Case Study: Historical Cast Iron Restoration for the Liepaja Tram Bridge with Reverse Engineering

Urban infrastructure projects often present complex challenges for engineers and craftsmen, especially when it comes to restoring historical sites. Restoration demands not only precision and authenticity but also the application of modern technologies to achieve durability and high quality.

One such exemplary project was the restoration of the cast iron railings for the Tram Bridge (Latvian – Tramvaju tilts) over the Trade Canal in the port city of Liepāja, Latvia. This bridge is a striking example of artistic and architectural casting and an integral part of a transport system unique to the Baltic States – the Liepāja tram, the oldest in the region, operating since 1899.

The bridge in its current form was opened in 1960, becoming a vital artery connecting Old and New Liepāja. Over time, its cast iron railings deteriorated due to heavy traffic and the harsh maritime climate. Our task was to recreate these elements, fully preserving their historical appearance while ensuring compliance with modern quality standards.

Stage 1: Preparation and Diagnostics

The initial condition of the original railing sections was disheartening. Decades of use, layers of old paint, and corrosion, exacerbated by the salty sea air, had concealed the true geometry and pattern details. To accurately assess their condition and prepare for further work, all surviving elements underwent thorough sandblasting. This process completely cleaned the surface of rust and buildup down to the bare metal, a critical step for subsequent precise measurement and scanning.

Stage 2: Reverse Engineering and 3D Modeling

The key stage of the project was creating an exact digital replica of the original railings. A combined approach was used for this:

• Manual Measurements: Each section and post was carefully measured by hand to determine its overall dimensions, pitch, and general proportions.
• 3D Scanning: To accurately reproduce complex decorative elements, such as the central ornament and intricate patterns, 3D scanning technology was employed. This allowed us to capture the finest details and irregularities inherent in historical castings.

Based on the collected data, solid 3D models of all railing components were created. The 3D model not only enabled a perfect recreation of the geometry but also allowed for an analysis of the future casting’s manufacturability, the addition of necessary draft angles and radii, and the final design approval from the client before production began.

Stage 3: Creating the Casting Tooling

After the 3D models were approved, the preparation for casting began. The project had a unique feature: while the railing sections shared the same height and pattern, they varied in length. Furthermore, the total number of castings was relatively small (up to 100 pieces).

To optimize costs and speed up production, an effective technological solution was adopted: creating a unified pattern plate. This tooling allowed for molding the main repeating pattern, while the length of the section was varied directly in the mold. This approach eliminated the need to produce individual patterns for each length, significantly reducing both the cost and the project timeline. Patterns for casting the railing posts were made separately.

Stage 4: Casting and Finishing

All railing elements were cast at a local machine-building foundry in Liepāja, which added a special symbolism to the project. Gray cast iron was chosen as the material – a traditional choice known for its excellent casting properties and durability.

The molding process was carried out using a no-bake sand molding process (chemically bonded sand) in large flasks, which were also custom-designed by our engineers for this specific order.

After casting and cleaning, all parts underwent final finishing. Considering the aggressive operational environment, a specialized protective coating resistant to sea air and moisture was applied to prevent corrosion. This ensures that the renovated railings will serve for many years to come, maintaining their aesthetic appeal.

Conclusion

The restoration project of the Tram Bridge railings in Liepāja is a prime example of how modern technologies like 3D scanning and modeling can be effectively integrated into traditional manufacturing processes such as cast iron founding. This approach made it possible not only to recreate the lost elements with historical accuracy but also to ensure their high quality and longevity.

Today, every resident and visitor to the beautiful city of Liepāja can admire the reconstructed bridge, which has once again become an adornment for the city and a testament to the careful preservation of its technical and cultural heritage.