From a structural standpoint it may seem counterintuitive that designing the footing size comes last in the design process.
In order to size a footing properly you must know the type of roof, the height of the building, the materials used, and the building forces acting on the structure. The phrase "from the ground up" is great rhetoric but only after a lot of study is a footing design appropriate to support a structure.
The term "footing" is often used interchangably with foundation especially in residential construction. Trevor Pan—Architect sees the footing as the lowest, shallow, spread structural element generally made from concrete—though other materials are permissible by this code section, namely gravel for wood foundations. The term "foundation" can be thought of as a system comprising: a footing, stemwall, or a turned-down footing combination.
Why do you conduct a site analysis?
Among many things, you want to be sure to build above a flood plain, or flood hazard area. Table R301.2(1) covers this; beware this table is set by the jurisdiction having authority (JHA). The table is empty in the IRC 2012, and is intentionally left empty for the JHA to give direction to their residents.
Prior to determining which foundation type is appropriate to your project a site analysis is required. This should happen at the very start of a project. At this point very little is invested in the project and it gives you the opportunity to discover.
This might sound obvious . . . build above the flood plain. It is obvious—if you've built projects before. However, if this is your first project you may not know to check this. You may buy property from an owner that does not disclose this (illegal). You may buy property from an owner who does disclose it, but you figure "that'll never happen to me" (your fault).
Think, you could do the entire set of plans, go to submit for a permit and find you must move the structure to a different part of the property, or that the lot you have is completely unbuildable. Do your site analysis. Call the local building department. Ask if there are flood zones you need to be aware of. They will help you. Don't try to skid by.
Selecting the proper foundation type
At this point:
site analysis should be complete
the number of floors, roof and basic plan is known
the type of construction is known (wood, masonry, steel stud)
The selection of a proper foundation is part art, mostly science.
Turned-down footings are optimal for flat sites, expedient projects, and poured against existing walls or other structures.
Cantilevered retaining walls optimally hold back earth for driveways, basements, patios on the side of a hill and stairs. These foundation types serve two functions: hold back earth, and support walls, floors or roofs above.
Footings with stemwalls are usefull for larger loads, or where the frost depth is so deep it is impractical to pour a turned-down footing, and the width of the footing is such that it would be a waste of material to pour wide enough and deep enough to satisfy structural requirements.
Wood footings have a guide all their own. This method of securing a building to the ground is expedient and saves considerably on material cost. Wood footings are easy to construct. They do not require concrete workers, or steel installers. This foundation type will likely become more popular in the 21st century. With material costs rising it makes for a less environmentally costly alternative to concrete.
For high seismic areas, consult R403.1.3 Seismic reinforcing.
If you want to stick with the IRC 2012, and not use the American Wood Council guide, this exception states the prescriptive methods outlined in Chapter 4 are valid.
- Buildings and roof—2 stories or less.
- Basement and foundation walls must be less than 50 feet in length.
Chapter 3 BUILDING PLANNING
- TABLE R301.2(1)
- Section R322
Chapter 4 FOUNDATIONS
- R401.4 Soil tests.
- R401.4.1 Geotechnical evaluation.
- TABLE R401.4.1 PRESUMPTIVE LOAD-BEARING VALUES OF FOUNDATION MATERIALS
- R403.1 General.
- TABLE R402.2
- FIGURE R403.1(1) CONCRETE AND MASONRY FOUNDATION DETAILS
- R403.1.3 Seismic reinforcing.
- TABLE R403.3(1) MINIMUM FOOTING DEPTH AND INSULATION REQUIREMENTS FOR FROST-PROTECTED FOOTINGS IN HEATED BUILDINGS
- R403.1.3.2 Slabs-on-ground with turned-down footings.
- R403.1.4 Minimum depth.
- R403.1.6 Foundation anchorage.
Chapter 5 FLOORS
- R506.1 General.
- R506.2 Site preparation.
- R506.2.1 Fill.
- R506.2.2 Base.
- R506.2.3 Vapor retarder.
- R506.2.4 Reinforcement support.
Chapter 6 WALL CONSTRUCTION
- R602.1 Identification.
- R602.3 Design and construction.
- TABLE R602.3.(1) FASTENER SCHEDULE FOR STRUCTURAL MEMBERS
- TABLE R602.3(3) REQUIREMENTS FOR WOOD STRUCTURAL PANEL WALL SHEATHING USED TO RESIST WIND PRESSURES
- TABLE R602.3(4) ALLOWABLE SPANS FOR PARTICLEBOARD WALL SHEATHING
- TABLE R602.3(5) SIZE, HEIGHT, AND SPACING OF WOOD STUDS
- FIGURE R602.3(1) TYPICAL WALL, FLOOR AND ROOF FRAMING
- FIGURE R602.3(2) FRAMING DETAILS
- TABLE R602.3.1 MAXIMUM ALLOWABLE LENGTH OF WOOD WALL STUDS EXPOSED TO WIND SPEEDS OF 100MPH OR LESS IN SEISMIC DESIGN CATEGORIES A, B, C, D o, D1, D2