One of the most challenging aspects of structural design, in my opinion, is ensuring that all relevant checks are accounted for. How many checks are necessary to confirm that a structural member can safely resist the imposed loads? The type of loads and their resulting effects often guide this process. For example, a member subjected to axial, shear, and bending loads will require significantly more design checks than a member experiencing only axial forces.
This thought process brought me to the topic of lateral-torsional buckling versus torsion. While working on the design of steel members using Prokon’s Combine module—commonly used for analyzing members under combined stresses—I noticed something intriguing. According to SANS 10162-1:2011, members are evaluated using several critical checks, including:
- Cross-sectional strength
- Overall member strength
- Lateral torsional buckling strength
- Additional check for different section classes
- Shear strength
- Slenderness ratio
Notably, these checks do not explicitly address torsion load effects. This observation raised an important question: What is the fundamental difference between lateral-torsional buckling and torsion?
As defined by IDEA StatiCa, “Lateral-torsional buckling is the deformation of an unrestrained beam due to applied loads that cause displacement and twisting away from its longitudinal axis.”(https://www.ideastatica.com/blog/lateral-torsional-buckling). Simply put, it is a stability issue that arises when a beam is inadequately supported laterally, causing it to twist and buckle under applied loads.
Torsion, however, is a separate phenomenon. It refers to the twisting of a structural member due to an applied torque or moment, independent of axial, shear, or bending forces. The design checks for torsion typically involve calculating the resultant shear stress caused by the twisting action and comparing it to the member’s allowable shear stress. This allowable stress is based on the material’s yield strength. Additional torsion-related checks may include considerations for warping torsion—commonly observed in “open” cross-sections like I-beams—and St. Venant torsion, which is prevalent in closed or symmetric sections.
In SANS 10162-1:2011, torsion design often involves torque-moment interaction diagrams. These diagrams, I presume, are specific to various profile types and assist in determining how torque interacts with bending and axial stresses. The complexity of torsion analysis lies in accurately accounting for these interactions, which can significantly impact the member’s overall strength and stability.
Given this complexity, a common approach is to design torsion out of your model whenever possible. This proactive strategy is effective in both steel and concrete structures and is often the preferred solution for simplifying the design process. By eliminating the need to address torsional effects directly, you reduce the number of checks and streamline the workflow.
Nevertheless, torsion remains a critical consideration for certain designs, especially where twisting forces are unavoidable. As such, this is far from the last time this topic will be explored. Its implications on design workflows and member performance ensure it will continue to be a significant subject of discussion in structural engineering.
Explore our blog series on calculating the Effective Width for various sections of a composite beam system. This series systematically builds on the topic, addressing increasing levels of complexity as the design challenges unfold.
