## Properties of Circular Cross-Section

This tool calculates the properties of a circular cross-section. Enter the shape dimensions R or D below. The calculated results will have the same units as your input. Please use consistent units for any input.

 R = or... D = Geometric properties: Area = Perimeter = Properties related to any centroidal axis: I = S = Z = Rgx = Iz =

## Definitions

### Geometry

The area A and the perimeter P of a circular cross-section, having radius R, are found with the next formulas:

### Moment of Inertia

The moment of inertia (second moment of area) of a circular section in respect to any axis passing through its centroid, is given by the following expression:

where R the radius of the section.

The moment of inertia (second moment or area) is used in beam theory to describe the rigidity of a beam against flexure. The bending moment M applied to a cross-section is related with its moment of inertia with the following equation:

where E is the Young's modulus, a property of the material, and κ the curvature of the beam due to the applied load. Therefore, it can be seen from the former equation, that when a certain bending moment M is applied to a beam cross-section, the developed curvature is reversely proportional to the moment of inertia I.

The polar moment of inertia, describes the rigidity of a cross-section against torsional moment, likewise the planar moments of inertia described above, are related to flexural bending. The calculation of the polar moment of inertia Izabout an axis z-z (perpendicular to the section), can be done with the Perpendicular Axes Theorem:

where the Ixand Iyare the moments of inertia about axes x-x and y-y that are mutually perpendicular with z-z and meet at a common origin. Because for a circular section Ix=Iy, the above equation becomes:

The dimensions of moment of inertia are .

### Elastic section modulus

The elastic section modulus Sxof any cross section about axis x-x (centroidal), describes the response of the section under elastic flexural bending. It is defined as:

where Ixthe moment of inertia of the section about x-x axis and Y the distance from centroid of a section point (aka fiber, typically the most distant one), measured perpendicularly to x-x axis. Application of the above formula to the circular cross-section, gives its elastic section modulus about any axis, taking into consideration that Y=R:

If a bending moment Mxis applied on axis x-x, the section will respond with normal stresses, varying linearly with the distance from the neutral axis (which under elastic regime coincides to the centroidal x-x axis). Along neutral axis the stresses are zero. Absolute maximum σ will occur at the most distant fiber, with magnitude given by the formula:

From the last equation, the section modulus can be considered for flexural bending, a property analogous to cross-sectional A, for axial loading. For the latter, the normal stress is F/A.

The dimensions of section modulus are .

### Plastic section modulus

The plastic section modulus is similar to the elastic one, but defined with the assumption of full plastic yielding of the cross section due to flexural bending. In that case the whole section is divided in two parts, one in tension and one in compression, each under uniform stress field. For materials with equal tensile and compressive yield stresses, this leads to the division of the section into two equal areas, Atin tension and Acin compression, separated by the neutral axis. The axis is called plastic neutral axis, and for non-symmetric sections, isn't the same with the elastic neutral axis (which again is the centroidal one). The plastic section modulus is given by the general formula:

where Ycthe distance of the centroid of the compressive area Acfrom the plastic neutral axis and Ytthe respective distance of the centroid of the tensile area At. For the case of a circular cross-section, the plastic neutral axis passes through centroid, dividing the whole area into two equal half-circles. The centroid of each one, is located at from the plastic neutral axis and the application of the above equation gives the following formula for the plastic section modulus, under any axis of bending:

### Radius of gyration

Radius of gyration Rgof a cross-section, relative to an axis, is given by the formula:

where I the moment of inertia of the cross-section about the same axis and A its area. The dimensions of radius of gyration are . It describes how far from centroid the area is distributed. Small radius indicates a more compact cross-section. For a circular section substitution to the above expression gives the following radius of gyration, about any axis:

Circle is the shape with minimum radius of gyration, compared to any other section with the same area A.