Support domain of the beam is the length of the supported bearing strip in section, usually one-quarter or one-half the floor area that would collapse if the beam failed.
In this example shown in cross section, the dashed line at the center of each drawing indicates a central bearing wall.
Think of joists resting on a header: if the header breaks,
the outside end of the joists drop, but the other half
of the joists are supported on the center bearing wall. The support domain is 1/4 of the building width.
If a girder breaks, joists on both sides would be affected. The support domain is 1/2 of the building width.
