bore large holes into the ground with augers, into load-bearing soil, and to fill these holes with compacted, crushed stone. In the foreground is a completed and filled rammed aggregate pier.
Photo 5 shows one of these completed piers inside and below the forms for the footing of the building. The load of the building will be carried by the footings, which will transfer the load by way of reinforcing steel and the mass of concrete to the rammed aggregate piers, which will then transfer the load to load-bearing soils below the surface of the ground.
No matter what the type of construction or the size or use of the building, its live and dead loads must transfer to load-bearing soils or bedrock by one of the foundation methods described above or as described in an earlier Construction Concerns article.
The more complex a foundation system must be to support a building, the higher its cost. This added cost must be justified to the owner by the builder, since most building owners prefer to spend money where it can be seen by the building’s occupants and visitors, rather than below the landscaping outside the building and below the floors in the lowest levels of the building, where the results are invisible.
For more information on different types of foundation systems for buildings and other types of structures, do an Internet search for the following:
- “Footings and foundations.”
- “Load-bearing soils.”
- “Rammed aggregate pier system.”
Contact your municipality’s building inspector or building department and use your quest for information as a step in building a relationship with these people, who can be a valuable resource rather than simply the enforcers of the building codes.
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Gregory Havel is a member of the Town of Burlington (WI) Fire Department; a retired deputy chief and training officer; and a 40-year veteran of the fire service. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College, has more than 40 years of experience in facilities management and building construction, and presents classes at FDIC and other venues.
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