WOOD BEAM SIZINGTM   Tributary area

SUPPORT DOMAIN

For uniformly distributed loads, support domain is the distance that loads directly affect a unit length of the beam: distance in feet, for example.

TRIBUTARY AREA

Tributary area is the area that uniform loads directly affect the entire beam. Beam span times support domain equals area in square feet.

Uniform loads outside the tributary area but within the collapse area are considered to be supported by adjacent beams or bearing walls.

COLLAPSE AREA

Collapse area is the total area that would be affected upon beam failure: area in square feet. Adding a concentrated load anywhere within the collapse area will change the total load on the girder.

For example: consider a central girder beam supporting one floor. Usual floor joists span at right angles to the beam on both sides, from outside wall to center support. Given: Beam span: 10 feet Building width: 32 feet Joists bear equally on central girder and outside wall. Support domain is therefore 16 feet Tributary area is 16 feet times beam span equals 160 sf Collapse area (any part of the floor that would move under beam failure) is building width times beam span equals 320 sf