Learn · guide

How to use the RC column calculator

Axial load and moments in, bars and links out: what each input means, how to read the N–M curve, and how the lighter options save you a few runs.

All guides Inputs Result Calc sheet Walk-through Mistakes

1. What the calculator does

The RC column calculator designs a reinforced concrete column for an axial load with moments about one or both axes. It works out the slenderness from the effective height, adds the extra moments a slender column picks up, designs the longitudinal bars and the links, and checks the section against its own N–M interaction curve. Rectangular and circular columns are covered, braced or unbraced, with tied or spiral links for circular columns.

The code routes are SANS 10100, BS 8110, Eurocode 2 (EN 1992-1-1) and SANS 51992-1-1 (SA Eurocode, 2023 draft annex). The badge under the title says which engine runs the checks.

RC column calculator header with the PASS badge and headroom — The header: PASS 69 %, governed by the N–M capacity. The headroom box gives the moment the same bars can take at this axial load.

2. The inputs, group by group

Input	What it means	Typical value
Column	A mark (Col 1...) and the design code.	Col 1, SANS 10100
Section	Rectangular b × h or circular D, cover to the links, and the link arrangement (auto picks the simplest that passes).	400 × 400, cover 40
ULS loads	Factored axial N and the design moments M_x, M_y and shear V. “+ load case” adds another combination.	N 1500 kN, M_x 80 kNm
Height	Effective height l_e, braced or unbraced, and the storey height (blank = l_e). More inputs sets the bracing per axis.	l_e 3 m, braced
Materials	Concrete f_cu and rebar f_y.	30 MPa, 450 MPa
Durability	Exposure and fire rating. “Not given” leaves those checks NOT CHECKED.	Moderate, R60
Bars	The engine designs the bars and links. Editing the bars yourself is part of the paid plan.	6Y12, Y10 @ 125

RC column inputs: column, section, ULS loads, height, materials, durability and bars — The inputs. Unlike the beam calculator, the loads here are ULS (already factored).

3. How to read the result

The reinforcement panel draws the section to scale with the bars, the axial load and the moment. The N–M interaction panel draws the engine’s own curve for the bars it designed, with your design point on it: inside the curve passes, outside fails. The distance to the curve is your reserve.

Column section with the bars, the axial load and the moment — The section to scale: 6Y12 with Y10 links at 125, N = 1500 kN and M = 80 kNm.

N-M interaction curve with the design point — The N–M interaction curve for the bars designed, with the design point inside it.

The badge names the governing check; here the N–M capacity at 69 %. “Headroom and lighter options” is the most useful card on the page: it gives M_Rd at your axial load (116.7 kNm with the same bars), and the engine designs lighter options again, such as 350 × 350 (PASS 99 %) or f_cu 25 MPa (PASS 96 %). Press Try to load one. Checks the engine could not run show as NOT CHECKED; they are not passes.

Design checks, headroom and lighter options, and the bending schedule card — Right column: design checks, headroom and lighter options, and the bending schedule card with “+ Add to a project”.

4. Calc sheet, drawing and schedule

The result (pass / fail and utilisation) stays free for everyone. Every check with its clause, the calc sheet and the bending schedule are open to signed-in users. Sign in free: 3 designs a month (one calculator, open for 24 hours). Unlimited with a calculator pack.

The bending schedule card reads: “Sign in (free): 3 schedules a month. CSV with a paid plan.” The full reinforcement detail drawing, sections and bar editing are part of the paid plan. Add more columns with “+ Add column” and the schedule collects all of them, with the starter bars.

Reinforcement detail panel marked paid plan — The reinforcement detail drawing is part of the paid plan.

Verification table comparing published column examples with the calculator — Verification: published SANS 10100 column examples run through the same engine, 9 of 9 within tolerance.

5. A 2-minute walk-through

Open the calculator: a braced 400 × 400 column, l_e 3 m, N 1500 kN and M_x 80 kNm, f_cu 30.
Read the badge: PASS 69 %, N–M capacity, with 6Y12 and Y10 links.
Press Try on the 350 × 350 option and see the bars go up to 8Y16 at 99 %.
Set the restraint to unbraced and the effective height to 6 m: the slenderness moments grow and the bars change.
Add M_y for a corner column with biaxial bending.
Sign in for the calc sheet and schedule, or add the column to a project.

6. Common mistakes

Characteristic loads in the ULS boxes. This calculator takes factored N and M; the beam calculator takes characteristic loads. Factor them first.
Effective height equal to the floor height without thinking about the end restraint. Braced columns with stiff beams have l_e below the clear height; unbraced columns above it.
No minimum moment. A column with “no moment” still gets the code’s minimum eccentricity; let the engine apply it rather than typing 0 and stopping.
Braced when the frame is not. A column is braced only if walls or bracing take the sway; otherwise pick unbraced.

7. Where it fits

The column loads come from the beam reactions or a load take-down; the column goes on to the pad footing calculator.

8. Learn more

Try it nowThe calculator opens with the walk-through inputs above. The result is free; sign in for the checks, calc sheet and schedule.
Open the RC column calculator

A guide to using the calculator, not a design. Check every value against the current code and your own project, and have the design checked and signed by a registered professional engineer.

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