Once you have a column’s factored axial load, a quick estimate gives a trial size: divide the load by the design stress the material can carry to get a required area, then pick a practical section at least that large. It is a preliminary step, not a final design, but it turns a number from a load takedown into a real cross section. StructLoads gives you the factored axial reaction; this page turns it into a first size.
Start from the factored axial load
You size a column on its factored axial load, not its service load, because strength design uses factored values. That load comes from a takedown plus a load combination: accumulate the floors, then apply the governing ASCE 7 case, usually 1.2 times dead plus 1.6 times live plus 0.5 times roof. The full route to that number is in how to calculate column loads. Take the worked value of about 1070 kN as the input here.
The required-area method
Divide the factored load by the stress the material can take, and that is the area you need.
| Material | Approx design stress | Required area for 1070 kN |
|---|---|---|
| Structural steel | About 200 MPa | About 5400 square millimetres |
| Reinforced concrete | About 12 MPa (gross, low rebar) | About 90000 square millimetres |
For steel, 1070000 newtons divided by 200 newtons per square millimetre is about 5400 square millimetres, which a modest universal column section provides. For concrete, the same load over a lower working stress needs a much larger gross area, roughly a 300 by 300 millimetre column or larger depending on the reinforcement. The stresses here are deliberately conservative placeholders for an early estimate.
Pick a practical section
Choose a standard section at least as large as the required area, rounded to a buildable size. For steel that means selecting a catalogue universal column whose area exceeds 5400 square millimetres; for concrete it means a sensible square or rectangular size with a reinforcement ratio you can detail. The point of the trial size is to check feasibility and feed the next design step, not to be the final answer.
What the quick method leaves out
The required-area estimate ignores several things that can increase the size: slenderness and buckling for tall columns, bending moments from frame action or eccentric loads, and the detailed capacity rules of the steel or concrete code. For steel the gap is explicit in the code itself: as SteelConstruction.info’s member design guide sets out, the cross-section resistance is the area times the yield strength, but for members in axial compression the buckling resistance, reduced by the slenderness factor chi, almost always governs instead. A short, axially loaded interior column is close to this estimate; a slender or moment-carrying column will need more. That is why this is a trial size, refined by a full check, and the service load is still used separately for serviceability, as in factored loads vs service loads.
Where it sits in the workflow
Sizing closes the loop that a structural load takedown opens: geometry to tributary area to load to reaction to size. A quick size on every column tells you early whether a grid is sensible or whether the columns are getting too large, long before a full model exists. If the trial sizes look wrong, the time to change the scheme is now.
Key takeaways: sizing a column from axial load
Divide the factored axial load by the material’s design stress to get a required area, then pick a practical section at least that large. It is a fast trial size that ignores slenderness, bending and detailed code rules, so a full check follows. StructLoads supplies the factored reaction that starts this, as a preliminary figure a qualified engineer confirms in design.
Quick answers
How do you size a column from its axial load?
Divide the factored axial load by the design stress the material can carry to get a required cross-sectional area, then choose a practical section at least that large. For a 1070 kN load and a steel design stress near 200 MPa, the required area is about 5400 square millimetres. This is a quick trial size for early design, refined later with full code checks.
What axial load do you use to size a column?
Use the governing factored axial load from a load takedown and a load combination, not the service load. For most gravity columns that is the ASCE 7 case of 1.2 times dead plus 1.6 times live plus 0.5 times roof, taking the largest result. The service load is used for serviceability, but the factored load sizes the strength of the column.
How accurate is a quick column size estimate?
It is a starting point, not a final size. The required-area method ignores slenderness, buckling, bending moments and the detailed capacity rules of the material code, all of which can increase the size. It is accurate enough to choose a sensible trial section and check feasibility, after which a full design either confirms it or steps it up.
When is a quick axial sizing not enough?
It is not enough whenever the column is slender, carries bending as well as axial load, is part of a moment frame, or resists wind or seismic loads. In those cases buckling and combined actions govern and the simple area method underestimates the size. Use it for the early trial, then a full code check, with a qualified engineer signing off.