Joist bearing under Georgia's R502.6
The Georgia DCA amendment to IRC R502.6 (effective January 1, 2020) requires the ends of each joist, beam or girder to have:
- not less than 3 in (76 mm) of bearing on masonry or concrete;
- not less than 1½ in (38 mm) of bearing on wood or metal.
The listed common error is applying the 1½ in wood or metal figure to concrete. There is no 4 in figure in R502.6.
Direct bearing or a 2 in sill
Georgia's change adds a rule on how the member bears on masonry or concrete: the bearing shall be direct, or a sill plate of 2-inch minimum (51 mm) nominal thickness shall be provided.
Traps here:
- 2 in is the sill plate's nominal thickness, not the bearing length. The bearing length on concrete is 3 in.
- A 1½ in sill plate mixes up the wood or metal bearing length with the sill thickness.
- The 1x4 ribbon strip nailed to adjacent studs is an alternative support for joist ends in framed walls, not a way to bear on concrete. Approved joist hangers are the other alternative.
- The amendment does not call for a steel bearing plate.
Concrete cover against earth
Concrete cast against and permanently in contact with earth, such as a footing, needs 3 in (76 mm) of specified cover over the reinforcement. ACI 318-19 Table 20.5.1.3.1 states this. The 2 in and ¾ in figures fall short of it, and 1½ in is a Georgia R502.6 bearing length, not a cover figure.
Which edition, which table
The 2018 IBC references ACI 318-14, and the table numbers differ between editions:
- ACI 318-14: specified cover is in Section 20.6.1, general requirements at 20.6.1.3, including Table 20.6.1.3.1.
- ACI 318-19: the cover table is Table 20.5.1.3.1.
The listed common error is citing the 318-19 table number when answering from the 2018 IBC. In 318-14, 25.4.2 is development of deformed bars in tension and 19.3.1 is exposure categories.