Foundations are meant to carry the structural frame loads to the underlying soil imposing a light compressive stress Foundation generally includes the footings and the pedestals. The simplest type of foundation is the spread foundation (pad foundation) i.e. isolated column footings. As a rule, spread footings consist only of a box and they are usually called flexible. Sometimes they may have a relatively large height and a sloped end, and they are called rigid.

Foundation consisting of flexible and rigid spread footings (no connecting beams)

Spread footing section

Footing dimensions depend on the soil quality and the loads transferred by the columns (column loads are determined by the distance between the columns along with the number and loads of the structure’s storeys). The usual footing dimensions range from 1.0×1.0 m to 3.0×3.0 m although sometimes they may be larger and their height varies between 0.5 and 1.0 m for flexible spread footings and between 0.7 and 2.0 m for rigid spread footings.

Foundation consisting of flexible spread footings and connecting beams

In order to ensure the proper behaviour of the foundation, the use of foundation beams (connecting beams) is mandatory. These beams tie together the column’s bases thus making the footings behave in an even way, especially during seismic incidents. Usually, their cross-section width ranges from 300 and 500 mm and their cross-section height from 500 to 1500 mm.

Footings are constructed symmetrically to their mass centre, except in those cases when due to building restrictions they are eccentrically constructed e.g. like boundaries of the building line or edge of the building land as shown below.

Side view and plan of flexible spread footing
The higher the footing’s construction eccentricity is, the strongest must the connecting beam in that direction should be. Spread foundation (pad foundation) is used in good quality soil. In the case of low soil capacity, strip foundation is used.
Foundation consisting of spread footings eccentrically constructed

Section and plan of a footing beam

Strip foundation as an inverted T beam consists of the web and the flange (the footing). The usual dimensions of the footing vary between 400 to 600 mm for its thickness and between 1000 to 2500 mm for its width. The typical web cross sections range from 300×800 to 500×1500.

Raft Foundation with connecting beams

In poor soil conditions a slab which extends over the entire loaded area, the so-called raft foundation is usually preferable. Frequently it is used in other soil conditions for practical reasons basically because of its fast and easy construction.
Raft foundation may have normal foundation beams as ribs, as shown in the bottom figure, or beams embedded into the foundation as hidden beams. The usual thickness of a raft foundation, ranges from 400 to 1000 mm, while the dimension of the raft foundation beams varies from 300×800 to 500×2000 mm.

Two-level foundation

Generally, the foundation should be preferably placed at one single level. However, in certain cases such as for example building on a sloped plot, the foundation is placed on more than one level.

Source: www.buildinghow.com

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