In engineering and structural design, the selection of sections’ shapes plays a critical role in determining how a component or structure will behave under various types of loads. When it comes to torsion, which is the twisting of an object due to an applied torque, choosing Rectangular Hollow Section (RHS), or Hollow Structural Section (HSS), or simply hollow tubes would be a great solution to take care of torsion forces. In this article, we’ll explore the reasons why RHS/hollow tubes are a great choice for torsion.

Geometric Efficiency of Hollow Section

Rectangular Hollow Sections are precisely what their name suggests – hollow tubes with a rectangular cross-section. This geometric shape provides a favorable combination of properties for torsion. Unlike solid sections or other shapes, the hollow nature of RHS results in a higher moment of inertia for the same amount of material, allowing for efficient distribution of stress during torsion. This increased moment of inertia enables the hollow tube to resist twisting and torsional deformation more effectively.

Let’s say, this is a beam. The shape of the cross-section can be anything, not necessarily a rectangular or circular section. If torsion is applied at the end of the beam like in the above picture, what will be the stress distribution? 

Yes, you guessed right. The maximum stress will occur at the outer-most point of this cross-section from the centroid and the minimum stress is at the centroid. If you consider another axis, the maximum stress point will be the same, the outermost point. In other words, maximum stress is at the periphery of the section.

Now if you are designing this member, you need to consider what are the shapes available for the design. As the maximum stress occurs at the periphery. Therefore, you need to keep the maximum material at the outer edge.

Let’s consider an I section. The maximum material is located at the outermost point of the section about the major axis.

But what about the minor axis? Yes, you got it. The is no material at the outermost point about the minor axis to carry the stress. Therefore, the I section is not stable under torsion.

So, what is the solution? The Box section, Yes! As the box section has the maximum material over its periphery, it can carry the applied torsion, unlike the I section. This also applies to circular hollow sections.

Therefore, the Structural Hollow Section (HSS). Especially RHS and Hollow tube is an excellent solution for torsion design.

Weight Efficiency

One thing that can be in your mind is that solid rectangular and circular sections also have materials in their periphery. So, why not a solid section?

The answer is simple. The solid section will increase the weight hence the cost of the project. One of the significant advantages of using RHS or hollow tubes for torsion is their weight efficiency. Due to their hollow nature, they have a lower material volume compared to solid sections of the same overall dimensions. This reduced weight can be cost-effective but also easier to transport and assemble, which makes the project economical.