Structures fail when the stresses generated by loads exceed what the structure, its material, or its geometry can safely resist. Failure may be sudden, such as fracture or buckling, or progressive, such as fatigue caused by repeated loading.
Main Causes of Structural Failure
| Cause | Mechanism of failure | Example |
|---|---|---|
| Overloading | A load produces stress beyond the material's elastic limit, yield strength, or ultimate strength. Stress is calculated using . | A shelf bends permanently because it carries more weight than its designed working load. |
| Material choice | The material lacks the required strength, stiffness, toughness, fatigue resistance, or environmental resistance. | A brittle material fractures under impact, while unprotected steel corrodes and loses effective cross-sectional area. |
| Size | Members or joints are too thin, short, slender, or small to carry the applied forces safely. | A thin column fails by buckling under compression even though its material has high compressive strength. |
| Shape | Poor geometry creates weak load paths, stress concentrations, bending moments, or instability. | A rectangular frame deforms under lateral loading, whereas triangulation increases rigidity. |
How These Factors Interact
Designers use a factor of safety so the failure load is greater than the expected working load:
For example, if a component fails at and its maximum working load is , its factor of safety is .
The common misconception is that simply making a structure larger always makes it safer. Increasing cross-sectional area can reduce stress, but it also increases material use, cost, and self-weight. Shape is often more efficient: I-beams place material away from the neutral axis, while arches transfer loads mainly through compression.
IB Exam Technique
For B3.2 Structural systems application and selection, do not merely identify a cause. Explain the load, resulting stress, failure mode, and suitable design improvement. For an evaluation question, balance structural performance against mass, cost, manufacturability, environmental exposure, and the required factor of safety.