What is tributary area?
Tributary area is the floor or roof area that delivers its load to one support — a beam, column, or footing — bounded by the midlines to adjacent supports. An interior column carries a full bay (column spacing one way × the other); an edge column about half, a corner about a quarter. Multiply by the load per square foot to size it.
Source: Structural load takedown (tributary area) · Verify against your AHJ and the adopted code edition.
Tributary area is the chunk of floor or roof whose load ends up on one support. Picture the framing draining like a watershed: the dividing line between two columns sits halfway between them, so each support “collects” everything out to those midlines.
For a regular column grid, that midline rule gives simple fractions of a bay:
- Interior column: a full bay — the spacing one way × the spacing the other. A 20 × 25 ft grid → 500 ft² per interior column.
- Edge column: about half a bay (it collects to the midline on only one side of one axis) → ~250 ft².
- Corner column: about a quarter bay → ~125 ft².
Turn area into load with the load takedown:
load on the support = tributary area × load per square foot
Add the dead load (structure + finishes) and live load per floor, and for a multistory column, sum the floors above. Big tributary areas qualify for live-load reduction under ASCE 7 §4.7. This is preliminary sizing — a licensed engineer signs off on the real design.
Work it out: the tributary area calculator figures the area for an interior, edge, or corner support and takes the load down for you.
Sources
- ASCE 7 — Minimum Design Loads for Buildings — ASCE 7 §4.7 (tributary area & live-load reduction)