Bearing walls use the double top plates to transfer loads from joists above through the wall studs, through the sole plates, through the floor system to the beams, columns, foundations and footings. Joints in top plates have to be located over the studs. The top and bottom double plate should not both have joints over the same stud. Joints in the top plates should be offset by at least one stud.
Holes or notches in top plates of bearing walls must have 2 inches of material intact, or the top plate should be reinforced with wood or metal.
If the floor or roof joists resting on the stud wall have the joists line up directly over or within 2 inches of the studs, a double top plate is not required, since there will be no load on the plate other than at the tops of studs. As a practical matter, this is rarely done, since it requires considerable care and layout. There is often a reason to move a joist one way or another slightly, and the double top plate provides the flexibility to do that.
If single top plates are used, the builder didn't use the double overlap method to lock wall corners and T intersections together. In this case, metal ties are used to secure the walls together. Metal ties are also used where lintels extend up flush with the top plate. This interrupts the continuity of the plate, and metal ties join each end of the lintel to the adjacent plate.
While we don't need
top plates on partition walls, these are common for a couple of reasons:
Partition walls can be secured to and brace loadbearing walls by overlapping the double top bearing plates, locking the walls together.
It is easier to cut all studs the same length to end up with an 8 foot high wall rather than to cut studs 1 1/2 inches shorter for bearing walls than for partition walls. It's faster and cheaper to build everything the same way.
Openings in bearing walls require
(horizontal beams) to carry the loads around the openings. Openings in partition walls do not need lintels.