Study Reveals Hidden Defenses in Steel Truss Bridges

Steel truss bridges, essential components of transport networks since the late 19th century, possess unexpected defenses that can prevent catastrophic collapses. A recent study published in the journal Nature on September 3, 2023, by researchers from Spain, sheds light on how these structures can withstand damage without immediate failure.

These bridges, built from interconnected steel bars, are designed to span long distances and support heavy loads, making them ideal for railways and highways. Notable examples include the Pamban, Howrah, and Saraighat bridges in India. Despite their strength, many of these structures face increasing challenges, such as higher traffic loads, extreme weather events, and accelerated material degradation due to climate change.

When a component of a truss bridge fails, the consequences can be dire. A sudden collapse can lead to human tragedies and significant economic impacts, with closures costing millions of rupees daily. While engineers understand how intact components manage regular loads, the reasons behind the resilience of some bridges after a failure have remained unclear until now.

To investigate this phenomenon, the research team constructed a scaled-down steel truss bridge based on a typical railway design known as the Pratt truss. They systematically simulated damage by severing specific components, such as chords and beams, to replicate sudden failures. Throughout these experiments, sensors monitored the structure’s response, while advanced computer models allowed the team to simulate over 200 different damage scenarios.

The findings revealed six crucial secondary resistance mechanisms that activate when a primary component fails. These mechanisms include panel distortions, torsion of the entire structure, hinged rotations, out-of-plane bending, bridging by nearby members, and uniaxial bending. Each of these adaptations helps reroute loads through alternative paths, similar to how a spider web adjusts to the loss of a thread. The dominant mechanism depends on the specific component that fails; for instance, losing a diagonal primarily triggers panel distortions, while the failure of a chord results in global torsion and rotation.

Remarkably, even when damaged, the bridge specimens could endure loads up to three times greater than their standard operational limits before collapsing. The nature of the failures varied based on the role of the original component. Compression-sustaining members, like upper chords, tended to lead to brittle failures, while tension-bearing members, such as lower chords, resulted in more gradual and ductile failures. In every scenario, the bridge ultimately collapsed only after a cascade of buckling failures spread throughout the structure.

These insights could significantly influence engineering practices. By understanding these secondary resistance mechanisms, engineers can refine designs for new bridges, enhancing their ability to withstand damage. Additionally, for existing structures, inspections and retrofits can focus on critical areas that help activate these “secret” defenses.

The study provides a comprehensive roadmap for increasing bridge resilience against accidents, natural disasters, and the passage of time. This knowledge not only promises to improve safety but could also lead to substantial cost savings associated with bridge maintenance and repairs.