Tributary Area Calculator
Free Tributary area Calculator for structural engineering projects. Enter dimensions to get material lists and cost estimates.
Reviewed by Abdullah, Technical Content Specialist
Formula
Interior: AT = Lx * Ly | Edge: AT = (Lx/2) * Ly | Corner: AT = (Lx/2) * (Ly/2)
The tributary area for an interior column equals the full bay dimensions in both directions. Edge columns use half the bay length perpendicular to the building edge. Corner columns use half the bay length in both directions. The column load equals the tributary area times the floor load intensity times the number of floors.
Worked Examples
Example 1: Interior Column Load
Problem:Calculate the tributary area and total load for an interior column in an 8m x 6m bay grid with 5 kPa floor load over 4 stories.
Solution:Tributary area = 8 * 6 = 48 m2\nLoad per floor = 48 * 5 = 240 kN\nTotal load = 240 * 4 = 960 kN\nInfluence area = 4 * 48 = 192 m2\nReduction = 0.25 + 15/sqrt(192) = 1.33 -> 1.0 (no reduction below 37.16 m2 threshold met, factor = 1.0 capped)
Result:Tributary area = 48 m2, total unreduced load = 960 kN
Example 2: Corner Column Load
Problem:Calculate tributary area for a corner column in the same 8m x 6m grid.
Solution:Tributary area = (8/2) * (6/2) = 12 m2\nLoad per floor = 12 * 5 = 60 kN\nTotal load = 60 * 4 = 240 kN
Result:Tributary area = 12 m2, total load = 240 kN (25% of interior column)
Frequently Asked Questions
What is a tributary area in structural engineering?
The tributary area is the floor area that is supported by a particular structural element such as a column, beam, or footing. For an interior column in a regular grid, the tributary area extends halfway to each adjacent column in both directions. It is used to calculate the axial load on the column by multiplying the tributary area by the floor load intensity. Accurate tributary area calculation is essential for proper sizing of columns and foundations.
How does column location affect tributary area?
An interior column supports the largest tributary area because load extends to it from all four surrounding bays. An edge column supports roughly half the area of an interior column because it has bays on only one side along the building edge. A corner column supports about one-quarter of the interior tributary area since it only receives load from one bay quadrant. This is why interior columns are typically the most heavily loaded and require the largest cross-sections.
What is the difference between tributary area and influence area?
The influence area is the tributary area multiplied by the live load element factor KLL. For interior columns KLL equals 4, for edge columns it equals 2, and for corner columns it equals 1. The influence area better represents the actual area over which load position affects the member force, and it is used in the ASCE 7 live load reduction formula. A larger influence area allows a greater reduction in the design live load.
References
Reviewed by Abdullah, Technical Content Specialist · Editorial policy