Bearing the Load: UIC Leaflet 702 Structure Design Diagrams

Introduction to UIC Leaflet 702

Before a single sleeper is laid, the ground beneath it—specifically bridges, viaducts, and earthworks—must be proven to support the immense weight of a moving train. UIC Leaflet 702, titled “Static loading diagrams to be taken into consideration for the calculation of rail carrying structures,” establishes the theoretical “worst-case” load scenarios used by civil engineers.

This standard provides the mathematical models (Load Models) representing standard traffic and heavy freight. It ensures that a bridge built in 1980 or 2020 can safely support not just today’s trains, but the heavier trains of the future.

Snippet Definition: What is UIC 702?

UIC Leaflet 702 is a technical standard that defines the static loading diagrams (such as Load Model 71) used for the structural analysis of railway bridges and infrastructure. It specifies the magnitude and positioning of vertical forces (axle loads and distributed loads), as well as horizontal forces (braking, centrifugal), that structures must withstand on international lines.

The Core Concept: Load Model 71 (LM71)

The heart of UIC 702 is Load Model 71. This is not a drawing of a real train, but a “fictional” envelope that creates stress greater than any normal operational train. It consists of:

• Four Axle Loads ($Q_{vk}$): Four point forces of 250 kN (approx. 25 tons) each, spaced 1.6 meters apart.
• Distributed Load ($q_{vk}$): A continuous load of 80 kN/m (approx. 8 tons/meter) extending indefinitely on both sides of the axles.

If a bridge is designed to survive LM71 multiplied by a safety factor (Dynamic Factor $\Phi$), it is deemed safe for standard international traffic (Line Category D4).

Heavy Load Models (SW/0 and SW/2)

For lines designated for heavy industry or military transport, UIC 702 defines heavier models:

• SW/0: Simulates continuous heavy freight wagons (133 kN/m).
• SW/2: Simulates heavy rail-mounted artillery or special transporters (150 kN/m).

Comparison: UIC 702 vs. Eurocode 1 (EN 1991-2)

UIC 702 is the ancestor of the modern European structural code.

Dynamic Effects and Centrifugal Force

A train crossing a bridge isn’t just a static weight; it bounces and sways. UIC 702 mandates the application of a Dynamic Factor ($\Phi$). This is a multiplier (typically between 1.0 and 1.67) applied to the static load to account for vibrations and track irregularities.

Additionally, on curved bridges, the Centrifugal Force must be calculated and applied horizontally to the bridge piers, ensuring the structure doesn’t twist or slide sideways under high-speed passage.