Structural failure modes—such as buckling, lateral-torsional buckling, local buckling, shear buckling, and shell stability—differ in their underlying causes, behaviors, and the structural components they impact. Thorough understanding and identification of these stability issues are crucial for conducting accurate analyses and designing robust structures. This article will assist you in that endeavor by offering a detailed explanation of each failure type, including its characteristics, main causes, and key features. Finally, a comparison table will be included to summarize the differences between the failure types, making it easier for you to identify and distinguish them.
As you go through this article, keep in mind that all these failure modes can be analyzed using Dlubal’s Structural Stability add-on. By utilizing this tool, you gain a unique opportunity to address the challenges associated with these failure types through advanced Finite Element Analysis (FEA). Each failure mode discussed here will be accompanied by an example, allowing a deeper dive into the topic. Additionally, Dlubal provides a wide range of valuable resources, including Knowledge Base articles, FAQs, webinars, and more, all available in the relevant section of Dlubal’s website to further enhance your understanding of the subject.
Flexural Buckling
Flexural buckling is a form of global instability that occurs in a structural member subjected to axial compression, causing the member to bend or "buckle" laterally due to the compressive force. This phenomenon happens when the critical load, beyond which the member loses its stability under compression, is exceeded. Buckling is most common in slender columns or members with a high length-to-radius of gyration ratio. This behavior is typically governed by Euler's buckling formula for elastic materials, but it can involve either elastic or inelastic buckling, depending on the material properties and geometry of the member.
Lateral-Torsional Buckling
Lateral-Torsional Buckling is an instability that occurs in beams subjected to bending, resulting in both lateral displacement and twisting. The primary cause of this phenomenon is compression in the top flange, coupled with inadequate lateral support. It is observed most often in flexural members such as beams and girders, where moment-induced stress plays a significant role. The occurrence of lateral-torsional buckling is influenced by factors such as unbraced length, cross-sectional shape, and moment gradient. It is typically analyzed using the elastic critical moment for lateral-torsional buckling.
Local Buckling
Local buckling is related to the failure of individual plate elements (e.g., flanges, webs) within a cross-section, without causing global instability of the entire member. The primary cause of local buckling is localized compressive stress that exceeds the critical buckling stress of the plate element. This type of buckling is prevalent in thin-walled sections like I-beams, box girders, and cold-formed steel members. It is important to consider because it affects the strength and stiffness of the members, leading to a potential reduction in load-carrying capacity.
Shear Buckling
Shear buckling refers to a type of structural instability that occurs in a member subjected to shear forces, causing the material to deform or "buckle" laterally. Shear buckling happens when the applied shear force exceeds the critical threshold, causing the structure to deform laterally or out of plane. This is particularly likely to occur when the member is thin and lacks adequate lateral support to resist the shear force. Hence, shear buckling is often observed in elements like plates, webs of I-beams, or other slender structures under shear stresses. The critical parameter governing shear buckling is the shear buckling stress, which is influenced by factors such as the element’s thickness, aspect ratio, boundary conditions, and material properties.
Shell Buckling
Shell buckling is the loss of stability in thin, curved structures (shells), such as cylindrical, spherical, or conical shapes (for example, tanks, silos, pipelines), when exposed to compressive or lateral loads. It occurs when these loads cause the shell to deform, reducing its ability to carry further load, which can lead to significant distortions or even collapse. The primary cause of shell buckling is a non-uniform distribution of stresses, typically resulting from axial forces, shear stresses, or external pressure.
Comparison Table
Now that each failure type has been defined and its characteristics outlined, the key differences between them are summarized in Table 1. This table provides a concise overview of each mode, highlighting the main structural element affected, the primary load conditions that lead to failure, the resulting deformation, and the critical factor that governs the mode.
| Failure Type | Main Structural Element | Primary Load | Deformation Mode | Critical Factor |
|---|---|---|---|---|
| Lateral-Torsional Buckling | Beams | Bending moments | Lateral displacement + twisting | Unbraced length |
| Shear Buckling | Plates | Shear forces | Diagonal buckling/wrinkling | Plate thickness |
| Local Buckling | Plate elements (e.g., flanges) | Local compressive stresses | Out-of-plane deformation | Plate slenderness |
| Shell Stability | Curved/shell structures | Axial, pressure, or shear | Complex buckling patterns | Imperfections, curvature |
Final Words
In structural design, several common stability issues can emerge, especially when they are not properly considered in the design process. This article provides an overview of five such issues: buckling, lateral-torsional buckling, local buckling, shear buckling, and shell stability. The information provided aims to help you understand their underlying causes, behaviors, and the structural elements they affect. This knowledge will enable you to identify and distinguish these stability concerns, giving you a solid foundation for integrating them into your analyses and designing resilient and safe structures.