Rectangular and circular columns
Tied or spiral transverse reinforcement, with bending about either axis of a rectangular section.
ACI 318-19 路 Columns
Free PM Interaction Diagram Calculator
Generate the axial load and moment (P-M) interaction diagram of rectangular and circular reinforced concrete columns in your browser, with the design and nominal curves, seven labeled key points, phi factors and a step-by-step calculation report.
Features
What PM Curve does
Flexible bar layouts
Top, bottom and any number of intermediate layers, or bars around a circle. Bar sizes #3 to #11, #14 and #18.
Design and nominal curves
phi Pn and phi Mn with the phi Pn,max compression cap, drawn over the nominal Pn and Mn curve.
Seven key points
Pure tension, pure bending, tension-controlled limit, balanced, half yield, decompression and the code cap on pure compression.
Compare two sections
Overlay a second section on the same chart to compare sizes or reinforcement at a glance.
Reports and exports
Step-by-step PDF report, CSV of points and curve, PNG and SVG chart, and models you can save and reopen. Imperial or metric units.
Method
How the curve is calculated
- Strain compatibility to ACI 318-19 Cl 22.2: a linear strain profile with a concrete crushing strain of 0.003 and the equivalent rectangular stress block of 0.85 f'c, with beta1 from Table 22.2.2.4.3.
- Reinforcing steel is elastic-perfectly plastic (fy and Es). Bars inside the compression block have the displaced concrete deducted.
- All moments are taken about the plastic centroid of the section, so unsymmetric reinforcement is handled correctly.
- The strength reduction factor phi follows Table 21.2.2: 0.65 (tied) or 0.75 (spiral) up to the yield strain, rising to 0.90 at a net tensile strain of ety + 0.003.
- The maximum axial strength is capped at 0.80 P0 for tied and 0.85 P0 for spiral columns (Cl 22.4.2.1). Pure tension strength is fy Ast (Cl 22.4.3).
Get started
How to use it
- 01
Choose the section shape and tied or spiral reinforcement.
- 02
Enter the dimensions, clear cover and the bars in each layer.
- 03
Enter f'c, fy and Es, in imperial or metric units.
- 04
Read the curve and key points, then download the PDF report or export the data.
FAQ
Frequently asked questions
What is a P-M interaction diagram?
It shows every combination of axial load (P) and bending moment (M) a reinforced concrete column section can carry. A factored load case that plots inside the design curve is within the section's capacity.
Which design code does it use?
ACI 318-19, using strain compatibility (Cl 22.2), the strength reduction factors of Table 21.2.2 and the axial strength limits of Cl 22.4.
Is it free?
Yes. The app is free to use in your browser while TekStructa is in beta. There is nothing to install.
Does it include slenderness or biaxial bending?
No. It calculates the uniaxial interaction diagram of the cross-section about one axis at a time. Check slenderness effects and biaxial bending separately.
Can I export the results?
Yes: a step-by-step PDF calculation report, a CSV file of the key points and curve, the chart as PNG or SVG, and the model as a file you can open again later.
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For use by qualified engineers. All results must be checked independently before use in any design. See the Terms of Use.