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Calculating the Moment of Inertia of a Beam Section

How to Calculate Moment of Inertia of a Beam Section
(Second Moment of Area)

Before we find the moment of inertia of a beam section (or second moment of area of a beam section) , its centroid (or center of mass) must be known. For instance, if the moment of inertia of the section about its horizontal (XX) axis was required then the vertical (y) centroid would be needed first (Please view our Tutorial on how to calculate the Centroid of a Beam Section).

Before we start, if you were looking for our Free Moment of Inertia Calculator please click the link to learn more. This will calculate the centroid, moi, and other results and even show you the step-by-step calculations! But for now, let’s look at a step-by-step guide and example of how to calculate the moment of inertia:

Step 1: Segment the beam section into parts

When calculating the area moment of inertia, we must calculate the moment of inertia of smaller segments. Try to break them into simple rectangular sections. For instance, consider the I-beam section below, which was also featured in our Centroid Tutorial. We have chosen to split this section into 3 rectangular segments:

SkyCiv, I-Beam, Moment of Inertia of a beam, how to calculate moment of inertia

Step 2: Calculate the Neutral Axis (NA)

The Neutral Axis (NA) or the horizontal XX axis is located at the centroid or center of mass. In our Centroid Tutorial, the centroid of this section was previously found to be 216.29 mm from the bottom of the section.

Try out our free Moment of Inertia Calculator:

Step 3: Calculate Moment of Inertia

To calculate the total moment of inertia of the section we need to use the “Parallel Axis Theorem”:

Calculating the Moment of Inertia of a Beam Section- 1, how to calculate moment of inertia

Since we have split it into three rectangular parts, we must calculate the moment of inertia of each of these sections. It is widely known that the moment of inertia equation of a rectangle about its centroid axis is simply:

Calculating the Moment of Inertia of a Beam Section- 2, how to calculate moment of inertia

The moment of inertia of other shapes is often stated in the front/back of textbooks or from this guide of the moment of inertia shapes. However the rectangular shape is very common for beam sections, so it is probably worth memorizing.

Now we have all the information we need to use the “Parallel Axis Theorem” and find the total moment of inertia of the I-beam section. In our moment of inertia example:

Calculating the Moment of Inertia of a Beam Section- 3, how to calculate moment of inertia

So there you have our guide on calculating the area of moment for beam sections. This result is critical in structural engineering and is an important factor in the deflection of a beam. We hope you enjoyed the tutorial and look forward to any comments you have. (Check out moment of inertia formula)

BONUS: Using our Moment of Inertia Calculator

There are many ways in terms of how to calculate moment of inertia, one of them is using software to make the process easier.

SkyCiv’s Account shows full calculations of the moment of inertia. This interactive module will show you the step-by-step calculations of how to find the moment of inertia:

SkyCiv, Section Builder, Hand Calculations, Moment of Inertia, how to calculate moment of inertia
Alternatively, you can see the results of our Free Moment of Inertia Calculator to check your work. This will calculate all the properties of your cross-section and is a useful reference to calculate the Centroid, Area, and Moment of Inertia of your beam sections!

SkyCiv Moment of Inertia Free Calculator, how to calculate moment of inertia

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