What are the specific mechanical properties that make hollow steel sections more resistant to bending and torsional loads?

Hollow steel sections (HSS) exhibit superior resistance to bending and torsional loads due to several mechanical properties:

1. Moment of Inertia:

The moment of inertia, denoted as ‘I’ in mechanics, is a measure of an object’s resistance to changes to its rotation. It is dependent on both the mass of the object and its distribution of mass around the axis of rotation. For a structural element, the axis of rotation is typically its neutral (centroidal) axis. In HSS, more material is located further away from this neutral axis compared to a solid beam of the same material and weight. This greater distance increases the moment of inertia, giving HSS better resistance to bending.

2. Section Modulus:

The section modulus, often symbolized as ‘S’, is another property that is indicative of a material’s strength against bending. It is defined as the moment of inertia divided by the distance from the neutral axis to the outer surface of the shape. Like the moment of inertia, the section modulus for HSS is larger than for a solid beam of the same material and weight, providing superior resistance to bending.

3. Torsional Constant:

The torsional constant, also known as the torsion constant, and represented as ‘J’, is a property that describes an object’s resistance to torsional deformation. For a circular HSS, the torsional constant is higher than for a solid circular bar of the same material and weight, providing superior resistance to torsion.

These mechanical properties are all influenced by the shape and distribution of material in a structural element. A hollow section places more material further from the neutral axis, which increases these properties and therefore improves resistance to bending and torsion. This is why, for the same weight, HSS can often be stronger than solid steel.