Adjust stories, tributary area, and floor loads — the building section and bar chart update in real time.
Gravity loads flow down through a building: roof loads → top floor beams → columns → lower beams → lower columns → foundations. A load takedown traces this path, accumulating loads story by story. Tributary area determines how much floor load each beam or column "catches." A typical interior column takes load from trib area × number of floors above. Getting the takedown right is the foundation (literally) of the entire design.
A load takedown traces the path of gravity from the top of the building to the bottom. Start at the roof: dead load + snow. Go to the top floor: add that floor's dead + live. Continue floor by floor, accumulating loads. At each level, the columns below must carry everything above. By the time you reach the foundation, a single interior column might carry hundreds of kips — the total weight of the building above its tributary area.
Tributary area is the floor area “assigned” to a member. For an interior column on a regular grid with bay sizes B₁ × B₂, the tributary area is simply B₁ × B₂. For an edge column, it's half that. The load per floor on the column is: P = (DL + LL) × A_trib. Tributary area is conceptually simple but is the single most common source of errors in a load takedown — always sketch it before calculating.
The final column load for design uses factored combinations. The typical gravity combo: Pu = 1.2D + 1.6L. For the roof: Pu_roof = 1.2(DL_roof × A_trib) + 1.6(LL_roof × A_trib). Total at foundation: sum of all floors. ASCE 7 also allows live load reduction for large tributary areas and multi-story columns — the probability that every floor is fully loaded simultaneously is low.