Section properties

• Introduction
• Centre of area (centroid)
• Second moment of area (I)
• Calculating the moment of inertia (I)
• Changing to significant figures
• Comparing beam A and beam B
• Section modulus (Z)
• Calculating the section modulus
• Radius of gyration (r)
• Calculating the radius of gyration

Calculating the moment of inertia (I)

For simple shapes such as squares, rectangles and circles, simple formulas have been worked out and the values must be calculated for each case.

Because millimetres are used, large numbers are generated in the calculation.

A standard method of denoting moment of inertia is to write the values as: number x 10⁶10⁶ is a compact way of writing 1,000,000 or 1 million. mm⁴.

The 10⁶ factor removes unwanted digits from the value.

The calculation is usually worked using four significant figures The number of significant figures is the number of digits used to represent the original number once it has been rounded to the given number of digits, eg 25.0658 is written as 25.07 when rounded to four significant digits. , so some rounding off is required and the decimal point may need to be moved to use the factor of 10⁶.

Example: 73,473,278 mm⁴ is written as 73.47 x 10⁶ mm⁴