Concrete foundations and slabs contain and release moisture that can be damaging to untreated wood products. If the moisture content in the wood gets too high, decay, mold and mildew growth can begin to break down the wood fibers.
Protecting untreated materials from direct contact with concrete can be done using galvanized steel flashing or using a 1/2" air gap for beams and girders sitting in concrete pockets. If TJI® joists are hung on a mud plate, only a slight 1/16" to a 1/8" gap is needed from the end of the joist to the concrete. The bottom seat of a Simpson hanger is indexed for that to be able to set the joist slightly away from the wall, but still have full seat bearing. Pressure preservative treated wood or steel shims will provide an adequate barrier at bearing surfaces.