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# Design a Concrete Column to AS 3600:2018
> Design rectangular and circular reinforced concrete columns to AS 3600:2018 (Amdt 2), with P-M interaction, biaxial bending, slenderness and shear.
Axial reactions from beams above link to this column automatically. Design rectangular and circular reinforced concrete columns to AS 3600:2018 (Amdt 2) with interactive P-M interaction diagrams, biaxial bending checks, slenderness magnification, and shear capacity.
How to Design a Concrete Column to AS3600:2018
The Calcs.com Concrete Column Calculator allows users to design concrete columns by specifying the desired load cases and dimensions of the column. In this article, each section of the calculator will be explained plus followed by a few worked examples.
The Column Calculator has 5 main sections:
1\. Key Properties.
2\. Longitudinal Reinforcement.
3\. Fitment Reinforcement.
4\. Loads.
5\. Summary and Graphs.
1\. Key Properties
## Method & scope
| Property | Detail |
| ---------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Design standards | AS3600:2018 (Amdt 2) |
| Regions | Australia & New Zealand |
| Who it's for | Australian structural and civil engineers designing circular reinforced concrete columns to AS 3600:2018. |
| Presets | Rectangular Column, Circular Column |
| What it checks | Section geometry and reinforcement, Axial squash and buckling capacity (AS 3600:2018, Cl 10.3 and 10.4), P-M interaction diagram (AS 3600:2018, Cl 10.6), Biaxial interaction check (AS 3600:2018, Cl 10.6.3), Shear capacity (AS 3600:2018, Cl 8.2) |
#### See the exact clause
Every check in this calculator links back to its governing clause in AS 3600:2018. Open the **Formula Reference** panel on any result and select the clause reference to read it. See [Viewing Clauses from Inside a Calculator](/docs/running-calculations/standards-and-codes/viewing-clauses-from-inside-calculator).
#### Calculation method
The Concrete Column calculator designs rectangular and circular reinforced concrete columns to AS 3600:2018 (Amendment 2) using limit state design. It generates P-M interaction diagrams for both axes, checks combined axial and bending demands, and evaluates buckling for slender columns.
##### Section geometry and reinforcement
For rectangular sections, the cross-section is defined by depth D and breadth B. For circular sections, the overall diameter is entered directly. Longitudinal reinforcement is specified by bar size and count, arranged in a circular pattern for circular sections or as corner and face bars for rectangular sections. Fitment (tie or helix) size and spacing are entered separately.
##### Axial squash and buckling capacity (AS 3600:2018, Cl 10.3 and 10.4)
The squash load is the maximum concentric axial load the section can carry without bending:
**phi × N\_uo = phi × \[alpha\_1 × f'c × (Ag, Ast) + fy × Ast]**
where alpha\_1 = 1.0, (f'c / 340) per Cl 10.6.2.2. The reduced squash load phi\_N\_u accounts for accidental eccentricity (Cl 10.3.1).
Buckling capacity is determined from the effective length kL and the section radius of gyration r:
**phi × N\_uc = pi² × E × I / (kL)²** (for slender columns per Cl 10.4)
##### P-M interaction diagram (AS 3600:2018, Cl 10.6)
The interaction diagram is constructed from three key points per axis:
* **Decompression point**: axial load at which tensile strain at the extreme fiber is zero (Cl 10.6.2.3)
* **Balanced point**: simultaneous concrete crushing and steel yielding (Cl 10.6.2.5)
* **Pure bending point**: zero axial load capacity per Cl 8.1
The rectangular stress block (Cl 10.6.2.4) is used throughout with gamma = 0.85, 0.007(f'c, 28) for the block depth factor. The phi factor transitions between 0.65 (compression-controlled) and 0.80 (tension-controlled) per Cl 2.2.2.
##### Biaxial interaction check (AS 3600:2018, Cl 10.6.3)
For combined biaxial bending and axial load, the utilization is checked against the interaction relationship:
**(Mu,x / phi × M\_ux)^alpha\_n + (Mu,y / phi × M\_uy)^alpha\_n ≤ 1.0**
where alpha\_n is determined from the axial load ratio N\*/N\_uo.
##### Shear capacity (AS 3600:2018, Cl 8.2)
Shear capacity about each axis is computed using the simplified method (Cl 8.2.4.3), valid for f'c up to 65 MPa without prestress, tension, or torsion. Results are reported for reference; columns with significant lateral shear demands should be verified separately.
##### Assumptions
No torsional demands are considered. Fitments are assumed horizontal and perpendicular to tensile reinforcement. Transmission of axial force through floor systems (Cl 10.8) and crack control are not checked within this calculator.
## How to use it
The figure given below illustrates the three parameters that need to be specified in this section.
* Cross-section height (D) must be specified in millimeters.
* Cross-section width (B) must be specified in millimeters.
* Total length of the column (L) must be specified in millimeters.
You can select the concrete strength from the drop-down menu below. All options are presented in Mega Pascals and have normal weight classification.
You can select the end restraint condition from the drop-down menu below.
2\. Longitudinal Reinforcement
You can choose the type of reinforcement for the column shown in the graphic in red, by selecting an option from the drop-down menu.
B. Additional Top / Bottom Reinforcement Bars (per side)
You can choose how many bars to add to the column to the top and bottom sides of the column, as illustrated in the graphic below in red, by entering the desired number of bars.
C. Additional Left / Right Reinforcement Bars (per side)
You can choose how many bars to add to the column to the left and right sides of the column, as illustrated in the graphic below in red, by entering the desired number of bars.
D. Concrete Cover to Fitments
Distance from concrete surface to ties. While concrete cover must in all cases be at least 20mm, the minimum cover requirement is often greater. See AS 3600:2018 - 2018, Cl 4.10 for specific requirements of minimum cover based upon exposure classification, concrete characteristic strength, and other factors.
3\. Fitment Reinforcement
The widget helps determine the configuration of fitment (stirrups or ties) reinforcement in concrete columns.
1. ### Fitment Type
You can choose the type of fitment reinforcement for the column, as illustrated in the graphic below, by selecting an option from the drop-down menu.
This is the minimum diameter of the fitment reinforcement bars in millimeters (mm). The minimum is typically determined based on structural requirements and code provisions.
This field specifies the actual spacing or pitch (vertical distance) between each fitment reinforcement (ties or spirals) in millimeters (mm).
This represents the maximum allowed vertical spacing between fitments, specified by AS 3600:2018 to ensure the required confinement.
This is the number of fitment legs (ties) that are placed perpendicular to the X-axis of the column's cross-section. In this case.
This input defines the number of fitment legs perpendicular to the Y-axis.
This input specifies the number of angled or diagonal fitment legs.
This shows the minimum number of required fitment legs in the X-axis direction as per the standard.
This field displays the area of reinforcement (in square millimeters per meter) provided by the fitment legs perpendicular to the X-axis.
Similar to the minimum number of legs in the X-axis direction, this shows the minimum required fitment legs in the Y-axis.
This shows the area of reinforcement provided by fitment legs perpendicular to the Y-axis.
This indicates whether or not a special confinement region (additional reinforcement) is required along the X-axis.
M. Special Confinement Region Required Perpendicular to Y-axis:
Similarly, this indicates whether a special confinement region is needed along the Y-axis.
4\. Loads.
In order to enter the loads please go to this link here for more information.[how to enter loads](/docs/running-calculations/setting-up-a-calculation/loads/enter-loads-patch-point-moment-axial-triangular-line)
5\. Summary.
In the summary, you can see that a column is safely designed when its demands (axial, shear, and moment) are well within the corresponding capacities, with utilization ratios staying below 100%. High utilization percentages in any area may warrant further analysis or investigation. The summary uses color-coding to highlight these utilization levels: percentages nearing 100% are shown in yellow as a warning, while values above 100% appear in red, indicating an unsafe condition.
Interaction Curve: Moment and Axial Interaction governing in the x and y axis.
* The curved line on the graphs, known as the interaction curve or interaction diagram, represents the combined capacity of the column for both axial load and bending moment. Any point on this curve shows the maximum combinations of axial load and moment that the column can sustain without failing.
* If the point were on or outside the curve, it would suggest that the column’s load condition is at or exceeds its safe capacity, which would indicate potential structural risk.
## Available presets
Each preset opens the calculator with a typical setup already entered.
| Preset | Open in Calcs.com |
| ---------------------------------------------------- | --------------------------------------------------------------------------------------------------- |
| Rectangular Column | [Run with preset](https://app.calcs.com/new/sheet/concreteColumn?presetCode=Rectangular%2520Column) |
| Circular Column | [Run with preset](https://app.calcs.com/new/sheet/concreteColumn?presetCode=Circular%2520Column) |
## Common questions
Limit state design to AS 3600:2018 (Amendment 2). Factored loads from AS 1170.0 load combinations are checked against design capacities. Section capacities are based on the rectangular stress block with concrete strain at the extreme fiber limited to 0.003, per Cl 10.6.
Both rectangular and circular cross-sections are supported. For circular sections, the diameter is entered directly. For rectangular sections, depth D and breadth B are entered. Longitudinal reinforcement is specified by bar size and count, circular arrangement for circular sections, face bars for rectangular sections.
The calculator checks: axial squash capacity (Cl 10.3 and 10.6.2.2), axial buckling capacity (Cl 10.4), X-axis and Y-axis P-M interaction (decompression, balanced failure, and pure bending points per Cl 10.6.2), biaxial moment and axial interaction (Cl 10.6.3), shear capacity about each axis (Cl 8.2), and special confinement region design (Cl 10.7.3) when required.
Biaxial bending is checked using the interaction formula in AS 3600:2018 Cl 10.6.3. Uniaxial P-M capacities about each axis (phi × Mux and phi × Muy) are computed at the design axial load Pu, and the combined utilization is checked against the code-prescribed interaction relationship.
The calculator determines whether the column is short or slender based on the effective length ratio kL/r (Cl 10.4). For slender columns, the moment magnification method is applied to increase the design bending moment, accounting for second-order effects due to member curvature under axial load.
Yes, axial load reactions from beam calculations above link directly to this column. When the beam load changes, the column demand updates automatically. Column axial reactions can also link to a footing calculation below, keeping the full gravity load path connected without manual re-entry.
## Next steps
Design concrete pad footings to AS 3600:2018 (Amdt 2) with instant moment, shear and punching shear results.
Design rectangular concrete beams to AS 3600:2018 (Amdt 2) with unlimited spans. Checks flexure, shear, and short- and long-term deflection.
Calculate tension and compression development lengths for reinforcement to AS 3600:2018 (Amdt 2), with code references and modification factors.
Changes in AS3600:2018 Amendment 2 (Concrete)
## Method & scope
| Property | Detail |
| ---------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Design standards | AS3600:2018 (Amdt 2) |
| Regions | Australia & New Zealand |
| Who it's for | Australian structural and civil engineers designing circular reinforced concrete columns to AS 3600:2018. |
| Presets | Rectangular Column, Circular Column |
| What it checks | Section geometry and reinforcement, Axial squash and buckling capacity (AS 3600:2018, Cl 10.3 and 10.4), P-M interaction diagram (AS 3600:2018, Cl 10.6), Biaxial interaction check (AS 3600:2018, Cl 10.6.3), Shear capacity (AS 3600:2018, Cl 8.2) |
#### See the exact clause
* Cross-section height (D) must be specified in millimeters.
* Cross-section width (B) must be specified in millimeters.
* Total length of the column (L) must be specified in millimeters.
2\. Longitudinal Reinforcement
B. Additional Top / Bottom Reinforcement Bars (per side)
You can choose how many bars to add to the column to the top and bottom sides of the column, as illustrated in the graphic below in red, by entering the desired number of bars.
C. Additional Left / Right Reinforcement Bars (per side)
You can choose how many bars to add to the column to the left and right sides of the column, as illustrated in the graphic below in red, by entering the desired number of bars.
D. Concrete Cover to Fitments
Distance from concrete surface to ties. While concrete cover must in all cases be at least 20mm, the minimum cover requirement is often greater. See AS 3600:2018 - 2018, Cl 4.10 for specific requirements of minimum cover based upon exposure classification, concrete characteristic strength, and other factors.
3\. Fitment Reinforcement
The widget helps determine the configuration of fitment (stirrups or ties) reinforcement in concrete columns.
1. ### Fitment Type
You can choose the type of fitment reinforcement for the column, as illustrated in the graphic below, by selecting an option from the drop-down menu.
* The curved line on the graphs, known as the interaction curve or interaction diagram, represents the combined capacity of the column for both axial load and bending moment. Any point on this curve shows the maximum combinations of axial load and moment that the column can sustain without failing.
* If the point were on or outside the curve, it would suggest that the column’s load condition is at or exceeds its safe capacity, which would indicate potential structural risk.
