Defining the Limit: Permissible Stresses in UIC 577 Chapter 5

UIC Leaflet 577, titled “Wagons – Stresses,” acts as the structural calculation bible for legacy freight wagon design. Chapter 5, specifically titled “Permissible Stresses,” defines the maximum allowable stress levels that the wagon’s steel structure can experience under various load conditions. It establishes the “Safety Factor” ($S$), ensuring that the calculated stress never exceeds the material’s yield point during exceptional impacts or its endurance limit during daily operation.

Defining the Safety Margins

The core philosophy of Chapter 5 is that a wagon must be lightweight enough to carry cargo but strong enough not to break. To achieve this balance, it assigns permissible stress values ($\sigma_{perm}$) based on the material’s mechanical properties. The engineer must demonstrate that the calculated stress ($\sigma_{calc}$) in every beam and joint is lower than this permissible limit.

Key Stress Criteria

• Static Limit (Yielding): For rare, high-load events (like buffer impacts), the stress must not cause permanent deformation. The permissible stress is typically set at the Yield Strength ($R_{eH}$) divided by a safety factor (often 1.0 or 1.1).
• Dynamic Limit (Fatigue): For repetitive loads (running on track), the permissible stress is much lower. It is derived from the material’s fatigue strength, adjusted for the type of welded joint (Notch Class).
• Shear Stress: Chapter 5 usually limits permissible shear stress ($\tau$) to roughly 58% ($1/\sqrt{3}$) of the permissible normal stress, following the Von Mises yield criterion.

Permissible Stress Ratios: A Guide

Chapter 5 differentiates between the base material (parent metal) and the welded connections, which are the weak points. The table below summarizes typical permissible limits relative to the material’s Yield Strength ($R_{eH}$):

Relation to Modern Standards (EN 12663)

While UIC 577 Chapter 5 was the standard for decades, modern European wagon design has largely transitioned to EN 12663-2. However, the logic remains identical. UIC 577 is still crucial for the maintenance of older fleets and for understanding the safety margins of wagons built before the 2010s. It teaches that “Safety” is not a fixed number, but a ratio between the Load applied and the Material’s capability.