On a regular grid an interior column carries about twice the load of an edge column and four times a corner column. The reason is geometry, not the columns themselves: each one carries the floor nearest to it, and an interior column simply has more floor around it. That one to one half to one quarter pattern is one of the fastest checks you have on a load takedown, and StructLoads makes it visible by showing every reaction at once.
Why position sets the load
A column carries the slice of floor closest to it, the tributary area, bounded by the lines halfway to its neighbours, which is the nearest-point partition of a Voronoi diagram. An interior column has neighbours on all four sides, so it draws a half bay in every direction and ends up with a full bay. An edge column is missing a neighbour on one side, so it loses that half. A corner column is missing two sides. Less floor means less load, and each lost side removes roughly half.
The three cases compared
On a six by five metre grid the areas and loads fall out directly.
| Position | Tributary area | Load per floor (at 7.5 kN/m2) | Load ratio |
|---|---|---|---|
| Interior | 30 m2 | 225 kN | 1.0 |
| Edge | 15 m2 | 112 kN | About 0.5 |
| Corner | 7.5 m2 | 56 kN | About 0.25 |
The geometry behind these tributary areas is set out in tributary area: edge vs corner column. Multiply by the floor load and the reactions follow the same ratio, because only the area changes. Ochshorn’s tributary areas chapter teaches the identical pattern, interior columns carrying twice the exterior and four times the corner, and adds a practical caveat: edge and corner columns can still end up physically larger, because they also carry cladding weight and lateral-frame stresses the gravity ratio does not count.
A worked comparison
Take a three-storey building with a combined 7.5 kN per square metre per floor. The interior column gathers 225 kN per floor, so about 675 kN over three floors before factors. The edge columns gather about 112 kN per floor, near 337 kN total, and the corners about 56 kN per floor, near 169 kN total. The interior column is close to twice the edge and four times the corner, exactly the expected pattern, computed in detail in how to calculate column loads.
Using the ratio as a check
The pattern is a quick test of a takedown. If an edge column comes out heavier than an interior column on the same grid, or a corner column is close to the interior, the geometry is almost certainly wrong, perhaps a misplaced support or a mis-drawn outline. The ratio check catches the kind of single-coordinate error that is otherwise easy to miss, and it complements the area balance check, where the tributary areas of all columns should sum to the floor area.
When the ratio breaks
The one to one half to one quarter ratio is a regular-grid approximation. Uneven bays, a skewed grid, an L-shaped floor, openings, or cantilevers past the perimeter columns all break it, because the tributary regions stop being neat fractions of a bay. There you compute each load from the geometry, which a tool does by clipping each region to the real outline, rather than trusting the ratio.
Key takeaways: column loads by position
Interior columns carry a full bay, edge columns about half, corner columns about a quarter, so the load ratio is roughly one to one half to one quarter. Use it as a fast sanity check on any takedown. It is exact on a regular grid and approximate on irregular floors, where StructLoads computes the real areas. The reactions are preliminary figures a qualified engineer confirms in design.
Quick answers
How much more load does an interior column carry than an edge or corner?
On a regular grid, about twice an edge column and four times a corner column. An interior column draws floor from all four sides, an edge from three, and a corner from two, so each step loses roughly half the load. The rough ratio of interior to edge to corner is one to one half to one quarter.
Why does an interior column carry more load?
Because more floor drains to it. A column carries the floor nearest to it, bounded by the lines halfway to its neighbours. An interior column has neighbours on all four sides and collects a full bay; an edge column is missing a neighbour on one side and a corner on two, so they collect less floor and therefore less load.
What is the load ratio between interior, edge and corner columns?
Roughly one to one half to one quarter on a regular grid. So if an interior column carries about 720 kN, the edge columns carry around 360 kN and the corners around 180 kN. The ratio is approximate, because edge and corner columns can pick up small extra strips, but it is close enough to flag a geometry error fast.
When does the 1, one half, one quarter ratio not hold?
It is a regular-grid approximation. It does not hold when bays are uneven, the grid is skewed, the floor is L-shaped or has openings, or cantilevers extend past the perimeter columns. There the tributary areas are not neat fractions of a bay, so you compute each load from the geometry rather than the simple ratio.