What is the difference between a “Grade Beam” and a “Spread Footing”?
By: Jason Robinson, PE–Associate Structural Engineer
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 rebar and form layout | Reference: http://pv14house.com
My goal is to give you a good foundational understanding between these two systems. Two foundation systems are exactly what we are talking about. 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. Let me explain each type of system.
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 wall overturning forces.
Spread Footing Diagram | Reference: www.teachengineering.org
Rebar Layout Diagram | Reference: http://rebar.ecn.purdue.edu
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.
Grade Beam Diagram | Reference: http://en.wikipedia.org/wiki/Grade_beam#/media/File:Grade_beam.jpg
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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Thanks for reading,
Jason works as a Senior Structural Engineer at Allegheny Design Services. His responsibilities include but are not limited to project management, production of construction documents with CAD and structural engineering design. He attended West Virginia University where he received his bachelor’s degree in civil engineering. He has extensive engineering experience as both a field engineer and a structural engineer. He has also acquired skills in the Autodesk Revit 3D modeling system for the production of construction documents.