IRC TableR403.1 Where Did Footing Width Numbers Come From???

[kar108]

According to the 20006 IRC Commentary, The Minimum Widths of Concrete Footings in Table R403.1 is "calculated based on 20 feet (6096 mm) of tributary roof area,16 feet (4877 mm) of tributary floor areas, 10-foot-high (3048 mm) first story walls, and 8-foot-high (2438 mm)second and third story walls. A 50-pound-per-square foot (2.4 kPa) snow load was used."

No mention is made of below grade foundation walls or the footing weight itself. I ran some spreadsheet calculations(see attachment) to verify the maximum allowable plf for the footing widths in the Table. What I found is that above grade loads appear to be accounted for, but not foundations and footings-- and we use 8 or 9 foot basement walls in our area. Am I making some mistaken assumption that I don't see?

 

[JStephen]

On the soils reports I see, allowable bearing is usually based on "net" bearing= bearing in excess of soil weight that was already there, which will reduce the below-grade weights considerably.