The Low-Profile Solution: UIC Leaflet 773 Bridge Design

Introduction to UIC Leaflet 773

In railway infrastructure, engineers often face a difficult geometric problem: building a bridge under an existing road or over a canal where there is almost no vertical space available. Standard concrete beams are too tall, and open steel girders require deep ballast trays. The solution is the “Joist-in-Concrete” (or Filler Beam) deck. UIC Leaflet 773, titled “Recommendations for the design of joist-in-concrete railway bridges,” provides the engineering rulebook for this unique composite structure.

This design involves placing rolled steel I-beams side-by-side and encasing them completely in concrete. The result is an incredibly robust, shallow deck that can support heavy train loads with a structural thickness of less than 1 meter.

Snippet Definition: What is UIC 773?

UIC Leaflet 773 is a technical guideline for the design and calculation of Joist-in-Concrete (filler beam) railway bridge decks. It specifies how to calculate the composite action between the steel profiles and the surrounding concrete without the need for mechanical shear connectors (studs), relying instead on bond and friction. It is widely used for short-to-medium span “underbridges” where construction depth is strictly limited.

The “Filler Beam” Concept

The structure defined in UIC 773 differs from standard reinforced concrete or steel composite bridges.

• Structural Core: The primary load is carried by a series of steel I-beams (e.g., HEB or HEM profiles) spaced closely together (typically 50-80 cm apart).
• Encasing: These beams are fully poured into a concrete slab. The concrete protects the steel from corrosion (fire and rust) and prevents the beams from buckling.
• Composite Action: Unlike modern composite bridges that use welded studs to join steel and concrete, UIC 773 bridges often rely on the natural bond and the “cage” effect of the transverse reinforcement passing through the web of the steel beams.

Design & Calculation Principles

UIC 773 outlines specific checks to ensure safety:

1. Construction Phase vs. Final Phase

The steel beams must be strong enough to support the weight of the wet concrete during construction (acting as their own formwork support). Once the concrete hardens, the entire section acts as a single composite unit to support the train traffic.

2. Bond and Shear

To prevent the steel beams from slipping inside the concrete, UIC 773 mandates:

• Lateral Reinforcement: Steel bars must pass through holes drilled in the webs of the I-beams (or pass closely under/over them) to lock the concrete core together.
• Surface Condition: The steel beams must be free of oil or loose mill scale to ensure adhesion.

Comparison: UIC 773 vs. Eurocode 4

While UIC 773 sets the historic best practice, Eurocode 4 is the modern regulatory standard.

Operational Advantages

Why choose a UIC 773 design?

• Shallow Depth: It offers the highest strength-to-depth ratio. You can fit a bridge carrying a 25-ton axle load train into a deck height of just 60-80 cm.
• Damping: The massive concrete volume provides excellent noise and vibration damping compared to an open steel bridge.
• Maintenance: Since the structural steel is encased, repainting is not required (except for exposed bottom flanges if not fully encased), reducing lifecycle costs.