Grade Beam or Spread Footing?

Grade Beam or Spread Footing?

What is the difference between a “Grade Beam” and a “Spread Footing”?
This is one of the most frequently asked questions during construction.  This is a legitimate question considering both usually have similar dimensions and similar rebar patterns.
Grade beam 
There are two major types of foundation systems:  Shallow foundations and deep foundations. A shallow foundation system utilizes spread footings and a deep foundation system utilizes grade beams and some type of drilled pier, micropile, augercast pile, etc.
Spread footings rest directly on the bearing soil and are resisted by an area load based on the soil bearing pressure (q) as seen in the diagram below. In the case of a wall footing, the transverse reinforcement (the rebar that is placed perpendicular to the wall) is the main flexural reinforcement for gravity loads. The longitudinal reinforcement is used for differential settlement and/or shear all overturning forces.

Grade beams are used for two purposes in deep foundation systems: To carry loads to the pile foundation and to brace the pile foundation. A grade beam is exactly what it sounds like, a beam. A beam by definition is a member carrying loads that span an unsupported length. Though a grade beam is cast on soil, it is actually designed to span from pile to pile not considering the soil underneath the beam for bearing. Therefore, the longitudinal reinforcement is the major flexural reinforcement in a grade beam unlike the spread footing. The transverse rebar ties are used for shear reinforcement.

In conclusion, a spread footing is designed to bear on an area of soil per the allowable bearing pressure. A grade beam is simply designed as a beam spanning from one point to another. Though they may look similar and fulfill the same purpose of supporting a building, they are two different types of structures.

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