8.2.
Tributary Width. This is the width of the area which contributes to the member's load. When
designing joists and rafters, tributary width is typically their On Center (O.C.) spacing. When designing a
beam which supports roof or floor loads, tributary width is typically half the span of the member (or
members) bringing its (their) load to the beam being designed.
8.2.1.
Examples:
•
If you are designing joists or rafters at 24" O.C.: Tributary width = 2 ft.
•
Joist or rafters at 16" O.C.: Tributary width = 1.3 ft.
•
Joists or rafters at 19.2" O.C.: Tributary width = 1.6 ft.
•
Header beam supporting one end of trusses of 30' span: Tributary width is 30/2 = 15 ft. If the
truss tails overhang 2 feet beyond the beam you were designing, the tributary width on the beam
would be 15 + 2 = 17 ft.
•
Beam supporting floor joists on one side (with 14' span) plus floor joist on the other side (with 10'
span): Tributary width is (14/2 + 10/2) = 12 ft.
•
Beam supporting the weight of an 8' tall wall on top of it: Tributary ‘width’ of the wall = 8 ft.